# Electric Drives Questions and Answers – DC Motors – Basic Characteristics

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “DC Motors – Basic Characteristics”.

1. Calculate the terminal voltage of Permanent Magnet DC motor having a resistance of 2 Ω and a full load current of 5 A with 20 V back e.m.f.
a) 30 V
b) 25 V
c) 20 V
d) 31 V

Explanation: Permanent Magnet DC motor is a special type of motor in which flux remains constant. The terminal voltage can be calculated using the relation Vt = Eb+IaRa = 20+5×2 = 30 V.

2. Armature reaction is demagnetizing in nature due to purely lagging load.
a) True
b) False

Explanation: Due to purely lagging load, armature current is in opposite phase with the field magneto-motive force. Armature magneto-motive force produced due to this current will be in the opposite phase with the field flux. It will try to reduce the net magnetic field.

3. The unit of Magento-motive force is Ampere-turns.
a) True
b) False

Explanation: The magneto-motive force is defined as the product of current and turns. It is mathematically expressed as F=NI.

4. Calculate the velocity of the wheel if the angular speed is 25 rad/s and radius is 10 m.
a) 250 m/s
b) 260 m/s
c) 270 m/s
d) 240 m/s

Explanation: The velocity of the wheel can be calculated using the relation V=ω×r. The velocity is the vector product of angular speed and radius. V=Ω×r = 25×10 = 250 m/s.

5. Displacement is a ____________ quantity.
a) Scalar
b) Vector
c) Scalar and Vector
d) Tensor

Explanation: Displacement is a vector quantity. It depends on the initial and final position of the body. It has both direction and magnitude. Distance is a scalar quantity.

6. When a UJT is used for triggering an SCR, then the wave shape of voltage obtained from the UJT circuit will be ____________
a) Square
b) Pulse
c) Trapezoidal
d) Saw-tooth

Explanation: UJT relaxation oscillator using RC circuit is used for SCR triggering and the wave shape obtained is an exponentially decaying pulse. So, we can say pulse waveform is obtained.

7. Calculate the value of the frequency if the time period of the signal is 0 sec.
a) infinity
b) 0.4 Hz
c) 0.2 Hz
d) 0.78 Hz

Explanation: The frequency is defined as the number of oscillations per second. It is reciprocal of the time period. It is expressed in Hz. F = 1÷0 = infinity. It signifies signal is aperiodic.

8. The slope of the I-V curve is 87°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
a) 0.0524 Ω
b) 0.0254 Ω
c) 0.0543 Ω
d) 0.0648 Ω

Explanation: The slope of the I-V curve is reciprocal of resistance. The slope given is 87° so R=1÷tan(87°)=.0524 Ω.

9. The most suitable device for high-frequency inversion in switching mode power supply is ______
a) GTO
b) BJT
c) MOSFET
d) IGBT

Explanation: MOSFET is fastest among all the power semiconductor devices and has the highest frequency range. MOSFET stands for metal oxide silicon field effect transistor.

Sanfoundry Global Education & Learning Series – Electric Drives.

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