ee22 part3 dc motor

8/9/2019 Ee22 Part3 Dc Motor

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DC Motors


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Types of DC Motors

Separately excited DC motor

Shunt DC Motor

Permanent-magnet DC motor

Series DC motor

Compound DC motor


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Separately Excited DC Motor


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Shunt DC Motor


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Torque-Speed Characteristic of a Shunt

DC Motor


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@ NO LOAD: EA1= VT= 250 V, n1= 1200 rpm


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Nonlinear Analysis of a Shunt DC Motor


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Speed Control of Shunt DC Motors


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Adjusting the Field Resistance RF

What will happen if the field resistance goes to infinity,

i.e., an open filed circuit?


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Adjusting the Terminal Voltage


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Speed Regulation of DC Motors


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The Permanent-Magnet DC Motor

- Poles are made of permanent magnets- Common in smaller fractional and subfractional

horsepower size

Advantages: No need for field excitation

No field circuit copper losses

Can be made smaller than corresponding shunt dc motors

Disadvantages:

Has a lower induced torque per ampere of armature currentthan a shunt motor of the same size and construction.

Has the risk of demagnetization.


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The Series DC Motor

Equivalent Circuit:

VT= EA+ IA(RA+ RS)

= cIA ind = K IA= KcIA

2


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The torque-speed characteristic of a

series dc motor:

Kc

RR

Kc

VSA

ind

T 1


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Speed Control of Series DC Motors.

there is only one efficient way to change the

speed of a series dc motor.

That method is to change the terminal voltage

of the motor.

If terminal voltage is increased, the speed will

increase for any given torque.


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Short-shunt connection:


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The EMF Equations

Long-shunt:

VT= EA+ IA(RA+ RS)

Short-shunt:

VT= EA+ IARA+ ILRS

Currents

IA= IL- IF

IF= VT/RF

The net mmf and the effective shunt field current Fnet= FF FSE- FAR

IF*= IF (NSE/NF) IAFAR/NF


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The Torque-Speed Characteristic of a Cumulatively Compounded DCMotor (CC)

There is a component of flux which is constant and anothercomponent which is proportional to its armature current (and thusto its load). Thus, CC motor has a higher starting torque than ashunt motor (whose flux is constant) but a lower starting torquethan a series motor (whose entire flux is proportional to armaturecurrent).

The CC motor combines the best features of both the shunt andseries motors. Like a series motor, it has extra torque for starting;like a shunt, it does not overspeed at no-load.

At light loads, the series field has a very small effect, so the motorbehaves approximately as a shunt dc motor. As the load gets verylarge, the series flux becomes quite important and the torque-speed curve begins to look like a series motors characteristic.


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Comparison of the torque-speed characteristic.:

The torque-speedcharacteristic of a cumulatively

compounded dc motor

compared to series and shunt

motors with the same full-load

rating.

The torque-speed

characteristic of acumulatively compounded

dc motor compared to a

shunt motor with the same

no-load speed.


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End of Part 3

ee22 part3 dc motor

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