O.E.M. Start Kits

As I mentioned in my last post, the reason most contractors do not use the OEM start kit is because they either don’t understand how it works or don’t know how to wire it into the circuit.

First, let’s see if we can explain how a real hard start kits work and then we’ll explain the wiring.

The START capacitor is always used with a start relay or potential relay.  Because it is designed to ONLY stay in the circuit while the motor is starting, the relay is necessary to “drop” the capacitor out of the circuit. Unlike the RUN capacitor, it is NOT designed to dissipate heat associated with staying in the circuit for prolonged periods.  The purpose of the START capacitor is to increase the phase angle between the start and run windings to create GREATER STARTING TORQUE. Because this is changing the phase angle, the start relay is installed to drop it out once motor comes “up to speed”.  The potential relay and start capacitor are wired in series with the start winding.

The operation of the potential starting relay is based on the increase in back electromotive force (back EMF) or a bucking voltage that is generated across the start winding as the motor increases in speed.

The large metal mass of the motor’s rotor turning at high speeds has a voltage generating effect. This generated back EMF opposes line voltage and can be measured across the start winding . The back EMF is usually a higher voltage than the line voltage and can be in the 400-V range. All motors have different magnitudes of back EMF.

The back EMF voltage generated across the start winding causes a small current to flow in the start winding and in the potential relay coil since they are in the same circuit. When the back EMF has built up to a high enough value, referred to as pick-up voltage, the contacts between terminals 1 and 2 will be picked-up opened. This will take the start capacitor out of the circuit. The pickup voltage usually occurs when the motor has reached about 3/4 speed. Each compressor needs a specific potential relay based on the pick up and drop out voltages.  There is no “universal” potential relay. It is important to choose the correct potential relay based on the recommendation of the compressor manufacturer.

Now, how do we wire this into the circuit? On the relay you need to locate terminals 1, 2, & 5.  Terminals 1 & 2 are the normally closed contacts that OPEN when the back EMF  is present at the coil.  Terminals 2 & 5 are the coil in the relay.  (I have attached a wiring diagram at the end of this post that you can print out for reference. )  The output of the start capacitor comes into terminal 1, and goes out on terminal 2. This is wired to the output of the run capacitor going to the start winding of the compressor. This also provides the back EMF voltage to terminal 2, which is 1 side of the relay’s coil. Terminal 5, the other side of the coil, is powered off the side the contactor feeding the common winding of the compressor.

The Start Capacitor is powered off the wire from the contactor going to the run winding and power side of the run capacitor.

It is a simple 3 wire hook up to tie it into the circuit.

So, hopefully you understand how the “true” hard start kit functions and how easy it is to wire it into the compressor circuit.   As I said in my last post, this is always the best start gear to use because it will cover all of the possible problem applications where the start assist kits may not be powerful enough to accomplish what your job requirements need.

hard start kit wiring


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
This entry was posted in HVAC Tech Support. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s