# Recent questions and answers in Electrical Machine

Interpoles are meant for

What will happen if you increase the current in the motor?

what is the difference between synchronous and asynchronous motor?

What is the difference between an AC and DC generator?

What is the difference between synchronous and asynchronous motors?

Split phase induction motor

Describe the reason of using dc series motor for electric trains.

UNIT 1 fundamentals of AC rotating machine Contents 1.1 Introduction 1.2 Advantages 1.3 Parts of AC machines 1.4 Synchronous speed 1.5 AC winding 1.6 Winding factor (Coil pitch and distributed winding) 1.7 Distribution factor 1.8 EMF equation of alternator or generator 1.9 Numericals 1.10 Harmonic 1.11 Slot harmonics 1.12 Harmonics minimization

Skew of rotor bar eliminates   1. The effect of space harmonics   2. The entire effect of crawling   3. Magnetic noise   4. Vibration due to unequal force developed on rotor

The fall in speed of a dc generator due to increase in load can be corrected by  (1) cooling the armature.. (2) increasing the excitation. (3) reducing the load voltage. (4) increasing the input to the prime mover

Equalizer rings are used in a dc machine to (A) provide mechanical balance (B) balance the flux produced by the poles (C) provide path for the circulating currents (D) overcome armature reaction

How is a three phase cage induction motor self starting?

How does a single phase induction motor work?

Analyze the differences of FOC and DTC. The switching frequency of FOC is 1.3 kHz and the average switching frequency of DTC is the same.

I got some grapic drawing questions guys and I need to see the answers immediately. Stator voltage curve for each four different start up methods.

Can you guys explain the advantages of frequency converter against the star delta or soft starter?

Which motor is generally used in Tap Recorders? A) universal motor B) reluctance motor C) split phase motor D) hysteresis motor

Which one is not a valid condition for the wave winding? A) Back pitch and front pitch are of the same sign B) Back pitch and front pitch are odd numbers C) Back pitch and front pitch may differ by two and may be equal also D) Commutator pitch is always unity

In star-star connection of 3-phase transformer, the triplen harmonics are suppressed by using (A) star connected tertiary windings (B) delta connected tertiary windings (C) neutral conductor (D) additional insulation for the phases

If two 3-phase transformers with unequal voltage ratios are connected in parallel, the result is (A) circulating current on no-load (B) no circulating current on no-load (C) dead short-circuit (D) equal no-load secondary emfs

In a 6-pole wave wound dc machine the number of conductors is 70. The back pitch and commutator segments, respectively, are (A) 11and 35 (B) 12 and 34 (C) 13 and 35 (D) 14 and 34

The number of brushes in a 6-pole double-layer lap wound dc machine is (A) 8 (B) 6 (C) 4 (D) 2

The demagnetizing AT/pole of a 4-pole dc generator is 480. The number of conductors is 480 and the current in each conductor is 40 A. Then the required brush shift in mechanical degrees (A) 9° (B) 7° (C) 5° (D) 3°

In a dc shunt generator critical field circuit resistance line is the line drawn (A) above the OCC (B) below the OCC (C) tangent to the saturation curve (D) tangent to the linear portion of OCC

A 200 V dc motor takes a field current of 2 A to generate a back emf of 180 V. If the field winding resistance is 80 Ω, the resistance of the shunt field regulator is (A) 10 Ω (B) 20 Ω (C) 100 Ω (D) 180 Ω

As the speed increases, the torque developed by a dc servo motor (A) decrease with a negative slope (B) increases with a positive slope (C) remain constant (D) becomes unstable

The starting torque of a V/f controlled inverter-fed 3-phase induction motor operating at 25 Hz as compared with the same motor operating at 50 Hz is (A) more (B) double (C) equal (D) less

Regenerative breaking of ac-dc converter fed dc motor is possible when firing delay angle of SCRs is (A) 30° (B) 60° (C ) 90° (D) 120°

In slip-energy recovery scheme of converter-fed 3-phase induction motor, super synchronous speeds are possible by (A) injecting voltage into the stator (B) injecting voltage into the rotor (C) extracting voltage from the stator (D) extracting voltage from the rotor.

The voltage collected by the pantograph of electric locomotive is (A) DC voltage (B) single-phase AC, 50 Hz (C) 3-phase AC, 50 Hz (D) single-phase AC, 25 Hz

Fast acceleration and high-speed cruising electric traction drive motor is (A) separately excited dc motor (B) DC series motor (C) AC series motor (D) 3-phase squirrel cage induction motor

As compared to a split phase motor, a capacitor start motor has _____.

A commutator with a diameter of 50 cm rotates at 1000 r.p.m. If brush width is 1.5 cm, the time of commutation will be (a) 0.168 m sec (b) 0.374 m sec (c) 0.481 m sec (d) 0.573 m sec

Shunt generators are most suited for stable parallel operation as their voltage characteristics are (a) drooping (b) identical (c) linear (d) rising

For very low speed and high power applications the best suited ac motor is:

If the current in the armature of do series motor is reduced to 5%, the torque of the motor will become: (1) 50% of the previous value (2) 25% of the previous value (3) 150% of the previous value (4) 125% of the previous value

The steady-state fault current during a 3 phase terminal fault on a generator is limited by: (1) Transient reactance of the generator (2) Sub-transient reactance of the generator (3) Synchronous reactance of the generator (4) DC off—set during the instant of the fault

Skewing of rotor bars eliminate the: (1) Effect of space harmonics (2) Entire effect of crawling (3) Magnetic noise (4) Vibration due to unequal force developed on rotor

A pole pitch in electrical machine is: (1) Equal to 180° electrical (2) Equal to 180° mechanical (3) Less than 180° electrical (4) Greater Than 180° electrical

The phase sequence of a three-phase alternator will reverse if  A) The field current is reversed keeping the direction of rotation same. B) The field current remains the same but the direction of rotation is reversed C) The field current is reversed and also the direction of rotation is reversed D) None of the above

A 4-pole, 3-phase, double-layer winding is housed in a 36-slot stator for an ac machine with 60o phase spread. Coil span is 7 short pitches. Number of slots in which top and bottom layers belong to different phases is  (A) 24 (B) 18 (C) 12 (D) 0

For a 1.8o, 2-phase bipolar stepper motor, the stepping rate is 100 steps/second. The rotational speed of the motor in rpm is  (A) 15 (B) 30 (C) 60 (D) 90

The armature resistance of a permanent magnet dc motor is 0.8 Ohm. At no load, the motor draws 1.5 A from a supply voltage of 25 V and runs at 1500 rpm. The efficiency of the motor while it is operating on load at 1500 rpm drawing a current of 3.5 A from the same source will be  (A) 48.0% (B) 57.1% (C) 59.2% (D) 88.8%

A rotating electrical machine its self-inductances of both the stator and the rotor windings, independent of the rotor position will be definitely not develop  (A) starting torque (B) synchronizing torque (C) hysteresis torque (D) reluctance torque

For a linear electromagnetic circuit, the following statement is true  (A) Field energy is equal to the co-energy (B) Field energy is greater than the co-energy (C) Field energy is lesser than the co-energy (D) Co-energy is zero

The following motor definitely has a permanent magnet rotor  (A) DC commutator motor (B) Brushless dc motor (C) Stepper motor (D) Reluctance motor