DC Motor Losses MCQ || Losses in DC motor Questions and Answers

DC Motor, MCQ / March 15, 2022

1. Overheating of DC motors is mainly due to:

  1. Rough commutator
  2. Overloads
  3. Loose parts
  4. Absence of brushes

Answer:2. Overloads

Explanation: 

The reason for DC motor overheating are

  1. Overload
  2. Poor power condition
  3. High effective service factor
  4. Frequent stops and starts
  5. Environmental reasons
  • Over-Heating in motors can cause a number of performance problems.
  • Overheating causes the motor winding insulation to deteriorate quickly.
  • For every ten centigrade rise in temperature, the insulation life is cut in half. It has been concluded that more than 55% of the insulating failures are caused by over-heating.
  • Every electric motor has a design temperature. If a motor is started off at a bad current value, it starts operating in a much warmer condition than the design temperature.
  • It is very important that the motors should be matched with their ideal current values.
  • Overheating also occurs when an electric motor is forced to operate in a high-temperature environment. This causes the rate at which heat can be conducted to reduce at an alarming rate.
  • The area where electric motors are operating must have a proper cooling system and a ventilation system should be there in case the cooling system stops working.

 

2. Which of the following is one of the categories of core loss in a DC machine?

  1. Friction loss
  2. Loss due to series field winding
  3. Windage loss
  4. Hysteresis loss

Answer:4. Hysteresis loss

Explanation: 

Core Losses or Iron Losses in DC Machine or Magnetic Losses:

  • As the iron core of the armature is rotating in the magnetic field, some losses occur in the core which is called core losses.
  • Normally, machines are operated with constant speed, so these losses are almost constant. These losses are categorized in two forms; Hysteresis loss and Eddy’s current loss.

Hysteresis Loss in DC Machine

  • Hysteresis losses occur in the armature winding due to the reversal of magnetization of the core. When the core of the armature is exposed to the magnetic field, it undergoes one complete rotation of magnetic reversal.
  • The portion of the armature is under S-pole, after completing half the electrical revolution, the same piece will be under the N-pole, and the magnetic lines are reversed in order to overturn the magnetism within the core.
  • The constant process of magnetic reversal in the armature, consume some amount of energy which is called hysteresis loss. The percentage of loss depends upon the quality and volume of the iron.

 

3. Iron losses in a D. C. machine are independent of variations in

  1. Speed
  2. Load
  3. Voltage
  4. Speed and voltage

Answer:2. Load

Explanation: 

Iron Loss or Core Loss in DC machine:

  • These losses occur in the armature of a d.c. machine and are due to the rotation of armature in the magnetic field of the poles.
  • Mechanical losses depend on speed, iron losses depend on field flux and speed, and copper losses depend on armature current. The field copper losses, iron, and mechanical losses are the constant types of losses for a d.c. shunt machine.
  • It depends on the frequency (Speed) and voltage and does not depend on load or load current

 

4. The loss that takes place due to winding resistance is called ______.

  1. Iron loss
  2. Hysteresis loss
  3. Eddy current loss
  4. Copper loss

Answer:4. Copper loss

Explanation: 

  • The winding losses taking place during the current flowing through the winding are known as Copper losses. 
  • Copper losses are directly proportional to the square of the current.
  • These are variable losses dependent on load current.
  • Windage losses are the losses sustained by a machine due to the resistance offered by air to the rotation of the shaft.
  • These are mechanical losses and independent of load.
  • As the armature core of the electrical machine is made of iron and rotates in a magnetic field, a small current gets induced in the core.
  • Due to this current, eddy current loss and hysteresis loss occur in the armature iron core.
  • Iron losses are also called core losses or magnetic losses.
  • These are constant losses and independent of load.

 

5. Torque developed by a motor while running at 1000 rpm is 106 N-m and the shaft torque available is 100 N-m. then the iron and mechanical losses in watts are:

  1. 300 π
  2. 50 π
  3. 200 π
  4. 100 π

Answer:3. 200 π

Explanation: 

Torque developed by a motor shaft is given by

Tsh = Psh/ω

Where

Tsh = shaft Torque or output torque

Psh = Shaft power or output power

ω = angular frequency = 2πN/60

N = shaft speed

Tloss = Td – Tsh

Where,

Tloss = Torque due to losses

Td = Developed Torque

Tsh = shaft or output Torque

Calculation:

Given:

Tsh = 100 N-m, Td = 106 N-m, N = 1000 rpm

Tloss = Td – Tsh  = 106 – 100 = 6 N-m

Therefore

Ploss = Tloss × ω

= 6 × 2π × 1000/60

Ploss = 200π W

 

6. In electric motor _________ is loss due to fans and the bearing friction.

  1. Iron loss
  2. Core loss
  3. Stray loss
  4. Friction and windage loss

Answer:4. Friction and windage loss

Explanation: 

Friction and Windage Losses in DC machine:

  • Friction and windage losses result from bearing friction, windage, and circulating air through the motor and account for 8 – 12 % of total losses. These losses are independent of load.
  • The reduction in heat generated by stator and rotor losses permits the use of a smaller fan.
  • The windage losses also reduce with the diameter of the fan leading to a reduction in windage losses.

 

7. _______ combined together are called stray losses.

  1. Frictional loss and copper loss
  2. Field copper loss and iron loss
  3. Iron and mechanical losses
  4. Copper and iron losses

Answer:3. Iron and mechanical losses

Explanation: 

Stray load loss:- If the machine is tested under load conditions, the measured total loss is consistently greater than the sum of the above losses, even using the most accurate computational and experimental methods available to determine these losses individually. To account for this additional loss, a fifth loss (called the stray load loss) is defined.

The combination of friction losses due to moving of bearing and shaft and windage losses due to air gap is called stray loss.

 

8. Which of the following is considered as mechanical loss in induction motor losses?

  1. Stator copper loss
  2. Iron loss
  3. Windage and friction losses
  4. Rotor copper loss

Answer:3. Windage and friction losses

Explanation: 

No-load test or open circuit test:

  • A no-load test or open circuit test is used to find the shunt branch parameters of the equivalent circuit and the constant losses.
  • We can separate these constant losses into iron losses and mechanical losses.

Constant losses = Iron loss + Mechanical losses

Windage and frictional losses come under mechanical losses.

Blocked rotor test or short circuit test:

  • It is used to find out series branch parameters (R01­ and X01) of equivalent circuit parameters when referred to the primary side (stator) and the full load copper losses (I2R).
  • By applying rated voltage under a blocked rotor test, the motor will take a very high short circuit current which is equivalent to 5 to 8 times the full load current.
  • We need to apply reduced voltage initially and increase the voltage up to the current drawn by the motor to reach the rated current.

Variable losses = Copper loss + Stray load Loss

 

9. In a D.C. motor the windage losses are proportional to

  1. Supply voltage
  2. Square of supply voltage
  3. Square of flux density
  4. Square of the armature speed

Answer:4. Square of the armature speed

Explanation: 

Friction and windage loss: Since parts of the machine are in motion, frictional forces must occur. The corresponding losses are localized at the bearing surfaces. In the wound rotor machine, additional friction loss is caused by the brushes sliding on the slip rings.

“Windage” is basically “wind friction” and occurs since machine motion occurs in an atmosphere. Machines with fans mounted on the shaft for forced ventilation cooling have greatly increased windage losses.

Additional windage loss is created by the rotor fan blades which circulate air within the motor, and, of course, there is some friction loss in the bearings.

Energy-efficient motors have less losses to be dissipated than a standard motor and, therefore, smaller external fans can frequently be used as less airflow is required. The smaller fans have a favorable effect on friction and windage loss and, therefore, contribute to increased motor efficiency.

  • If N is the speed, then the windage loss in the dc motor is proportional to the square of armature speed.
  • Windage losses occur due to the air inside the rotating coil of the machine.
  • Eddy’s current losses are proportional to the square of speed.

 

10. Which of the following is NOT one of the categories of copper loss in a DC machine?

  1. Field winding loss
  2. Brush contact resistance loss
  3. Armature loss
  4. Eddy current loss

Answer:4. Eddy current loss

Explanation: 

The primary I2r is the ohmic loss caused by current passing through the stator winding.

Increasing the cross-sectional area of copper to reduce the primary resistance is an effective means of reducing the primary I2r loss and this method is used in nearly all energy-efficient motor designs.

Type of Copper Loss in DC machine

Armature copper losses:

  • Armature copper losses = I2aRa
  • These losses are about 30% of the total full load losses.
  • Armature copper losses in a DC generator vary significantly with the load current.

Field copper losses:

  • Field copper losses = I2shRsh
  • These losses are about 25% theoretically, but practically it is constant.

Brush Contact Loss

Brush contact loss is attributed to the resistance between the surface of the brush and the commutator. It is not a loss that could be calculated separately as it is a part of variable losses. Generally, it contributes to both types of copper losses.

Note:- Eddy current Losses is a type of core or Rotational loss

 

11. For a dc motor operating under the condition of maximum transfer of power, the efficiency of the motor is

  1. 100 %
  2. about 90%
  3. 75%
  4. less than 50%

Answer:4. less than 50%

Explanation: 

 Maximum power condition:

If the value of $\frac{dP_m}{dI_a}=0,$ the motor will deliver maximum power

$\frac{dP_m}{dI_a}= V – 2I_aR_a = 0$

Ia Ra = V / 2 ….(2)

From equation (1)&(2)

Eb = V / 2

The efficiency of the motor is = Output Power(Pm) / Input power(P)

$\eta= \frac{E_b I_a }{VI_a }\times 100=\frac{E_b }{V}\times 100=50$%

∴ For a dc motor operating under the condition of maximum transfer of power, the efficiency of the motor is less than or equal to 50%

 

12. Which of the following losses occur due to the induction of current in a DC Machine?

  1. Copper Loss
  2. Iron Loss
  3. Mechanical Loss
  4. Windage Loss

Answer: 2. Iron Loss

Explanation: 

Core Losses or Iron Losses in DC Machine or Magnetic Losses:

  • As the iron core of the armature is rotating in the magnetic field, some losses occur in the core which is called core losses.
  • Iron Loss occurs due to the induction of current in a DC Machine.
  • Normally, machines are operated with constant speed, so these losses are almost constant. These losses are categorized in two forms; Hysteresis loss and Eddy’s current loss.

Hysteresis Loss in DC Machine

  • Hysteresis losses occur in the armature winding due to the reversal of magnetization of the core. When the core of the armature is exposed to the magnetic field, it undergoes one complete rotation of magnetic reversal.
  • The portion of the armature is under S-pole, after completing half the electrical revolution, the same piece will be under the N-pole, and the magnetic lines are reversed in order to overturn the magnetism within the core.
  • The constant process of magnetic reversal in the armature, consume some amount of energy which is called hysteresis loss. The percentage of loss depends upon the quality and volume of the iron.

Eddy Current Loss in DC Machine

  • According to Faraday’s law of electromagnetic induction, when an iron core rotates in the magnetic field, an emf is also induced in the core.
  • Similarly, when armature rotates in the magnetic field, a small amount of emf is induced in the core which allows the flow of charge in the body due to the conductivity of the core.
  • This current is useless for the machine. This loss of current is called eddy current. This loss is almost constant for the DC machines. It could be minimized by selecting the laminated core.

Related Posts

Scroll to Top