Permanent Magnet DC (PMDC) Motor MCQ

Permanent Magnet DC (PMDC) Motor MCQ

1. In PMDC motor we use______instead of salient pole.

  1. Permanent magnet

  2. Temporary magnet

  3. Armature Winding

  4. Resistance winding

Answer.1. Permanent magnet

Explanation:-

A permanent-magnet DC motor is like an ordinary shunt motor except that its field is provided by a permanent magnet instead of a salient-pole, wound-field structure.

All kinds of DC motors work on the same principles. For constructing a DC motor it is essential to establish a magnetic field. The magnetic field is established, obviously, by means of a magnet. The magnet can be of many types including an electromagnet or permanent magnet. When a permanent magnet is used to create a magnetic field in a DC motor, the motor is referred to as a permanent-magnet DC or PMDC motor.

 

2. In PMDC motor the material used for making permanent magnet is/are

  1. Alnico

  2. Ceramic

  3. Rare earth material

  4. All of the above

Answer.4. All of the above

Explanation:-

There are three classes of permanent magnet materials (or hard magnetic materials) used for permanent magnet dc (PMDC) motors: Alnicos, ceramics (ferrites) and rare-earth materials.

1. Alnico magnets have good magnetization/demagnetization properties but very low coercive force and the B—H loop is too square-shaped. The permanent demagnetization occurs too easily. Alnico magnets are used in motors up to 200 kW

2. Ceramic ferrite magnets use barium and strontium. They are low-cost and have a moderately high-service temperature at 400°C. This type of magnet is represented by a straight-line demagnetization curve Br (residual). Flux density is low and hence the machine volume and size are typically large. Ceramic magnets are most economical in fractional kW motors.

3. Rare-earth magnets, using neodymium-iron-boron (NdFeB), have very good demagnetization properties except when at Curie temperature of 150°C. A range of B-H for each material and ordering measures for useful applications are determined. In general, rare-earth materials are expensive but cost-effective and useful in very small resistors. The rare-earth magnetic materials are very costly but are the most economic choice in very small motors.

 

3. The qualities required to obtain a good permanent magnet for PMDC motors are

  1. High residual flux

  2. Low coercivity

  3. High coercivity and High residual flux 

  4. High coercivity

Answer.3. High coercivity and High residual flux 

Explanation:-

Two important qualities of a permanent magnet (PM) are defined below with reference to the second quadrant of its hysteresis loop.

Permanent magnetization or residual flux density (Br)

It is the flux density trapped in closed magnetic structure if the applied MMF (and therefore the magnetic field intensity, H) were reduced to zero.

Coercivity

When a permanent magnet has been magnetized once, it will remain magnetized even if its magnetic field intensity is decreased to zero. The magnetic flux density at this point is called magnetic remanence. Furthermore, when the magnetic field intensity is increased in the opposite polarity along the demagnetization curve, the flux density will eventually become zero. Here, this field intensity is called the magnetic coercive force or coercivity.

The high permeance and coercivity allow marked reductions in motor frame size for the same output compared to motors using ferrite (ceramic) magnets. For very high-temperature applications Alnico or rare-earth cobalt magnets must be used.

4. The type of field winding required in PMMDC is

  1. Shunt Winding

  2. Series Winding

  3. Armature winding

  4. None of the above

Answer.4. None of the above

Explanation:-

A PMDC motor consists of a stator, rotor, brushes, and so on. The stator has permanent magnets fixed on its frame and produces the main flux of the motor. The rotor has an armature core and windings. A commutator is mounted on the shaft and its segments are connected to the armature windings.

A traditional commutator motor in which the electromagnetic field system is replaced by permanent magnets is called a PMDC motor. The use of permanent magnets considerably reduces the diameter of the stator and hence the overall size of the motor is smaller compared to a conventional motor of the same rating.

5. In PMDC motor the permanent magnet is placed is

  1. Rotor

  2. Stator

  3. Either Rotor or Stator

  4. Armature

Answer.2. Stator

Explanation

In permanent magnet dc (PMDC) motors, the stator wound poles of a conventional dc motor are replaced by permanent magnets. The rotor has a conventional dc armature, with commutator segments and brushes. Unlike the salient-pole field structure of the conventional dc motor.

 

Ques.6. The characteristic of PMDC motor is similar to_______

  1. DC series

  2. DC shunt

  3. Compound Motor

  4. DC generator

Answer.2. DC Shunt

Explanation:-

The working of the PMDC machines is similar to the DC shunt motors except for the fact that the stator fields are replaced by the permanent magnets to provide a necessary flux by which the motor operates. There is no need for separate field excitation for the stator. So, PMDC acts as a shunt motor with a fixed field. Because of the less interaction of the armature field with the main field, the motors experience linear speed-torque characteristics. If the torque increases, the speed will be decreasing proportionally for a given armature voltage. By controlling the armature, the speed of the motor could be controlled.

 

7. The type of Armature structure of PMDC motor is/are

  1. Slotted

  2. Slotless

  3. Moving coil

  4. All of the above

Answer.4. All of the above

Explanation

The armature is the part of the PMDC motor in which the emf is induced and to which the external source is connected through a commutator and brushes. There are three types of armature structures available: slotted, slotless, and moving coil. The constructional details of them are as follows.

Slotted armature: Slotted armature is made of silicon sheet steel or carbon steel sheet which are punched together and mounted on the shaft. The armature has slots on its outer periphery. Armature conductors are placed in these slots and properly connected to form the armature winding.

Slotless armature: There is another type of construction of rotor in which the conductors are fixed on the outer periphery of the core. There are no slots. The effective air gap length is larger in this armature compared to that of the slotted armature and hence Alnico or rare earth permanent magnets are used. An advantage of this construction is the reduction in torque ripples.

Moving coil armature: In this construction, the iron core is replaced by a non-magnetic core which is usually made of glass fiber. This has the advantages of low inertia and no iron loss in the armature. The commutator and brushes are very small and made of precious metals like gold, silver, platinum, and others. Small size commutator and brushes help in stable commutation.

 

8. Which of the following rule is used to determine the direction of rotation of the PMDC motor?

  1. Columb’s Law

  2. Lenz’s Law

  3. Fleming’s Right-hand Rule

  4. Fleming’s Left-hand Rule

Answer.4. Fleming’s Left-hand Rule

Explanation:-

The working of a PMDC motor is based on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a force whose direction is given by Fleming’s left-hand rule and magnitude is given by F = BII, where B is the flux density, I is the current through the conductor and I is the length of the conductor.

 

9. The main advantages of PMDC motor are

  1. Compact in size

  2. Good Acceleration and De-acceleration

  3. High Efficiency

  4. All of the above

Answer.4. All of the above

Explanation

PMDC motors have several advantages. Because of the absence of field windings, the copper loss is absent and this increases efficiency.

No space is required for field windings; consequently, these motors are smaller than corresponding wound-pole motors and, in some cases, cheaper as well. Although permanent magnet materials tend to be more expensive, the reduction in the size of the motor reduces the cost of other materials, which compensates, at least in part, for the extra magnet cost. PMDC motors have a definite cost advantage in the smaller size range.

Because of the less interaction of the armature field with the main field, the motors experience linear speed-torque characteristics. If the torque increases, the speed will be decreasing proportionally for a given armature voltage. They exhibit excellent acceleration and deceleration.

Ques.10.  The speed of the PMDC motor can be controlled by

  1. Fluxed controlled Method

  2. Field Voltage Method

  3. Can’t be controlled Externally

  4. Frequency controlled Method

Answer.3. Can’t be controlled Externally

Explanation:-

Since flux remains constant, the speed of a PMDC motor cannot be controlled by using the flux control method. The only way to control its speed is to vary the armature voltage with the help of an armature rheostat or electronically by using choppers. Consequently, such motors are found in systems where speed control below base speed only is required.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top