BASIC ELECTRICAL TECHNOLOGY

DET 211/3

Chapter 7

Direct Current (DC) Motor

&

Direct Current (DC) Generator

DC Motor

There are four major types of DC motor in general use:

• Separately excited DC motor

• Shunt DC Motor

• Series DC Motor

• Compounded DC Motor

Speed Regulations

• DC motors are often compared by their speed regulations.

• Speed Regulations (SR) of the a motor is defined by

100% xSRfl

flnl

100% xn

nnSR

fl

flnl

The Equivalent Circuit of a DC Motor

Armature circuit represented by voltage source, EA and a resistor RA.

The brush voltage drop represented by battery, Vbrush opposing the direction of current flow in the machine.

The field coils, which produce the magnetic flux are represented by inductor LF and resistor RF.

The separate resistor, Radj represents an external variable resistor used to control the amount of current in the field circuit.

RA

The Equivalent Circuit of a DC Motor

The brush voltage drop is often only a very tiny fraction of the generated voltage in a machine so the voltage drop may be left out or approximately included in the value of RA.

The internal resistor in the field coils is sometimes lump together with the variable resistor, and the total is called RF.

The Equivalent Circuit of a DC Motor

The internal generated voltage in DC motor is

KEA

The induce torque developed by DC motor is

Aind IK

These two equations, the KVL equation of the armature circuit and the machine’s magnetization curve are all the tools necessary to

analyze the behavior and performance of the dc motor.

Separately Excited and Shunt DC motors

Separately excited DC motor

Shunt DC motor

F

FF R

VI

AAAT RIEV

AL II

F

TF R

VI

AAAT RIEV

FAL III

Example

1. the speed of this motor when its input current is 100A2. the speed of this motor when its input current is 200A3. the speed of this motor when its input current is 300A.4. the induced torque of this motor for above conditions.

A 50hp, 250V, 1200rpm DC shunt motor with compensating windings has an armature resistance of 0.06Ω. Its field circuit has a total resistance of 50Ω, which produces a no load speed of 1200rpm. There are 1200 turns per pole on the shunt field winding. Find:

Solution

KEA

V250VT

rpm1200n

1rads67125x2

60

n2

.60

1200

The relationship between the speeds and internal generated voltages of the motor at two different load conditions is

11A KE (1)

22A KE (2)

The flux is constant and no armature reaction

Solution

2

1

2A

1A

K

K

E

E

At no load, rpm1200n

IA = 0A

V250VE T1A

1)

A9550

250100

R

VIIII

F

TLFLA

V3.244)06.0(95250RIVE AATA EA at this load will be

The resulting speed of this motor is

2

1

2A

1A

K

K

E

E

1

1A

12A2 rads8.122

250

67.125x3.244

E

E

rpm11732

8.122x60

2

60n2

Solution

Answer

2)

IA = 195A,

EA = 238.3V,

n2 = 1144rpm

IA = 295A,

EA = 232.3V,

n2 = 1115rpm

3)

Solution

4)

indAAconv IEP

AA

ind

IE

Nm1898.122

95x3.244ind

At IL = 100A

At IL = 200ANm388ind

At IL = 300ANm587ind

Series DC Motor

)RR(IEV sAAAT

SAL III

Compounded DC Motor

A compounded DC motor is a motor with both a shunt and a series field.

Current flowing into a dot produces a positive magnetomotive force.

If current flows into the dots on both field coils, the resulting magnetomotive forces add to produces a larger total magnetomotive force. It is called cumulative compounding.

Equivalent circuit of compounded DC motor

)RR(IEV sAAAT

FLA III

F

FF R

VI

ARFnet F- F FF SE

F

ARA

F

SEF

*F N

IN

NII

F

Long shunt connection

Short shunt connection

The net magnetomotive force

Effective shunt field current

Applications of DC motor types

Separately-excited dc motor applications:i) Golf cars (buggy)ii) Forkliftiii) Aerial lift equipment

DC GeneratorThere are five major types of DC generators:

1. Separately excited generator. In a separately excited generator, the field flux is derived from a separately power source independent of the generator itself.

2. Shunt generator. In a shunt generator, the field flux is derived by connecting the field circuit directly across the terminals of the generator.

3. Series generator. In a series generator, the field flux is produced by connecting the field circuit in series with the armature of the generator.

4. Cumulatively compounded generator. In a cumulatively compounded generator, both a shunt and a series field are present, and their effects are additive.

5. Differentially compounded generator. In a differentially compounded generator, both a shunt and a series field are present, but their effects are subtractive.

DC Generator

DC generators are compared by their voltages, power ratings, efficiencies, and voltage regulations.

Voltage regulation, VR is defined by

100%x V

VVVR

fl

flnl

Separately Excited Generator

AL II

F

FF R

VI

AAAT RIEV

Example

If no load voltage of a separately-excited dc generator is 135V at 850 r/min, what will be the voltage if the speed is increased to 1000 r/min? Assume constant field excitation

Solution

V135V1 rpmn 8501 rpmn 10002

VV2 ?

2

1

2

1

2A

1A

n

n

nK

nK

E

E

Constant field excitation means; if1 = if2 or constant flux; 1 = 2

1A

1

22A E

n

nE V8158135

850

1000.)(

Shunt DC Generator

AAAT RIEV

LFA III

Series DC Generator

LSA III

)RR(IEV SAAAT

The Cumulatively Compounded DC Generator

Cumulatively compounded dc generator with a long shunt connection

Cumulatively compounded dc generator with a short shunt connection

ARFnet F- F FF SE

Total Magnetomotive force

ARASEFF*FF INININ F

F

AR

F

ASEF

*F NN

INII

F

LFA III

)RR(IEV SAAAT

F

TF R

VI

Differentially Compounded DC Generator

With a long shunt connection

ARFnet F- F FF SE

ARASEFF*FF INININ F

F

AR

F

ASEeq NN

INI

FEquivalent shunt field

current,

F

AR

F

ASEF

*F NN

INII

F

Example

A short-shunt compound generator delivers 50A at 500V to a resistive load. The armature, series field and shunt field resistance are 0.16, 0.08 and 200, respectively. Calculate the armature current if the rotational losses are 520W, determine the efficiency of the generator

Solution

W520Pu W25000A50Vx500Pout

A52200

500If . A552A50A52III Lfa ..

Armature Copper Loss: W441160552RIP 2a

2aca ).().()(

Series Field Copper Loss: W5220080552RIP 22f

2a2cf .).().()(

Shunt Field Copper Loss: W125020052RIP 21f

2f1cf )().()(

Friction + Stray + windage + etc: W520Pu

So,Total Losses = W5243152012505220441 .).(

Continued…

Efficiency, η =sTotallossePout

Pout

Pin

Pout

%.@..

1391911305243125000

25000

Assignment 6

The equivalent circuit of the separately-excited dc motor

Figure above shows fixed field voltage VF of 240V and armature voltage VA that can be varied from 120 V to 240 V. What is the no-load speed of this separately-excited dc motor when Radj =

175 and

a) VA = 120V, b) VA = 180V, c) VA = 240V ?

QUESTION 1

Assignment 6

The equivalent circuit of the shunt dc motor

QUESTION 2

a) If the resistor Radj is adjusted to 175, what is the rotational speed of the motor at no-load conditions?

b) Assuming no armature reaction, what is the speed of the motor at full load? What is the speed regulation of the motor?

The magnetization curve for the dc motor of Question 1 and Question 2. This curve was made at a constant speed of 1200 r/min.