What are the possible causes of a split-phase motor running too slowly?

A split-phase motor may run slower than planned due to a number of issues, including:

Electrical Problems

Voltage Issues: Inadequate voltage supply can result in lower motor speed. To operate properly, ensure that the motor obtains the correct voltage between its prescribed range.

Faulty Capacitor: For starting, split-phase motors employ capacitors to induce a phase shift. The speed of the motor can be affected if the starting capacitor is faulty or has decayed.

Incorrect Wiring: improperly wired connections (or) incorrect connections during installation might have an impact on the operation of the motor.

Winding Problems: Faulty (or) damaged windings can degrade motor performance and cause it to operate slower. This could be due to electrical issues, insulation deterioration, or physical damage.

Mechanical Problems

Overload: If the motor is asked to perform more work than it is capable of, it will slow down. Check the load to ensure it is within the capacity of the motor.

Worn or damaged bearings: Worn or damaged bearings increase friction and drag on the motor shaft, slowing it down.

Rotor issues: Rotor damage, such as bent bars or damaged laminations, can also affect motor speed.

Mechanical Pressure: Excessive mechanical pressure on the motor shaft (or) bearings might strain the motor and cause it to run slower than usual.

Aging or Wear: Components inside the motor may degenerate or wear out over time, reducing efficiency and speed.

Other Problems

Temperature: Due to thermal expansion, excessive heat might cause the engine to slow down. Maintain appropriate ventilation & avoid exceeding the operating temperature of the motor.

Dust and Debris: The accumulation of dust & debris inside the motor can obstruct its operation and limit its speed. Clean & maintain the motor on a regular basis.


Troubleshooting slow-running split-phase motors require evaluating these parameters in a systematic manner to discover the root cause and correct the problem.

  • At the motor terminals, measure the voltage & frequency.
  • Examine the capacitor for the bulges, leaks, or improper capacitance.
  • Examine the wiring and connectors for any signs of damage or looseness.
  • Listen for strange noises coming from the motor, that include grinding or screeching, which could indicate a problem with the bearings.
  • Reduce the motor’s load to observe if the speed increases.

A qualified electrician (or) technician may be required for an accurate diagnosis and repair.