Power rating for electrical machines indicates the required supply voltage for smooth running of that machine, it also shows the permissible maximum amount of current which can easily flows through the machine. And there will be a chance of breakdown in the machine if those parameters goes beyond this limit.
Similarly when we discuss about motor power rating, we are looking for the suitable conditions where maximum efficiency is obtained from the electric motor. When the motor have insufficient rating, there will be frequent damages and shut downs due to over loading, and this is not intended. On the other hand, if the power rating of a motor is decided liberally, the extra initial cost and then loss of energy due to operation below rated power makes this choice totally uneconomical.
Another essential criteria of electrical motor power rating is that, during operation of motor, heat is
produced and it is inevitable due to I2R loss in the circuit and
friction within the motor. So, the ventilation system of the motor should be
designed very carefully, to dissipate the generated heat as quickly as
possible. The output power of the motor is directly related with the
temperature rise, that's why it is also called thermal loading. The thermal
dissipation will be ideal when the ventilation system is designed in such a way
that the heat generated during the operation is equal to or less then heat dissipated
by the motor to the surrounding. Now, due to the design of motors, temperature
is not same everywhere inside the motor. There is a high amount of heat
produced in the windings because, windings cause higher heat generation. The
insulating materials used in the winding are also chosen depending on the
amount of heat generated inside the motor during operation. So in the end it
can be said that the main objectives of selecting and finding out motor power
rating are-
1. To obtain the
suitable thermal model of motor and design the machine properly.
2. Finding out motor duty class.
3. Calculating motor ratings for various classes of duty.