DC Motors Starters and Breaking
Methods
KL3073
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
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.
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.
Solution to the problem of excess current
 Shunt motor with an extra starting resistor.
shunt motor with a
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
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
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
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
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
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?
DC Motor Starting Circuits
 Devices commonly used in motor-control circuits
DC Motor Starting Circuits
 One common motor-starting circuit
DC Motor Starting Circuits
 One common motor-starting circuit
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.
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.
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
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 WardLeonard motor-control system
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
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
DC MOTOR BREAKING METHODS
There are three kinds of electric breaking, namely:
 Rheostatic or dynamic breaking
 Plugging and
 Regenerative breaking
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.
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.
Electric breaking for shunt motors
Regenerative Breaking
 In regenerative breaking, Eb is greater thanVt.
 The direction of IA and the armature torque Tb are reversed
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.
Electric breaking for series motors
Plugging or Reverse Current Breaking
 it is similar to that of shunt motor.