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Compressor Start Assists

Recently we had a customer ask if there was a way to stop the lights from flickering when the air conditioning system came on.  In providing a solution to the customer, it dawned on us that this topic also would be a very good blog post for our website.

To get started, we’ll have to discuss a couple of concepts about electrical motors and their starting characteristics as a compressor is an electrical motor for the purposes of analyzing the issues in this post.  We are not going to get too deep into these concepts and will instead grossly oversimplify to make this post easier to understand.  We are going to use the term, “motor” and “compressor” interchangeably, although we acknowledge they are only the same in the broadest sense of these terms.  We apologize in advance if these oversimplifications cause any distress to our technically-advanced readers.

The first thing we need to cover is what type of motor is in the compressor in a residential air conditioning or heat pump system.  The type of motor found in most cases is a single-phase, permanent split capacitor motor (or simply a “PSC motor”).  With a PSC motor, there is a run capacitor that is wired in series with the start windings of the motor.  This capacitor provides the electrical current boost needed to overcome the starting torque of the motor at rest.  This starting torque draws for milliseconds electrical current that is much higher than the rated circuit breaker for the motor, but typically does not last long enough to trip the circuit breaker.  This “inrush” current or amperage (which is designated by “A”) is what causes the lights to flicker whenever the air conditioner starts.

Components that help the compressor motor with overcoming its starting torque are called start assists.  Some examples of a compressor start assist are a positive temperature coefficient device (PTC), a potential relay with a start capacitor (hard start) and a soft start. 

A PTC is a resistor that drops out of the starting circuit of the compressor, by becoming non-conductive, once a certain temperature is reached.  At Atlas, we typically don’t recommend this type of start assist unless the manufacturer of the compressor specifically recommends it.  Our reasoning is that once the temperature of the PTC reaches its non-conductivity point, it can take several minutes for the device to reset.  In the summertime during peak usage of an air conditioner or heat pump, the cycle times of the compressor may not allow the device to reset.  If the PTC is not reset, then it is not helping the compressor start, and is, in effect useless. 

A hard start is a start capacitor that provides a boost to the start circuit of the run capacitor and compressor to get it started.  This is a critical component for compressors that start under uneven loads, such as having uneven pressures in the compressor.  Most compressor manufacturers have a recommended hard start system for their compressors.  Although a hard start does not reduce “inrush” current, it reduces the time it takes for the compressor to get up to speed and can therefore eliminate the perception of issues caused by the “inrush” current (such as flickering lights in the home when the compressor starts up).

Soft starts work by providing less inrush current to get the motor started without sacrificing the start-up torque necessary to get the compressor started.  The reduction of “inrush” current can eliminate light flicker at compressor startup, reduce noise at compressor startup, and is easier on the motor windings of the compressor - meaning it can extend the life of the compressor.  It is also a vital component of the electrical system if there is an emergency generator powering the air conditioner or heat pump.  The only instance where a soft start is not a viable option is in the instance where the compressor is multi-staged.  

In the case of our customer, we recommended a soft starting device to reduce “inrush” current for the compressor to eliminate the flickering of the lights.  Please see below some pictures of the readings of the “inrush” current before and after the soft start was installed.  The soft start was installed on a 4-ton heat pump.  The manufacturer of the outdoor unit recommended a minimum circuit breaker size of 35A and a maximum circuit breaker size of 45A.  A 40A circuit breaker was installed for the unit in the electrical panel.


The first photo is of the inrush current without any start assist:



                    Total Inrush Current of 126.9A


The second photo is of the reading of the inrush current after the soft start was installed on the 4-ton heat pump.



                  Total Inrush Current of 4.9A



The last photo is of how the soft start looks in the control cabinet on the system.



        White box is the Soft Start device


As seen from the photos, the inrush current is reduced significantly.  The customer now does not have any issues with lights flickering.  The customer also has reported that the noise of the compressor when it starts up has been reduced.  For more information on this subject, please feel free to contact us at (407) 538-9672.

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