In capacitor-start induction-run motors, at approximately what percent of rated speed is the start winding disconnected by the centrifugal switch?

In capacitor-start induction-run motors, at approximately what percent of rated speed is the start winding disconnected by the centrifugal switch?

• A. 25%
• B. 50%
• C. 75%
• D. 100%

Answer: (C)

In capacitor-start induction-run motors, the start winding with the timing capacitor provides a strong initial torque to get the rotor turning. A centrifugal switch driven by the rotor speed disconnects that start circuit once the motor is up to speed, so the motor runs with the run winding only. That disconnect happens at about three-quarters of the motor’s rated (full) speed. The idea is to have enough speed reached so the motor can continue running reliably on the run winding after the start winding is removed, while still ensuring the starting torque is available when needed. If the switch opened too early, starting torque would be insufficient; if it opened too late, the start winding would waste energy and generate extra heat. So, the start winding is disconnected when the rotor reaches roughly three-quarters of its normal running speed.

In capacitor-start induction-run motors, the start winding with the timing capacitor provides a strong initial torque to get the rotor turning. A centrifugal switch driven by the rotor speed disconnects that start circuit once the motor is up to speed, so the motor runs with the run winding only.

That disconnect happens at about three-quarters of the motor’s rated (full) speed. The idea is to have enough speed reached so the motor can continue running reliably on the run winding after the start winding is removed, while still ensuring the starting torque is available when needed. If the switch opened too early, starting torque would be insufficient; if it opened too late, the start winding would waste energy and generate extra heat.

So, the start winding is disconnected when the rotor reaches roughly three-quarters of its normal running speed.