Induction Motor Performance

The performance characteristics of an induction motor are the plots of slip S, rotor speed n₂, torque T, stator current I₁, consumed power P₁, power factor cosφ, and efficiency ηas functions of useful power P₂ taken off the motor. Fig. 7.7 illustrates these characteris­tics plotted under natural operating conditions, i.e. the conditions in which the uncontrollable motor carries varying loads at the con­stant frequency f₁ of stator current and constant impressed voltage V₁.

An increased load on the motor shaft leads to an increased slip; the rate of increase of the slip grows with the load. At no load, the slip is very small, S≈0or n_2≈n₁ At the rated full load the slip commonly ranges be­tween 3 and 5%. The rotor speed is n₂ = n₁ (1–S) = (60f₁/p)(1–S).

As the slip increases with the load, the rotor speed decreases. Вut the rate of change of the speed with increasing load from 0 to the rated value is very insignificant and does not exceed 5%. That is why the speed characteristic of an induction motor is essentially flat, i.e. the characteristic curve only slightly drops toward the x-axis.

The torque T developed by the motor is balanced out by the braking (load) torque T_b and the torque T₀ required to compensate for mechanical losses: T = T_b +T₀ = P₂/v₂ + T₀, where P₂ is the useful power of the motor, and v₂ is the angular velocity of the rotor.

At no load, the torque T is equal to T₀. As the load thrown on the shaft grows, so does the torque, which increases faster than the power P₂ because of a certain decrease in the rotor speed.

The current I₁ drawn from the supply varies nonuniformly with increasing load on the motor. When the motor runs without load, cosφ is small and the reactive component of current is large. At small loads, the active component of I_x is less than its reactive com­ponent, therefore the former has an insignificant effect on I₁ as against the latter. At large loads, the active component becomes higher than the reactive component and variations in the load ap­preciable change the stator current I₁.

The curve of power P₁ consumed by the motor approaches the straight line that insignificantly bends upward at large loads because the power losses in stator and rotor windings grow with loading.

The power factor varies with the load on the motor shaft in the following manner. At no load, cos q> is small, in the order of 0.2, because the stator current's active component due to the power loss in the machine is small in comparison with the reactive component that produces the magnetic flux. With an increase of the load on the shaft, cosφ rises, reaching its maximum value of 0.8 or 0.9, because of the increased active component of the stator current. At very large loads, cosφ slightly decreases since the reluctance of the rotor winding grows at too high values of the slip and rotor frequency.

The curve of the efficiency n has the shape specific to any machine or a transformer. At no load, the efficiency is zero. When the load increases, the efficiency sharply goes up and then declines. The efficiency reaches the highest value at a definite load such that the load-independent iron losses and mechanical losses are equal to load-dependent copper losses in stator and rotor windings.