Q6 (10 Marks) Electrical Circuits & Calculations
MET • Written Exam

(a) Describe the effect of the following loads on power factor. (6)

(i) Induction motors

(ii) Transformers

(iii) Partly loaded motors

(iv) Cage type motors

(b) In a 50-kVA, star-connected, 440V, 3-phase, 50Hz alternator, the effective armature resistance is 0.25 ohm per phase. The synchronous reactance is 3.2 ohm per phase and leakage reactance is 0.5 ohm per phase. Determine at rated load and utility power factor (10)

(i) Internal EMF

(ii) No-load EMF E0

(iii) Percentage regulation on full-load

(iv) Value on synchronous reactance which replaces armature reaction

Appeared In: Oct 2022

Verified Model Answer (Text Solution)

Structured for DG Shipping MEO Class II examination scoring criteria.

Exam Ready
Part (a)

Effect of power factor on:

(i) Induction motors: Induction motors typically have a lagging power factor, meaning the current lags behind the voltage. This is because of the inductive nature of their windings. The power factor is generally between 0.7 and 0.9, and it varies depending on the motor's load and design. A fully loaded motor will usually have a higher power factor than a lightly loaded one.

(ii) Transformers: Ideally, transformers should have a unity power factor (1.0), meaning the current and voltage are in phase. However, in reality, they exhibit a slightly lagging power factor due to the core losses (hysteresis and eddy currents) and magnetizing current. These losses are relatively small, resulting in a power factor close to unity.

(iii) Partly loaded motors: Partly loaded induction motors operate at a lower power factor than fully loaded ones. This is because the magnetizing current (which is reactive and doesn't contribute to real power) forms a larger proportion of the total current when the motor is lightly loaded. The power factor decreases as the load decreases.

(iv) Cage type motors: Cage type induction motors (squirrel-cage motors) are a common type of induction motor. Their power factor characteristics are similar to those of induction motors in general—a lagging power factor that varies with the load, typically lower at lighter loads. They are known for their simplicity, robustness, and relatively low cost. However, their starting torque is typically lower compared to other types of motors, which may limit their use in certain applications.

← Back to MET Question Bank Upload Recent Question Paper →