There are three types of overcurrent relay (OCR) used for motor protection:
Electronic Thermal Electromagnetic
Electronic OCIT (overcurrent inverse time) relays have largely superseded
electromagnetic types as they have no moving parts (except for their output trip relay) and their very reliable tripping characteristics can be closely matched to the motor circuit.
Such relay is are robust, smaller and lighter than the equivalent electromagnetic type.
The block diagram of the electronic OCIT relay shows that the current and time settings can be adjusted over a limited range to match the motor FLC and run-up time. A self-test of the OCR performance can usually be applied with a fixed setting of, typically, 6 x FLC and the tripping-time can be measured and compared against the manufacturers current/time characteristics.
Although electromagnetic devices with time delays can give adequate protection against large, sustained overloads to motors which are operated well below their maximum output and temperature, they have been found to be inadequate for continuous maximum rated (CMR) motors.
Most LV motors are protected by less expensive thermal OCRs. Inverse-time thermal OCRs usually work with bi-metal strips.
The strips are heated by the motor current and bend depending on the temperature. If the motor takes an overload current, the strips operate a normally-closed (NC) contact which trips out the line contactor to stop the motor.
The minimum tripping current of such a device can be adjusted over a small range.
This adjustment alters the distance the strips have to bend before operating the trip contact.
For larger motors, the heaters do not carry the full motor current. They are supplied from current transformers (CTs) which proportionally step-down the motor current so that smaller heater components may be used to operate correctly, induction motors must be connected to a three phase a.c. supply.
Once started they may continue to run even if one of the three supply lines becomes disconnected. This is called single-phasing and can result in motor burn-out.
Single-phasing is usually caused when one of the three back-up fuses blows or if one of the contactor contacts is open-circuited.
The effect of single-phasing is to increase the current in the two remaining lines and cause the motor to become very noisy due to the uneven torque produced in the rotor.
An increase in line current due to single-phasing will be detected by the
The three thermal elements of an OCR are arranged in such a way that unequal heating of the bi-metal strips causes a differential movement which operates the OCR switch contacts to trip out the motor contactor.
For large HV machines a separate device, called a negative phase sequence (NPS) relay, is used to measure the amount of unbalance in the motor current.
For star connected motor windings the phase and line currents are equal to the line connected OCR is correctly sensing the winding current.
If the overcurrent setting is exceeded during a single-phase fault the motor will be tripped off.
The situation is not so simple with a delta connected motor. Normally the line current divides phasorally between two phases of the motor windings.
The phase current is just over half the line current.
When one of the lines becomes open circuited a balanced here phase condition no longer exists. Now the sets of line and phage currents are no longer balanced.
Look at the condition where the motor is at 50% of full load when single-phasing occurs: the line currents are 102 % of the full-load value but the current in winding C is 131% of its full-load value. The 102 % line current will probably not activate a line connected OCR and the motor remains connected.
However, the local overheating in winding C of the motor will quickly result in damage.
Motors can he protected against this condition by using a differential type relay which trips out with unbalanced currents.
In fact, most modern thermal OCRs for motors have this protection against single-phasing incorporated as a normal feature.
If single-phasing occurs when in operation on light load, the motor keeps on running unless the protection trips the contactor.
If the motor is stopped, it will not restart.
When the contactor is closed, the motor will take a large starting current but develop no rotating torque. The OCR is set to allow the starting current to flow long enough for the motor, under normal conditions, to run up to speed.
With no ventilation on the stationary motor, this time delay will result in rapid and severe overheating. Worse still, if the operator makes several attempts to restart the motor, it will burn out.
If a motor fails to start after two attempts, you must investigate the cause.
Undervoltage protection is necessary in a distribution system that supplies motors.
If there is a total voltage loss or black-out, all the motors must be disconnected from the supply.
This is to prevent all the motors restarting together which would result in a huge current surge, tripping out the generator again.
Motors must be restarted in a controlled sequence after a supply failure.
Undervoltage (UV) protection for LV motors is simply provided by the spring-loaded motor contactor because it will drop out when the supply voltage is lost.
For large HV motor the UV protection function will be covered by a relay separate from the OCR function or it may be part of a special motor relay which incorporates all of the necessary protection functions.
When the supply voltage becomes available, the motor will not restart until its contactor coil is energised.
This will usually require the operator to press the stop/reset button before initiating the start sequence.
For essential loads, the restart may be performed automatically by a sequence restart system.
This system ensures that essential service are restarted automatically on restoration of supply following a blackout. Timer relays in the starters of essential motor circuits are set to initiate start-up in a controlled sequence.