Principles of Chopper Control (PWM Torque control)

The preferred scheme for Chopper Control is called Constant-Frequency. The chopping frequency is kept constant the on time (t_on) is varied. The amount of time that t_on is on divided by the chopping period is the duty cycle. The duty cycle for our application will most likely run from 25% - 90% depending on how much current needs to be delivered to the motor for proper torque.

To make the drive reversible, power needs to be able to flow both directions to the motor. To achieve this, a Two-quadrant type B chopper needs to be implemented.


Commutation needs to be researched and understood for chopper control. The controller needs some way to make the current flowing through the thyristor to equal zero for a certain period of time. This can be done with either load commutation or forced commutation.

In Load Commutation, the current flowing through becomes zero or is transferred to another device (besides the thyristor)
In Forced Commutation, there is a force that is forcing the current to become zero.

• Voltage commutation, a charged capacitor momentarily reverse biases.
• Current commutation, a current pulse is made to flow in the reverse direction making the net current equal zero.

This type of controller is ideal for use with DC motor require continuous motor current that needs to be varied. It is costlier than other control methods, however, because there are more components necessary for the commutation circuitry.

In order to reduce ripple from supply current, a capacitor can be placed in shunt with the chopper circuit. An LC filter can also be used to reduce the size of the capacitor and has an additional function of providing transient isolation between supply and load during a short circuit.

The chopper circuit will supply the current to the armature. The field current will be fixed in this application. The controller will measure the speed of the motor and compare it to a control setting which will generate the proper signaling.

A microcontroller will control the circuit, thyristors will provide the power electronics needed for switching and passive components capable of handling high power will provide filtering/chopper circuitry.

1. http://www.freepatentsonline.com/3845379.html
2. http://books.google.com/books?id=s0k9kGs5bHYC&lpg=PA290&ots=kB9BqiJTMG&dq=chopper%20circuit%20forklift&pg=PA290
3. http://books.google.com/books?id=4-Kkj53fWTIC&lpg=PA210&ots=959jgndlmD&dq=chopper%20circuit%20forklift&pg=PA210
4. http://books.google.com/books?id=WwXi9LI5W1sC&lpg=RA2-PA249&ots=K5z0SAJOfM&dq=chopper%20circuit%20forklift&pg=RA2-PA249
5. http://www.lgep.supelec.fr/uploads/images/lgep/Mocosem/Cocodi/Pages_perso/gremy/Hebron2009/Practice4_DC_Motor_Chopper_Control.pdf
   

Library: TK2851 is a good place to look for motor control books.