Question

What is the maximum speed of a synchronous motor?

Answer 1

The maximum speed of a **synchronous motor** is determined by the frequency of the power supply and the number of poles in the motor. The motor operates at synchronous speed, which is the speed at which the magnetic field created by the stator rotates.

The formula to calculate the synchronous speed (**Ns**) is:

\[
N_s = \frac{120 \times f}{P}
\]

Where:
- **Ns** = synchronous speed in revolutions per minute (RPM),
- **f** = frequency of the AC supply in hertz (Hz),
- **P** = number of poles in the motor.

### Example:

For a 50 Hz power supply and a 2-pole motor:
\[
N_s = \frac{120 \times 50}{2} = 3000 \, \text{RPM}
\]

Thus, for a 2-pole motor on a 50 Hz supply, the maximum synchronous speed is 3000 RPM.

For a 60 Hz power supply and a 2-pole motor:
\[
N_s = \frac{120 \times 60}{2} = 3600 \, \text{RPM}
\]

Hence, the maximum synchronous speed is higher (3600 RPM) on a 60 Hz supply with the same number of poles.

### Conclusion:
The maximum speed of a synchronous motor depends on the frequency of the supply and the number of poles, with higher frequencies and fewer poles resulting in higher speeds.

Answer 2

The maximum speed of a synchronous motor is determined by the synchronous speed, which is the speed at which the motor's magnetic field rotates. This speed is given by the formula:

\[ \text{Synchronous Speed} (N_s) = \frac{120 \times f}{P} \]

where:
- \( N_s \) is the synchronous speed in revolutions per minute (RPM).
- \( f \) is the frequency of the AC supply in Hertz (Hz).
- \( P \) is the number of poles in the motor.

### Key Points to Consider:

1. **Frequency and Number of Poles**:
   - The synchronous speed depends on the frequency of the AC power supply and the number of poles in the motor.
   - For a 60 Hz power supply, the synchronous speeds are 3600 RPM for a 2-pole motor, 1800 RPM for a 4-pole motor, and so on. For a 50 Hz power supply, these speeds are 3000 RPM for a 2-pole motor and 1500 RPM for a 4-pole motor.

2. **Fixed Speed**:
   - Unlike induction motors, synchronous motors run at a constant speed, which is exactly the synchronous speed, as long as the frequency and the number of poles are constant. They do not slip as induction motors do.

3. **Load Conditions**:
   - The speed of a synchronous motor does not change with the load. However, maintaining synchronization can be challenging under heavy loads or fluctuating conditions, which can affect the stability of the motor.

4. **Practical Limitations**:
   - In practical applications, the actual operational speed of synchronous motors is often lower than the theoretical synchronous speed due to mechanical and electrical losses.

5. **Special Designs**:
   - Some synchronous motors are designed to operate at higher speeds, but these designs may require special considerations and materials to handle the stresses involved.

So, the theoretical maximum speed of a synchronous motor is directly calculated from the frequency and number of poles, but practical considerations may limit the achievable speed.