ETO ETO Exam papers 📅 Mar 2024

Exam Question

(a) Describe the effects of changes in speed, rotor current and torque as load is applied to an induction motor. How does the motor adjust its stator current with changes in mechanical load?

(b) A 20kVA, 2000/220V, single-phase transformer has a primary resistance of 2.1Ω and a secondary resistance of 0.026Ω . The corresponding leakage reactances are 2.5Ω and 0.03Ω. Estimate the regulation at full load under power-factor conditions of

(i) unity

(ii) 0.5 (lagging) and

(iii) 0.5 (Leading).

Reference Answer

### Part (a): Induction Motor Load Characteristics
An induction motor operates on the principle of a rotating magnetic field (RMF) in the stator inducing currents in the rotor, which in turn create a rotor magnetic field. The interaction between these two fields produces torque. The motor's characteristics change significantly as mechanical load is applied.
#### 1. Effect on Speed and Slip
* **Synchronous Speed ($$N_s$$)**: This is the speed of the stator's RMF, determined by the supply frequency (f) and the number of poles (P): $$N_s = \frac{120f}{P}$$. It is constant for a fixed supply.

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