DC Machines Multiple Choice Questions on “PMDC Motors”.
1. In PMDC motors we use magnetic material with ___________
a) Low residual flux
b) Low coercivity
c) High residual flux
d) All magnetic materials
Answer: c
Clarification: In market magnetic materials with various properties are available, but for DC motor application we need magnetic materials with high residual flux in order to get high no load voltage. Along with residual flux, coercivity of material should be high.
2. In PMDC magnetic material is placed instead of _____________
a) Armature
b) Rotor
c) Stator
d) Can be placed anywhere
Answer: c
Clarification: In PMDC motors we keep magnetic material or magnetic poles in the place of field winding of original DC machine. That is we are changing stator windings with two permanent magnetic poles bounded on casing.
3. PMDC motors are _________________
a) Smaller than corresponding rated field wound motor
b) Larger than corresponding rated field wound motor
c) Almost same size that of corresponding rated field wound motor
d) Can be smaller or greater than corresponding rated field wound motor
Answer: a
Clarification: PMDC motors are always of smaller size compare to field wound motors of same rating. Here, field winding is replaced by small block of magnet so that power output remains same. PMDC motors are costly compare to DC motor of same rating.
4. PMDC motor offers ___________
a) Cumulative compound type characteristic
b) Differential compound type characteristic
c) Series characteristic
d) Shunt characteristic
Answer: d
Clarification: The stator is made of permanent magnet in the PMDC motor hence, there is no need for separate field excitation for stator. So, PMDC acts as a shunt motor with fixed field. This can be seen from the construction also.
5. Which speed control method is employed in PMDC?
a) Field control method
b) Armature control method
c) Armature Field control method
d) Cannot be determined
Answer: b
Clarification: Unlike in DC motors, field is provided by permanent magnets in PMDC. This implies that field provided in PMDC motor is permanent and can’t be altered unless whole construction is modified. Thus, armature voltage control method is employed here.
6. What is the air gap flux density for PMDC motor?
a) Bg= µ0(tm/lg) Hm
b) Bg= – µ0Hm
c) Bg= – µ0(tm/lg) Hm
d) Bg= – µ0(lg/tm) Hm
Answer: c
Clarification: Since, flux crosses the air gap length lg two times a thickness of permanent magnet two times tm. We get, 2*lg* Hg + 2*tm Hm = 0 and we know that air gap flux density is equal to µ times the magnetic field intensity.
7. Which of the following is most favoured choice for PMDC motor?
a) Neodymium-iron-boron
b) Iron-nickel
c) Nickel-neodymium
d) Cannot be given
Answer: a
Clarification: Obvious choice of PMDC motor is neodymium-iron-boron which has high coercivity and high retentivity. Its characteristic is almost a straight line. Hence, neodymium-iron-boron combination is preferred over the other materials.
8. Speed-torque characteristic of PMDC motors is _______________
a) Starting from zero increasing continuously
b) Starting from some positive value and remaining constant
c) Starting from some positive value and decreasing exponentially
d) Starting from some positive value and decreasing on straight line
Answer: d
Clarification: The material chosen for PMDC motor is such that speed torque characteristic of a given motor remains straight line for wider range of torque, for wider range of armature voltage. The straight line is such that it makes positive intercepts on both axes.
9. PMDC produces high torque on ______________
a) At low speeds
b) Only at maximum speed
c) Torque is constant
d) Cannot say
Answer: a
Clarification: PMDC produces high torque even at very low speeds. In DC shunt motor torque produced at very low speeds is lower than the torque produced at the same speeds in PMDC motor.
10. PMDC gives better speed regulation than DC shunt motor.
a) True
b) False
Answer: a
Clarification: PMDC motor exhibits better speed regulation and efficiency than dc shunt motor. The main problem of dc shunt motor is going to run-away when the field terminals are opened. But in PMDC there is no run-away problem, so it gives practical benefit to the industry applications.
11. If higher terminal voltage is applied to PMDC what will happen?
a) Speed at 0 torque will be reduced
b) Torque at 0 speed will be reduced
c) Torque at 0 speed will be increased
d) Torque will remain constant
Answer: c
Clarification: The straight-line characteristic of PMDC is such that it produces positive intercepts on both axes. If terminal voltage is increased both of these intercepts increases, slope remaining constant. Thus, speed and torque at initial conditions both will increase.