Basics for Practical Operation – Motor Protection

The motor’s protection requirements

Temperature Rise

In line with applicable standards, every motor manufacturer guarantees that critical machine parts remain within the permissible temperature range during rated operation and that short-term overloads do not damage the motor.

The motor protection device, on the one hand, has to allow full use and thus the efficient operation of the motor and, on the other hand, react quickly enough in case of an overload.

Operational behaviour

Electrical motors are energy transformers. They take in electrical energy and transform it into mechanical energy. This process causes energy losses, which takes the form of heat.

The total energy loss comprises of two components:

• Current-independent losses: they are practically constant, this means they also occur at no-load.
  – core losses caused by polarity changes and eddy currents
  – mechanical losses caused by friction and aeration
• Current-dependent losses: they increase with load, i.e. with increased current.
  – I²R losses in the stator
  – I²R losses in the rotor

The latter is almost proportional to the motor’s slip. According to Figure 2.2.1, for a stalled , static rotor, the maximum starting current in the stator is 4…8 In.

The total input power is transformed into heat. If the rotor remains stalled, the temperature of the stator and rotor winding increases considerably, as part of the heat can only flow into the motor casing after a delay. If the motor is not switched off in time, the stator and rotor winding can burn out.

After run-up, the temperature increases further in an e-function, as shown in Figure 2.2.2, until it reaches final temperature level. For a higher load, the final temperature will be correspondingly higher.

Electrical motors are thermally non-homogenous systems. The windings, stator iron and rotor have a different heat capacity and heat conductivity.

After start and during load changes, a temperature compensation takes place between the different machine parts. The heat flows from the hot winding to the cooler iron until a temperature balance has been achieved.