# Objective Questions on Special Machines

1. What happens if supply is given at rotor side in any machine?

When supply is given to rotor side then after some time load may increase or decrease and speed will decrease or increase then frequency should decrease or increase but it is not possible. So that reason we are going to supply only at stator side but not at rotor side.

2. A motor armature conductor of length 0.5 m is exposed to a flux density of 4 Wb / m2. Calculate the force experienced by the conductor, when the armature electric current is 5 Amp?

The force experienced by a electric current carrying conductor placed within a magnetic field is given by F = BIL.
∴ F = 4 × 5 × 0.5 = 10 N, which is not given in the options.

3. A 12 pole machine will pass through electrical degree is one revolution of value

We know that electrical degree = ( P / 2 ) × Mechanical degree.
Mechanical degree is 360°.
So electrical degree in one revolution = ( 12 / 2 ) × 360° = 2160°.

4. For a 1.8°, 2 phase bipolar stepper motor, the stepping rate is 100 step / second. The rotational speed of the motor in r.p.m is

Stepping rate is 100 steps /sec. Steps required for one revolution = 360° / 1.8° = 200. So time required of one revolution 200 / 100 = 2 sec. Hence, Rev / sec = 0.5 r.p.s. So rev / minute = 30 r.p.m.

5. Which instrument measures the rotations speed of shaft in a motor?

Tachometer is an instrument that measures the rotation speed of a shaft of a motor. It is small AC or DC generator, that gives the output as voltage which is proportional to the rotational speed of a motor shaft. It displays digitally.

6. A stepper motor has a step-angle of 2.5°. Determine the resolution?

Resolution = 360° / Step angle = 360° / 2.5 = 144.

7. Both stator and rotor there are eight teeth of a 3 stack four pole stepper motor, when the excitation is changed from one stack to another to the next. Determine the step angle?

Step angle = 360° / ( No. of stack × No. of rotor teeth ) = 360° / ( 3 × 8 ) = 15°.

8. Which special type of motor has rotor movements in discrete steps?

Stepper motor has its rotor movements in discrete steps.

9. A 3 - Φ, three stacks, variable reluctance step motor has 20 poles on each rotor and stator stack. The step angle of this motor is

Step angle = 360° / ( 3 × 20 ) = 6°

10. In domestic uses such as for sewing machines which type of motor is used?

Universal motor is a motor which can work at both ac and dc supplies. A universal motor is a series wound motor, compensated or uncompensated, specifically designed to operate on direct or single-phase alternating-current of the same voltage at approximately the same speed & output. So, universal motors uses in domestic purpose such as for sewing machines.

11. In cloth washing machines and refrigerators which motors are preferred?

Small universal motors usually series type motor are preferred in washing machines and refrigerators.

12. In a stepper motor the detente torque means

In a stepper motor the torque means-minimum of the static torque with the phase winding unexcited.

13. A load of 900 lbs is to be raised with a lift car weighting 855 lbs at a speed of 110 feet per minute, the over balance being 40 % and percentage efficiency 45 %. Find the horse power of the motor without counterweight

Horse power the motor without counter weight = [ ( Contract load + Weight of car) / 33000 ] × Car speed × [ 100 / Percentage efficiency ] = [ ( 855 + 900 ) / 33000 ] × 110 × [ 100 / 45 ] = 13 HP.

14. Magnetic circuit with cross sectional area of 20 cm2 is to be operated at 50 Hz from 120 Vrms supply. The number of turns required to active a peak magnetic flux density of 1.8 T in the core flux is

B = Φ/A
⇒ 1.8= Φ / ( 20 × 10−4 ) = Φmax = 3.6 mWb
E = 4.44fΦmaxN = 4.44 × 50 × 3.6 Times; N = 120
N = 150.

15. Mechanical energy is supplied to DC generator at the rate of 4200 J / s. The generator delivers 32.2 Amp at 120 V. What is the percentage efficiency of the generator?

Input power power (Pi) = 4200 J / s = 4200 W, output Power = PO = EI = 120 × 32.2 = 3864 W, Efficiency ( η ) = Po / Pi × 100 = 3864 / 4200 × 100 = 92 %.

16. If a shunt generator delivers 100 A at 200 V, and the resistances of the shunt field and armature are 100 ohms and 0.01 ohms respectively, the generator e.m.f will be

Ish= V / Rsh = 200 / 100 = 2 A Ia= 100 + 2 = 102 A.
Eg = 200 + 102 × 0.01 = 201.02 V

17. We can apply source transformation to voltage source when a resistance is connected to it in

Source transformation is applied to a voltage source when an impedance is connected in series to it and it is applied to a current source when an impedance is connected in parallel to it.

18. In a 3 phase induction motor maximum torque at a slip of 3 % it the rotor resistance is 0.13 ohms the value of rotor reactance is

Maximum torque condition R2 = sX2
X2 = R2 / s = 0.13 / 3 / 100 = 13 / 3 ohm.

19. Universal motor have which of the following application?

Universal motor is used in food mixer because of high speeds about 1500 - 15000 rpm .

20. A 3 stack stepper motor has 12 teeth on stator as well as on rotor. Determine the step size when each stack excited?

Step size = ( 360 degree / 3 × 12 ) = 10 degree.

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