And when it is motor winding theory pdf possible to accurately define their likely occurrences or magnitudes, it becomes desirable for the motor to have some in-built reserve capacity. With this service factor, no more derating would normally be necessary. Any service factor greater than one 1. The service factor is not intended as a general increase in the motor horsepower rating.

In most cases, a 10 kW motor with 1. 15 service factor should not be treated as an 11. 5 kW motor and it is not considered acceptable for application to a pump or fan with this service factor to operate continuously at 11 kW. NEMA MG-1, establishes the operating conditions required for proper performance and normal life expectancy of ac motors.

Many facilities may experience occasional periods of low voltage during peak load periods. Under this condition a motor operating within its service factor rating may experience an unacceptable increase in current and temperature. The motor is permitted under the standard to operate at a 10 degrees C higher winding temperature rise under service factor load than under its normal rated load. As a rule of thumb life expectancy is halved for each 10 degree increase in operating temperature. Thus continuous operation at service factor load can be expected to a higher rate motor failure due to insulation breakdown.

The increased load may also reduce bearing life. Relying on the service factor of motor under continuous duty operations would be unwise in most cases. The appropriate use of this rating is to handle short term or occasional overloads. It is suitable only where supply voltage is highly stable as in case of industrial premises having self generation units. Motor life expectancy is not a concern. The former category includes unbalanced supply voltages, Under Voltage, over voltage, under frequency, over frequency, single phasing and reverse phase conditions.

The latter category includes the bearing failures, Internal Shunt faults, which are commonly earth faults and Over loads. The Protection applied to a particular machine depends on its size and the nature of load to which it is connected. All motors will have a thermal with stand curves which indicates the thermal withstand time at various over load currents. The Heating of the machine is because of the thermal current which can be derived from the positive and negative sequence components of the Motor current as follows. Negative sequence Component of the Input Current.

It can be seen that the negative sequence plays proportionately more than positive sequence current in the heating of the machine. The relay should computes the positive and negative sequence components and calculate the thermal current. The ideal relay should calculate sequence component on the basis of vector calculation. The thermal withstand time should be provided by manufacturer or alternatively calculated as per logarithmic formula. Constant is selectable from 1 to 60 minutes depending on the motor Characteristics. This will help to avoid the tripping of the motor in a continuously processing industry.

There is a Restart inhibition Temperature level which ensures the next starting of the machine after the Motor cooled down to the safe temperature level. The trip time delay is selectable from 1 to 120 seconds. The trip time characteristics are inverse time characteristics. This is because each time when a motor is getting started, the motor will see 3 to 6 times motor rated current for the duration of starting time, thus increasing the thermal status of motor considerably. These higher currents will cause the thermal stress in the winding, which leads to the winding failure. Ideal motor protection relay should have repeat start protection element.

If the number starts exceeded the set value, the relay inhibits the next start for a set time. This time is called restart inhibition time, which is selectable from 1 to 60 minutes. The trip time delay can be set as either instantaneous or time delayed. Ideal motor protection element is provided with the above element in which the pickup range is selectable from 1 to 5 times Ist. The delay is selectable from 0. In such cases fuses provided in the power circuit will provide the short circuit protection.

The pickup level is selectable from 0. 15 to 1Im and the trip time delay is adjustable. This element is disabled if the current is below 0. This element can be connected in residual connection or by using a CBCT.