DC Motor Types MCQ Quiz in मराठी - Objective Question with Answer for DC Motor Types - मोफत PDF डाउनलोड करा

• In dc series motor, an increase in armature circuit resistance will cause more voltage drop in the armature circuit, so speed will be reduced.
• The torque which is proportional to the product of the flux and armature current will also be reduced with the increase in armature circuit resistance as the increase in armature circuit resistance will reduce the armature current and the flux which also depends upon the armature current.
• Therefore, in this method of speed control, reduced speeds are obtained by inserting resistance in the armature circuit.

We have,

V = 200 volts

R_a = 0.5 Ω

I_a = 20 A

From Equation of DC Motor:

V = E + I_aR_a

200 = E + 20 × 0.5

⇒ E = 200 - 10 = 190 volts

The correct answer is (option 2)  

DC series motor.

Concept:

The speed regulation of a DC motor is defined as the change in speed from no load to full load. It is expressed as a fraction or a percentage of the full load speed.

Percentage speed regulation \(= \frac{{{N_{nl}} - {N_{fl}}}}{{{N_{fl}}}} \times 100\)

No load speed of a DC series motor is very high. So, it has the poorest speed regulation.

Additional Information

All the DC motors have positive regulation except the Differential compound motor.

Speed regulation maximum to the minimum:

Series motor > Cumulative compound motor > Differential compound motor > Shunt motor

The speed-torque characteristics of different DC motors are as shown in the below figure.

• In DC series motors, torque increases as the square of armature current. Hence these motors are used in high starting torque and variable speed applications.
• In DC shunt motors, torque is directly proportional to armature current. It is a constant speed motor. It is used for constant speed applications.
• The compound motors are used where higher starting torque and fairly constant speed are required.

Key Points

Applications of different DC motors are given below.

DC Series wound motor: 

• It has a very high starting torque.
• Hence it is used for heavy-duty applications such as electric railways, mine hoists, continuous conveyors cranes, rolling mills, metallurgical works, etc.

 

DC Shunt-wound motor: 

• This is an almost constant speed motor and its torque varies nearly as the current.
• Hence, it is used for driving constant speed line shafts, lathes, constant speed head centrifugal pumps, fans,  woodworking machines, reciprocating pumps, laundry washing machines, milling machines, grinders, small printing presses, paper making machines, metal cutting machines, etc.

 

DC Cumulative compound motor:

• It has high starting torque and has varying speed within limits i.e. it has self-adjustable speed with changing the load.
• Hence, it is used for driving compressors, pressure blowers, door lifts, circular saws, passenger elevators, freight elevators, etc.

 

DC Differentially compound motor: 

It has low starting torque and has a constant speed, but the dangerously high speed at no load under the circumstances it is seldom used.

Intermittent loads: The load on the motor occurs periodically in an identical duty cycle, each duty cycle having a period of application of load and rest.

Cumulative compound motors develop good starting torque and also its no-load speed is limited. Therefore these motors are used for intermittent load applications. I.e. the load operation consists of load and no-load operation intervals continuously.

The torque produced in DC motor is,

T ∝ ϕIa

ϕ is the main field flux

Ia is the armature current

DC series motor:

In the case of DC series motor, the field and armature winding are connected in series. If the polarity of the input voltage or supply terminals are interchanged, then it changed the direction of the field and armature current both get changed so the direction of speed will remain the same.

The direction of rotation of DC series motor may be reversed by changing the polarity of either field or the armature winding.

Separately excited DC motor:

The direction of the torque and the speed can be reversed by changing the direction of either the field current or armature current by changing the polarity of the DC voltage. However, this method is used for separately excited DC motor where it is possible to change the polarity of the field or armature.

Concept:

EMF generated in the shunt generator is given by

Eg = V + IaRa

Where, Ia = armature current

V = Terminal voltage

Ra = armature resistance

Rsh = Shunt resistance

IL = Load current

Field current in shunt generator is given by

\({I_f} = \frac{{V}}{{R_{sh}}} \;\)

Ia = IL + If

Calculation:

Given -

V = 240 V

Load current (IL) = 400 A

Rsh = 40 Ω

Ra= 0.04 Ω

According to the question, the circuit diagram of DC shunt generator can be drawn as

       

∴ Field current, \({I_f} = \frac{{240}}{{40}} = 6\;A\)

∴ Armature current, Ia = If + IL = 400 + 6 

Ia = 406 A

Now generated emf is calculated as

Eg = 240 + 406 × 0.04 = 256.24 V

Applications of DC Motors:

1. Shunt Motor: The characteristics of a shunt motor reveal that it is an approximately constant speed motor.

It is, therefore, used

• Where the speed is required to remain almost constant from no-load to full-load
• Where the load has 10 be driven at a number of speeds and any one of which is required to remain nearly constant.
• Application: Lathes, drills, boring mills, centrifugal and reciprocating pumps, shapers, spinning and weaving machines, etc.
   

2. Series Motor: It is a variable speed motor i.e., speed is low at high torque and vice-versa.

However, at light or no-load, the motor tends to attain dangerously high speed.

The motor has a high starting torque.

It is, therefore, used,

• Where large starting torque is required e.g., in elevators and electric traction
• Where the load is subjected to heavy fluctuations and the speed is automatically required to reduce at high torques and vice-versa
• Application: Electric traction, cranes, elevators, air compressors, vacuum cleaners, hair drier, sewing machines, etc.
   

3. Compound Motor:

• Differential-compound motors are rarely used because of their poor torque characteristics.
• However, cumulative-compound motors are used where a fairly constant speed is required with irregular loads or suddenly applied heavy loads.
• Application: Presses, shears, reciprocating machines, etc.

DC Motor Connection Diagram:

The below-given figure shows a basic notation of symbols to understand the connection of the DC motor

There are three general types of DC motors, classified according to the method of field excitation as follows:

DC Shunt Motor:

A shunt DC motor uses a comparatively high resistance shunt field winding, made up of many turns of fine wire, connected in parallel (shunt) with the armature.

DC Series Motor:

A series DC motor uses a very low resistance series field winding, made up of very few turns of heavy wire, connected in series with the armature.

DC Compound Motor:

A compound DC motor uses a combination of a shunt field (many turns of fine wire) in parallel with the armature, and a series field (few turns of heavy wire) in series with the armature.​

Additional Information

Connections of DC motors:

Connections of DC compound motors: