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Motor Start Capacitors Role in HVAC System

by Team Techvilly

The motor may be the heart of your HVAC system, but like a car battery, it’s nothing without quality capacitors to keep the motor and system running properly. How well do you understand the critical function of motor start capacitors in HVAC systems?
This article will help you understand some of the industry standards that have been set for capacitor quality, safety, and performance, and provide insights for choosing capacitors in the field.

Capacitor Function

Almost all motors are equipped with motor start capacitors, a running capacitor, or both.
The starting capacitor is connected to the electrical circuit of the motor when it is at rest. This gives the motor an initial “push” when starting, temporarily increasing the starting torque and allowing the motor to turn on and off quickly. Typical starting capacitor ratings range from 25 µF to 1,400 µF and 110 Vac to 330 Vac.
When the motor reaches a certain speed, a switch (or relay) disconnects the starting capacitor from the winding circuit. When the motor drops below that speed, the capacitor is switched into the electrical circuit, bringing the motor up to the required speed.
A run capacitor designed for continuous use remains energized and connected to the motor’s electrical circuit at all times. Typical working capacitor ratings range from 2 microfarad to 80 microfarad with ratings of 370 Vac or 440 Vac.
Properly sized run capacitors increase the efficiency of motor operation by providing the proper “phase angle” between voltage and current to produce the required rotating electric field in the motor.

Proper installation/replacement of motor start capacitors

Importance of Capacitance Rating Specified on the motors? In short, this is very, very important. Always use exactly the nominal capacitance rating specified on the motor nameplate to ensure proper motor operation as designed by the manufacturer and to prevent damage to the motor.
Microfarad (µF) ratings always have a tolerance level. The standard tolerance for capacitance of motor run capacitors in HVAC applications is 6% give or take. This means that a 40 microfarad capacitor is rated between 37.6 and 42.4 microfarad and is still considered a passing capacitor.
When engineers design motors, they take this type of tolerance range into consideration. They specify a nominal (40 microfarad) rating and a tolerance (+/-6%) to ensure that the motor will provide the same performance as it was designed for if the capacitors are replaced. From the tolerance discussion above, using 35 microfarad instead of 40 microfarad is not recommended.

Disadvantages Of An Improperly Sized Capacitor

Improperly sized capacitors can have a variety of adverse effects on motors. If the microfarad rating of the capacitor is less than the motor design value, the motor winding current will be too high. If the microfarad rating of the capacitor is higher than the motor design value. The motor winding current will be too low. In any of the scenario given above the following can happen:

  • Decreased motor speed
  • Reduce system airflow/cooling
  • Increases system noise
  • Temperature rise
  • Causes bearing wear and lubrication loss
  • lead to insulation breakdown
  • increase noise
  • Reduced motor efficiency
  • increase energy consumption
  • Reduce system and motor life
  • Improper equipment operation
  • lead to improper cycling
  • Increased noise
  • stress other components
  • Motors are designed with specific nominal ratings and tolerances.

If there is anything outside of that rating, the motor will run faster or slower. Either way, the machine will eventually stop working properly, putting additional stress on the motors, capacitors, or other components in the machine, causing damage and noise, and requiring repair.
There was also a question about the voltage to use when replacing capacitors. A rule of thumb is to always use more than the rated voltage your motor requires. The required voltage is always indicated on the motor’s nameplate. Do not use voltages lower than necessary as this will exponentially degrade capacitor life. Using a capacitor with a lower voltage rating will not damage the system, but it will shorten the life of the capacitor.


Rated voltage is the working voltage at which the capacitor can be used for 60,000 hours. If the heating or air conditioning unit increases the voltage to the capacitor (for example, if the capacitor is rated at 370 Vac and 440 Vac is being detected from the unit), the life of the motor start capacitors will be significantly reduced. Conversely, if the heating or air conditioning unit is derating the voltage to the capacitor (for example, the capacitor is rated for 440 Vac but the unit sees 370 Vac), the capacitor will last longer.
Capacitors are inexpensive components, but installing the wrong size can have dramatic effects on the entire system!

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