DC Machines Week
Transcript of 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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Solution to the problem of excess current
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
Solution to the problem of excess current
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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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How many steps are required to
accomplish the current limiting?
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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How many steps are required to
accomplish the current limiting?
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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DC Motor Starting Circuits
One common motor-starting circuit
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DC Motor Starting Circuits
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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THE WARD-LEONARD SPEED
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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THE WARD-LEONARD SPEED
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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THE WARD-LEONARD SPEED
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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Electr ic breaking for shun t motors
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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Electr ic breaking for shun t motors
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