### Electrical Machines - Mixed bag

**Why are the pole shoes of d.c. machines laminated?**

Normally the pole core is a solid piece made of cast iron or cast steel but the pole shoe is laminated because flux near the pole shoe fluctuates and hence laminated pole shoe reduces the iron losses.

**If the magnetization characteristic were a straight line passing through the origin, the shunt generator will not be able to build voltage. Discuss.**

When the magnetisation characteristic is a straight line passing through the origin then there is no residual magnetism and hence without flux in the air gap, no emf will be induced and hence no build up of emf will take place.

**In the field control method of speed control, the speed can only be increased above normal. Explain.**

We know that E

_{b}= φZNP/60A

Hence , i.e. speed is inversely proportional to φ or I

_{f}the field current. Hence by addition of any resistance in field current reduces the field current and hence speed will increase.

**What is the effect of increasing the frequency on hysteresis loss with voltage magnitude remaining the same?**

Hence

**How dues a delta-connected tertiary winding of a transformer suppress harmonic voltages?**

A delta connected tertiary winding reduces the impedance offered to the zero sequence currents. It limits voltage imbalance. It mainly permits the third harmonic current to flow thereby reducing third harmonic voltage which is the main component of harmonics.

**What is the phase difference between fifth harmonic voltages present in the three-phase windings and their phase sequence?**

The phase difference between the phase sequences of three-phase winding and 5th harmonic voltages present would be 72°.

**What is the design change made in an a.c. servo motor to obtain linear torque-speed characteristic?**

If the rotor of an a.c. the servo motor is excited with a special kind of voltage unbalance, the angular phase difference of 90° is maintained while the magnitude of phase a voltage varies. The corresponding torque-speed characteristics are nearly linear with respect to motor speed and voltage of phase a.

**Why is a compensating winding provided in a.c. series motor?**

Compensating winding in a.c. series motor neutralizes the armature reaction ampere turns and hence it neutralizes the effect of armature reaction.