Difference between Synchronous Motor and Induction Motor
AC motors are divided into two types, synchronous motors and asynchronous motors which are also called induction motors. The biggest difference between synchronous motors and asynchronous motors (induction motors) is whether the speed of rotor is consistent with the speed of the rotating magnetic field in the stator. If the rotor’s rotation speed and the stator’s field speed are the same, it is called synchronous motor; if not, it is asynchronous motor. Furthermore, there are big differences specific to the performance parameters and applications between the two.
Difference in Construction
The stator windings of synchronous and induction motors are alike, and the main difference lies in the structure of the rotor. There are DC field windings in the rotor of synchronous motor, which need to be provided with the external excitation power introduced through the slip ring. However, the rotor windings of induction motor is short-circuit, which produce current by electromagnetic induction. In contrast, synchronous motors are more complex and costly.
- Stator
Stator components of synchronous motor are basically the same as ones of induction motors, playing a role of receiving, outputting the electric energy and producing rotating magnetic field. There is not much difference in the form of the result. The stators of both synchronous motor and induction motor are made of magnetic stator core, conductive three-phase AC windings, the base to fixing core, the terminal cover etc. - Rotor
Synchronous motor: the pole core of rotor is laminated by the steel sheets which are punched into by steel plates. The pole core is put on by excitation windings which are wound with insulated copper wires.
For permanent magnet synchronous motor, the permanent magnet on the rotor is the key factor to distinguish it from other motors.
Induction motor: the rotor is composed of iron core and windings, and it is made of steel sheets laminated, and it is installed on the rotating shaft. There are two types of the rotor: squirrel-cage type and wound type. The wound-type induction motor is also equipped with a slip ring and a brush mechanism.
Difference in Working
1. Synchronous motor
The synchronous motor rotates for the interaction between the rotating magnetic field produced by the powering-on stator windings and the magnetic field the rotor generated. For PM synchronous motor, it rotates by 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 motor.
During the steady-state operation of synchronous motor, there exists a constant relationship between the rotor rotating speed and the grid frequency:
N = Ns = 120f / p
f - the grid frequency, p - the motor pole number, Ns - synchronous speed.
2. Induction Motor
Three-phase induction motor’s stator core is embedded with three-phase symmetrical windings. After the power on, between the stator and rotor produce a rotating magnetic field which rotates at a synchronous speed. The rotor bar is cut by the rotating magnetic field in which produce the induced current. The powering-on rotor bar is subjected to electromagnetic force in the rotating magnetic field, thus the rotor overcomes the rotation of load torque and accelerates its rotation. When the electromagnetic torque is equal to the load torque, the motor rotates at a constant speed.
Induction motor’s rotating speed (stator speed) is slower than the rotating magnetic field speed, and this difference is called "slip", expressed by the percentage of synchronous speed:
S = (Ns-N) / Ns.
S - slip, Ns - the magnetic field speed, N - the rotor speed.
Difference in Applications
Synchronous motors are mostly used in large generators, while induction motors are almost used as the motors to drive machines.
For synchronous motor, its power factor can be flexibly adjusted by excitation. However, induction motor power factor is not adjustable, so in some large factories, for the more applied induction motors, a synchronous motor can be added as a phase modifier, to adjust the power factors of the factory and grid interface. However, due to the high cost of synchronous motors and much maintenance, capacitors are now commonly used to compensate for power factor.
Synchronous motor’s operation is not easy as induction motor because the synchronous motor has the excitation winding and slip ring in need of high-level controlling the excitation. In addition, compared with the maintenance-free of induction motor, the work for maintaining the synchronous motor is large. Therefore, as a motor, the induction motor is the choice at the most time.
Three phase AC machines are widely used for generation of electrical energy and for driving loads in industries and homes. The two types of AC rotating machines that are widely used are induction machines and synchronous machines. The main differences between the two two types of AC rotating machines in terms of 1. Construction, 2. Operating Principle, 4. Operating characteristics, 4. Excitation, and 5. Applications are given below.
1. IN TERMS OF CONSTRUCTION
The stator of both machines are similar is construction. They are made of laminated steel sheets. The rotor of synchronous machines are DC excited whilst those of induction machines are short-circuited and are excited by AC. Induction machines are self-starting while synchronous machines require auxiliary starting equipment.
2. IN TERMS OF OPERATION PRINCIPLE
Induction machines operate on the principle of electromagnetic induction. A rotating magnetic field is produced in the stator windings as the alternating current flows through it. The speed of this magnetic field is at proportional to the supply frequency and is known as the synchronous speed. The rotor windings are short circuited on one end. There are two types of Induction Machine Rotors, namely; squirrel cage induction machines and wound-rotor induction machines.
As the magnetic field rotates, they cut rotor windings, and eventually induce a voltage in them. This induced voltage causes current to flow in the windings, which produces its own magnetic field. The magnetic field of the rotor tries to catch up with the magnetic field of the stator. If the two magnetic fields rotate at the same speed, the rotor stops, its speed decreases and therefore tries to catch up with the synchronous speed. The speed of the rotor is known as the rotor speed and the difference between the synchronous speed and the rotor speed is known as the slip speed.
The rotor speed will always be less than the synchronous speed in induction machines.
In synchronous machines, the speed of the rotor is always equal to the speed of the rotating magnetic field. This is because the rotor magnetic field locks with the stator magnetic field and the two fields rotate at the same speed.
3. IN TERMS OF EXCITATION
Synchronous machines are doubly excited whereas induction machines are singly excited machines. In synchronous machines, the rotor is excited from a DC source supplied through brushes or mounted on the shaft of the machine. The DC excitor produces a stationary magnatic field that locks with the rotating magnetic field produced by the field windings when they are supplied by an AC source.
Induction machines have both their stator and rotor windings excited by AC.
4. IN TERMS OF OPERATING CHARACTERISTICS
Induction machines are inductive loads. They take inductive current and have a lagging power factor. Synchronous machines can be made to behave as inductive loads or capacitive loads by varying the excitation voltage. If the terminal voltage is less than the excitation voltage, an synchronous machine used as a motor is said to be overexcited and behaves like a capacitive load by taking in a leading current. When the terminal voltage is greater than the excitation voltage, the synchronous motor is said to be underexcited and behaves like and inductor.
5. IN TERMS OF APPLICATION
Synchronous machines are usually used as large generators in generating power stations such as in steam plants (using Round Motor Synchronous Machines) and hydro-electric plants (using Salient Pole Rotor Synchronous machines). Induction machines are widely used as motors in industries to drive loads such as convey belts, cranes etc. In addition, synchronous machines can be used for power factor improvement when they are connected in parallel with a load, operating on NO LOAD and OVEREXCITED.