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Week 6: DC Motors

Ibrahim AL-Zughaibi

202333100

Video Question 1

The field coils can be connected to the rotor winding in the

two different ways, mention them?

1. Series connection

2. Parallel connection

Question 2

The result of connection between the field coils and the

rotor winding is two different kinds of DC Motors

construction, mention them?

1. A Shunt Motor

2. A Series Motor 30/12/2014 Capacitors 2

Methods of Starting

Introduction

Thus, to avoid the above dangers while starting a DC motor, it is necessary to limit the starting current. For that purpose, starters are used to start a DC motor.

There are various starters like, 3 point starter, 4 point starter and DC series motor starter.

The main concept behind every DC motor starter is, adding external resistance to the armature winding at starting.

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Methods of Starting

1- 3 Point Starter

When lever touches point 1, the field

winding gets directly connected

across the supply, and the armature

winding gets connected with

resistances R1 to R5 in series.

When the motor is overloaded

beyond a predefined value,

overcurrent release electromagnet D

gets activated, which short circuits

electromagnet E , and hence releases

the lever and motor is turned off.

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Methods of Starting

2- 4 Point Starter

The main difference between a 3

point starter and a 4 point starter is

that the no voltage coil is not

connected in series with field coil.

The electromagnet pull of the hold-on

coil will always be sufficient so that

the spring does not unnecessarily

restore the lever to the off position.

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Partner Work

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Search by using google website to find

information about methods of starting on DC

Motor.

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Googling

Methods of Starting

3- DC series motor starter

A start arm is simply moved towards

right to start the motor.

Thus at first maximum resistance is

connected in series with the armature

and then gradually decreased as the

start arm moves towards right.

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(1) All of three types of starting play a very significant role

in limiting starting current of DC motor for proper

starting and running of the DC moto ( ) 10 points

(2) These three types couldn’t use in industry ( ) 10

points

(3) The 3 point starter and 4 point starter Used for the

starting of shunt wound DC motor and compound

Wound DC motor ( ) 10 points

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Team Work

There are two types of losses occur in DC motors. These

losses are,

• Constant or fixed losses,

• Variable losses.

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Losses and Efficiency

Constant or Fixed Losses

Constant losses are those losses which are considered to

remain constant over normal working range of DC motor.

The fixed losses can be easily obtained by performing no-load

test on DC motor. These losses are further classified as:

1. Iron or core losses.

2. Mechanical losses.

3. Brush friction losses.

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Losses and Efficiency

Losses and Efficiency 1. Iron or Core Losses

Iron or core losses are further divided into hysteresis

and eddy current losses. Eddy current losses are

minimized by using lamination.

The core losses depend upon frequency.

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Losses and Efficiency

2. Mechanical and Brush Friction Losses

Mechanical losses occur at the bearing and brush friction

loss occurs in wound rotor DC motor.

These losses occurs with the change in speed. In DC

Motor the speed usually remains constant. hence these

losses almost remains constant.

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Googling Use Google website to find more information about the

third type of losses which is Brush friction losses.

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Partner Work

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Losses and Efficiency

Variable Losses

These losses are also

called copper losses.

These losses occur due

to current flowing in

stator and rotor windings.

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Losses and Efficiency

Efficiency is defined as the

ratio of the output to that of

input

Efficiency = 𝑂𝑢𝑡𝑝𝑢𝑡

𝐼𝑛𝑝𝑢𝑡

DC Motor efficiency Equal

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Losses and Efficiency Electric motor has input power consumption of 50 watts.

The motor was activated for 60 seconds and produced work of 2970 joules.

Find the efficiency of the motor.

Solution:

Pin = 50W

E = 2970J

t = 60s

Pout = E / t = 2970J / 60s = 49.5W

η = 100% * Pout / Pin = 100 * 49.5W / 50W = 99%

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Alone Work

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