Troubleshooting motor capacitors
problem encountered in the service end of the electrical construction industry is trou bleshooting capacitor-type motors. What is a fast, sure method of checking the motor capacitor?
a number of procedures for test ing capacitors used with single phase capacitor-type motors.
Here are a few.
Method 1. One way to solve this problem is to replace the capacitor with a known, good capacitor. If the motor runs, the original capacitor was at fault; if not, the trouble could be the capacitor or elsewhere in the motor.
Method 2. Appliance servicemen check capacitors quickly with an instrument known in their jargon as a “cappy.” This instrument is actually a bridge-circuit tester that indicates whether or not the capacitor is good.
Method 3. Another fast method of testing a capacitor is with a multitester using the ohms scale. With the capacitor out of the circuit, place the ohmmeter leads on each terminal of the capacitor. If no reading is obtained, the capacitor is open. If an immediate reading of continuity is obtained, the capacitor is shorted. If the pointer jumps up to approxi mately mid-scale and slowly returns towards infinity, the capacitor is probably good. (Since the ohmmeter impresses only 1.5 volts de on the capacitor, it may check out good on this test. However , it may break down when placed on line voltage. Line-voltage tests are described in the following two methods.) Also check whether or not the capacitor is grounded by placing the ohmmeter leads between one terminal and the capacitor case. A zero ohms reading indicates a ground.
Method 4. Connect the capacitor in series with a 10-amp fuse across a 120-volt, 60-cycle ac line as shown in sketch A. If the fuse blows, the capacitor is shorted and must be replaced with a new one. If the fuse does not blow, the capacitor will be charged up to some voltage. This charging will only take a few seconds. If the line is then disconnected from the capacitor and the capacitor terminals are carefully shorted by a screwdriver, a spark will be drawn at the terminals (sketch B). If a capacitor has wire leads, the leads can be touched together. If a spark cannot be drawn, the capacitor is either open or has decreased in capacity. Test should be repeated a few times.
Method 5. A reliable method of testing a capacitor is shown by the circuit in sketch C. The fuse should be sized slightly higher than the capacitor-rated current. The object of this test is to obtain the capacitor’s microfarad value by measuring voltage and current in the circuit and inserting these values in an established working formula. The capaci tor should be energized only for a short time because some capacitors are rated for intermittent duty. Make measure ments quickly using a snap-on volt-ohm-ammeter to obtain voltage and current readings. Then the approximate capac ity in microfarads can be computed by substituting the readings in the following formula (a 60-Hz supply IS assumed):
Microfarads = 2650 X amps + volts
This computed value should be compared with the value marked on the capacitor. If the capacitance in microfarads is not within 5%, the capacitor should be replaced.
