Welcome to the solutions page – April 2023

By Grant Laidlaw | All images supplied by Grant Laidlaw

Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavour to enlighten. I am going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in industry is not in place.

Hi Ryan, picking up from the past issue where we looked at direct on line and star delta, let us look into the Variable Speed Drives (VSDs) also known as a Variable Frequency Drive (VFD) and the reasons for their use.

Burnt motor winding

A variable speed drive takes fixed-frequency AC supply and converts this to a variable-frequency AC supply through a DC link. Power use and mechanical power output is controlled so that the motor can run at the most efficient speed for the motor and the equipment being driven.

Thanks you for the question, Riaan. Speaking to some of the industries suppliers, this appears to be a common problem. Unfortunately the incorrect wiring of a single phase compressor will almost certainly result in damage / burnt windings.

This in turn leads to warranty claims and some ill feelings as this type of burnout is not covered by warranty. Make one hundred percent sure your connections are correct prior to starting the compressor. In addition, do not supply power to a hermetic / semi hermetic compressor while the system is in deep vacuum as electrical damage may occur.

You should have a multimeter and an insulation tester available (MEGA). A multimeter does not normally have the functionality to perform insulation tests. In addition, a clamp type tong tester used to test amperes is a useful tool when testing motors.

Using a multimeter set on the ohms scale and the compressor electrically disconnected, test the motor windings for continuity, see figure below.

Please note: The readings given below are an example. Testing between the Run (R) to Common (C) connections will give you the lowest reading. Testing between the Start (S) to Common will give you the reading with the in-between value and testing between Run and Start will result in the highest reading. This is the highest reading as you are testing across both start and run windings.

At the risk of repeating myself: different compressor makes and sizes will have different readings but the running winding will always have a lower resistance than the starting winding. The two readings added together give the total resistance. This reading identifies the common connection. The resistance of the running and starting windings when added together must be the same as the total resistance. If not, the windings are damaged and the compressor will have to be replaced.

EXAMPLE:

R = Running winding
S = Starting winding
C = Common winding
E = Earth terminal

R to C = 3.5 ohms
S to C = 10.2 ohms
R to S = 13.7 ohms

3.5 + 10.2 = 13.7 ohms

Note: The resistance of some undamaged windings can be as low as 0.5 ohm. Zero resistance indicates a shorted winding and an infinite or very high resistance indicates an open winding.

Use an insulation tester when checking for a grounded winding. Current is passed through the weakness / breakdown in the insulation material to the other winding or to earth. Unless you have specific information indicating otherwise, any hermetic motor of one horsepower or under can be considered suspect if the resistance between the windings and the housing is less than 0.5 meg-ohm.

If all the readings test correct the compressor can be run. Using a tong tester take the ampere reading while it is running. The amp reading should correspond with the ampere rating on the compressor while under operating conditions.

The above is extremely important, so I would recommend that you check all compressors (even new) prior to connecting the power supply. If a fault is found prior to installation of the compressor the warranty claim should be simplified. An incorrectly connected motor can burn out within a few seconds, so double check your connections to safeguard against burning out motors.

With regards to the second part of your question:

The function of a multiple-winding motor is to provide a selection of running torques which, when driving a load such as a blower wheel, will result in varying speeds and thus affects air flow volumes.

Multi Speed motor windings.

The connections shown are for low speed.  For higher speeds switch the input from A to B, C or D.

The capacitor in series with the motor windings serves a dual function: it improves starting torques and provides better running efficiency. Of late, Variable Speed Drives with associated controls are becoming more and more common place. (See previous RACA Journal issue)

The name plate or data contains all the information required for the installer. You may find some variation between manufactures, so always check.

This information is important in motor section and application.

Looking at your issue with circuit breakers, we need to understand that whilst all circuit breakers will trip when the load exceeds their rated amps some will trip instantly while others have a delay. There are different classes of circuit breaker referring in this instance to the curve ratings. Certain circuit breakers will allow excess current for a limited specified time to cater for higher amperage drawn – for example during motor start-up.

On the other hand, when a circuit breaker is used in conjunction with electronic circuitry a rapid or instantaneous response is required. Different suppliers use different symbols, numbers or letters when designating various curve ratings. A general example is as follows:

When connecting a system to a Curve 0 or Curve P you may experience tripping, as when the motors start up their starting inrush current is high. When selecting a circuit breaker, attention should be given as to which curve rating would best suit your application. On the front of the circuit breaker you may find 20A – C2, the C2 indicating a curve 2 circuit breaker. Check the breakers’ manufacturer’s specifications to determine the circuit breakers’ suitability for your application.

Thank you, Riaan. I hope you will experience success with your compressor connections and circuit breaker issues.

References:

1. ACRA
2. USFDA / FEMCO
3. Electricalsolutions

About Grant Laidlaw

Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale. He holds a Bachelor of Business Administration and an associate degree in educational administration. He has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical. He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018. Currently he is the SAIRAC national treasurer.

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