DC Machines Week

8/11/2019 DC Machines Week

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KL3073

DC Motors Starters and Breaking

Methods


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DC MOTOR STARTERS

In order for a dc motor to functionproperly it must have some special

control and protection equipment

The purposes of this equipment are.

To protect the motor against damage

due to short circuits in the equipment

from long-term overloads from excessive starting currents

To provide a convenient manner in which

to control the operating speed of the


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DC Motor Problems on Starting

It must be protected from physicaldamage during the starting period.

At starting conditions, the motor is not

turning, and so EA= 0 V.

The full-load current of this motor is:

Since the internal resistance of a normal

dc motor is very low a very high currentflows.

This current is over many times the

motor's rated full-load current. This may


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Solution to the problem of excess current

Insert a starting resistor in series with thearmature to limit the current flow.

Resistor must not be in the circuit

permanently.because of: excessive losses

torque speed characteristic to

drop Resistor must be removed again as the

speed of the motor builds up.


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Shunt motor with an extra starting resistor.

In designing the starter it is important toproperly pick the size and number of

resistor segments.

Shuts the resistor bypass contacts at the

ro er time

shunt motor with astarting resistor


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Selected Rstart so that the current flowequals say twice the rated current.

the increasing EA decreases the IA in the

motor. When the IA falls to rated current, a

section of the starting resistor must betaken out to increase the starting current

back up to 200 percent of rated current the increasing EA decreases the IA in the

motor.

Repeat until all segments are out



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How many steps are required to

accomplish the current limiting?

Rtotas the original resistance in the

starting circuit

The total resistance left in the starting

circuit after stages 1

Initial starting resistance must be

resistance R, must be switched out at 1st

stage


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After switching that part of the resistance

out, the armature current must jump to

Equating previous 2 equation

By direct extension, the resistance left in

the circuit after the nth stage is switched

out is


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At the boundary where RA= Rtot,n

Equating previous 2 equation

Solving for n yields


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Example

Example 6-7. Figure 6-24 shows a 100-hp 250-V 350-A shunt de motor with an armature

resistance of 0.05 ohms. It is desired to design

a starter circuit for this motor which will limit the

maximum starting current to twice its rated

value and which will switch out sections of

resistance as the armature current falls to its

rated value. (a) How many stages of starting resistance will

be required to limit the current to the range

specified?

(b) What must the value of each segment of the


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DC Motor Starting Circuits

Devices commonly used in motor-control

circuits


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One common motor-starting circuit


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One common motor-starting circuit


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THE WARD-LEONARD SPEED

CONTROLLER

The speed of a separately excited, shunt, orcompounded dc motor can be varied in one of

three ways:

by changing the field resistance, changing the armature voltage, or

changing the armature resistance.


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CONTROLLER

figure below shows an ac motor serving as a

prime mover for a dc generator, which in turn is

used to supply a dc voltage to a dc motor by

changing the field resistance. This system is called Ward-Leonard system.


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CONTROLLER

Controlling the field current of the dc generatorarmature voltage can be controlled

This allows the motor's speed to be smoothly

varied between a very small value and the basespeed.

Higher speed can be achieved by reducing the

motor's field current


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CONTROLLER

if the field current of the generator is reversed,polarity of generated voltage also reversed.

This reverse the motor's direction of rotation.

If the torque or the speed alone of the motorreverses while the other quantity does not, then

the machine serves as a generator.

The operating range of a Ward-

Leonard motor-control system


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SOLID-STATE SPEED CONTROLLERS

The average voltage applied to the armature ofthe motor can be controlled by fraction of the

time the supply voltage is applied to the

armature.

fast on and off of the supply can be done by

modern solid state devices such as SCR.

A simple dc armature voltage controller circuit

using SCR is shown below

A two-quadrant solid-state dc motor controller


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SOLID-STATE SPEED CONTROLLERS

A more advanced circuit capable of supplyingan armature voltage with either polarity is

shown below. This armature voltage control

circuit can permit a current flow out of the

positive terminals of the generator, so a motor

with this type of controller can regenerate

A 4-quadrant solid-state dc motor controller


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DC MOTOR BREAKING METHODS

There are three kinds of electric breaking, namely:

Rheostatic or dynamic breaking

Plugging and

Regenerative breaking


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Electr ic breaking for shun t motors

The armature of the shunt motor is disconnectedfrom the supply and it is connected across avariable resistor R.

The field winding is kept undisrupted and thisbreaking is controlled by varying the seriesresistor R.

This method used generator action.

Rheostatic or dynamic breaking


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the armature terminals are reversed to rotate themotor in the reverse direction

VT and the back Eb start acting in the samedirection.

Plugging or Reverse Breaking


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In regenerative breaking, Ebis greater than Vt.

The direction of IA

and the armature torque Tbare reversed

Regenerative Breaking


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Electr ic b reaking for ser ies moto rs

it is similar to that of shunt motor.

Plugging or Reverse Current Breaking

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