__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 φ_{n} = 0.83

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