ELECTRICAL ENGINEERING DEPARTMENT�Semester: 4^{th�AY: 2021 – 22} �Subject: Energy conversion – I�Topic: D. C. Motor �CHAPTER-2

PREPARED BY

ER SUBHALAXMI ROUT

(FACULTY IN ELECTRICAL ENGINEERING BRANCH

Principle of operation

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The direction of this force is given by Fleming’s left-hand rule and magnitude is given by, F = BIL Newtons

Back E.M.F.

Torque Equation

Let 

r = average armature radius

L = effective length of each conductor

Z = total number of armature conductors

A = number of parallel paths

I_a = armature current

I = current through each conductor= I_a  / A

B = average flux density

Φ = flux per pole

P = number of poles

a = cross-sectional area of flux path per pole at radius, r = (2πrL/P)

Force on each conductor = BIL         

Torque due to one conductor = BILr   

Torque Equation

Torque Equation

Characteristics of DC motors

Characteristics of dc series motors

Characteristics of dc series motors

Characteristics of dc series motors

Speed Vs. Torque (N-Ta):

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From the two characteristics of DC series motor, it can be found that when speed is high, torque is low and vice versa. Also series motor develops high torque at low speed and vice-versa.

It is because an increase in torque requires an increase in armature current, which is also the field current. The result is that flux is strengthened and hence the speed drops.

Applications of dc series motor:

Electrical Cranes, Trolley Cars, Conveyors Belt Drives, Electric locomotives

Characteristics of dc shunt motors

Characteristics of dc shunt motors

Characteristics of dc shunt motors

Characteristics of dc Compound motors

Cumulative compound motor:

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• Cumulative compound motors are used where series characteristics are required but the load is likely to be removed completely.
• Series winding takes care of the heavy load, whereas the shunt winding prevents the motor from running at dangerously high speed when the load is suddenly removed.
• These motors have generally employed a flywheel, where sudden and temporary loads are applied like in rolling mills.

Characteristics of dc Compound motors

Differentially compound motor:

• Since in differential compounded motors, series flux opposes shunt flux, the total flux decreases with increase in load.
• Due to this, the speed remains almost constant or even it may increase slightly with increase in load.
• Differential compound motors are not commonly used, but they find limited applications in experimental and research work.

Armature Reaction

Commutation

Speed control of D.C. shunt motors

Speed control of D.C. shunt motors

1. Flux Control Method

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Initially, when the variable resistor is kept at its minimum position, the rated current flows through the field winding due to a rated supply voltage, and as a result, the speed is kept normal.

When the resistance is increased gradually, the current through the field winding decreases. This in turn decreases the flux produced. Thus, the speed of the motor increases beyond its normal value.

Speed control of D.C. shunt motors

Speed control of D.C. shunt motors

3 – point motor starter

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3 – point motor starter

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3 – point starter

Drawbacks of a Three-Point Starter:

• To increase the motor speed, the field resistance must be increased to decrease the shunt field current.
• Whenever adding high resistance to get a high speed will make the field current very low.
• When NVC (no volt coil) is associated in series with shunt field, the current will decrease the power of the electromagnet.
• This magnet may liberate the arm of the H-handle through the usual motor operation as well as detach it from the power supply.
• Therefore, the 4 Point Starter can be used, where no volt trip coil is connected in the parallel with the field winding.

4 – point starter

Testing of D.C. machines

Open Circuit Characteristic (OCC):

• The OCC is a graph showing the variation of the induced emf as a function of excitation current, when the speed is held constant, with the load current being zero. It is also called the no-load saturation curve or no load magnetization characteristic.
• This is experimentally determined by running the machine

as a separately excited generator on no-load at a constant speed and noting the terminal voltage as a function of the excitation current.

• This curve can be used to find the OCC at other speeds and also the self excited voltage when the machine works as a shunt generator.

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Testing of D.C. machines

Short circuit characteristics:(SCC)

• In the case of short circuit test the armature is kept short circuited through an ammeter.
• The machine is demagnetized and an extremely small field current is passed through the field.
• The variation of the short circuit current as a function of excitation current is plotted as the SCC.
• The speed is to be held constant during this test also.
• The short circuit test gives an idea of the armature drop at any load current.

Testing of D.C. machines

Load test:

• To assess the rating of a machine a load test has to be conducted.
• When the machine is loaded, certain fraction of the input is lost inside the machine and appears as heat, increasing the temperature of the machine.
• The load test gives the information about the efficiency of a given machine at any load condition. Also, it gives the temperature rise of the machine.
• The maximum continuous load that can be delivered by the machine without exceeding the temperature rise for the insulation used, is termed as the continuous rating of the machine.
• Thus the load test alone can give us the proper information of the rating and also can help in the direct measurement of the efficiency.

Losses

Calculation of efficiency

Condition for maximum efficiency

THANK YOU