AC Motor Control Basics

 An AC motor controller is known as a device that controls the speed of the AC motor. An AC controller can also be referred to as a variable frequency drive, adjustable speed drive, frequency converter and more. The AC motor receives power, which the AC motor controller converts into an adjustable frequency. This adjustable output allows the motor speed to be precisely controlled.
Typically, an AC motor controller consists of three basic parts: the rectifier, inverter, and the DC link to connect the rectifier and inverter. The rectifier converts AC input into DC (direct current), while the inverter switches the DC voltage to an adjustable frequency AC output voltage. The inverter can also be used to control output current flow if needed. Both the rectifier and inverter are directed by a set of controls to generate a specific amount of AC voltage and frequency to match the AC motor system at any given point in time. 

Figure 1 -Motor control speed Diagram

 What are AC Motor Controllers?

 The operation of AC motors is explained thoroughly in our article on induction motors and is a good review on how AC motors convert electromagnetism into useful mechanical energy.
An important note is that the speed of an AC motor is dictated by its AC supply frequency, as the alternating current in its stator coils generates a rotating magnetic field (RMF) which electromagnetically induces an opposing magnetic field in the rotor, and produces output rotation. Also note that in asynchronous models, the phenomenon of slip exists, which is a discrepancy between the input frequency and the output rotational frequency. It makes intuitive sense that, if some kind of device can either control the AC signal frequency, or change the slip between the rotor and the AC frequency, then speed control can be attained.
This is where AC motor controllers come into play; these “motor drives” alter the frequency of the electrical supply to the motor and allow operators to choose the frequency, and therefore the resulting speed of the motor. These drives are often called inverters, variable frequency drives (VFDs), or adjustable speed drives (ASD), and they allow speed control by converting the AC current to DC current (known as a “rectified” AC current) and imitating/simulating the AC frequency to the motor. This way, the rectified AC current can be set to any desired frequency, which changes the effective motor speed.

How do AC Motor Controllers work?

As previously stated, AC controllers convert an input AC frequency into a DC current that simulates an AC frequency, which can then be set by the user, thus controlling the speed of the motor. This is accomplished using three main components: a rectifier, an inverter, and a DC circuit that connects the two.
The rectifier is directly connected to the AC electrical supply and is the component which switches, or “rectifies”, the AC current to DC current. The connecting DC circuit then stores this energy (usually via high power capacitors) for the inverter component to use. The inverter is powered by the connecting DC circuit and modulates the rectified frequency to the motor, effectively changing the speed of the motor. Figure 1 above visualizes this explanation via a basic circuit diagram, and shows the transfer of frequencies between components.

There are also methods for increasing the slip of an AC motor, which will change the speed of rotation in relation to the AC frequency, thus controlling speed. Increasing the slip can be done by either varying the resistance through the motor coils (variable series resistance, see our article wound rotor motors), or varying the voltage to the coils (variable voltage transformers); however, AC drives are more commonplace, as they are more modular and have been engineered to give the user precise digital control over their motor without using integrated components such as resistors or rheostats in the motor windings.

Types of AC motor controllers

There are some common ways in which AC drives take incoming AC current and transform it into a simulated, controllable frequency. In each method, the goal is to allow operators to input frequency commands to the stator RMF, and as such change the rotation speed of the rotor. There are three main methods for doing this: variable voltage inversion (VVI), pulse-width-modulation (PWM), and flux vector drives.

Variable Voltage Inversion

In this method, the AC frequency of the power source is rectified to a DC current, which is then increased and decreased in discrete steps to imitate a sine wave (or how a true AC current oscillates). This way, operators can regulate these steps to effectively change the motor speed, and these are commonly referred to as six-step inverters (though different steps exist). Figure 2 below shows an example of how a VVI controller sends stepwise power to a motor, simulating a true sine wave:

Figure 2 - Voltage over time graph representing how VVIs allow speed control in AC motors

Note: Notice that, if more steps were added, it would give a better approximation to the true sine wave.

Pulse-Width-Modulation

Pulse width modulator circuits, or PWMs, are a popular method to simulate AC oscillations, as they often provide more precise control than variable voltage inverters. They do so by rapidly switching the current (or “pulsing” it) to match the area under the curve of the true sine wave. Recall from calculus that the integrals of any two continuous graphs are equal if their areas under their curves are equal; the goal of PWM is to more accurately approximate the area of a sine wave using many pulses of voltage, and the density of these pulses will dictate the size of the simulated sine wave (and thus change the motor speed). Figure 3 visualizes these spikes on a voltage over time graph:

Figure 3 - PWM graph compared to a true sine wave

Note: Notice how the pulses change frequency as they move along the sine wave, but do so in an orderly, equidistant fashion.

Vector flux

Flux vector drives not only vary the frequency of the current to change the speed of the motor but can also change the motor’s torque as well. They can do this by taking advantage of the motor’s magnetic flux (the number of magnetic field lines passing through a surface, in this case, the coils) as a portion of the current in all induction motors must generate the magnetic coupling between the rotor and stator. The remaining current is the portion that generates torque, and flux vector drives hold the coupling current to a minimum while allowing operators to adjust this torque-producing current. This is easier said than done, as controlling both torque and flux current is analytically intensive, and requires continuous transformations between coordinate systems. Flux vector drives, therefore, require microprocessor-based controllers, software, and oftentimes encoded sensors to precisely tune the independent currents. These drives typically provide speed accuracy of up to 0.3%, and while more impressive than the other two options, comes at a heavier installation and operational cost.

Applications and Selection Criteria

AC motor controllers have upgraded traditional AC motors, and have allowed them to serve functions in even more areas of industry. One of the most common areas where AC drives are implemented is in HVAC systems, specifically in fans, pumps, and blowers. Designers can optimize the airflow through a building by electrically signaling these components to quickly increase or decrease their speeds, conditioning the building with more efficiency, and saving the owner money on such costs. They also save manufacturers from excess wear and tear on their motors, as AC drives regulate the inrush of starting current, which can damage the motor over repeated cycles of successive starting and stopping. They pair well with integrated motor systems, and anyone looking to control many motors at once should seriously consider AC motor controllers.
However, before buying one of these devices, your specifications should be detailed. Below are some basic questions that designers should consider. Note that these questions are great to bring to suppliers, as they will have the most up-to-date information on what kind of AC motor controllers are available to you:

1. At how many speeds will the motor be run, or will it be constantly changing?
2. Can the machine tolerate the stress of a large inrush current/voltage?
3. What is the safe speed range of the motor, and how accurate should speed control be?
4. Would control via digital systems be useful? If not, how many input/output (I/O) points will be needed to control the motor without digital communication systems?
5. Does your motor require special programming functions, other than simple speed and direction commands?
6. Consider cost: How many AC drives will you need, and how much will they cost to both purchase, install, and maintain?

Taking some time to seriously consider these questions will not only help you to find a better suited AC drive but also to optimize its performance at a reasonable cost.

 STAR – DELTA Connection

Difference Between Star And Delta Connections

STAR – DELTA Starter

The Star Delta starter is the most basic starting method for minimizing the inrush starting current of an induction motor. For typical operation, the starter can be utilized with all squirrel-cage induction motors that are delta connected.

The decrease of the high motor current reduces the motor’s starting torque. Star-delta starting is thus particularly ideal for drives that are not loaded until after starting. The starting time is longer than with direct starting, which is noticeable when driving higher inertial loads. For more details see Figure 3 below

Figure 3 - Star-Delta Connection including devices

To limit starting current, a reduced voltage starter is required to start a large motor. The star delta starter is a lower voltage starter that applies 1/√3 voltage during the starting process.

Star Delta Starter Components

The following are the primary components of a wye-delta starter:

1. Contactors
2. Overload Relay
3. Timer
4.  The fuse unit
5. MCB
6. Push-button Start (NO)
7. Push button Stop (NC)

Figure 4 - Components Installation

Contactors

A contactor is a high-current-rated heavy-duty relay used to power the induction motor. Contactors have current ratings ranging from 10A to several hundred amps.
Usually, most often use three contactors in wye-delta starters, which are as follows:

• The primary contactor
• Delta contactor
• Star contactor

The main contactor & delta contactor are AC3 type and rated at 58% of the motor’s current rating. During the operation of the induction motor, these contactors are closed.

Figure 5 - Contactor

A star contactor, on the other side, only carries star current when the motor is turned on. Because the starting current is 1/3 the rated current, the star contactor can be of the AC3 type and 33% of the full load current rating.

Because these contactors only control winding current, which is 33% of the full load current in the star delta starter, the three contractors used in the star delta starter are smaller than the single contactor used in the DOL starter.

Overload Relay

As with induction motors, the majority of winding failures are caused by overloading, operating on an unbalanced voltage supply, and single phasing owing to phase loss, which can lead to excessive heating and motor winding insulation degradation.

Figure 6 - Overload Relay

To prevent windings against overloading and short-circuiting in the internal winding, overload protection is required. As a result, the 3-pole thermal overload relay prevents all of these circumstances.

Timer

The timer in the wye-delta starter’s fundamental function is to switch the contactor from star to delta when the motor reaches almost 80% of full load speed.

Figure 7 - Omron Timer

The fuse unit

Three fuses are connected in series with the motor circuit to protect it against external overcurrent and short circuit faults. In addition, one fuse is used to protect the wye-delta starter’s control circuit.

MCB

A circuit breaker is an electrical switch that operates automatically to protect an electrical circuit from harm caused by excess current from an overload/short circuit. Its primary function is to interrupt current flow when a problem is identified. In addition a fuse, which can only be used once before needing to be replaced, a circuit breaker can be reset to resume regular operation.
In the star delta starter motor circuit, an MCB is utilised to protect the motor from short circuit circumstances and to prevent motor winding damage.

Push-button Start (NO)

This is an open (NO) type push button that is used to start the motor.

Push button Stop (NC)

This is an NC type push button for stopping the motor.

Star-Delta Starter Circuit Diagram and Working Principle 

The diagram illustrates the connection of a three-phase induction motor with a star-delta starter. The star-delta starter is a common type of starter that is widely used to start squirrel cage induction motors. It is used to start a squirrel cage induction motor that is meant to function normally on a stator winding that is delta linked.

 Figure 8 - Star-Delta Starter Circuit Diagram

The stator windings are connected in star when the switch S is in the START position. When the motor reaches 80% of its rated speed, the changeover switch S is thrown to the RUN position, connecting the stator windings in the delta.

 Figure 9 - Classic Circuit Diagram

The line current generated by the motor at starting is decreased to one-third of the starting current with the windings linked in delta by connecting the stator windings first in star and then in delta. When the stator windings are connected in a star at the start, each phase receives a voltage equal to 𝑉𝐿/√3, where VL is the line voltage.
Because an induction motor’s torque is exactly proportional to the square of the applied voltage, the star-delta starter reduces the starting torque by one-third when compared to direct-delta starting.

 Stator windings are connected in a star connection, and when speed hits 80%, it flips from star to delta using a triple pole double throw (TPDT) switch.
This toggle switches the stator winding from the star to delta. During the starting condition, the stator windings are connected in the shape of a star. Now we’ll look at how a star delta starter can assist a 3-phase induction motor to minimize its starting current.

Consider the following:

• V_L – Supply Line Voltage.
• I_LS – Supply Line Current.
• I_PS -Winding Current per Phase, &
• Z – Impedance per phase winding (At stationary).

The winding current per phase (IPS) equals supply line current (ILS) since the winding is star-connected.

I_PS=I_LS

Due to the star connection, the voltage across the winding’s individual phases is

V_L/√3

Thus, each phase winding current equals

I_PS =V_L/√3Z

Since the supply line current (ILS) and the winding current per phase (IPS) are equal in this case, we may generate,

I_PS = V_L/√3Z↔I_LS = V_L/√3Z

Let’s now examine the condition in which the motor is started using the same three-phase supply terminals and a delta-connected stator winding.

Consider the following:

• I_LD – Supply Line Current.
• I_PD – Winding Current per Phase, &
• Z – Impedance per phase winding (At stationary).

  Figure 10 - Star-Delta Starter Circuit Diagram

Due to the delta-connected winding, the supply line current (I_LD) is equal to 3 times winding current per phase (I_PD).

I_LD= √3I_PD

The voltage across the winding’s individual phases is because the winding is delta connected.

V_L

Consequently, each phase winding current is

I_PD= V_L/Z

Now that can be expressed as,

I_LD= √3I_PD= √3V_L/Z

By comparing the currents drawn from the supply line by an induction motor with the star & delta-connected winding, we are able to obtain the following:

I_LD/I_LS= √3V_L/Z/V_L/√3Z=3

I_LS= (1/3)I_LD

Thus, in the case of a star delta, the starting current from the primary is one-third that of direct switching in delta.
Again, we know that an induction motor’s starting torque is directly proportional to the square of the voltage applied to the winding per phase.

(Starting torque in a stator winding motor with a star connection) /(Starting torque in a stator winding motor with a delta connection)

=(V_L/√3)²/VL²=1/3

According to the equation, the starting torque generated by the DOL starter is reduced to one-third by the star delta starter. The star-delta starter can be considered as a 57.7% tapping autotransformer.
Thus, when starting, the current in the motor’s winding is decreased to one-third of its rated value, and the starting torque is reduced to 33%.

Transient States of Star Delta Starter

The operation of a star delta starter can be simply understood using three different states:

Star connected state

During the start-up process, the main contactor and star contactor will be near together to complete the power circuit. In this state, the induction motor stator winding will be connected in star, reducing the voltage across the motor winding to 1/3 of the line voltage. When the motor reaches 80% of its full load speed, the timer circuit disconnects the star contactor first and then connects the delta contactor to the circuit.

Open transition state
The contactor circuits open during the transfer from star to delta, and the motor remains neither in star nor in delta mode. As a result, this is referred to as an open transition state.

Delta connected state
The motor shifted from star to delta when the timer was activated. The phase voltage of a delta-connected state stator is equivalent to the line voltage. As a result, line voltage appears across the stator winding, and the motor operates normally at its rated full-load speed.

Star-Delta starter Types

• Manual Star-Delta Starter
• Semi-Automatic
• Automatic Star-Delta Starter

Manual Star-Delta Starter

The Manual Star Delta Starter is an ideal choice for oil mills, flour mills, and agricultural industries. Superior design by a contractor, with a transient Star Position, assures a smooth transition.
This is the most cost-effective alternative to conventional starters while maintaining the same reliability and protection. Because of the wide band operation coil, it is suited for field circumstances with significantly changing supply voltage.

Semi- Automatic Star-Delta Starter

Semi-Automatic Starter (SASD) is a time-tested and extremely reliable starter that is ideal for a wide range of agricultural and industrial applications. It protects the motor from overload, single phasing, phase reversal, and severe unbalanced voltage circumstances.

Automatic

Automatic Star Delta Starters have three contactors, an overload relay or circuit breaker, and a timer for setting the Star position (starting position). The Star Delta Starter requires a motor to be in delta connection during normal operation. When the motor is idle, all the rotor bars create a closed route, like a short-circuited transformer. This will increase rotor current flow. Start-up draws 8-10 times the rated current from the stator. Reduce the motor’s voltage before starting it. In a Star connection, the line voltage is three times the phase voltage. So, starting a motor in star reduces voltage and current. In delta connection, the voltage is the same as phase voltage, so the motor receives full voltage. The Star/Delta Starter is comprised of three contactors, a pneumatic timer, and a thermal overload relay or circuit breaker for 440-volt, 50-Hz 3 phase motors.

Star-Delta starter (motor starting) Characteristics

• 33% of maximum load current is available as starting current.
• Peak starting current ranges from 1.3 to 2.6 times full load current.
• Peak starting torque is 33% of maximum torque.
• Star-delta starting is recommended for motors that start on no load or have a low load torque with a progressive build-up.
• In a star connection, the starting torque is reduced to one-third of the direct starting torque, or approximately 50% of the rated torque.

Figure 11 - Star-Delta starter (motor starting) Characteristics

Features of the Star-Delta starter

• Star delta starters can only be used for low to high power ratings3-phase squirrel cage induction motor
• The induction motor terminal box requires six connection cables.
• Reduced starting torque.
• Reduced starting current.
• Current peak on the change-over from Star to Delta.
• Mechanical load during the Star-to-Delta changeover.

Advantages of a Star-Delta starter

• It is less expensive than other low-voltage induction motor starting methods.
• The torque and current characteristics of the star delta starter perform better than any other starting methods.
• The starting current is one-third that of the direct online (DOL) starting current.
• There is no need for a tap changing device to be used.
• It generates a lot of torque per ampere of line current.
• The star delta starter’s design and operation are simple and efficient.

Disadvantages of a Star delta starter

• Star delta starters are only useful for motors with six motor terminals.
• For Delta connections, the supply voltage should be the same as the rated motor voltage.
• Because the starting current is lowered to approximately 1/3 of the nominal current, the starting torque is also reduced to 1/3.
• From the starter to the motor, two sets of cables are required.
• If the motor is overloaded, there will not be adequate torque to accelerate the motor to the appropriate speed before switching to the delta position.

Application of a Star delta starter

• For low to medium voltage and low torque squirrel cage induction motors, the star delta staring method is commonly preferred (SCIM).
• Centrifugal compressors also use star-delta starters.
• It is suitable for application where the starting current is low and the line current drown is low.
• It can be utilised in industries such as textiles, food and beverage, sugar plants, paper and printing, cold storage etc.

What are the reasons for not utilizing a star delta starter with HT motors?

For heavy-duty machinery, HT motors are typically recommended. To do that, a soft start and a high starting torque are required. Wound type induction motors with external resistance are often preference in these conditions.
Although high stator voltage squirrel cage motors are still usable, the corresponding Star-delta system would need to be extremely expensive and complex because it involves high voltage switching inside the circuit.

Figure 12 - star delta starter with HT motors

When it comes to a star-delta starter, what is the intended function of the delay?

A star-delta starter’s delay is designed to make it easier for high-torque induction motors to move from the star connection which permits lower voltage and, consequently, lower starting current to the delta connection which provides full voltage for the normal operation.
In order to control starting current, minimize torque, and protect against unexpected mechanical strain on the motor and any attached equipment, the motor is first connected in the star configuration during the starting phase. In the starting, there is less voltage and current, enabling it to avoid overheating and damage.

The star-delta starter’s delay gives the motor time to attain a specific speed by allowing it to accelerate progressively while in the star connection. The starter converts the motor from star to delta configuration following the delay period. The motor can function well and deliver the necessary torque for its intended use by switching to full voltage & full power in a delta connection.

Figure 13 -delay system

A star-delta starter’s delay essentially provides a smooth and regulated transition from low voltage starting to high voltage operation, enhancing motor performance and shielding it from unexpectedly high currents during starting.

 Induction Motor Working Priciple (Asynchronous Motor)

The motor serves the purpose of converting electrical energy into mechanical form. Motors are categorized into AC motors and DC motors based on the type of power supply. In this post, we will talk about the different types of three-phase induction motors with working and applications.
The induction motors especially three-phase induction motors are by and large used AC motors to convey mechanical power in modern applications. 80% of the motor is a three-phase induction motor among all motors used in undertakings. Consequently, the induction motor is the fundamental motor among any excess types of motor.

What is a 3 Phase Induction Motor?

A three-phase induction motor is a type of AC induction motor that deals with a phase supply when diverged from the single-phase induction motor where the single-phase supply is supposed to work. The three-phase supply current makes an electromagnetic field in the stator winding which prompts the production of the torque in the rotor bending of the three-phase induction motor having a magnetic field. Next, we will be Going Through Faraday’s Law of Induction Motor.

Faraday’s Law of Induction Motor

Faraday’s law of electromagnetic induction is a significant decision that figures out the fundamental movement of a three-phase induction motor. It was sorted out by Michael Faraday and portrays the association between a changing magnetic field and the prompted electromotive force (EMF).
The basic idea of Faraday’s regulation is summarized as follows
Exactly when the attractive field around a conductor changes, it induces an electromotive power (EMF) in the conductor. The size of the induced EMF corresponds comparatively with the speed of progress of magnetic flux through the conductor.

•  Rotating Magnetic Field: In a 3-phase induction motor, three arrangements of windings are set 120 degrees separated in space. At the point when a three-phase AC voltage is applied to these windings, it delivers a rotating magnetic field.
• Interaction with Rotor Conductors: The rotating magnetic field induces an EMF in the rotor conductors because of Faraday’s law. As the magnetic field rotates, it cuts across the rotor conductors, causing an adjustment of attractive motion.
• Generation of Current in Rotor: The induced EMF in the rotor conductors causes the progression of current in the rotor. This current in the rotor conductors connects with the magnetic field, creating a torque that makes the rotor turn.
• Rotor Movement and Synchronization: The rotor begins to follow the rotating magnetic field, endeavoring to find its rotational speed. The asynchronous development (slip) between the rotor and the turning magnetic field permits the motor to ceaselessly create torque and rotate.

What is 3 Phase Power ?

Three-phase power is a type of electrical power transmission or distribution wherein three sinusoidal voltages of a similar recurrence are produced or utilized. It is a typical strategy utilized in electrical systems, industrial systems, and huge electrical motors. The three-phase system is more proficient and gives a more steady power supply contrasted with a single-phase system.

Figure 1 - Three phase wave

Here are the key components and attributes of three-phase power:

• Phases: In a three-phase system, there are three separate electrical waveforms, frequently alluded to as phases. These phases are assigned as Phase 1, Phase 2, Phase 3. The voltage waveforms for these three phases are counterbalanced by 120 degrees from one another, making a balanced system.
• Voltage: The voltage in a three-phase system is estimated between any two phases. This voltage is known as the line voltage or phase-to-phase voltage. There is likewise the stage voltage, which is the voltage estimated between phases and the neutral point in systems where a neutral is available.
• Waveforms: The voltage waveforms in each phase are sinusoidal and are equally separated in time. The 120-degree phase shift between the phases guarantees a nonstop and almost consistent power conveyance.
• Balanced System: Basically, in a decent three-phase system, the load and impedance across the three phases are equivalent. This equilibrium assists in keeping a smooth and consistent power with streaming.
• Power Generation: Three-phase power is normally generated including three coils or windings in the generator, each adding to one of the phases. Power generation and scattering in three stages are ordinarily more effective than in a single-phase framework, as it thinks about an even more even dissemination of power.

Construction of 3 Phase Induction Motor

The construction of an induction motor is exceptionally basic and robust. It has predominantly two sections :

Figure 2 - Induction motor

 Stator of 3 Phase Induction Motor

As the name recommends, the stator is a fixed piece of the motor. The stator of the induction motor comprises three primary parts;

• Stator Casing
• Stator Core
• Stator Winding

Stator Casing

The stator outline is the external piece of the motor. The stator outline can offer help to the stator core and stator winding. It gives mechanical strength to the inward pieces of the motor. The casing is balanced on the external surface for heat dissemination and cooling of the motor.
The enclosure is cast for small machines and is produced for a large machine. Depending on the applications, the enclosure is constructed from die-cast or fabricated steel, aluminium/aluminium alloys, or stainless steel.

Stator Core

The stator core’s function is to carry the alternating magnetic flux, leading to hysteresis and eddy current losses. To limit these losses, the core is coated with high-quality steel stampings with a thickness ranging from 0.3 to 0.6 mm.
These stampings are protected from one another by stain. All stampings stamp together looking like the stator centre and fixed it with the stator outline.
An inward layer of the stator core has various numbers of slots.

Stator Winding

The stator winding is positioned within the available spaces inside the stator core. The stator winding consists of three phases, and a three-phase power supply is provided to it.
The motor’s pole count is determined by the internal arrangement of the stator winding, and it Controls the motor’s speed. When the number of poles is higher, the speed is lower, whereas if the number of poles is fewer, the speed is higher. The poles are generally two by two. Subsequently, the absolute number of poles is generally high. The connection between simultaneous speed and number poles is as displayed in the below equation

Ns = 120f / P

Where;

f = Supply Frequency

P = Total Number of Poles

Ns = Synchronous Speed

As the end of winding connected with the terminal box. Consequently, there are six terminals (two of each phase) in the terminal box.

As per the application and sort of turning over techniques for motors, the stator winding is associated in star or delta and it is finished by the association of terminals in the terminal box. For more details see Figure 3

 Figure 3 - Stator and Rotor

Rotor of 3 Phase Induction Motor

As the name proposes, the rotor is a rotating part of the motor. As indicated by the type of rotor, the induction motor is delegated:

• Squirrel Cage Induction Motor
• Phase Wound (Wound Rotor) induction motor / Slip-ring induction motor

The development of the stator is the same in the two kinds of induction motors. We will examine the kinds of rotors utilized in 3-phase induction motors in the accompanying part of types of three-phase induction motors.

Types of 3 Phase Induction Motor

Three-phase motors are arranged mainly in two classes in light of the rotor winding (Armature coil winding) for example squirrel cage and slip ring (wound rotor motor).

• Squirrel Cage Induction Motor
• Slip-ring or Wound Rotor Induction Moto

Squirrel Cage Induction Motor

The condition of this rotor is seeming to be the condition of the enclosure of a squirrel. As a result, this motor is referred to as an induction squirrel cage motor. This kind of rotor is easy to make and durable to make. In this way, basically 80% of the induction motor is a squirrel confine induction motor. See Figure 4 below

 Figure 4 - Squirrel Cage Induction Motor

The covered centre of the rotor is shaped like a tube, and the outside edges have spaces. The spaces are not equivalent anyway it is skewed eventually. It helps with magnetic locking between the stator and rotor teeth. It achieves smooth action and diminishes the mumbling commotion. Because it increases the length of the rotor guide, the rotor resistance gets bigger.
Instead of a rotor winding, the squirrel cage rotor is made up of rotor bars. Aluminium, metal, or copper make up the rotor bars. End rings permanently shorten rotor bars. It creates a complete closure in the rotor circuit in this manner. Mechanical support is provided by welding or propping the rotor bars to the end rings. The rotor bars are short-circuited. Along these lines, it is ridiculous to hope to add external assurance from the rotor circuit. In this sort of rotor, the slip rings and brushes are not used. As a result, the construction of this kind of motor is simpler and more durable.

 Advantages

1. It has very simple and extremely rugged, almost unbreakable construction (especially squirrel-
   cage type).
2. Its cost is low and it is very reliable.
3. It has sufficiently high efficiency. In normal running condition, no brushes are needed,
   hence frictional losses are reduced. It has a reasonably good power factor.
   
4. It requires minimum of maintenance.
5. It starts up from rest and needs no extra starting motor and has not to be synchronised. Its
   starting arrangement is simple especially for squirrel-cage type motor.

Disadvantages

1. Its speed cannot be varied without sacrificing some of its efficiency.
2. Just like a d.c. shunt motor, its speed decreases with increase in load.
3.  Its starting torque is somewhat inferior to that of a d.c. shunt motor.

Slip-Ring or Wound Rotor Induction Motor

Slip-ring Induction Motor are generally called wound rotor motor. The rotor is made up of a tube-shaped center with slots on the outside edges. The rotor winding is placed inside the slots. In this type of rotor, the winding is arranged in such a way that the number of rotor winding slots matches the number of stator winding poles. The rotor winding can be related as a star or delta. The slip-rings are connected to the end terminals of the rotor windings. As a result, it is referred to as a slip-ring induction motor engine. The external block can without a doubt interact with the rotor circuit through the slip-ring and brushes. Likewise, it is uncommonly useful for controlling the speed of the motor and dealing with the starting force of the three-phase Induction Motor. For more details see Figure 5 below

 Figure 5 - Slip-Ring or Wound Rotor Induction Motor

The outside opposition is used solely for the starting reason. The rotor copper loss will be increased if it remains connected while the system is operating. For the initial condition, a high rotor resistance is extremely advantageous. During the initial state, the rotor circuit is linked to the outside resistance in this manner. The metal collar short-circuits the slip rings when the motor is running close to the actual speed. By this arrangement, the brushes and outside obstruction are taken out from the rotor circuit.
Copper loss from the rotor and brush friction are both reduced as a result of this. The rotor advancement is fairly perplexing and stood out from the squirrel keep engine in light of the presence of brushes and slip-rings. This motor requires more maintenance. As a result, this motor might be used in applications requiring high starting force and variable speed control. In any case, the slip-ring induction motor is superior to the squirrel confine induction motor.

Advantages

• Variable Speed Control: One of the main advantages of slip-ring motors is the ability to control the speed and power by moving the external block related with the rotor windings. Because of this, they are suitable for applications that require variable speed.
• High Starting Torque Control: Slip-ring motors can give high starting torque, and the external devices can be changed as per control the starting torque.
• Smooth Acceleration: The ability to control the rotor resistance results in less mechanical pressure during starting and a smoother speed increase.

Disadvantages

• Complex Construction: Slip-ring motors have a more marvelous improvement as a result of the external securities and slip rings, inciting higher gathering and backing costs.
• Higher Maintenance: The slip rings and brushes in the rotor require high maintenance, and the external devices are require.
• Lower Efficiency: Due to the complexity of the rotor development and the additional losses in the external protections, slip-ring motors typically have lower performance than squirrel confine motors.

Working Principle of 3 Phase Induction Motor

The stator winding is covered at 120˚ (electrically) to one another. When the stator winding is supplied with a three-phase power source, it results in the induction of a rotating magnetic field (RMF) within the stator circuit.
The speed of the rotating magnetic field is known as synchronous speed (NS).
As per Faraday’s law, EMF is induced in the guide because of the pace of progress of transition (dφ/dt). The rotor circuit cuts the stator magnetic field and an EMF is actuated in the rotor bar or rotor winding.
The rotor circuit is a nearby way. Thus, because of this EMF current will move through the rotor circuit.
Presently, we realize that the current-conveying guide actuates the attractive field. In this way, the rotor current prompts a second magnetic field. In the general movement between the stator flux and rotor flux, the rotor begins to rotate to diminish the reason for relative movement. The rotor attempts to get the stator transition and starts rotating. The heading of revolution is given by Lenz’s law. Furthermore, is toward the rotating magnetic field induced by the stator.

Here, the rotor current is created because of inductance. Hence, this motor is known as the induction motor.
The speed of the rotor is not exactly the speed of synchronous speed. The rotor attempts to get the alternating attractive field of the stator. Yet, it never gets it. Consequently, the rotor speed is slightly less than the synchronous speed. The synchronous speed is determined by the number of poles and the supply frequency. The difference between the rotor’s actual speed and the synchronous speed is referred to as slip.

Why the slip is never zero in an induction motor?

When the rotor’s actual speed matches the synchronous speed, the slip is zero. However, in the case of an induction motor, this scenario is never encountered. This is because, at zero slip, both speeds are equivalent, resulting in no relative movement. Consequently, no electromotive force (EMF) is induced in the rotor circuit, and the rotor current becomes zero. As a result, the motor cannot operate. Despite this limitation, the induction motor is extensively used in industries due to its numerous advantages outweighing its disadvantages.

Advantages of 3 Phase Induction Motor

• Reliability: These motors are known for their strong and solid activity. They have a with less moving parts, decreasing the probability of mechanical disappointment.
• High Power Output: 3-phase motors can convey high power yield, making them appropriate for modern applications that require significant mechanical power.
• Cost-Effective: 3-phase induction motor are for the most part financially savvy to produce and keep up with, adding to their far reaching use in different industries.
• Efficiency: 3-phase induction motor are exceptionally effective. They convert a high level of electrical power into mechanical power, making them reasonable for a great many applications.
• Smooth Operation: These motors give a smooth and consistent activity because of the steady power conveyance from the three phase.
• Low Maintenance: Contrasted with different types of motors, 3-phase induction motor have lower support necessities. They are self-starting and don’t need extra starting devices.

Disadvantages of 3 Phase Induction Motor

• Starting Torque: The starting torque induction motor is lower contrasted with a few other motor types. This can be a constraint in applications where high beginning torque is essential.
• Complex Control: Accomplishing exact speed control can be more intricate with 3-phase induction motor contrasted with a few other engine types. Extra gear might be expected for calibrated control.
• Power Factor: The power component of 3-phase induction motor may not be ideal under all working circumstances, prompting power misfortunes in the electrical system.
• Harmonics Generation: 3-phase enlistment engines can bring sounds into the electrical framework, which might require extra separating or rectification devices.
• Speed Control: While variable frequency drives (VFDs) can be used to control the speed of 3-phase induction motor, accomplishing exact command over a wide speed reach can be more difficult contrasted with a few other motor types.
• Size and Weight: In specific cases, 3-phase induction motors can be bigger and heavier than identical DC motors, particularly in applications where size and weight are basic elements.

Applications of 3 Phase Induction Motor

• Wastewater Treatment Plants: Pumps and other hardware in wastewater treatment plants frequently depend on 3-phase Induction motors for their operation. These motors can deal with the constant and variable loads related with wastewater treatment.
• Oil and Gas Industry: Induction motors are utilized in different cycles inside the oil and gas industry, including driving siphons, blowers, and boring equipment.
• Electric Traction: Various electric trains, cable cars, and electric vehicles utilize 3-phase induction motor for drive. These motors give a decent overall influence and effectiveness for transportation applications.
• Air Conditioning and HVAC Systems: Induction motors power the fans and blowers in cooling and warming, ventilation, and cooling (air conditioning) system. Their effectiveness and capacity to deal with differing loads make them ideal for these applications.
• Water Pumping Stations: Induction motors are generally utilized in water siphoning stations for providing water to urban communities and horticultural regions. Their dependability and capacity to work under changing burden conditions make them appropriate for water pumping applications.
• Mining Operations: In mining, 3-phase Induction motors power rollers, transports, and other equipment. Their hearty plan and capacity to work in testing conditions make them appropriate for mining applications.
  Manufacturing Plants: Induction motors are used in an extensive variety of assembling processes, including material taking care of, bundling, and sequential construction systems. Their unwavering quality and capacity to deal with weighty burdens add to their ubiquity in assembling.
• Industrial Machinery: 3-phase induction motor are broadly utilized in modern apparatus like pumps, fans, blowers, transports, and machine devices. Their capacity to give high power yield and solid activity makes them reasonable for driving these types of equipment.

Conclusion

In conclusion, the three-phase induction motor stands as fundamental equipment and is broadly used in electrical machines in different industrial applications. Its robust design, simplicity, and efficiency make it a favoured decision for controlling a huge number of devices, from pumps and fans to conveyer systems. The motor’s operation depends on the principles of electromagnetic induction, where the cooperation between the stator and rotor magnetic fields induces a rotating motion.
The main advantage of the three-phase induction motor is its self-starting capability, so for this advantage, there is no need for external devices. Its ability to work at a constant speed under varying loads adds to its dependability in practical applications. Additionally, the absence of brushes in the design of the motor reduces the maintenance requirements, which contributes to its durability and longevity.
Despite these merits, it’s fundamental to consider the motor’s limitations, such as the difficulties in speed control and its inherent tendency to operate at synchronous speed. Engineers and analysts constantly investigate advancements, including the coordination of modern-day control techniques and materials, to improve the motor’s performance, efficiency, and flexibility in advancing modern scenes. In conclusion, the three-phase induction motor stays a foundation in electrical engineering, balancing conclusion with viability and consistently developing to meet the needs of different applications.

Single-phase, two-phase and three-phase motors

Electric motors let us obtain mechanical energy in the simplest and most efficient manner. Depending on the number of supply phases, we can find single-phase, two-phase and three-phase motors with coiled startup winding and with coiled startup winding with a capacitor. And choosing one or the other will depend on the necessary power.
If you are involved in a project and don’t know what type of motor you should use, in guide we tell you about each motor and their differences. Let’s get to it!

 What is a single-phase motor?

A single-phase motor is an electrically-powered rotary machine that can turn electric energy into mechanical energy.
It works by using a single-phase power supply. They contain two types of wiring: hot and neutral. Their power can reach 3Kw and supply voltages vary in unison.
They only have a single alternating voltage. The circuit works with two wires and the current that runs across them is always the same. For more details see Figure 1 below 

Figure 1 - sine curve on single phase motor

In most cases these are small motors with a limited torque. However, there are single-phase motors with a power of up to 10 hp that can work with connections of up to 440V.
They do not generate a rotating magnetic field; they can only generate an alternate field, which means that they need a capacitor for startup.
They are easy to repair and maintain, as well as affordable.
This type of motor is used mainly in homes, offices, stores and small non-industrial companies. Their most common uses include home appliances, home and business HVAC and other appliances such as drills, air conditioners and garage door opening and closing systems.

What is a two-phase motor?

A two-phase motor is a system that has two voltages 90 degrees apart, which is no longer in use nowadays. The alternator is composed of two windings placed at 90 degrees from each other.
They require 2 live and one ground wire that work in two phases. One increases the current up to 240v for the motion, and the other one maintains the fluidity of the current for the use of the motor.

What is a three-phase motor?

A three-phase motor is an electric machine that transforms electric power into mechanical energy by means of electromagnetic interactions. Some electric motors are reversible – they can transform mechanical energy into electric power acting as generators.
They work by using a three-phase power source. They are driven by three alternating currents of the same frequency, which peak at alternating moments. They can have a power of up to 300KW and speeds ranging between 900 and 3600 RPM.
Three conductor lines are used for transmission, but the final use requires 4-wire cables, which correspond to the 3 phases plus neutral. For more details see Figure 2 below

 Figure 2 - sine curve on three phase motor

Three-phase electric power is the most common method in use in electric grids around the world since it transfers more power, and it sees significant use in the industrial sector.

After a general explanation we go into specifics

What is a 3-Phase Motor?

A 3-phase motor is an electric motor that operates on a three-phase power supply, utilizing three alternating currents to generate a rotating magnetic field, resulting in a smoother and more efficient power delivery compared to single-phase motors. They are a type of AC motor that is a specific example of a polyphase motor. These motors can be either an induction motor (also called an asynchronous motor) or a synchronous motor. The motors consist of three main components – the stator, the rotor, and the enclosure.
The stator consists of a series of alloy steel laminations around which are wound with wire to form induction coils, one coil for each phase of the electrical power source. The stator coils are energized from the three-phase power source.
The rotor also contains induction coils and metal bars connected to form a circuit. The rotor surrounds the motor shaft and is the motor component that rotates to produce the mechanical energy output of the motor.
The enclosure of the motor holds the rotor with its motor shaft on a set of bearings to reduce the friction of the rotating shaft. The enclosure has end caps that hold the bearing mounts and house a fan that is attached to the motor shaft which spins as the motor shaft turns. The spinning fan draws ambient air from outside the enclosure and forces the air across the stator and rotor to cool the motor components and dissipate heat that is generated in the various coils from the coil resistance. The enclosure also typically has raised mechanical fins on the exterior that serve to further conduct heat to the outside air. The end cap will also provide a location to house the electrical connections for the three-phase power to the motor. For more details see Figure 3 below

Figure 3 - Motor internal parts

How Does a 3-Phase Motor Work?

A 3-phase motor works by harnessing three alternating currents to produce a rotating magnetic field, which, in turn, drives the motor’s rotor and induces rotation. This design enhances efficiency and performance, making 3-phase motors suitable for a wide range of industrial applications.
Three-phase motors operate by the principle of electromagnetic induction which was discovered by the English physicist Michael Faraday back in 1830. Faraday noticed that when a conductor such as a coil or loop of wire, is placed in a changing magnetic field, there is an induced electromotive force or EMF that is generated in the conductor. He also observed that current flowing in a conductor such as wire will generate a magnetic field and that the magnetic field will vary as the current in the wire changes in either magnitude or direction. This is expressed in mathematical form by relating the curl of the electric field to the rate of change in time of the magnetic flux:

These principles form the basis for understanding how a three-phase motor works.
Figure 4 below is an illustration of Faraday’s law of induction. Note that the presence of an EMF depends on the motion of the magnet which results in a changing magnetic field to exist.

Figure 4 - Principle of Electromagnetic Induction

For induction motors, when the stator is powered from a three-phase electrical energy source, each coil generates a magnetic field whose poles (north or south) change position as the AC current oscillates through a complete cycle. Since each of the three phases of the AC current are phase-shifted by 120o, the magnetic polarity of the three coils are not all identical at the same instant of time. This condition results in the stator producing what is known as an RMF or Rotating Magnetic Field. As the rotor sits in the center of the stator coils, the changing magnetic field from the stator induces a current in the rotor coils, which in turn results in an opposing magnetic field being generated by the rotor. The rotor field seeks to align its polarity against that of the stator field, the result being a net torque is applied to the motor shaft and it begins to rotate as it seeks to bring its field into alignment. Note that in the 3-phase induction motor, there is no direct electrical connection to the rotor; magnetic induction causes the motor rotation.

With three-phase induction motors, the rotor seeks to maintain alignment with the RMF of the stator, but never achieves it, which is why induction motors are also called asynchronous motors. The phenomenon which causes the rotor speed to lag the speed of the RMF is known as slip, as is expressed as:

where Nr is the speed of the rotor, and Ns is the synchronous speed of the rotating field (RMF) of the stator.

Synchronous motors operate in a similar fashion to induction motors except that in the case of a synchronous motor, the stator and rotor fields are locked into alignment so that the stator RMF will cause the rotor to turn at the exact same rate of rotation (in synch – therefore the slip is equal to 0). For more information on how this is accomplished, refer to these articles on reluctance motors and brushless DC motors (BLDC motors). Note that synchronous motors, unlike induction motors, need not be powered by AC power.

Motor Controllers for 3-Phase Motors

The speed that is generated by a three-phase AC motor is a function of the AC supply frequency since it is the source of the RMF in the stator coils. Therefore, some AC motor controllers operate by using the AC current input to generate a modulated or controlled frequency input to the motor, thereby controlling the speed of the motor. Another approach that can be used to control motor speed is by altering the slip (described earlier). If the slip increases, the motor speed (i.e. the speed of the rotor) decreases.

To learn more about the approaches for motor control, review our article on AC Motor Controllers.

3-Phase Motor Types

There are several types of 3-phase motors, each designed for specific applications:

• Induction Motors: Also known as, asynchronous motors, are widely used for general-purpose applications due to their simplicity and reliability. Two common types of induction motors are squirrel cage and wound rotor.
• Synchronous Motors: Maintain synchrony with the supply frequency, making them suitable for precision applications such as clock motors.
• Brushless DC Motors: Utilize electronic commutation for improved efficiency and longevity, often found in applications like robotics and electric vehicles.

When Would a Three-Phase Motor Be Used?

A three-phase motor would be used in various applications, including:

• Industrial machinery and manufacturing equipment
• HVAC systems for large commercial buildings
• Water pumping stations and wastewater treatment plants
• Oil and gas processing facilities
• Electric vehicles and transportation systems
• Large-scale air compressors and pumps