Synchronous Motors: Types, Operation, and Control

Posted on Jan 15, 2025 in Audiovisual Communication and Journalism

Synchronous Motor Construction

Synchronous motors are very similar to alternators. They run at synchronous speed and remain stationary. The speed can be varied by changing the supply frequency because the synchronous speed (Ns) = 120f/P. Due to the unavailability of economical variable frequency sources, this method of control was not used in the past, and they were mainly used for constant speed applications. The development of semiconductor variable frequency sources, such as inverters and cycloconverters, has allowed the use of synchronous motors in variable speed applications. They are not self-starting and must be run up to synchronous speed by some means before they can be synchronized with the supply. Starting methods usually involve auxiliary motors or damper windings.

Types of Synchronous Motors

• Wound field synchronous motor
• Permanent magnet synchronous motor
• Synchronous reluctance motor
• Hysteresis motor

All these motors have a stator with a three-phase winding connected to an AC source. Fractional horsepower synchronous reluctance and hysteresis motors employ a single-phase stator.

Operation of Wound Field Synchronous Motors

The rotor is provided with a DC field winding and a damper winding. The stator contains a three-phase winding and is connected to an AC supply. The stator produces the same number of poles as the DC field. When a three-phase supply is given to the stator, a rotating magnetic field revolving at synchronous speed is produced. DC excitation in the rotor produces a field that interacts with the rotating magnetic field. Initially, the torque is pulsating and not unidirectional, so the synchronous motor is not self-starting. Normally, the motor is made self-starting by providing a damper winding on the rotor. Due to the presence of the damper winding, the motor will start as an induction motor. When the speed of the motor is near synchronous speed, DC excitation is applied to the rotor. The rotor locks with the rotating magnetic field of the stator and continues to rotate at synchronous speed.

Load Angle/Power Angle/Torque Angle

The rotor locks with the stator’s rotating magnetic field, and both run at synchronous speed in the same direction. When the load on the motor increases, the rotor falls back in phase by some angle, known as the load angle. The value of the load angle depends on the load.

Pull-Out Torque

The torque produced by a synchronous motor is given by Pm = 3VE/Xs * sinδ, and the torque T = P/ω = 3VE/ωXs * sinδ. The maximum torque, Tmax = 3VE/ωXs, is known as the pull-out torque. Increasing the torque beyond this value causes the motor to slow down, and synchronization is lost. This phenomenon is called pulling out of step.

Variable Frequency Control of Synchronous Motors

Synchronous speed is directly proportional to frequency. By varying the frequency, the speed can be controlled. Similar to an induction motor, at the base speed and below, the V/f ratio is kept constant. Above the base speed, the terminal voltage is maintained at the rated value, and the frequency is varied. Variable frequency control of synchronous motors can operate in two modes: true synchronous mode and self-controlled mode.

True Synchronous Mode

In this mode, the stator supply frequency is controlled by an independent oscillator. The frequency is initially set to a low value and then varied gradually. The difference between the synchronous speed and the actual speed is always small. The frequency command is applied to the VSI (Voltage Source Inverter) through a delay circuit. The rotor speed is able to track the changes. Flux control is achieved by adjusting the stator voltage with frequency to maintain constant flux below the base speed and constant terminal voltage above the base speed. Under steady-state conditions, a gradual increase in frequency causes the synchronous speed to be greater than the actual speed, and the torque angle increases. To follow the change in frequency, the motor accelerates and settles at the new speed after some oscillations, which are damped by the damper winding. A gradual decrease in frequency causes the synchronous speed to become less than the actual speed, and the torque angle becomes negative. To follow the change in frequency, the motor decelerates in regenerative braking and settles down at the new speed after hunting oscillations. The frequency must be changed gradually to allow the rotor to track the change in the revolving field; otherwise, the motor may pull out of step. This method is employed in multiple synchronous motor drives requiring accurate speed tracking.

Self-Controlled Mode

A machine is said to be self-controlled when it gets its variable frequency from an inverter whose thyristors are fired in sequence using information about the rotor position.

I) Rotor Position Sensor: A rotor position sensor is used to measure the position of the rotor with respect to the stator and send pulses to the thyristors. The frequency of the inverter output is decided by the rotor. The supply frequency changes with the synchronous speed and that of the rotor. Hence, the rotor cannot pull out of step. Hunting and oscillations are eliminated. A self-controlled motor has the properties of a DC machine, both under steady-state and dynamic conditions. It is also called a commutatorless motor.

II) Stator Voltage Sensor: Firing pulses for the inverter are derived from the stator-induced voltage. A synchronous machine with an inverter can be considered similar to a line-commutated converter, where firing pulses are synchronized with the line voltage. Variable-speed synchronous motor drives are generally operated in self-controlled mode.

VSI-Fed Synchronous Motor Drive

VSI-fed synchronous motor drives can be classified as follows:

1. Self-control mode using a rotor position sensor and stator voltage sensor: Here, the output frequency is controlled by the inverter, and the voltage is controlled by a controlled rectifier. If the inverter is a PWM (Pulse Width Modulation) inverter, both frequency and voltage can be controlled within the inverter. Below the base frequency, the V/f ratio is kept constant, and above the base frequency, the voltage is varied while keeping it at the rated value.
2. True synchronous mode: The speed of the motor is determined by an external independent oscillator. Here, the output frequency and voltage are controlled within the PWM inverter. If the inverter is not a PWM converter, then the voltage is controlled by using a controlled rectifier, and the frequency is controlled by the inverter.

Advantages and Disadvantages of True Synchronous Mode

• Multi-motor drive is possible.
• Involves hunting and stability problems.
• Implemented using VSI and CSI (Current Source Inverter).
• Power factor control is possible in wound-field synchronous motors by controlling field excitation.

Advantages and Disadvantages of Self-Controlled Mode

• Eliminates hunting and stability problems.
• Good dynamic response.
• Can be implemented using VLSI and CSI.
• Load commutation of the inverter is possible, and there is no need for forced commutation.
• Power factor can be controlled in wound-field synchronous motors by controlling field excitation.

Self-Controlled Synchronous Motor Drive with Load-Commutated Thyristor Inverter

A CSI-fed synchronous motor drive may employ a load-commutated thyristor inverter. A synchronous motor fed from a CSI can be operated in self-controlled mode. When fed from a CSI, a synchronous motor operates at a leading power factor, so the inverter will work as a load-commutated inverter. The source-side converter is a six-pulse line-commutated thyristor converter. For firing angles between 0 and 90 degrees, it works as a line-commutated fully controlled rectifier, delivering positive Vd and Id. For firing angles between 90 and 180 degrees, it works as a line-commutated inverter, delivering negative Vd and Id. When the synchronous motor operates at a leading power factor, the load-side converter can be commutated by the motor-induced voltage in the same way that thyristors in a line-commutated converter are commutated by line voltages. Commutation aided by induced voltage is known as load commutation. The load-side converter will work as an inverter.

• Motoring Operation: For firing angles between 0 and 90 degrees, the source-side converter works as a rectifier, and the load-side converter works as an inverter, causing power to flow from the AC source to the motor.
• Generating Operation: For firing angles between 90 and 180 degrees, the load-side converter works as a rectifier, and the source-side converter works as an inverter, causing power to flow from the motor to the AC source.

The DC link inductor produces a ripple in the DC link current. The load-side converter works as a CSI. When operating in self-controlled mode, the rotating magnetic field should be the same as that of the rotor. This condition is achieved by making the frequency of the load-side converter output voltage equal to the frequency induced in the armature. Hall sensors are used to obtain rotor position information. The main difference between CSI-fed induction motors and synchronous motors is that an induction motor drive uses forced commutation, while a synchronous motor drive uses load commutation.

Advantages of Load Commutation

• High efficiency.
• Four-quadrant operation is possible with regenerative braking.
• Higher power output rating.
• Ability to run at higher speeds.

Permanent Magnet Synchronous Motor (PMSM) Drive

In a permanent magnet synchronous motor, the DC field winding in the rotor is replaced by a permanent magnet.

Advantages of Using Permanent Magnets

• Elimination of field copper loss.
• Higher power density.
• Lower rotor inertia.
• More robust construction of the rotor.
• Higher efficiency.

Drawbacks of Using Permanent Magnets

• Loss of flexibility in field flux control.
• Demagnetization effect.
• Higher cost.

Commonly Used Materials for Permanent Magnets

• Alnico
• Ferrite
• Cobalt Samarium
• Neodymium Iron Boron
• Barium and Strontium Ferrite

Construction of PMSM

The main parts are the stator and the rotor. A three-phase winding is placed in the stator slots. The rotor contains permanent magnets. Based on the construction of the rotor, permanent magnet synchronous motors are classified into two types:

• Surface-Mounted Permanent Magnet Motor: Permanent magnets are mounted on the surface of the rotor using epoxy resin. The rotor has an iron core made up of laminations. This type of motor is used for high-speed applications.
• Interior Permanent Magnet Motor: Permanent magnets are placed inside the slots of the rotor. Since the permanent magnets are placed inside the rotor, it has a non-salient pole structure. This type of motor is used for high-speed applications.

Types of PMSM Drives

Based on the nature of the voltage induced in the stator, PMSM drives are classified into sinusoidal PMAC motors and trapezoidal PMAC motors. The speed of PMAC motors is controlled by feeding them from a variable-frequency voltage or current source inverter. They are operated in self-controlled mode. Rotor position sensors are employed for operation in self-controlled mode. Alternatively, stator-induced voltage can be used for achieving self-control. MOSFET inverters are used for low-voltage and low-power applications, while IGBTs are used for high-voltage and high-power applications.

Sinusoidal Permanent Magnet AC (PMAC) Motor

The stator has a three-phase winding that is sinusoidally distributed in the slots. The stator winding is excited from a three-phase supply to produce a rotating magnetic field. The rotor contains permanent magnets embedded in the steel rotor to create a constant magnetic field. The rotor is shaped so that the voltage induced in the stator phases is sinusoidal. Permanent magnet AC motors are not self-starting, like wound-field synchronous motors. Damper windings cannot be used on the rotor to make the motor self-starting. The motor requires a variable AC source for starting.

Speed Control of Sinusoidal PMAC Motors

The speed of the motor can be varied by changing the stator supply frequency. For speed control below the base speed, the V/f ratio is kept constant. For speed control above the base speed, the voltage is kept at its maximum rated value, and the frequency is varied. Permanent magnet AC motors are fed from a variable-voltage, variable-frequency inverter. The inverter switches are fired according to the rotor position information. The inverter can operate in either 120-degree or 180-degree conduction mode.

• 120-Degree Conduction Mode: At any given point in time, only two switches conduct, and six switch combinations are possible.
• 180-Degree Conduction Mode: At any given point in time, at least three switches are on.

Trapezoidal PMAC Motor (BLDC Motor)

This motor is also known as a brushless DC (BLDC) motor. The stator has a three-phase concentrated winding. The rotor contains permanent magnets with a wide pole arc so that the rotor-induced voltage has a trapezoidal shape. BLDC motors are supplied from an inverter. The motor is operated only in self-controlled mode. Rotor position information is required, which is obtained by using Hall sensors or from the stator current and voltage.

Working of BLDC Motor

The motor is supplied from an inverter. The inverter switches are turned on and off in sequence to ensure proper commutation. Two phases are energized at any instant. Permanent magnets create rotor flux, and energized stator windings create electromagnetic poles. The rotor is attracted by the energized stator poles. By applying the appropriate sequence of steps for the supply, a rotating magnetic field of the stator is created and maintained. The rotor follows the stator’s rotating magnetic field. The stator has concentric windings, so the induced EMF is trapezoidal in nature. Here, two switches are on at any instant so that two phases are energized. Each switch conducts for 120 degrees. The switching sequence and waveforms are similar to those of a DC motor without a commutator and brushes. In a DC motor, the voltage induced in the stator is proportional to the speed, and the torque is proportional to the current. The stator and rotor magnetic fields remain stationary with respect to each other. In a BLDC motor, commutation is achieved by proper switching of the inverter switches.

Comparison Between Sinusoidal and Trapezoidal PMAC Motors

Microcontroller-Based PMSM Driver

This is a 16-bit microcontroller with a 28-pin configuration. It has six default PWM output channels. RB3, RB4, and RB5 are input/output ports used to give the Hall sensor output signals to the controller. RC14 is a digital input port used to give the on/off command. AN1 and AN2 are used to give analog input.

Working Principle

According to the rotor position information from the Hall sensors, the controller will generate PWM signals to control the inverter. The inverter output is given to the motor, and the motor operates according to the voltage from the inverter.