Start Capacitor with Potential Relay

How Does Start Capacitor Help Start Motor - Permanent Split Capacitor Motors or PSC Motors use run capacitors to help the motor be more efficient while running. The start capacitor gives the motor a boost on startup. The run capacitor helps the motor run more efficiently. Many types of HVAC equipment use PSC motors as blowers and pumps.

So in HVAC, you will have motors in most applications, including residential and commercial, that use capacitors for the motor no matter the application or purpose. These motors are a step above the shaded pole type of motor, which was the original type of motor used for many applications. A shaded pole motor could have power applied to it and not move or turn backwards from its intended direction.

With the advent of PSC motors, those problems went away. PSC motors will always start in the direction for which it is engineered to start with a few exceptions. Even in those few exceptions, the PSC motor will fight to turn in the intended direction.

How Does Start Capacitor Help Start Motor - The Basics

While the PSC motor uses a run capacitor to increase its efficiency while running there are also some HVAC motors used that also require a start capacitor to help it start. A start capacitor is used in applications where the motor needs a boost on start-up to overcome pressure or some force. Remember, an AC induction motor goes from zero RPM’s to full speed (RPM’s) in a split second.

Therefore, if the motor has to start up against some kind of force like the high pressure that a refrigeration circuit would have for a compressor to start against, then some help will be needed to give the compressor motor a little boost on start-up.

We introduce the start capacitor, which will only be in the circuit for a split second. Then a relay will take it out of the circuit. The relay is the potential relay or less commonly a current relay. A potential relay works off of back EMF while a current relay works off of current. So after covering the basics, how does a capacitor work to help the motor?

The answer is that a capacitor, whether it is a start capacitor or a run capacitor, changes the phase angle when the motor is powered up. By changing the angle, you give the motor a little more push. If you push straight down on the rotor, you are expending energy in the wrong way to get the most out of the rotor. However, if you push from an angle, then you change the ease of which you apply the energy. See the diagram below.

How Does Start Capacitor Help Start Motor - Illustrated

In the illustration above, imagine you have a wheel and a stick. The wheel and stick on the left you are going to push straight down. Or apply pressure straight down on the wheel. How easy do you think it will be to turn that wheel by pushing straight down on it? Now see the illustration on the right. Imagine you put the stick at an angle and then push the wheel.

Consequently, the wheel will roll far easier rather than be pushing straight down. That is how a motor capacitor works in the motor circuit. It changes the phase angle of the EMF, therefore, giving it a boost and making the motor more efficient. Whether it has a start capacitor or run capacitor, both change the phase angle to help the motor. As illustrated above. I hope I answered your question about How Does Start Capacitor Help Start Motor

Why is a capacitor used in a single-phase motor?

Because the motor has little to no torque on start up. It needs a boost for torque purposes especially compressors. Refrigeration systems are presurized and to overcome the pressure on start up a start capacitor is used in some cases. A refrigeration system needs to overcome pressures of up 100 PSI or more depending what the pressure requirements are at static for the refrigeration system. If the system didn’t equalize from the low to the high side the pressure could be much higher. The answer is: to provide more torque for the motor on start up.

How Does Start Capacitor Help Start Motor | Recap:

• A shaded pole electric motor does not use a run or start capacitor.
• Shaded pole motors are inefficient.
• PSC motors are used extensively in HVAC.
• PSC Motors can be replaced with the more efficient variable speed ECM Motor.
• Run capacitors help the PSC run more efficiently.
• Start capacitors help the motor start.
• A potential or current relay takes the start capacitor out of the circuit after start-up.
• Capacitors change the phase angle of the EMF.
• Changing the phase angle gives the motor a better boost (see illustration above).
• Finally, a 3-phase motor does not use capacitors.

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How Does Start Capacitor Help Start Motor

Technical Resource: Refrigeration and Air Conditioning Technology