single-phase motor

Causes of Single-phase Motor Failures

The frequent failures with single-phase motors include a centrifugal switch, thermal switch, or the capacitor. Though these problems can be easily be addressed, it is essential that you check if the model you have is aged 10 years or less or more than 1 HP, and if not, then it’s time for an upgrade.


How to troubleshoot split-phase motors?

The split-phase motor has both starting and running winding. The starting winding is automatically removed by a centrifugal switch as the motor starts kicking though in usual cases, a thermal switch that has a manual and automatic reset is often used as it automatically turns off the motor when it starts overheating. The risk could be that the motors may restart at any time.

Troubleshooting procedures:

  1. Turn off the power and inspect the motor. Immediately change the motor if it is damaged or burned.
  2. Check if the thermal switch is at manual mode and if it is, restarts it and turn motor on.
  3. If the motor opts to start, use a voltmeter to check for voltage if it is within 10% of the listed voltage and other motor terminals. If it is incorrect, you must troubleshoot the circuit leading to the motor or if correct, simply turn motor off and test. We recommend you use our Fluke 87V Industrial Multimeter for full efficiency.
  4. Turn off the combination starter and lockout and tag starting mechanism per the company’s policy.
  5. With the power off, connect Fluke 87V to the same motor terminals the incoming power leads were disconnected from. The ohmmeter will then read determine the resistance of the starting and running windings. Since the windings are parallel, the combined resistance is less than the resistance of either winding alone. It is indicative that you immediately change the motor if it reads zero, then there’s a shortage or reads infinity, then an open circuit is present.
  6. Check the centrifugal switch for signs of burning or broken springs. If it has, changes the switch and if none, inspect it with an ohmmeter.


Troubleshooting capacitor motors

Troubleshooting capacitor is the same way as troubleshooting split-phase motors, only the additional device which is the capacitor has to be considered. A capacitor motor is a split-motor with another one or two capacitors. These capacitors give the motor more starting and running torque.

The common issue with capacitors is that it deteriorates fast. It has a limited life and without you knowing, it could already have a short circuit or an open circuit. So it is obvious that you will have to change it more often. You must be cautious that if these failures with capacitors are not immediately addressed, it may cause your motor to burn out or not start at all.

Capacitors are either oil or electrolytic and made with two conducting surfaces separated by a dielectric material. It is a medium to maintain an electric field with little to none outside energy supply. It is usually used to insulate the conducting surfaces of a capacitor.  Furthermore, an oil capacitor is sealed in a metal container and the oil serves as the dielectric material.

Between oil and electrolytic capacitors, electrolytic are more often used. It is formed by winding two sheets of aluminum foil separated by pieces of thin paper impregnated with an electrolyte. The electrolyte is used as the dielectric material as it acts as the conducting medium through a current flow by ion migration.

The aluminum foil and the electrolyte is sealed in a cardboard or aluminum cover but note that it must have a vent hole to prevent possible explosion if the capacitor is overheated or shorted.

Troubleshooting procedures:

  1. Turn off the combination starter and lockout and tag starting mechanism per the company’s policy.
  2. Measure voltage using Fluke 87V at the motor terminals to make sure the power is dead.
  3. Capacitors are found on the outside frame of the motor. Remove the cover but in this process, be cautious as the capacitor may hold charge though the power is off.
  4. Check for leakage, cracks, or bulges. Replace if found.
  5. Remove the capacitor from the circuit and discharge it. In order to safely discharge it, place a 20,000-ohm, 2 W resistor across the terminals for five seconds.
  6. After discharging, connect Fluke 87V leading to the capacitor terminals. The device will determine the condition of the terminal to be either good, shorted, or open.


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