Above 90% of motors used in an industry are induction motors, because they are cheap, robust and easy to maintain. For higher HP (>250HP) motors we prefer high voltage, because it will reduce operating current and the size of the motor.
Why We Require Protection of Motors?
To understand this we need to know the cost associated with the failure of motor, i.e.
- Loss of production (Cost of production)
- Replacement of motor (Replacement cost)
- Cost of repair
- Cost of man hours due to this emergency
The basic function of a protective relay is to identify the fault and isolate faulty part from the healthy part of the system. This will improve the reliability of power system.
For protection of motor, we have to identify the various causes of failure and to address the same. The various causes of failure are as below
- Thermal stress on winding
- Single phasing
- Earth fault
- Short circuit
- Locked rotor
- Number of hot starts
- Bearing failure
Brief descriptions of the different failures are given below:
- Thermal Stress on Winding –
If a motor runs continuously more than its rated capacity then this will over heat the winding and insulation. Subsequently deteriorate the winding insulation resulting in failure of motor. If the voltage is less than the designed value then also it will over heat the winding at rated load and failure of the motor can occur.
- Single Phasing –
Loss of one phase supplied to the motor (in case of 3-phase motor) leads to single phasing. If we start the motor on load, then the motor will fail due to imbalance.
- Earth Fault –
If any part of winding comes in contact with the ground then we can say the motor is earthed. If we start the motor then it will lead to failure of motor.
- Short Circuit –
If there is a contact between two phases of a three phase winding or between the turns of a phase, and then this will be termed as short circuit.
- Locked Rotor –
If the driven equipment is in jammed condition or the motor shaft is jammed, then this is known as locked rotor. If we start the motor then it will fail.
- Number of Hot Start –
Each motor is designed to withstand a certain number of hot starts. Consider a motor is in running condition, if we stop the motor and immediately start the same, then this is called as a hot start. Depending upon the thermal curve of a motor we have to give certain time to bring down the temperature of winding.
- Bearing Failure –
If bearing fails then rubbing of rotor on stator will occur, resulting physical damage of insulation and winding. The bearing failure can be avoided by monitoring the bearing temperature. Bearing temperature detector (BTD) is used for monitoring and tripping of motor in case of abnormality.
All motor protection relays operate on the basis of current taken by the motor. Motor protection relay is used for high voltage area having the following features
- Thermal overload protection
- Short circuit protection
- Single phasing protection
- Earth fault protection
- Locked rotor protection
- Number of start protection
For setting of the relay we require the CT ratio and full load current of the motor. The setting of different element is listed below
- Thermal over Load Element –
To set this element we have to identify the % of Full load current on which the motor is running continuously.
- Short circuit Element –
The range available for this element is 1 to 5 times of starting current. Time delay is also available. We normally set it at 2 times of starting current with a time delay of 0.1 second.
- Single Phasing Element –
This element will operate, if there is an unbalance in current of three phases. It is also called as unbalance protection. The element is set for 1/3rd of starting current. If it tripped during starting, then the parameter will changed to 1/2 of starting current.
- Earth Fault Protection –
This element measures the neutral current of star connected CT secondary. The range available for this element is 0.02 to 2 times of CT primary current. Time delay is also available. We normally set at 0.1 times of CT primary current with a time delay of 0.2 seconds. If tripped during starting of motor, then the time setting can be raised to 0.5 sec.
- Locked rotor protection –
The range available for this element is 1 to 5 times of full load current. Time delay is also available. We normally set at 2 times of FLC (Full Load Current). The time delay will be more than the starting time of the motor. “Starting time means the time require by the motor to reach its full speed.”
- Number of hot start protection –
Here we will provide the number of start allowed in specified time duration. By this we will limit the number of hot starts given to the motor.
The schematic diagram to connect a motor protection relay is as below
Modern digital motor protection relays are having some extra features, i.e. protection against no load running of a motor and thermal protection.
In case of no load running, the relay senses the motor current. If it is less than the specified value then it will trip the motor. We can also connect the temperature probe to the relay, which will monitor the bearing and winding temperature and trip the motor if it exceeds the specified value of temperature.