Electrical Component Quiz

Ten questions on contactors, capacitors, compressors, transformers and motors. Pick an answer to see whether you are right and why. Related reading: The Code: controls.

1. A contactor in an outdoor unit is:

   1. A switch with a low-voltage coil that opens and closes the line-voltage circuit to the compressor and fan
   2. A device that stores electrical charge
   3. A refrigerant pressure sensor
   4. A transformer

   The 24-volt coil pulls in contacts that carry line voltage to the loads. That is how a thermostat's small signal controls large loads.

2. The run capacitor on a PSC motor:

   1. Only helps it start
   2. Stays in the circuit while it runs, creating the phase shift that helps the motor run efficiently
   3. Converts AC to DC
   4. Limits current to the motor

   A run capacitor feeds the start winding for the whole run cycle. A weak one can make a motor run hot, draw more current or fail to start.

3. On a single-phase compressor, the three terminals are:

   1. Line 1, Line 2 and Ground
   2. Common, Start and Run
   3. Hot, Neutral and Ground
   4. R, C and G

   C, S and R connect to the start and run windings inside the compressor.

4. Between which two compressor terminals is the resistance highest?

   1. Common to Run
   2. Common to Start
   3. Start to Run
   4. Each terminal to ground

   Start to Run goes through both windings in series, so it reads the sum of the other two readings.

5. You measure continuity from a compressor terminal to the compressor's copper or shell. This means:

   1. Normal operation
   2. A grounded winding, and the compressor should not be energized
   3. A bad capacitor
   4. A welded contactor

   The windings should be insulated from the shell. A ground fault is dangerous and usually means the compressor has failed.

6. A transformer rated 40 VA with a 24-volt secondary can supply about:

   1. 0.6 amps
   2. 1.7 amps
   3. 10 amps
   4. 40 amps

   Volt-amps divided by volts gives amps: 40 ÷ 24 is about 1.7 amps. Overload it and it overheats.

7. A hard start kit is used to:

   1. Make a compressor run faster
   2. Give a compressor extra starting torque with a start capacitor that drops out once it is running
   3. Replace the contactor
   4. Lower head pressure

   The start capacitor boosts starting torque and is then taken out of the circuit. It can help a compressor that struggles to start, but it does not fix a mechanical failure.

8. An ECM blower motor differs from a PSC motor because it:

   1. Has no electronics
   2. Uses an electronic control module to vary its speed and is typically more efficient
   3. Needs a run capacitor
   4. Runs at only one speed

   ECMs are brushless DC motors with a built-in control. They can hold airflow or torque across conditions and use less power than PSC motors.

9. Pitted, burned contactor contacts can cause:

   1. Higher refrigerant pressure only
   2. Voltage drop across the contacts, overheating and hard starting
   3. Lower indoor airflow
   4. A short in the thermostat

   Damaged contacts add resistance, which drops voltage to the compressor and heats the contacts. Measure voltage across the closed contacts under load to find it.

10. A dual run capacitor has terminals marked C, FAN and HERM. HERM connects to:

    1. The condenser fan motor
    2. The compressor
    3. The thermostat
    4. Ground

    HERM refers to the hermetic compressor. FAN goes to the condenser fan motor, and C is common to both.