Synchronous and Asynchronous Motors Details

Each Synchronous and Asynchronous Motor type has its own strengths and suitable applications depending on the requirements.

Synchronous Motor

Definition of Synchronous Motor: A synchronous motor is an AC motor that runs at a speed directly proportional to the frequency of the supply current.

Synchronous Motor operation: It operates by synchronizing the rotation of the rotor with the magnetic field of the stator.

1. Magnetic Field Creation: When a three-phase AC supply is given to the stator, it produces a rotating magnetic field. This magnetic field rotates at synchronous speed, given by:

                            Ns=(120×f)/P

        Where $N_{s}N\_s$ is the synchronous speed in RPM, $ff$ is the frequency of              the AC supply, and $PP$ is the number of poles.

2. Rotor Alignment: A direct current (DC) is supplied to the rotor winding, which creates a fixed magnetic field around the rotor.

3. Magnetic Locking: The rotor magnetic field interacts with the stator’s rotating magnetic field. To achieve synchronous speed, the rotor is pulled into alignment with the rotating magnetic field of the stator and locks in step with it, rotating at the same speed as the field.

4. Constant Speed Operation: Once synchronized, the motor runs at a constant speed, regardless of the load, as long as the load does not exceed the pull-out torque.

Advantages of Synchronous Motor: High efficiency, constant speed under varying loads, and better power factor correction.

Applications of Synchronous Motor: Used in power plants, industries, and applications requiring precise speed control. Synchronous motors are highly efficient and are used in applications requiring precise speed control and a high power factor, such as in power generation stations, industrial applications, and large mechanical systems.

Asynchronous Motor (Induction Motor)

Definition of Asynchronous Motor: An asynchronous motor, commonly known as an induction motor, is an AC motor where the rotor is not synchronized with the stator's rotating magnetic field.

Operation of Asynchronous Motor: The motor operates on the principle of electromagnetic induction, where the rotor induces a current due to the rotating magnetic field of the stator.

1. Rotating Magnetic Field: When an AC supply is given to the stator windings, it produces a rotating magnetic field at synchronous speed ($N_{s}N\_s$). The speed is given by:

$${\mathrm{<b><span\; style="font-size:\; x-large;">N</span></b>}}_{\mathrm{<b><span\; style="font-size:\; x-large;">s</span></b>}}<b><span\; style="font-size:\; x-large;">=</span></b>\frac{\mathrm{<b><span\; style="font-size:\; x-large;">120</span></b>}<b><span\; style="font-size:\; x-large;">\times </span></b>\mathrm{<b><span\; style="font-size:\; x-large;">f</span></b>}}{\mathrm{<b><span\; style="font-size:\; x-large;">P</span></b>}}N\_s\; =\; \backslash frac\{120\; \backslash times\; f\}\{P\}$$

Where $N_{s}N\_s$ is the synchronous speed in RPM, $ff$ is the frequency of the AC supply, and $PP$ is the number of poles.

2. Induced Current in Rotor: The rotating magnetic field cuts across the rotor conductors. According to Faraday's Law of Electromagnetic Induction, this induces a current in the rotor. The frequency of the induced current in the rotor is known as slip frequency, and it depends on the difference between the stator's magnetic field speed and the rotor speed.

3. Torque Production: The induced current in the rotor generates its own magnetic field. According to Lenz's Law, the rotor's magnetic field opposes the change that induced it, trying to follow the rotating magnetic field of the stator. This interaction creates a torque that causes the rotor to turn. However, the rotor can never catch up with the stator's rotating magnetic field; hence, it always runs at a speed slightly less than the synchronous speed.

4. Slip: The difference between the synchronous speed ($N_{s}N\_s$) and the actual rotor speed ($N_{r}N\_r$) is called slip. It is expressed as a percentage:

$$\text{<b><span style="font-size: x-large;">Slip</span></b>}<b><span\; style="font-size:\; x-large;">(</span></b>\mathrm{<b><span\; style="font-size:\; x-large;">s</span></b>}<b><span\; style="font-size:\; x-large;">)</span></b><b><span\; style="font-size:\; x-large;">=</span></b>\frac{{\mathrm{<b><span\; style="font-size:\; x-large;">N</span></b>}}_{\mathrm{<b><span\; style="font-size:\; x-large;">s</span></b>}}<b><span\; style="font-size:\; x-large;">-</span></b>{\mathrm{<b><span\; style="font-size:\; x-large;">N</span></b>}}_{\mathrm{<b><span\; style="font-size:\; x-large;">r</span></b>}}}{{\mathrm{<b><span\; style="font-size:\; x-large;">N</span></b>}}_{\mathrm{<b><span\; style="font-size:\; x-large;">s</span></b>}}}<b><span\; style="font-size:\; x-large;">\times </span></b>\mathrm{<b><span\; style="font-size:\; x-large;">100</span></b>}\mathrm{<b><span\; style="font-size:\; x-large;">\%</span></b>}\backslash text\{Slip\}\; (s)\; =\; \backslash frac\{N\_s\; -\; N\_r\}\{N\_s\}\; \backslash times\; 100\backslash \%$$

Slip is essential for torque production; if there were no slip, no relative motion would exist, and no torque would be generated.

Advantages of Asynchronous Motor: Simple and rugged construction, low cost, low maintenance, and good starting torque.

Applications of Asynchronous Motor: Widely used in household appliances, industrial machinery, and electric vehicles. Asynchronous motors are widely used due to their robustness, simplicity, and cost-effectiveness. They are found in household appliances, industrial machinery, pumps, fans, and electric vehicles.

Main Difference Between Synchronous and Asynchronous Motors

Here are the main differences between synchronous and asynchronous (induction) motors:

Feature	Synchronous Motor	Asynchronous Motor
Speed	Constant (synchronous speed)	Varies with load (less than synchronous speed)
Rotor Speed	Matches stator's rotating magnetic field	Always less than the stator's rotating magnetic field
Starting	Requires external mechanisms	Self-starting
Power Factor	Unity or leading	Lagging
Efficiency	Higher	Lower
Applications	Power plants, synchronous clocks, precise speed control	Household appliances, industrial machinery, pumps

Here’s a quick rundown of the main differences between synchronous and asynchronous motors:

Synchronous Motors

1. Speed: The rotor speed is synchronized with the supply current's frequency. In other words, the rotor's speed is constant and matches the frequency of the power supply.

2. Construction: These motors often have a more complex construction involving slip rings or permanent magnets.

3. Applications: They are used where precise and constant speed is required, such as in clocks, record players, and synchronous clocks.

4. Starting: They often require an additional starting mechanism to bring the rotor up to synchronous speed.

5. Efficiency: Generally, they are more efficient due to the absence of slip losses.

Asynchronous Motors (Induction Motors)

1. Speed: The rotor speed is not synchronized with the supply current's frequency. The speed of the rotor is always less than the synchronous speed.

2. Construction: These motors have a simpler and more rugged construction, usually with squirrel cage rotor or wound rotor.

3. Applications: They are widely used in industrial applications because of their robustness and simplicity, such as in fans, pumps, and compressors.

4. Starting: They are easier to start because they do not require any additional starting mechanism.

5. Efficiency: They may have lower efficiency compared to synchronous motors due to slip losses.

Key Difference between Synchronous and Asynchronous Motors

• Synchronization: The most significant difference is that synchronous motors operate at a speed that is synchronized with the power supply frequency, while asynchronous motors operate at a speed that is always less than the synchronous speed.