DC motor and open loop speed control. Closed loop speed control of DC drives. Closed loop speed control with inner loop current control Closed loop field control Closed loop armature control.

1. 6th Sem
Electrical Engineering Department
Batch – B1 (2014 Batch)
Abhishek Choksi
(140120109005)
Control of Electric Drives (2160913)
ALA Presentation
On
“Closed loop speed control”
Prepared By: Guided By:
Prof. Jaydeep Vanapariya
Gandhinagar Institute OfTechnology

2. Index
■ DC motor and open loop speed control.
■ Closed loop speed control of DC drives.
■ Closed loop speed control with inner loop current control
■ Closed loop field control
■ Closed loop armature control.

3. DC motors
 DC motor is extensively used machine where the wide range of speed
control is desired.
 The operation of DC motor in different steps are easy compared to AC
motors.

4. Open loop speed control
 By the open loop speed control the DC motor can be operated at any
intermediate speed by changing the voltage, armature current etc.
 But in open loop control accuracy cannot be obtained i.e. speed will
not be constant for load variations on the motor.
 There will not be any feedback to the controller to indicate the change
in speed due to load .

5. Closed loop speed control of DC drive
■ To avoid the disadvantage that is caused due to open loop speed control
closed loop speed control technique is implemented.
■ Here the output speed measured is feed back to the speed controller.
■ In closed loop controller the speed can be maintained by adjusting
terminal voltage according to the speed difference caused by the load
torque i.e. a fine control of speed can be obtained using closed loop
speed control.

6. Closed loop speed control of DC drive
■ The potentiometer adjusts the voltage between 0- 10 volts which
represents the reference speed.
■ A tacho-generator is used for producing the speed feedback signal.it
produces a DC voltage which is proportional to actual speed of the
motor. This voltage is called speed feedback signal.
■ Reference speed is compared with the speed feedback signal via
comparator.

7.  Its output is the error signal which is applied to the speed controller.
 It produces a dc voltage such that error is brought down to zero.
 This DC voltage is applied to the firing pulse generator. The firing angle
α will vary depending on the value of control voltage.
 The firing pulses at desired value of α is applied to the SCR’s connected
in the power circuit.
 The firing angle is thus automatically adjusted in order to operate the
motor at the desired speed.

8. Closed loop speed control with inner loop current
control.
■ Here speed output signal generates a corresponding armature current
signal and this signal is compared with the existing armature feedback
to the comparator circuit.
■ The difference in the current drive the current loop controller and
produces a control signal to the power convert.

9. ■ This introduces a very smooth increase in terminal voltage and thus
armature current by providing a dual controller. the current controller can
be proportional or proportional integral controller.
■ Most probably PI is used because of less steady state error and smooth
response compared to proportional controller.

10. Closed loop field control
 This method is used for varying speed of DC shunt motor above the
rated speed. The armature voltage is kept constant and field current is
varied.
 The reference voltage is passed through the ramp unit to provide soft
starting of the motor.
 The o/p of ramps unit acts as the speed reference, this reference signal is
compared with the speed feedback signal obtained from tachogenerator.

11. ■ The o/p of comparator is called as speed error signal.
■ The speed controller will produce a control voltage from error
signal i/p and this control signal changes the firing angle α in
such a way that the field voltage is automatically increased or
decreased.

12. Closed loop armature control
■ In this method the field winding is excited by a constant d.c. voltage and
the armature voltage is varied in order to vary the speed of the motor.
■ The working of armature control scheme is basicaly same as the field
control scheme.
■ The control signal changes the firing angle α in such a way that the
armature voltage is automatically increased or decreased so that the speed
error gets reduced and the armature voltage is varied to vary the speed of
a DC shunt motor.

13. References
■ International Conference on Science, Technology, Engineering &
Management
■ M.Nedeljkovic and Z.Stojiljkovic, Fast current control for thyristor
rectifiers, IEE Proceeding. Electr. Power Appl, 150(6), 2003, 636-
628.
■ Wikipedia -- https://en.wikipedia.org/wiki/Pulse-
width_modulation#Principle
■ D.A. Staton, M.I.McGilp and T.J.E.Miller, DC machine
teaching experiment, in proceedings of the European
Power Electronics Association EPE, Brighton, 1993, 35-40.