Motor Protection Guide


This information is intended to guide the domestic consumer, the small commercial businessman and the small industrial manufacturer in seeking professional advice for the protection of motors, especially those that drive compressor loads. It is hoped that this information will make these consumers aware that protective devices are available and that costly repairs to fridges, deep freezers, air conditioning equipment, etc., can be avoided by judiciously installing such protective devices.

If you have electric motors which drive:-

  • Compressors for Air Conditioning Equipment
  • Refrigeration Equipment
  • Printing Machines
  • Pumps
  • Industrial Machinery

then you should ensure that they are adequately protected against conditions which may cause their useful service life to be shortened or which may cause premature failure.


By protecting your electric motors against conditions which –

  • may cause their useful service life to be shortened; or
  • may cause premature failure

You can save yourself the cost of expensive rewinds and repairs; unnecessary down time of your production machinery; and undue inconvenience due to the unavailability of your machinery. In this way your investment in protection equipment for your electric motors can save you time and money.


The electricity supply conditions which may occur and affect your machinery or appliance are not unique to Trinidad and Tobago as they are experienced all over the world. The solution is to seek professional advice on the type of electrical protection to install. There are devices which will ensure that your motors are automatically disconnected from the system as soon as conditions exist which may cause damage to them. The cost of these devices should be looked upon as an insurance premium against having to repair, rewind or replace a burnt-out motor, as well as the consequential loss as a result of the machine down time while the motor is being repaired, rewound or replaced.

The following conditions may affect your motors adversely:-

  • Low voltage
  • Automatic auto-reclosing on distribution feeders to restore electricity supply quickly
  • Unbalanced voltages
  • Single phasing
  • Phase reversal
  • Overloading
  • Stalling

It may not be necessary for you to protect your motors against all of these conditions, but it is advisable that you should seek professional advice concerning those conditions which are most likely to affect you and so shorten the useful service life of your motors or cause premature failure.


Every protective device is designed to serve a specific purpose or to meet a special need. There is no “gimmick” or “cure all” for every problem.

It is important, therefore, that your problems be entirely understood and that protective devices be wisely and judiciously applied.

It is possible to be too cautious and to spend too much money needlessly on protection as well as to be too conservative and have your motors burn out in spite of having installed some protection.

You should, therefore, be wary of gimmicks and have your problems properly identified.

This can be done by seeking professional advice so that you can apply the correct solutions to your protective problems.


The following is a summary of adverse conditions affecting proper motor performance and a guide to the protective devices used to safeguard the motor and its associated machinery. In each case the device is inserted in the line or circuit of the motor to be protected as shown in the diagram below:

Trinidad and Tobago Electricity Commission

The disconnection switch and sensing device (shown within the dotted box) may be available as a single unit or a combination of units depending on the complexity of the protective scheme.

Low VoltageThis condition may cause the motor to labour or stop and thus cause overheating. This would result in deterioration of motor life or burn out of the motorLow voltage detection relay that would disconnect the motor from the faulty supply and enable the supply to be restored when voltage conditions return to normal.
Reclosing of supply shortly after initial loss of electricity.This condition may be specially dangerous to motors driving compressor loads as re-energizing the motor under heavy load could cause overheating resulting in deterioration of motor life or burn out.Time relay that would prevent the re-energization of the motor even though the supply of electricity is available. This relay must be timed to ensure return to acceptable compressor conditions before restarting can be done.
Unbalanced voltagesThis condition causes temperature increases in the motor windings which result in deterioration of motor life and also a waste of energy.Three phase unbalance relay (negative phase sequence relay) to disconnect supply. Setting of this relay may require professional advice.
Single Phasing (loss of supply on one phase)This is the worst condition of phase unbalance. It may occur when protective fuses are blown. Again this condition will result in overheating and subsequent deterioration of motor life. If undetected for too long it will most likely cause burn out.Loss of phase relay to disconnect supply until conditions are returned to normal.
Phase ReversalThis condition will cause motors to rotate in the opposite direction and may cause injury to operational personnel and damage to equipment.Phase reversal relay to disconnect supply on detection of condition.
OverloadingExcessive current in circuit resulting in overheating and deterioration of machine life.Overload relay to alarm or to disconnect the supply until conditions can be returned to normal.
StallingThis condition occurs when the motor cannot drive its connected load. Excessive heating again occurs causing machine life deterioration and/or burn out.Stalling relay to disconnect supply.