What is Synchronous Motor?
A synchronous motor is an electromagnetic motor that operates at a constant speed known as the synchronous speed. The synchronous speed is determined by the frequency of the power supply and the number of poles on the motor's stator. The synchronous speed is typically expressed in revolutions per minute (RPM).
What is Induction Motor?
An induction motor is an electromagnetic motor that operates based on the principle of electromagnetic induction. Electromagnetic induction is the process by which an electrical current is induced in a conductor when it is placed in a rotating magnetic field.
Difference between Induction Motor and Synchronous Motor
| Content | Synchronous Motor | Induction Motor | |
|---|---|---|---|
| 01 | Definition | The electromagnetic motor which converts electrical energy into mechanical work at constant speed | The electromagnetic motor which converts electrical energy into mechanical work at variable speed |
| 02 | Motor Speed | Runs at synchronous speed | Runs at non-synchronous speed |
| 03 | Principle | Works on principle of magnetic locking | Works on principle of electromagnetic induction |
| 04 | Starting Position | Non-self starting | Self-starting (especially three-phase induction motor) |
| 05 | Connected Supply | Stator connects to three-phase AC supply, rotor connects to DC supply | Stator connects to three-phase AC supply |
| 06 | Excitation System | Requires DC excitation system (or prime mover) to start | Does not require excitation system to start |
| 07 | Construction | More difficult | Relatively easy |
| 08 | Relative Motion | No relative motion between stator and rotor | Relative motion required between stator and rotor |
| 09 | Slip | Zero slip | Different slip |
| 10 | Motor Speed | Runs from 150 to 1800 rpm | Runs at less than 1500 rpm |
| 11 | Power Factor | Unity, lagging, or leading power factor (mostly unity) | Lagging power factor |
| 12 | Efficiency | More efficient due to unity or leading power factor | Less efficient than synchronous motor |
| 13 | Uses | Used for power factor correction, constant speed load service, voltage regulation of transmission lines, etc. | Mostly used in industries |
| 14 | Cost | More expensive | Cheaper |
| 15 | Maintenance | Requires more maintenance | Requires less maintenance |
| 16 | Example | Fan, blowing fan, dryer | Conveyor belt, rolling mill, mixer, grinder |
Key Difference between Induction Motor and Synchronous Motor
- A three-phase synchronous motor is a doubly excited machine, meaning that both the armature winding and the field winding are energized by external sources. An induction motor is a single excited machine, meaning that only the stator winding is energized by an external AC source.
- The armature winding of a synchronous motor is energized from an AC source, while the field winding is energized from a DC source. In contrast, the stator winding of an induction motor is energized from an AC source.
- Synchronous motors always run at a constant speed, called the synchronous speed, which is independent of the load. Induction motors, on the other hand, always run at a speed slightly lower than the synchronous speed, and the speed decreases as the load increases.
- Synchronous motors are not self-starting, meaning that they require some external means to bring them up to synchronous speed before they can be synchronized to the AC supply. Induction motors, on the other hand, have self-starting torque and can start on their own.
- Synchronous motors can be operated with both lagging and leading power factors, depending on the excitation. Induction motors, on the other hand, operate only at a lagging power factor and have a poor power factor at high loads.
- Synchronous motors can be used for both driving mechanical loads and correcting power factor, while induction motors are only used for driving mechanical loads.
- Synchronous motors are generally more efficient than induction motors of the same output and voltage rating, but they are also typically more expensive.
Other Key Points of difference between induction motor and Synchronous motor:
- Operating principle: An induction motor operates on the principle of electromagnetic induction, whereas a synchronous motor operates on the principle of electromagnetic attraction.
- Construction: An induction motor has a stator and a rotor, with the stator being the stationary part and the rotor being the rotating part. A synchronous motor has both a stator and a rotor, but the rotor is magnetically coupled to the stator and rotates at the same speed as the stator field.
- Speed: The speed of an induction motor is dependent on the frequency of the applied voltage and the number of poles. The speed of a synchronous motor is determined by the frequency of the applied voltage and the number of poles, as well as the number of stator teeth.
- Starting torque: An induction motor has a relatively low starting torque compared to a synchronous motor. This means that it requires more time to reach its full speed when starting.
- Efficiency: Induction motors are generally more efficient than synchronous motors, especially at low speeds.
- Power factor: Induction motors have a power factor that is less than unity (i.e., less than 1), whereas synchronous motors have a power factor that is equal to unity.
- Excitation: An induction motor is self-excited, meaning that it generates its own magnetic field. A synchronous motor requires an external source of excitation to generate its magnetic field.
- Load characteristics: An induction motor is suitable for use with variable loads, as it can adjust its speed and torque to meet the changing demand. A synchronous motor is best suited for constant load applications, as it has a constant speed and torque output.
- Maintenance: An induction motor requires minimal maintenance compared to a synchronous motor, which requires more frequent maintenance due to the need for external excitation.
- Cost: Induction motors are generally less expensive to purchase and operate than synchronous motors.
- Slip: An induction motor has a slip, which is the difference between the synchronous speed and the actual speed of the rotor. A synchronous motor has a slip of zero, as the rotor rotates at the same speed as the stator field.
- Starting method: An induction motor can be started using a starter or by connecting it directly to the power supply. A synchronous motor requires a special starting method, such as using a motor-generator set or a static starter.
- Power transmission: Induction motors are commonly used for power transmission applications, such as in pumps, fans, and conveyors. Synchronous motors are less common for power transmission applications due to their high cost and maintenance requirements.
- Torque-speed characteristics: An induction motor has a nonlinear torque-speed characteristic, meaning that the torque output varies with the speed. A synchronous motor has a linear torque-speed characteristic, meaning that the torque output is constant over a wide range of speeds.
- Load ability: An induction motor can handle a wide range of load conditions and can operate at any load. A synchronous motor is limited in its load ability and can only operate at a specific load.
- Power factor correction: Induction motors are often used in power factor correction applications to improve the power factor of a system. Synchronous motors are not commonly used for power factor correction due to their constant power factor.
- Voltage regulation: Induction motors have a poor voltage regulation, meaning that their speed and torque output can be affected by changes in the applied voltage. Synchronous motors have a good voltage regulation, as they are less sensitive to changes in the applied voltage.
- Torque ripple: An induction motor generates a torque ripple, which is the fluctuation of the torque output. A synchronous motor generates a smooth torque output with minimal torque ripple.
- Vibration: Induction motors can generate vibration due to the fluctuating torque output and the mechanical imbalance of the rotor. Synchronous motors have a smooth torque output and a balanced rotor, resulting in minimal vibration.
- Power range: Induction motors are available in a wide range of power ratings, from fractional horsepower to several thousand horsepower. Synchronous motors are typically available in a narrower power range, from several hundred horsepower to several thousand horsepower.