Advanced Motor Speed Control Techniques & Power Electronics

Posted on Sep 1, 2025 in Electronics

Introduction to Induction Motor Speed Control

Historically, induction motors were primarily used in applications requiring constant speed, as variable speed applications were dominated by DC drives. However, conventional methods for induction motor speed control were often expensive or highly inefficient. With the availability of thyristors, power transistors, and GTOs, the development of variable speed induction motor drives became feasible.

Unlike DC drives, which require frequent maintenance due to the presence of commutators and brushes and cannot be used in explosive environments, induction motors (particularly squirrel-cage types) are cheaper, lighter, smaller, and more robust. They require less maintenance and can operate in dusty and explosive environments. Due to these advantages, the three-phase induction motor is now the most common machine in industry, accounting for more than 90% of mechanical power used. While variable speed induction motor drives can be expensive, their applications are widespread, including:

  • Fans and blowers
  • Cranes and conveyors
  • Traction systems
  • Underground water installations

Principle of 3-Phase Induction Motor Operation

When a three-phase AC supply is given to the three-phase stator winding, a rotating magnetic field (RMF) is produced. The speed of rotation of this magnetic field is called the synchronous speed (Ns).

This rotating magnetic field links the stationary rotor windings, producing an induced electromotive force (EMF) in them. If the rotor windings are short-circuited (as in a squirrel-cage rotor), a large current flows through the rotor windings. This situation is exactly like a current-carrying conductor placed in a magnetic field, which experiences a mechanical force. These forces cause the rotor to rotate in the same direction as the RMF.

According to Lenz’s Law, the direction of the rotor current will be such that it opposes the force producing it (i.e., the relative speed between the rotating magnetic field and the rotor conductors). If the rotor were to run at synchronous speed (the same speed as the rotating magnetic field), the rotor conductors would appear stationary to the rotating magnetic field, and thus, the magnetic field would not cut the rotor conductors, preventing induced EMF and torque. Therefore, the rotor always runs at a speed less than synchronous speed, allowing the magnetic field to cut the rotor conductors and produce torque.

Induction Motor Speed Control Methods

Different methods employed for speed control of induction motors include:

  • Pole Changing
  • Stator Voltage Control
  • Supply Frequency Control
  • Rotor Resistance Control
  • Slip Power Recovery

Pole Changing Control

The synchronous speed (Ns) of an induction motor is directly proportional to the supply frequency (f) and inversely proportional to the number of poles (P) (Ns = 120f/P). By changing the number of poles, the synchronous speed, and therefore the motor’s speed, can be changed. Provisions for changing the number of poles must be incorporated during the machine’s manufacturing stage. Such machines are called pole-changing motors or multi-speed motors.

Stator Voltage Control

Reducing the stator voltage can reduce the speed of an induction motor, which is a viable option for speed control in some applications. Torque is directly proportional to the square of the voltage. Efficiency varies with voltage and speed. Speed control often involves significant power dissipation in the rotor resistance, leading to heat. Variable voltage for speed control is obtained by an AC voltage controller. Stator voltage control is primarily effective for speed control below the base speed.

AC Voltage Controller for Stator Voltage

An AC voltage controller produces AC voltage of variable magnitude from a fixed voltage, but its frequency remains constant. This method is commonly used for controlling the speed of fans and pumps. Domestic fans and pumps typically use single-phase AC voltage controllers, while industrial fans and pumps often use three-phase AC voltage controllers, employing either triacs or antiparallel thyristors to control the motor’s speed by varying the firing angle. Since AC voltage controllers allow smooth control of voltage from zero, they are also used for soft starting.

Integral Cycle Control

In integral cycle control, thyristors are switched ON to connect the control circuit to the source voltage for a few cycles, then disconnected for another few cycles. This method is used in applications where the mechanical or thermal time constant is high.

Phase Control Techniques

The real power flow to the load is controlled by varying the firing angle of the thyristor power controller. This method introduces harmonics into the supply current. Different configurations of phase control AC voltage controllers include:

  • Single-phase half-wave AC voltage controller
  • Single-phase full-wave AC voltage controller
  • Three-phase half-wave AC voltage controller
  • Three-phase full-wave AC voltage controller

Closed-Loop Control of Induction Motor

A closed-loop control system for an induction motor using an AC voltage controller typically consists of a three-phase induction motor, a power circuit, a tachogenerator, and control circuits. The tachogenerator generates a voltage proportional to the actual motor speed. Its output is compared with a reference speed. The difference (error) is fed to an amplifier and then to a firing control circuit. The firing angle changes according to the error signal, which in turn changes the motor terminal voltage and thus the motor speed.

Stator Voltage Control: Advantages & Drawbacks

Advantages of Stator Voltage Control

  • Simple circuit
  • Compact and less weight
  • Quick response
  • Provides smooth control for voltage
  • Can be used for soft starting

Drawbacks of Stator Voltage Control

  • Input power factor is low.
  • Voltage and current waveforms are distorted due to harmonics.
  • Maximum torque decreases because stator voltage decreases.
  • At low speeds, motor current is high and requires limiting.
  • Regeneration is not possible.

Supply Frequency Control for Induction Motors

The synchronous speed of an induction motor, and thus its operating speed, can be controlled by varying the supply frequency. Since the supply frequency is standard, to change frequency, a power electronic converter is needed. Commonly used circuits for frequency variation include cycloconverters and inverters.

Cycloconverter for AC Motor Control

A cycloconverter converts AC supply frequency to a variable frequency. They are often used for low-frequency, high-power applications. Harmonics can increase with frequency, and output frequency is typically restricted to 40% of the input frequency to limit harmonics. They provide a smaller range of frequency variation, which is well-suited for low-speed, larger power applications like ball mills, cement kilns, etc. They offer regenerative braking capacity, which is achieved by reversing the phase sequence of the motor terminal voltage.

Inverter for AC Motor Control

Inverters involve the conversion of a DC supply to AC. There are two main types: voltage source inverters (VSI) and current source inverters (CSI). The rectifier section can be controlled or uncontrolled. Inverters can use Pulse Width Modulation (PWM) for sinusoidal or square wave output, and their frequency can be controlled.

V/f Control of Induction Motors

The speed of an induction motor can be controlled by changing the supply frequency. However, simply varying the supply frequency will affect the motor’s performance. An induction motor’s terminal voltage is similar to a transformer. An increase in supply frequency without a change in terminal voltage will cause an increase in flux, leading to saturation of the core. Magnetizing current increases, line current becomes distorted, and excessive noise is produced. Conversely, if frequency is increased to increase speed while keeping voltage constant, flux decreases, and as a result, the torque produced by the motor decreases. Any increase or decrease in flux beyond the rated value is undesirable.

Variable frequency control below base speed is carried out by varying the terminal voltage proportionally with frequency, maintaining a constant V/f ratio. For speeds above rated speed, the V/f ratio cannot be maintained constant since the terminal voltage cannot be increased above its rated value. In this region, the terminal voltage is kept at its rated value, and frequency is varied to achieve speeds above rated speed. As frequency increases, flux decreases, and the torque produced by the motor decreases.

Variable frequency control provides good running transients and performance. Speed control and braking operation are available from zero to maximum speed. It offers low losses, high efficiency, and a high power factor from zero to full load.

Constant Torque & Constant Power Operation

The variation of maximum torque and power with frequency defines the operating regions of the motor. From zero to base speed, the motor operates at constant torque. Above base speed, the machine operates at constant power. Both constant torque and constant power regions are present in V/f control of induction motors, below and above base speed.

Constant Torque Mode

An induction motor is operated in constant torque mode for speed control below base speed by maintaining a constant V/f ratio. To achieve a desired speed, the supply frequency is decreased while keeping V/f constant. The motor current remains constant at its maximum rated value. Since the voltage is varied to keep V/f constant, the power factor produced by the motor is high, and hence the flux produced by the machine remains constant, resulting in constant torque operation.

Constant Power Mode (Field Weakening)

An induction motor is operated in constant power mode for speeds above base speed. To get speeds above base speed, the supply frequency increases. However, the supply voltage cannot be increased above its rated value, so the V/f ratio cannot be kept constant. Current remains at its maximum rated value. Since motor current and voltage are constant, the power produced by the motor remains constant, resulting in constant power operation. When frequency increases, flux in the machine decreases, and the torque produced by the machine delivering constant power also decreases. This mode is also called field weakening. Above base speed, the machine operates at constant power and maximum current. This region is often called the high-speed region or field-weakening region. Variable speed and frequency for induction motors can be controlled using inverters or cycloconverters.

Rotor Resistance Control for SRIM

The speed of induction motors can be controlled below rated speed using rotor resistance control. This method is only applicable to Slip Ring Induction Motors (SRIM). When the machine is near synchronous speed, slip is very low and can be neglected. If torque remains constant, then slip and hence the rotor field can be controlled by varying rotor resistances. It also increases starting torque. However, it causes additional power wastage in the rotor resistance, which follows a square law. The variation of torque-speed characteristics with rotor resistance is significant.

Static Rotor Resistance Control (Chopper)

Instead of mechanically varying the rotor resistance, it can be varied statically using the principle of a chopper. The AC output voltage of the rotor is rectified by a diode bridge rectifier and connected to a parallel combination of fixed resistances. When a switch (e.g., a chopper) operates, the resistances are periodically connected and disconnected to the supply. When the switch is ON, the resistor is disconnected, and when the switch is OFF, the resistor is connected. By controlling the ON-time (duty cycle) of the switch, the effective rotor resistance can be varied.

Principle of Slip Power Recovery Schemes

The power flow diagram of an induction motor shows the total stator input power. Some power is wasted in the stator winding as stator copper losses. The remaining power crosses the air gap to reach the rotor. A portion of this is wasted in rotor resistance as rotor copper loss. The remaining air gap power is converted into mechanical power. The portion of the air gap power which is not converted into mechanical power is called slip power. Normally, in an induction motor, slip power is wasted in rotor resistance as heat.

In rotor resistance control used for speed control of SRIM, we are controlling slip power to control the speed of the motor. As rotor resistance increases, the speed of the motor decreases, and as a result, the mechanical power developed decreases. Instead of wasting slip power in rotor resistance, we can recover this power and feed it back to the supply. This method of speed control is called a slip power recovery scheme.

The magnitude of the phase voltage can be controlled. This method allows the motor to retain its natural torque characteristics, though it can reduce the power factor as motor speed decreases. Speed control below synchronous speed is achieved by recovering the slip power produced in the rotor resistance. For speed control above synchronous speed, additional power must be supplied to the rotor. These methods of speed control are collectively called slip power recovery schemes. There are two main types:

  • Static Kramer Drive
  • Static Scherbius Drive

Static Kramer Drive

In a Static Kramer Drive, slip frequency power from the rotor is converted to DC voltage, then to line frequency AC, and pumped back to the AC source. Power can flow only in one direction. This drive is for speed control below synchronous speed. The input power is the difference between the motor’s mechanical power and the recovered slip power. The slip power from the rotor is rectified to DC voltage using a diode bridge. The rectified DC voltage is then converted to line frequency AC by a line-commutated inverter and fed back to the AC supply. A three-phase transformer may be required if the rotor circuit voltage is less than the supply voltage. The power fed back to the source is controlled by varying the firing angle of the inverter. Analysis shows that slip is directly proportional to cos(alpha), with the maximum value of alpha restricted to 165 degrees. This drive is suitable for applications like fan and pump drives which require below synchronous speed operation in a small range. Instead of wasting slip power in rotor resistance, it is fed back to the source, resulting in higher drive efficiency.

Static Scherbius Drive

In a Static Kramer Drive, the speed of an SRIM can be varied only below synchronous speed. The Static Scherbius Drive, however, allows for both subsynchronous and supersynchronous speed control. It consists of two fully controlled thyristor bridges. Both bridges can work as a rectifier or an inverter depending on their firing angle. Using this drive, speed control above and below synchronous speed is possible. It has two modes of operation:

Mode 1: Subsynchronous Speed Operation

The induction motor’s slip power is removed from the rotor and pumped back to the AC supply. Bridge 1 works as a rectifier, and Bridge 2 works as an inverter. Slip power flows from the rotor circuit, through Bridge 1 (rectifier), the DC link, Bridge 2 (inverter), a transformer, and back to the supply.

Mode 2: Supersynchronous Speed Operation

Additional power is supplied to the rotor at slip frequency. For supersynchronous speed, Bridge 1 works as an inverter (with a specific firing angle), and Bridge 2 works as a rectifier. Power now flows from the supply, through the transformer, Bridge 2, the DC link, Bridge 1, and into the rotor circuit. This drive is more expensive than the Kramer drive because it requires two controlled bridges and more complex control circuitry. When motor speed approaches synchronous speed, the magnitude of the induced voltage in the rotor may not be sufficient to provide line commutation for the thyristors, so forced commutation might be required.

Cycloconverter Scherbius Drive

In a cycloconverter Scherbius drive, the bridge converter system is replaced by a three-phase line-commutated cycloconverter. This allows slip power to flow in both directions, enabling both subsynchronous and supersynchronous operation without a DC link.

Chopper-Fed DC Motor Drives

A chopper is a DC-DC converter that provides variable DC voltage from a fixed DC source. Self-commutated devices like MOSFETs, IGBTs, and power transistors are preferred for building choppers because they can be controlled by low-power control signals and do not require complex commutation circuits. These devices can be operated at higher frequencies, leading to improved motor performance. Regenerative braking can also be carried out at low speeds.

Control Strategies for Choppers

Time Ratio Control (PWM & FM)

As the name suggests, the time ratio is varied. This is realized using two different strategies:

  1. Constant Frequency System (Pulse Width Modulation – PWM): Here, the ON-time (Ton) is varied while the total period (T) is kept constant.
  2. Variable Frequency System (Frequency Modulation – FM): Here, Ton is kept constant, and T is varied.

Current Limit Control

The chopper circuit is switched ON and OFF directly by a preset value of the load current. The chopper is switched off when the load current falls to a lower limit and switched on when it rises to an upper limit.

Single Quadrant Chopper-Fed DC Motor

First Quadrant Chopper (Forward Motoring)

The transistor (switch) of the chopper-fed DC motor drive is operated periodically with period T. It remains ON during Ton. During Ton, the motor terminal voltage is Vs (supply voltage), and the motor current increases. When the transistor is turned OFF, the motor current freewheels through a diode, and the output voltage becomes zero. The motor current decreases. The duty ratio (or duty cycle) is given by D = Ton / T. The average output voltage Vavg = D * Vs. By varying the duty cycle, the chopper output voltage can be controlled, and hence the speed of the motor.

Second Quadrant Chopper (Regenerative Braking)

In this mode, energy from the motor is fed back to the supply (regenerative braking). The transistor is operated periodically within a period T. During the ON period (Ton), the motor current increases. The motor works as a generator, converting mechanical energy to electrical energy. When the transistor is turned OFF, the motor current flows through a diode, and the current increases. From the waveform, the average output voltage Vavg = D * Vs.

Two Quadrant Chopper-Fed DC Motor

A two-quadrant chopper can provide motoring and regenerative braking in the forward direction. Transistor TR1 and diode D1 from the chopper circuit provide control for forward motoring operation. Transistor TR2 and diode D2 from the chopper circuit provide control for regenerative braking.

First Quadrant Operation (Forward Motoring)

If TR1 is ON and TR2 is OFF, current flows from the DC supply to the load, and the voltage across the load is positive. When TR1 is OFF, the motor current freewheels through diode D1.

Second Quadrant Operation (Regenerative Braking)

If TR1 is OFF and TR2 is ON, the motor works as a generator, producing electrical energy. This energy is stored in the inductor. When TR2 is turned OFF, the stored energy is released to the supply through diode D2.

Four Quadrant Chopper-Fed DC Motor

A four-quadrant chopper (H-bridge) can provide motoring and regenerative braking in both forward and reverse directions. It typically uses four transistors as switches and four diodes. Both the load voltage and current can be either positive or negative. Care must be taken to ensure that switches S1 and S2, as well as switches S3 and S4, are not turned on simultaneously; otherwise, the supply voltage will be short-circuited.

Quadrant 1 Operation (Forward Motoring)

Switch S1 is operated (pulsed), S4 is kept ON, and S2, S3 are OFF. When S1 is ON, point A is connected to the positive terminal of the DC supply, and point B is connected to the negative terminal through S4. The machine operates as a motor in the forward direction. When S1 is OFF, the current in the circuit decreases, and the inductor’s reverse polarity causes diode D2 to turn ON, allowing current to freewheel through S4 and D2. This provides first quadrant operation.

Quadrant 2 Operation (Forward Regenerative Braking)

Switch S2 is operated (pulsed), and S1, S3, S4 are kept OFF. When S2 is ON, diode D4 is forward biased, and armature current freewheels through S2 and D4. The inductor stores energy. When S2 is OFF, the current in the circuit decreases suddenly, and the inductor’s reverse polarity causes diodes D1 and D2 to turn ON. Power flows from the motor back to the source (regenerative braking). The inductor voltage adds to the motor’s back EMF, creating a voltage higher than the supply voltage. The machine operates in regenerative braking mode in the forward direction.

Quadrant 3 Operation (Reverse Motoring)

Switch S3 is operated (pulsed), S2 is kept ON, and S1, S4 are OFF. When S3 is ON, point B is connected to the positive terminal of the DC supply, and point A is connected to the negative terminal through S2. The machine operates as a motor in the reverse direction. When S3 is OFF, the current in the circuit decreases suddenly, and the inductor’s reverse polarity causes diode D4 to turn ON, allowing armature current to freewheel through S2 and D4. This provides third quadrant operation.

Quadrant 4 Operation (Reverse Regenerative Braking)

Switch S4 is operated (pulsed), and S1, S2, S3 are kept OFF. When S4 is ON, diode D2 is forward biased, and armature current freewheels through S4 and D2. The inductor stores energy. When S4 is OFF, the current in the circuit decreases suddenly, and the inductor’s reverse polarity causes diodes D1 and D3 to turn ON. Power flows from the motor back to the source (regenerative braking). The inductor voltage adds to the motor’s back EMF, creating a voltage higher than the supply voltage. The machine operates in regenerative braking mode in the reverse direction.

Cycloconverter Fundamentals & Types

A cycloconverter is a power electronic circuit that converts a fixed voltage and frequency AC supply into a variable voltage and frequency AC supply. Traditionally, AC-to-AC conversion is done in two different ways:

  1. Indirect AC-DC-AC conversion (DC link converter)
  2. Direct AC-AC conversion (cycloconverter)

There are different types of cycloconverters classified according to the frequency and supply voltage:

  • According to Frequency:
    • Step-up cycloconverter: Where output frequency is greater than input frequency.
    • Step-down cycloconverter: Where output frequency is less than input frequency.
  • According to Supply Voltage:
    • Single-phase to single-phase cycloconverter
    • Three-phase to three-phase cycloconverter
    • Three-phase to single-phase cycloconverter

Cycloconverters are used for high-power applications where output voltage and frequency can be controlled. Thyristors are commonly used as switching devices.

Cycloconverter Applications

  • Speed control of high-power AC drives
  • Induction heating
  • Static VAR compensation

Single-Phase to Single-Phase Cycloconverter

Here, both input and output AC voltages are single phase. The converter can work as both a step-up and step-down cycloconverter. Two configurations are possible:

Midpoint Centre-Tapped Cycloconverter

The configuration of a midpoint center-tapped cycloconverter typically involves two groups of thyristors: a P-group (positive) and an N-group (negative).

Step-Up Cycloconverter Operation

For step-up operation, the output frequency is greater than the input frequency. The P-group and N-group thyristors are fired in a sequence to synthesize an output voltage waveform with the desired higher frequency. For example, to achieve an output frequency four times the input frequency, a specific firing sequence of SCRs (e.g., T1, T4, T1, T4, etc., for positive and negative half-cycles) is used, with forced commutation often required.

Step-Down Cycloconverter Operation

For step-down operation, the output frequency is less than the input frequency. During the positive half-cycle of the supply, P-group thyristors (e.g., T1) are turned ON, allowing current to flow through the load. These thyristors are naturally commutated when the supply voltage reverses. Similarly, during the negative half-cycle, N-group thyristors (e.g., T3) are turned ON. By controlling the firing angles and the number of input cycles contributing to each output half-cycle, a lower output frequency is achieved. For example, an output frequency of one-third the input frequency can be generated.

Single-Phase Bridge Type Cycloconverter

In this configuration, a transformer with a center tap is not required. The circuit configuration typically involves SCRs T1, T2, T3, T4 forming the positive group and SCRs T5, T6, T7, T8 forming the negative group. Care must be taken to ensure that the positive and negative groups do not conduct simultaneously, as this would short-circuit the supply. This type of cycloconverter can also work as both a step-up and step-down converter.