There are two different types of capacitors associated with electric motorsâ€”motor starting capacitors and motor running capacitors. Motor Start Capacitors and Motor Run capacitors are not the same type of capacitors. Motor Start capacitors are Dry Electrolytic Capacitors and Motor Run Capacitors are Oil Filled Capacitors. They serve different purposes in the circuit and cannot be used interchangeably. Testing Motor Start Capacitors will not be covered in this article.
Motor Start Capacitors and Motor Run Capacitors serve two very different purposes. Motor Start Capacitors creates a slight phase shift in the current flowing the the motor's “Start” and “Run” windings. This slight phase shift in currents creates the increase in torques necessary to start the motor's rotor turning. As you can see in this diagram, the Start Capacitor is connected in series with the Start Winding through a centrifugal switch or relay (Potential or Current).
Once the motor has reached two-thirds its normal running speed i.e. 2,376 RPM for a 3,600 RPM motor, the centrifugal switch or the relay opens, disconnecting the Start Capacitor and the Run Winding” from the circuit. The motor will approach its running speed in about one-third of a second so the Start Capacitor remains in the circuit for a very brief period of time. How Potential Relays and Current Relays work is beyond the scope of this article and will be covered in a future article devoted to them.
In this diagram, I have added a Run Capacitor.
With the addition of the Run Capacitor, the Start Winding remains in the circuit even after the Centrifugal Switch or Relay opens and disconnects the Start Capacitor from the circuits. The purpose of the Run Capacitor is to increase the motors efficiency.
Commercial Test Equipment
There are Capacitor Checkers available but those that provide accurate test results are very expensive and not something the average DIY Electrician will want to invest in. The tester that are affordable are also unreliable. The good news is that for less than $10 in parts, a few minutes of your time, and a DMM (Digital Multimeter), which you most likely already have, you can build a very accurate capacitor testing jig.
Tools that you will need.
DMM (Digital Multimeter)
Supplies that you will need.
Rosin Core Solder
Two Alligator Clips
130-Volt, 100-Watt, Rough Service Light Bulb
Pigtail Lamp Socket
Cord set with molded plug
Constructing the testing jig.
Connect the pigtail lamp socket in series with one side of the cord set as shown here. Solder it in series about 18 to 24 inches from the end of the wire.
Solder the alligator clips to the ends of the wires. Do not tape or insulate the soldered connections on the pigtail socket. You will be using those soldered connections as your meter test points.
Performing the actual test.
If your DMM is an auto-ranging meter, simply set it to read AC-Voltage. If you have a manual meter set the function switch to read AC-Volts of 150 volts or higher. Using alligator test leads on your meter attach them to the soldered connections on the pigtail socket.
Attach the alligator clips on your test jig leads to the terminals on the Run Capacitor under test. After, and only after attaching the alligator clips to the capacitor's terminals, plug the test jig into a 110-Volt wall receptacle.
Compare the voltage indicated on the DMM with those given on this chart for various size Run Capacitors.
If the voltage reading that you get is significantly higher or lower than the figures given in this chart, replace the capacitor with a new one of the same rating.
WARNING: Capacitor will hold their charge after being unplugged fro the wall receptacle, so always discharge them by shorting their terminals together with the screwdriver before touching the alligator clips.
Single-Phase Motor Wiring Diagrams