KL3073

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DC Motors Starters and Breaking Methods

• KL3073

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DC MOTOR STARTERS

• In order for a dc motor to function properly 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 Motor

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DC Motor Problems on Starting

• It must be protected from physical damage 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 current flows.
• This current is over many times the motor's rated
  full-load current. This may damage the motor.

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Solution to the problem of excess current

• Insert a starting resistor in series with the
  armature 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 to
  properly pick the size and number of resistor
  segments.
• Shuts the resistor bypass contacts at the proper
  time

shunt motor with a starting resistor
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Solution to the problem of excess current

• Selected Rstart so that the current flow equals
  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 be taken 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?

• Rtot as 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
  resistor be? At what voltage should each stage of
  the starting resistance be cut out?

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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, or
  compounded 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 generator
  armature voltage can be controlled
• This allows the motor's speed to be smoothly
  varied between a very small value and the base
  speed.
• 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 motor
  reverses 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 of
  the 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 supplying an
  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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Electric breaking for shunt motors
Rheostatic or dynamic breaking

• The armature of the shunt motor is disconnected
  from the supply and it is connected across a
  variable resistor R.
• The field winding is kept undisrupted and this
  breaking is controlled by varying the series
  resistor R.
• This method used generator action.

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Electric breaking for shunt motors
Plugging or Reverse Breaking

• the armature terminals are reversed to rotate the
  motor in the reverse direction
• VT and the back Eb start acting in the same
  direction.

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Electric breaking for shunt motors
Regenerative Breaking

• In regenerative breaking, Eb is greater than Vt.
• The direction of IA and the armature torque Tb
  are reversed

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Electric breaking for series motors
Rheostatic or dynamic breaking

• In this method the motor is disconnected from
  supply. The field connection is reversed and the
  motor is connected through a variable resistance
  R.

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Electric breaking for series motors
Plugging or Reverse Current Breaking

• it is similar to that of shunt motor.