In this article, we will discuss the differences between synchronous motor and asynchronous motor. But before that, let’s see what an electric motor, synchronous motor, and asynchronous motor are.
What is an Electric Motor?
Electric motors are defined machines or devices that transform electrical energy into mechanical energy.
The electric motor can be classified into two broad categories based on the type of input current supplied as follows:
- AC motor
- DC motor
DC motors can be further divided into four main types based on the arrangement or construction as follows:
- Series DC motor
- Shunt/parallel DC motor
- Compound DC motor
- Permanent magnet DC motor
Furthermore, AC motors can be subdivided into two broad categories as follows:
- Synchronous motor
- Asynchronous motor
The above-mentioned types of AC motors share some similarities, but also have some major differences that set them both apart based on their construction, working principles,s and efficiency. In this article, we will be focusing on synchronous motor and asynchronous motor and the major differences between them.
What is a Synchronous Motor?
A synchronous motor is defined as an electric machine whose rotor rotates at the same speed as the stationary rotatory magnetic field (i.e., the stator magnetic field) at synchronous speed. The synchronous motor is comprised of a stator and rotor as its main parts, along with slip rings, and winding. AC power is supplied to the stator, while DC power is supplied to the rotor winding.
Therefore based on the type of excitement, synchronous motors are of two types:
- Non-excited motor
- DC current excited motor
The working principle of the synchronous motor is as follows:
In the synchronous motor, the AC power supplied to the stator winding of the motor generates a rotating magnetic field around it. DC power is supplied to the rotor winding of the synchronous motor via the slip rings attached to the rotor or a permanent magnet is placed near the rotor, in order to generate a magnetic field around the rotor as well. Another name for a synchronous motor is a ‘dual-excited machine’ as it is supplied with both AC and DC currents to the stator and rotor winding of the motor respectively.
At the start, the synchronous motor does not start at the synchronous speed (rotational magnetic field) because of load inertia. The damper winding is used to provide starting torque. Thus, the synchronous motor is not self-start and needs an extra starting mechanism.
The speed of the synchronous motor depends on the supply frequency and the number of poles of stator windings. The load on the motor does not affect the speed of the motor. The number of poles is fixed and therefore the speed of the motor can be governed by a variable frequency drive.
What is an Asynchronous motor?
An asynchronous motor is defined as an electric machine whose rotor does not rotate at the same speed as that of the stationary rotatory magnetic field (i.e. the stator magnetic field) at synchronous speed. This difference in the speed of the rotor and stator magnetic field is termed a slip. The asynchronous motor is also comprised of a stator winding, rotor, and stator core. as its main parts. Practically, the speed of the rotor is comparatively less as compared to the speed of the stator’s rotating magnetic field.
Based on the type of excitement, the rotor of the asynchronous motor is of two types:
- Squirrel cage rotor
- Wound or slip ring rotor
The squirrel cage rotor is supplied with only one kind of input supply (either AC or DC), and they are also referred to as ‘singly-excited machines’. But the wound-type rotor is supplied with two different kinds of input power. Hence they are termed ‘dual-excited machines’.
The working principle of the asynchronous motor is as follows:
While the working of the asynchronous motor is similar to that of the synchronous motor, the excitement mechanism of the rotor in the asynchronous motor makes it different from the synchronous motor. The rotor of the asynchronous motor is not excited by any external DC supply, but rather the speed of rotation of the asynchronous rotor depends on the magnetic field induced by the stator winding.
Therefore, the asynchronous rotor’s rotation is based on the electromagnetic induction phenomena. An asynchronous motor is also known as an Induction motor because it operates on the principle of electromagnetic induction. The difference in speed of rotation between the stator and the rotor windings of the asynchronous motor is termed as ‘slip’.
The induction motor can not run at a synchronous speed. The motor runs always slower than the synchronous speed and it depends on the slip of the motor. The rotor current does not flow if there is no slip, hence no torque is produced. The slip cause voltage to induce in the rotor and it cause current to flow in the rotor. The torque produced in the motor depends on the rotor current and magnetic flux in the air gap. Thus, the existence of slip is a must for the operation of the asynchronous motor.
The speed of the motor depends on the load. The higher load on the motor causes more slip and subsequently, it lowers the speed of the motor.
Difference between Synchronous and Asynchronous Motor
The comparison table given below highlights all the key differences between a synchronous motor and an asynchronous motor.
| Key | Synchronous Motor | Asynchronous Motor |
| Definition | A synchronous motor is an AC machine whose rotor speed is synchronized with the rotatory magnetic field of the stator winding. | An asynchronous motor is an AC machine whose rotor speed is not synchronized with the rotatory magnetic field of the stator winding. |
| Speed of Rotor | The rotor speed of the synchronous motor is the same as that of the rotational speed of the stator magnetic field. | The rotor speed of the synchronous motor is less than that of the rotational speed of the stator magnetic field. |
| Working Principle | The synchronous motor works on the principle of interlocking of rotor and stator magnetic fields. | The asynchronous motor operates on the electromagnetic induction mechanism existing between the magnetic fields of the stator and rotor. |
| Symbolic Representation | Resistance is represented by the letter ‘R’. | Impedance is represented by the letter ‘Z’. |
| Formula for rotor speed | The rotor speed of the synchronous motor is equal to the synchronous speed; thus, it is given by, Ns=120f/P Where NS is the synchronous speed, f is the supply frequency, and P is the number of stator poles. | The rotor speed for an asynchronous motor is given by, NR= NS(1-s) Where s is the slip, NS is the synchronous speed, and NR is the rotor speed. |
| Factors affecting motor speed | The number of stator poles and the supplied input AC power is the deciding factors of the synchronous motor speed. | The motor slip, mechanical load, and the resistance of the rotor circuit are the deciding factors of the asynchronous motor speed. |
| Unit of measurement | The SI unit of measurement of synchronous speed is radian per second (Rad/s) or revolution per minute (rpm). | The SI unit of measurement of asynchronous speed is also radian per second (Rad/s) or revolution per minute (rpm). |
| Motor slip | The slip of the synchronous motor is zero, i.e., the speed of rotation of both rotor and stator are equal. | The slip of the asynchronous motor is ranging between 0 and 1. The slip of the asynchronous motor is never zero. |
| Load dependence on speed | Change in mechanical load does not affect the speed of the synchronous motor. | Change in mechanical load results in variations in the speed of the asynchronous motor. |
| Power supply to rotor | To generate a rotor magnetic field, DC power is supplied to the rotor. | The asynchronous rotors do not need any form of excitation and can start rotating on their own. |
| Motor speed control | To control the speed of the synchronous motor, the variable frequency drive (VFD) is used to vary the frequency. | To control the speed of the asynchronous motor, the variable frequency drive (VFD) and variable rotor resistance (VRR) are used to vary the frequency. |
| Capital Expenditure | The expense of managing synchronous motors is higher. | The expense of managing asynchronous motors is comparatively lower. |
| Performance | The efficiency of the operation of a synchronous motor is comparatively higher. | The efficiency of the operation of asynchronous motors is comparatively lower. |
| The magnitude of motor speed | The average motor speed is 300 RPM. | The average motor speed is slightly above 600 RPM. |
| Power factor | The power factor of a synchronous motor is either unity, leading, or lagging. | The power factor of the asynchronous motor is always lagging. |
| Applications | Synchronous motor finds its application in power factor correction and operation of mechanical loads. | An asynchronous motor is used only to operate mechanical loads. |
Conclusion
In conclusion, synchronous and asynchronous motors are vital in industrial applications for driving heavy mechanical loads. It is observed from this article that, although synchronous motors are comparatively more efficient in terms of performance, they have a high cost of maintenance and installation.
The most significant difference between them is that synchronous motors have their rotor and stator magnetic fields rotating at the same speed, while asynchronous motors have them running at different speeds, commonly referred to as slip. Moreover, the ease of operation of asynchronous motors in comparison to synchronous motors is an added advantage of asynchronous motor operation.