Induction motor:Current/Speed Curve of an Induction Motor

Current/Speed Curve of an Induction Motor

It is a V-shaped curve having a minimum value at synchronous speed. This minimum is equal to the magnetising current which is needed to create flux in the machine. Since flux is purposely kept constant, it means that magnetising current is the same at all synchronous speeds.

Fig. 34.24. shows the current/speed curve of the SCIM discussed in Art. 34.28 above. Refer Fig. 34.23(b) and Fig. 34.24, As seen, locked rotor current is 100 A and the corresponding torque is 75 N-m. If stator voltage and frequency are varied in the same proportion, current/speed curve has the same shape, but shifts along the speed axis. Suppose that voltage and frequency are reduced to one- fourth of their previous values i.e. to 110 V, 15

breakdown torque (Fig. 34.25). It means that by reducing frequency, we can obtain a larger torque with a reduced current. This is one of the big advantages of frequency control method. By progressively increasing the voltage and current during the start-up period, a SCIM can be made to develop close to its breakdown torque all the way from zero to rated speed.

Another advantage of frequency control is that it permits regenerative braking of the motor. In fact, the main reason for the popularity of frequency-controlled induction motor drives is

1 their ability to develop high torque from zero to full speed together with the economy of

2 regenerative braking.

Torque/Speed Characteristic Under Load As stated earlier, stable operation of an induction motor lies over the linear portion of its torque/ speed curve. The slope of this straight line depends mainly on the rotor resistance. Higher the resistance, sharper the slope. This linear relationship between torque and speed (Fig. 34.26) enables us to establish a very simple equation between different parameters of an induction motor. The parameters under two different load conditions are related by the equation

The only restriction in applying the above equation is that the new torque T2 must not be greater than T1 (V /V )2. In that case, the above equation yields an accuracy of better than 5% which is sufficient for all practical purposes.

Example 34.24. A 400-V, 60-Hz, 8-pole, 3-f induction motor runs at a speed of 1140 rpm when connected to a 440-V line. Calculate the speed if voltage increases to 550V.

Solution. Here, s1 = (1200 - 1140)/1200 = 0.05. Since everything else remains the same in Eq. (i) of Art. 34.30 except the slip and voltage, hence

Example 34.25. A 450.V, 60.Hz, 8-Pole, 3-phase induction motor runs at 873 rpm when driving a fan. The initial rotor temperature is 23°C. The speed drops to 864 rpm when the motor reaches its final temperature. Calculate (i) increase in rotor resistance and (ii) approximate temperature of the hot rotor if temperature coefficient of resistance is 1/234 per °C.

Plugging of an Induction Motor

An induction motor can be quickly stopped by simply inter-changing any of its two stator leads. It reverses the direction of the revolving flux which produces a torque in the reverse direction, thus

applying brake on the motor. Obviously, during this so-called plugging period, the motor acts as a brake. It absorbs kinetic energy from the still revolving load causing its speed to fall. The associated Power Pm is dissipated as heat in the rotor. At the same time, the rotor also continues to receive power P2 from the stator (Fig. 34.27) which is also dissipated as heat. Consequently, plugging produces rotor I 2R losses which even exceed those when the rotor is locked.