Difference between Synchronous Motor and Induction Motor

AC motors are divided into two types, synchronous motors and asynchronous motors also called induction motors. The biggest difference between synchronous and asynchronous motors (induction motors) is whether the speed of the rotor is consistent with the speed of the rotating magnetic field in the stator. If the rotor speed of rotation and the stator field speed are the same, this is called a synchronous motor; otherwise, it is an asynchronous motor. In addition, there are major differences specific to the performance and application parameters between the two.

Difference in construction

The stator windings of the synchronous and induction motors are similar, and the main difference is in the rotor structure. There are DC field windings in the rotor of the synchronous motor, which must be supplied with the external excitation power introduced through the slip ring. However, the rotor windings of the induction motor are short-circuited, which produce current by electromagnetic induction. On the other hand, synchronous motors are more complex and expensive.

Stator

The components of the synchronous motor stator are basically the same as those of induction motors, playing a role in receiving, producing electrical energy and producing rotating magnetic fields. There is not much difference in the form of the result. The stators of the synchronous motor and the induction motor are made of the magnetic stator core, conductive three-phase AC windings, the base of the fixing core, the terminal cover etc.

Rotor

• Synchronous motor: the core of the rotor pole is laminated by steel sheets perforated by steel sheets. The pole core is placed by excitation windings that are wound with insulated copper wires. Structure of the PM synchronous motor For permanent magnet synchronous motors, the permanent magnet on the rotor is the key factor to distinguish it from other motors.
• Induction motor: the rotor consists of iron core and windings, is made of laminated steel sheets and is installed on the rotary shaft. There are two types of rotor: squirrel cage and wound type. The wound type induction motor is also equipped with a slip ring and a brush mechanism.

Difference at work

1. Synchronous motor

The synchronous motor rotates for the interaction between the rotating magnetic field produced by the drive stator windings and the magnetic field generated by the rotor. For the PM synchronous motor, it rotates due to the driving torque generated by the interaction between the rotating magnetic field of the stator and the secondary magnetic field of the rotor. As for the rotor winding, it does not induce current during the normal rotation of the motor and also does not participate in the work. It only serves to start the engine.

During the steady state operation of the synchronous motor, there is a constant relationship between the rotor rotation speed and the grid frequency:

N = Ns = 120f / p

f - the network frequency, p - the number of the motor pole, Ns - synchronous speed.

2. Induction motor

The stator core of the three-phase induction motor is incorporated with symmetrical three-phase windings. After the connection, between the stator and the rotor produces a rotating magnetic field that rotates at a synchronous speed. The rotor bar is cut by the rotating magnetic field in which it produces the induced current. The rotor drive bar is subject to electromagnetic force in the rotating magnetic field, therefore, the rotor overcomes the rotation of the load torque and accelerates its rotation. When the electromagnetic torque is equal to the load torque, the motor rotates at a constant speed.

The rotation speed of the induction motor (stator speed) is slower than the speed of the magnetic field of rotation and this difference is called "slip", expressed by the percentage of synchronous speed:

S = (Ns-N) / Ns.

S - slip, Ns - the speed of the magnetic field, N - the speed of the rotor.

Difference in applications

• Synchronous motors are used mainly in large generators, while induction motors are almost used as motors to drive machines.
• For synchronous motors, their power factor can be flexibly adjusted by excitation. However, the power factor of the induction motor is not adjustable; therefore, in some large factories, for the most applied induction motors, a synchronous motor can be added as a phase modifier, to adjust the power factors of the factory and network interface. However, due to the high cost of synchronous motors and a lot of maintenance, capacitors are now commonly used to compensate for the power factor.
• The operation of the synchronous motor is not as easy as the induction motor because the synchronous motor has the excitation winding and the slip ring in need of a high-level control of the excitation. In addition, compared to the maintenance-free induction motor, the work to keep the motor synchronous is great. So, like an engine, the engine

Synchronous Motor Excitation System

Before understanding this excitation of the electrical motor, it must be remembered that any electromagnetic device must draw a magnetizing current from the AC source to supply the required workflow. This magnetizing current is nearly 90o at the supply voltage. In other words, the function of this magnetizing current or delayed VA attracted by the electromagnetic device is to align the flow within the magnetic circuit of the device. So it comes under an electromagnetic device. It receives three-phase electrical supply to the armature winding and DC power to the rotor winding.

Synchronous motor excitation refers to the DC power supplied to the rotor, which is employed to provide the required magnetic flux.

One of the most and unique characteristics of this motor is that it's frequently operated on any electric power factor that drives, is delayed or with a unit, and this feature is predicated on the excitation of the electrical motor. When the electric motor is running at constant applied voltage V, the resulting air gap flow, as needed by V, remains substantially constant. This resulting air gap is established by the cooperation of the AC supply of the armature winding and therefore the DC supply of the rotor winding.

Synchronous motor excitation refers to the DC power supplied to the rotor, which is employed to supply the required magnetic flux.

One of the most and unique characteristics of this motor is that it is often operated on any electric power factor it's carrying, with delay or unit, and this feature is predicated on the excitation of the electric motor. When the electric motor is working with constant applied voltage V, the resulting air gap flow, as needed by V, remains substantially constant. This resulting air gap is established by the cooperation of the AC supply of the armature winding and therefore the DC supply of the rotor winding.

CASE 1: When the sector current is sufficient to supply the air gap flow, as needed by the constant supply voltage V, the magnetization current or the delayed reactive VA required from the AC source is zero and therefore the motor operates with the factor the unit's power. the sector current, which causes this unit power factor, is named normal excitation or normal field current.


CASE 2: If the sector currently isn't sufficient to supply the required air gap flow, as needed by V, the extra magnetization current or delayed reactive VA are going to be drawn from the AC source. This magnetizing current produces the deficient flow (constant flow configured by the winding of the DC supply rotor). Therefore, during this case, the engine is claimed to work under a delayed power factor and is claimed to be under excitation.

CASE 3: If the sector current is bigger than the traditional current, the motor is excited. This excess current within the field produces excess flow (flow configured by the winding of the DC supply rotor - flow resulting from the air gap) that has got to be neutralized by the armature winding.

Therefore, the armature winding absorbs the most reactive VA or the initial voltage of the demagnetizing current at almost 90o from the AC source. Therefore, during this case, the engine operates under the most power factor.

This whole concept of excitation and power factor of the electric motor is often summarized within the following graph. this is often called the V curve of the electric motor.

Image Source- Google

Conclusion: An overexcited electric motor operates with the main power factor, an under excited electric motor operates with a lagged power factor and a traditional excited electric motor operates with one power factor.