This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Permanent Magnet DC (PMDC) Motors”.
1. The qualities aspired to obtain a good permanent magnet is/are ____________
a) high residual flux
b) low coercivity
c) high coercivity
d) high residual flux and high coercivity
Explanation: Its both high residual flux as well as coercivity is desired for a good permanent magnet.
2. Which type of field winding required in PMMDC?
a) series winding
b) shunt winding
c) cumulative winding
d) none of the mentioned
Explanation: There is no field winding used in PMMDC to create the flux.
3. PMMDC are smaller in size due to __________
a) ansence of field winding
b) presence of smaller field winding
c) present of magnets
d) Any of the mentioned
Explanation: There is no field winding used in PMMDC to create the flux. So it is lighter.
4. PMMDC offers ______ characteristics.
Explanation: As the field remains constant due to the constant field, it has shunt characteristic.
Explanation: The magnetic field has a positive value when the magnetic field intensity, H has negative value due to coercivity property. We use second quadrant of hysteresis loop.
Explanation: It is linear in nature and also analogous to shunt machine.
7. PMDC had no ________ excited machine.
c) none of the mentioned
Explanation: It has no electrical excitation.
8. How to manage run-away issue for dc shunt motor in industry?
a) By using PMDC motor
b) By using constant field motors
c) This can not be avoided
d) None of the mentioned
Explanation: A PMDC motor can be used to replace the dc shunt motor in industries as they have quite identical characteristics.
9. A PMMDC motor has an armature resistance of 4.2 ohms. When 6 V supply is applied to motor it runs at 1215 rpm drawing 14.5 mA on no-load. The rotational losses is?
a) 86.1 mW
b) 86.1 W
c) 8.6 W
d) 8.6 mW
Explanation: Rotational losses = E*Ia = 5.939* 14.5/1000 = 86.1 mW.
10. PMMDC can only be armature controlled.
Explanation: Due to absence of the field winding there is no option of field control.
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