Explain the difference between synchronous and asynchronous motors.

Explain the difference between synchronous and asynchronous motors.

• A. Synchronous motors always run faster than the supply frequency; asynchronous are always slower
• B. Synchronous motors rotate at a speed tied to the supply frequency; asynchronous (induction) slip relative to the magnetic field
• C. Both run at the same speed regardless of frequency
• D. Synchronous motors have slip; asynchronous do not

Answer: (B)

The key idea is how rotor speed relates to the supply and the rotating magnetic field. A synchronous motor keeps its rotor speed locked to the speed of the stator’s rotating magnetic field. That field speed is set by the electrical supply frequency and the motor’s number of poles, so changing the frequency or pole count changes the rotor’s locked speed. In other words, the rotor runs at the same speed as the magnetic field, not based on any slip. An induction (asynchronous) motor, on the other hand, must run a little slower than the rotating field. The difference between the field speed and the rotor speed is called slip, and it’s what induces currents in the rotor to produce torque. The amount of slip changes with load, so the rotor speed remains below synchronous speed and adjusts to deliver the required torque. That’s why the correct statement emphasizes that synchronous motors rotate at a speed tied to the supply frequency, while asynchronous motors operate with slip relative to the magnetic field. The other ideas don’t fit: speeds aren’t defined as simply “faster than” or “slower than” the frequency, and motors don’t always run at the same speed regardless of frequency. Also, synchronous motors don’t rely on slip to generate torque, whereas asynchronous motors do.

The key idea is how rotor speed relates to the supply and the rotating magnetic field. A synchronous motor keeps its rotor speed locked to the speed of the stator’s rotating magnetic field. That field speed is set by the electrical supply frequency and the motor’s number of poles, so changing the frequency or pole count changes the rotor’s locked speed. In other words, the rotor runs at the same speed as the magnetic field, not based on any slip.

An induction (asynchronous) motor, on the other hand, must run a little slower than the rotating field. The difference between the field speed and the rotor speed is called slip, and it’s what induces currents in the rotor to produce torque. The amount of slip changes with load, so the rotor speed remains below synchronous speed and adjusts to deliver the required torque.

That’s why the correct statement emphasizes that synchronous motors rotate at a speed tied to the supply frequency, while asynchronous motors operate with slip relative to the magnetic field. The other ideas don’t fit: speeds aren’t defined as simply “faster than” or “slower than” the frequency, and motors don’t always run at the same speed regardless of frequency. Also, synchronous motors don’t rely on slip to generate torque, whereas asynchronous motors do.