6th Sem Electrical Engineering Department

Batch – B1 (2014 Batch)

Abhishek Choksi(140120109005)

Control of Electric Drives (2160913)

ALA PresentationOn

“Closed loop speed control”Prepared By: Guided By:

Prof. Jaydeep Vanapariya

Gandhinagar Institute Of Technology

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.

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.

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 .

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.

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.

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.

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.

■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.

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.

■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.

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.

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.