What is a synchronous motor?

The synchronous motor is an AC electric type motor where at a steady-state the shaft rotation is synchronized with the supply current variable-frequency. To an integral number of AC cycles, the rotation period is exactly equal. The stator of the motor in the synchronous motor contains multi-phase AC electromagnets due to which a magnetic field is created that rotates in time with the oscillation of the line current. The rotor contains a permanent magnet and the rotor rotates in synchronization with the stator field at the same rate and provides the second synchronized magnetic field which is rotating in nature as a result. When the synchronous motor is supplied with independently excited multi-phase AC electromagnets on both the rotor and the stator, it is termed as a doubly-fed machine. The most widely used AC-type motors are the synchronous AC motor and the induction AC motors.

An induction motor is a type of asynchronous motor. To induce a current in rotor winding the rotor must rotate slightly slower than AC alternations. In timing applications such as in timers in appliances, synchronous clocks, tape recorders, etc. small synchronous motors are used. Synchronous motor is available in sub fractional horsepower sizes which is self-excited to high power industrial sizes. Wherever the precise constant speed motor-drive is required the fractional horsepower range synchronous motors are used. These machines are usually used in timers, electrical clocks which are analog in nature, and other devices where correct time is required.

Two important functions are provided by the synchronous motor which are in high power industrial sizes.

• Firstly, it converts AC energy into work.
• Secondly, it can provide power factor correction because it can operate at leading or unity power factor. The power factor of the motor can be varied by varying the DC or direct-current excitation of the motor.

The difference between the induction motor and the synchronous motor is that since the synchronous motor does not rely on current induction to produce the rotor magnetic field it rotates at a rate locked to the line frequency whereas the induction motor does.

Synchronous speed

The synchronous rotational speed of a synchronous motor is given in RPM by:

$N_S = \frac{120 x f}{p}$

where f = frequency in Hz and p = number of poles

and in rad/second it is given by :

${\omega }_s = \frac{4\pi f}{p}$

where f = frequency in Hz and p = number of poles

Example

Consider a single phase, 4-pole synchronous motor is operating at an AC frequency of supply of 50 Hz so the synchronous speed will be given by $N_S = \frac{120 x 50}{4} = 1200 rpm$.

Construction

The stator and rotor are the principal components of the synchronous motor. The stator of both induction and the synchronous motor is similar in construction. As an exception, the stator frame contains a wrapper plate with the wound-rotor synchronous doubly-fed electrical machine. The keybars and circumferential ribs are attached to the wrapper plate. Frame mount and footings are required to carry the machine. To connect to the excitation supply brushes and slip rings are required when the field winding is excited by DC excitation but when it is non-excited it would not get connected. With the help of a brushless exciter also the field winding can be excited. For up to six poles the cylindrical round rotors are used.

Unlike a non-salient pole rotor, a salient pole rotor is used when a large number of poles are needed in some machines. To that of the synchronous alternator, the construction of synchronous motors is almost similar. The stationary armature and rotating field winding are used in construction by most synchronous motors. This type of construction is an advantage to the DC motor where the armature used is of rotating in nature.

Operations

Due to the interaction of the stator and rotor field, a synchronous motor operates. The stator winding which is present in the stator is provided with a three-phase supply and the rotor is provided with a DC supply. A three-phase rotating magnetic flux is produced by the three-phase stator winding carrying a three-phase current. The rotor gets locked with this rotating magnetic field and rotates accordingly with it. The motor is said to be in synchronization once the rotor field gets locked with the stator field. A stator having single-phase winding is also possible but here the direction of rotation is not defined because the machine may start to rotate in either direction unless prevented from doing so with the help of starting arrangements.

The speed of the motor is dependent only on supply frequency once the motor is in operation. The field winding no longer follows the rotating magnetic field and the motor falls out of synchronization when the motor load is increased beyond the breakdown load. If it falls out of synchronization since the motor can not produce torque, the practical synchronous motors have windings called damper windings to stabilize the operation and to facilitate the starting.

The torque is produced by the phase difference between the rotating magnetic field and the rotor. When there is a small phase difference between the rotating rotor and the rotating magnetic field, the torque produced is low in value. The torque which is developed in the starting is called starting torque and the full load torque of the motor is called a running torque. Then that of an equivalent induction motor since these winding's are smaller and on long operation can overheat and in the rotor excitation winding's since large voltages are induced, the protection devices of synchronous motors can sense these condition and cut out the power supply.

Starting methods

Synchronous motors are those kinds of motors that are not self-start. This is due to the inertia of the rotor so it cannot instantly follow the rotation of the stator magnetic field. No inherent torque is produced by the synchronous motor during standstill conditions. So, without some supplement mechanism, it cannot accelerate to synchronous speed.

The squirrel-cage induction motor is included in the large motors operating on commercial power frequency which provides sufficient torque for acceleration and in motor speed operation which also serves to damp oscillations. The field winding gets excited and the motor pulls out of the synchronization once the rotor nears the synchronous speed.

Before the load is applied the very large motors system may include a pony motor which accelerates the synchronous machine which is unloaded. By changing the frequency of stator current, the motors that are controlled electronically can be accelerated from zero speed. Inline powered electrical mechanical clocks or very small synchronous motor timers are used, which uses the line power frequency to run the gear mechanism at a correct speed. If the moment of inertia of the rotor and its mechanical load is sufficiently small then such small synchronous motors are able to start without assistance.

To address different starting methods the operational economics is an important parameter. To solve the motor starting issue the excitation of the rotor is the possible way. The running torque in the synchronous motor is from 0.3 to 16 oz-in. A shaded-pole synchronous motor is a type of AC motor which uses field coils with additional copper shading coils to produce a rotating field that is weak. Nidec motors corporation makes the best synchronous motors.

Context and Applications

This topic is significant in the professional exams for graduate and postgraduate courses.

• Bachelors in Electrical Engineering
• Masters in Electrical Engineering

Practice Problems

Q1. Which power factor the synchronous motors can operate?

1. Unity power factor
2. Leading power factor
3. Both a and b
4. None of these

Answer: Option c

Explanation: It can operate leading and unity power factor because the synchronous motors convert AC energy into work and also provide power factor correction.

Q2. Induction motors are which type of motors?

1. Synchronous motors
2. Asynchronous motors
3. DC motors
4. None of these

Answer: Option b

Explanation: The induction motors are the types of synchronous motors.

Q3. What is the running torque in synchronous motors?

1. 0.3 to 16 oz-in
2. 17 to 20 oz-in
3. 50 to 100 oz-in
4. None of these

Answer: Option a

Explanation: The running torque in synchronous motors is from 0.3 to 16 oz-in.

Q4. What happens to synchronous speed as load changes?

1. Increases as the load increases
2. Decreases as the load decreases
3. Always remains constant
4. None of these

Answer: Option c

Explanation: Synchronous speed is a constant speed that depends on the supply frequency and the number of poles.

Q5. What does the rotor contain?

1. Permanent magnet
2. Electrical magnet
3. Electromagnet
4. None of these

Answer: Option a

Explanation: The rotor contains a permanent magnet and the rotor rotates in synchronization with the stator field at the same rate and provides the second synchronized magnetic field which is rotating in nature as a result.

Related Concepts

• Clock drive
• Doubly fed electrical machine
• Short circuit ratio