# Important MCQ Question of DC Motor

#### No-load speed of which of the following motor is highest?

Correct! Wrong!

At the time of starting, flux is very less in DC series motor. From the inverse relationship between speed and flux i.e flux decrease when speed increase. Hence the speed of DC series motor is high during starting. Therefore we never start DC series motor at no load.

#### Which of the following load normally need starting torque more than the rated torque?

Correct! Wrong!

• Starting torque is also called the locked-rotor torque and the motor current at this point is referred to as the 'locked rotor current. At this stage, the rotor reactance is higher than its resistance, as the rotor frequency is at its highest (equals the supply frequency).
• Starting torque is the amount of torque required to overcome the inertia of a standstill motor.
• Conveyors require high starting torque because the material is placed before the starting of the Conveyors belt.
• So obviously we require more torque to overcome the load at the starting period.
• Therefore conveyor required high starting torque and constant speed.
• Series motor is suitable for conveyor belts operating in a particular region of its torque-speed characteristics.
• In industries  BLDC is used, as BLDC provides PWM speed controlling method which is easier to vary the speed as well as with low maintains.

#### Which of the following rule is used to determine the direction of rotation of D.C motor?

Correct! Wrong! • When a current-carrying conductor such as a wire attached to a circuit moves placed in a magnetic field, an electric current is induced in the wire due to Faraday's law of induction.
• The right hand is held with the thumb, first finger and second finger mutually perpendicular to each other than
• The thumb is pointed in the direction of the motion of the conductor relative to the magnetic field i.e direction of the force.
• The forefinger is pointed in the direction of the magnetic field.
• The middle finger represents the direction of the induced or generated current within the conductor.

#### Which part of the DC motor can sustain maximum temperature rise?

Correct! Wrong!

• The commutator is made up of copper segments.
• This copper segment is insulated from each other with the help of mica.
• Mica has a very high dielectric stress of about 1180 kV/cm and the next best is Diamond.
• It can withstand high temperatures, up to 600 deg C.

#### The ratio of starting torque to full load torque is least in

Correct! Wrong!

• In the differential compound, motor two field windings i.e shunt and series windings oppose each other.
• This causes a reduction in flux and consequences decrease in torque. #### The number of the pole in Small Dc Motor Up to 5 H.P are

Correct! Wrong!

• Small H.P motors require only 2 poles because as the number of poles is inversely proportional to the speed therefore 2 pole motor runs at high speed than 4 pole motor.
• Two pole motor has better efficiency.
• Two pole motor has better rpm and noise performance.

Correct! Wrong!

#### Why efficiency of D.C motor is below 50%

• The DC motor develops maximum power when the Back EMF is half the applied voltage is Eb = V/2.
• Practically it is not possible to develop an exact 50% of maximum power because in that case, the current would be much beyond the rated current of the motor.
• Some of the energy is wasted in the form of heat and other losses. Therefore motor efficiency is below 50%.

#### In which of the following application DC series motor is used?

Correct! Wrong!

• In DC series motor Torque (Ta) increase as the Square of armature current (Ia) Ta ∝ Ia2.
• So DC motor provides high starting torque which is required to start a car.

#### In the DC machine the fractional pitch winding is used

Correct! Wrong!

```In full pitched coils, as one conductor of a turn in a coil cuts N pole, the other conductor of the same turn cuts S pole resulting in the production of induced emf(E), i.e phase angle is 180 degreeIn short-pitched coils, both conductors of the same turn in a coil don't cut the respective poles simultaneously. so phase angle is slightly less than 180 degree As a result, the magnitude of induced emf gets reduced to E × Cos(nα/2) Where E × Cos(nα/2) is called a pitch factorFor eliminating 3rd harmonic from Generated EMF,
Cos(3α/2) = 0
3α/2 = π/2
α = π/3 = 60°```

#### A three-point starter is suitable for

Correct! Wrong!

```3 point starters in DC Shunt and Compound machines serve for the following purposes.
It limits the high starting current into the armature by having the resistance high at the time of starting and reducing it during the running conditions.It also protects the motor from overload and under-voltage conditions.
```

1. Browin says:

Answer of question number 48 and 49 are wrong.
Flux density is increasing for trailing pole tips in generator action and leading pole tips in motor action.

1. admin says:

Yes you are right @browin the flux density increases in the case of dc generator under trailing pole tips but the question is about motor flux density … motor work opposite to that of DC generator so the answer is right

2. SHRUTI YADAV says:

i have doubt in qstn no. 18..why ans is series not differential motor? if self loading and self releiving properties are same or different? pls explain..

1. admin says:

3. Veesam ajay gopal says: