5.3.2. Rating

Speed behaviour depending on the torque

Operating an asynchronous motor presupposes a certain speed for a given torque. This ratio is given for any one motor.

Figure 55 - Rotational behaviour of an asynchronous motor in dependence on the torque

1 Speed, 2 Torque, 3 Rated speed n_n, 4 Starting torque M_a, 5 Rated torque M_n, 6 Breakdown torque M_k

The initial torque M_a is the torque yielded by the motor in no-load operation. The breakdown torque M_k, is the greatest possible motor torque. If this torque is exceeded, the motor comes to a standstill.

The torque at rated load M_n is the motor torque yielded during rated load and rated speed.

M_n = torque at rated load
P_n = rated power in kW
n = rated speed in rpm.

Every asynchronous motor must be able to accept at least a 160 per cent rated torque for short load spells without motor breakdowns, that is to say the rotor stops. The speed does not vary greatly given considerable load variations.

Operating characteristics of an asynchronous motor

The various motor data about the yielded torque are cited in order to provide an overview of the behaviour of the asynchronous motor between no-load and rated load operation.

This figure indicates the operational curves of an asynchronous motor by means of the following data:

P_n = 4 kW, U = 380 V, I_n = 8.8A, n = 1.430 rpm.

cos j_n = 0.83

Figure 56 - Operating curves of a 4 kW squirrel cage motor (380 V; 8.8 A; cos j = 0.83)

1 Current in amperes
2 Power in kilowatts
3 Power curve
4 Current curve
5 Nominal power

Where asynchronous motors are driven at subload, efficiency and the power factor assume lesser values.

Therefore, asynchronous motors should not be overdimensioned or run in no-load operation.