Question: Why do motors and compressors need capacitors?
If you ask 10 HVAC technicians you'll get 10 different answers. In fact, I participated in a webinar hosted by a major HVAC manufacturer and the trainer even got it wrong. I will answer this question in detail, but they're a few things you must know first.
1. Ohm's Law states Watts (power) = Voltage x Current. In the example below, if a device (in this case a motor) is operating properly in an HVAC system...
... the power supplied to the motor is 115v x 1.9a = 218.5 watts. As you can see the voltage and current are in phase, giving the motor maximum power.
2. Most HVAC motors and compressors are inductive motors. What happens with an inductive motor, or any inductive device is that the inductance shifts the phase or timing so that the voltage is out of phase with the current. Let's see what happens to our power now.
With our voltage and current out of phase, our power to the motor is now 115v x .95a = 109.25w. This is not enough power to turn the motor and you will have an air conditioner that won't work.
3. A capacitor, shifts the phase of the voltage and current the opposite direction as an inductive device. In electronic theory there is a way to remember this phase shift. it is "ELI the ICE man". E = voltage, L = inductance, I = current. So ELI - voltage in an inductive circuit leads current and ICE - current leads voltage in a capacitive circuit. You don't need to remember this.
4. When the properly sized capacitor is matched with an inductive motor the phase shifting of both of these device cancel each other out and bring everything back into phase. This gives the motor or compressor maximum power.
5. It is important to remember this, for two reasons:
- If you check a capacitor and it is more than +-10% out of tolerance it lowers the power to the motor by 10%. This is hard on the motor and the capacitor should be replaced.
- Do not replace a 5 ufd capacitor with a 7.5 ufd capacitor or vice versa. I have seen this in many cases. This lowers the power to the motor by 33% and will damage the motor and cause higher electric bills.
6. Air conditioning systems usually have a dual capacitor, one side for the fan motor and one side for the compressor. Hear are symptoms of a bad capacitor:
- Outdoor fan motor is buzzing and/or is hot to the touch.
- Outdoor fan motor not turning.
- Fan motor turning and the compressor turning on and off.
- The capacitor is bulging
- The condensing unit has power but the fan and compressor are not running
by Ron Walker