The Motor Doctor Diagnoses Motor Capacitors

May 1, 2001
A squirrel cage induction motor must sense two or more electric phases to cause the magnetic field to start spinning, thus turning the motor’s rotor and shaft. Once the motor starts, the interaction of rotor and stator keeps the field spinning.

Whenever the motor is “fed” with just single-phase current, there must be some way to create an artificial second phase.

Capacitors are electrical components that help create and strengthen the artificial second phase in single-phase electric motors, and always work with the secondary, or phase, winding. They work by storing and releasing a charge of electricity that is out of phase with the oscillating current that is fed into the motor.

A capacitor contains two metal plates separated by a special insulating material called the “dielectric.” If you look inside the capacitor of a small motor, you would see that the metal plates are actually sheets of metal foil, and the dielectric is a flexible, paper-like metal. The two sheets of foil, with the dielectric in between, are rolled up and placed inside a housing. Each sheet of foil is connected to a terminal on the housing.

Some capacitors are just used to help the motor start. Starting-type capacitors (also called electrolytic capacitors) are frequently found in applications where you need a large amount of torque to begin moving the load, such as commercial garage door openers, belted blowers, and mechanical devices like conveyors. They generally have a high storage capacity of 100 or more microfarads (mfd) and are found in a round, plastic case.

The second type of capacitor, more commonly found in hvac units technicians come across, is constantly energized when the motor is running. It is called a run capacitor. These devices are generally placed in oil-filled, oblong metal cases that help dissipate heat. Run capacitors typically have a storage capacity of about 25 mfd.

Run capacitors are permanently connected in the starting circuit (as in a permanent split capacitor, or PSC, motor). These are commonly found in air-handling applications, residential garage door openers, and increasingly in water pumps.

In these applications you need enough torque to get the load moving, but want the added efficiency of a PSC motor without mechanical switches.

Capacitor Troubleshooting

Here is a quick way to tell whether or not a capacitor is defective using an ohmmeter.

1. Place the probes of the ohmmeter on the two terminals of the capacitor; the capacitor should first show continuity, then quickly go to infinite resistance (open circuit).

The reason for these readings is that the ohmmeter contains a dc power source, its battery. When you first connect the ohmmeter, current will appear to flow through the capacitor as electrons in the battery begin to accumulate on one of the two plates. When the plate has all of the electrons it can hold (a fully charged capacitor), the test current will cease to flow and the capacitor will appear as open-circuited to the ohmmeter.

2. When you reverse the probes on the capacitor terminals, current should again flow as electrons accumulate on the other plate.

The reason is that reversing the polarity of the probes also reverses the polarity of the battery in the ohmmeter.

There are also a number of symptoms that will tell you if the capacitor on a motor is faulty:

  • The motor will not start its load, but if you spin the load by hand, the motor will run properly.
  • On air-handling equipment, the motor may start but will always fall short of normal operating speed.
  • You may encounter excess noise, high amps, and high temperatures in combination with either of the first two symptoms.
  • It’s relatively easy to diagnose common capacitor defects. A capacitor that is open-circuited will show no current movement when tested across its terminals with an ohmmeter.

    In most cases, motors with open capacitors won’t start at all. Short-circuited capacitors will show zero resistance when measured with an ohmmeter. A motor with a short-circuited capacitor often will start and run, but with less starting torque and lower full-load rpm than normal.

    Capacitors in metal housings also may become grounded. Grounded capacitors show a range of symptoms, from not starting to weak starting and running.

    To determine if the capacitor is grounded, touch the ohmmeter probe between the metal case and each terminal in succession. You will measure zero resistance from a grounded capacitor. It should be replaced immediately — even if the motor is running properly — because a grounded capacitor is an electrical hazard.

    Capacitor Replacement

    When replacing capacitors, always try to find a replacement with the same value as measured in mfd’s. If the appropriate replacement is not available, however, you can add the value of two capacitors by wiring them in parallel. For example, wiring two 5-mfd capacitors in parallel gives you the same value as one 10-mfd capacitor.

    A Motor Doctor hint: If you have an application where the capacitor fails frequently due to constant motor starting, try using two capacitors of half the required value wired in parallel. The reason is that using two capacitors in parallel increases the thermal capacity of the capacitor because the greatest surface area of the two units increases the capacitor’s ability to dissipate heat.

    Capacitors also have voltage ratings. This relates to the maximum voltage the capacitor can stand across its terminals, and is determined by the strength of the dielectric. When making replacements, it’s acceptable practice to use capacitors with higher voltage ratings than the original unit.

    It also is generally acceptable to use a replacement capacitor that is one standard rating size higher than the original as a temporary solution in the field. For example, it’s OK to replace a 7.5-mfd with a 10-mfd capacitor. But if you make such a substitution, always check to make sure the output amps do not exceed the motor’s nameplate amps.

    A final safety precaution: Since capacitors store an electrical charge, they can give a surprising jolt if you accidentally come into contact with the terminals of a recently charged unit.

    The best approach is to discharge the capacitor through a 10- to 20-ohm resistor. Some technicians discharge the capacitor by placing a tool, such as a screwdriver, across the terminals. While this may work, it can also damage the capacitor and force you to make a replacement in the field, wasting time and money.

    Simon is with A.O. Smith Corp., Milwaukee, WI; 414-359-4104; 414-359-4064 (fax).

    Publication date: 05/01/2001