DC Motor Drives Dr. Ahmad Harb

Outline General overview of motor drives Mechanical relay based motor drives Semiconductor based motor drives Speed Control Issues Protection Issues Interfacing Issues

General Overview What is a DC Motor? A machine that converts electrical to mechanical energy. What to control in a DC motor? Speed Direction Start/Stop Positional?

General Overview – Speed Control Can be achieved by controlling voltage or current. The most simplest method of control is by inserting a resistor (high power resistor – aka cement resistor) in series with the DC motor. This method is simple but not efficient due to high power loss (heat) at the resistor.

General Overview – Speed Control Variable voltage approach By reducing/increasing the supply voltage of the DC motor will decrease/increase the DC motor speed. The most practical method to control voltage is via the PWM method.

General Overview – Direction Basic concept The only way to control direction is change the ‘polarity’ of the DC motor. The ‘polarity’ of the DC motor will determine the direction of current flow hence will determine the direction of rotation of the DC motor.

General Overview - Start Starting a DC motor The most easiest way to start a DC motor is by switching in the power supply. However, it would introduce high current spikes into the motor circuit due to the motor inertia, Inductive characteristics and also start with load condition. Consequences: ‘fried’ controller, mosfets, stripboards, break down of insulation and also ‘fried’ circuits!

General Overview - Start Solution By reducing the supply voltage during starting (ramp start). Introducing high resistance during starting. All this can be achieved by using a controller – atmel, PIC, OOPic, etc etc.

General Overview - Stop Stopping a DC motor Simplest method is by switching off the power supply. However, if the motor has inertia, instantaneous stop is not possible.

General Overview - Stop Solution Sudden reversal of polarity Regenerative braking / Electronic Braking These solution would require the use of a controller.

General Overview - Positional Is positional possible? Yes – Servo motor / Stepper Motor. Servo motor – position is based upon the pulse width of PWM signal going into the servo. Stepper motor – position is possible by counting the number of pulse sequence given to the stepper. All this is possible by incorporating controller.

H bridge DC Motor Drive Circuit Shaped like the letter ‘H’ Advantage – ability to control direction, speed, start and stop. Disadvantage will be dependent upon the component used.

H Bridge Components Relay Mechanical switch Very robust Types of relay – 5V, 9V, 12V.. Etc etc Relay types will be dependent upon supply voltage and the voltage at which pulse is given to the relay contacts. Speed control is not possible. Safety – interface to relay must be protected, free wheeling diode must be installed to protect from the ‘inductive’ kick.

H Bridge Components Mosfet Semiconductor relay Capable of withstanding high current flow Eg: IRF, IRFZ series For optimized H bridge configuration, must have ‘p’ and ‘n’ channel mosfets. Disadvantage, gate is sensitive to high currents.

H Bridge Example Circuits

H Bridge Example Circuits

H Bridge Example Circuits

H Bridge Example circuits

H Bridge Example circuits

H Bridge Example Circuits

H Bridge Example circuits

H Bridge Example circuit

H Bridge example circuit

MosFet Driver circuit – Totem Pole Driver

MosFet Driver circuit – Totem Pole Driver with optoisolator

Speed Control Issues All relay driver Speed control is not possible All relay driver with Mosfet Speed control is possible Semiconductor based driver

Speed Control Issues Speed control can be achieved by giving a PWM pulse train to the semiconductor swtich. This will determine the duration at which the motor receives voltage. If the switching is done fast enough, the average voltage received by the motor may be controlled. Therefore, the average voltage received by the motor will be dependent upon the ‘duty ratio’ (time ratio at which the switch is ‘on’) of the switch.

Protection Issues Motor If the motor only turns in one direction, free wheeling diode is recommended to discharge any remaining voltage stored in the motor when the motor is not rotating. Mosfet A free wheeling diode is also recommended to discharge the internal capacitance in the Mosfet during turn off and turn on.

Protection Issues Fuse It is also recommended to put a fuse to generally protect the circuit in the event of a current surge. Remember, fuse is cheap, mosfets and relays are expensive!. Connections All connections to the motor must be ‘wired’, DO NOT use the copper stripboard as the connectors to motors. Copper stripboard do not have current capacity.

Interfacing Issues MicroC via ULN 2003 vs MicroC via Optoisolators? Using ULN2003 is simple but it has a disadvantage of a common ground for both MicroC and Motor circuit. In the event of a current surge during start, stop, direction change, excess current will go to ground and could ‘fry’ the microcontroller. Solution is by using an optoisolator (4n25) to separate the ground for both MicroC and Motor circuit. Cost wise, optoisolator would cost about RM1 each while a MicroC would cost a lot!!.

Thank you :o) Questions?