DC Motor DrivesDC Motor Drives

Dr. Ahmad HarbDr. Ahmad Harb

OutlineOutline

General overview of motor drivesGeneral overview of motor drivesMechanical relay based motor drivesMechanical relay based motor drivesSemiconductor based motor drivesSemiconductor based motor drivesSpeed Control IssuesSpeed Control IssuesProtection IssuesProtection Issues Interfacing IssuesInterfacing Issues

General OverviewGeneral Overview

What is a DC Motor?What is a DC Motor? A machine that converts electrical to mechanical A machine that converts electrical to mechanical

energy.energy.

What to control in a DC motor?What to control in a DC motor? SpeedSpeed DirectionDirection Start/StopStart/Stop Positional?Positional?

General Overview – Speed General Overview – Speed ControlControl

Speed ControlSpeed ControlCan be achieved by controlling voltage or Can be achieved by controlling voltage or

current.current.The most simplest method of control is by The most simplest method of control is by

inserting a resistor (high power resistor – inserting a resistor (high power resistor – aka cement resistor) in series with the DC aka cement resistor) in series with the DC motor.motor.This method is simple but not efficient due to This method is simple but not efficient due to

high power loss (heat) at the resistor.high power loss (heat) at the resistor.

General Overview – Speed General Overview – Speed ControlControl

Variable voltage approachVariable voltage approachBy reducing/increasing the supply voltage By reducing/increasing the supply voltage

of the DC motor will decrease/increase the of the DC motor will decrease/increase the DC motor speed.DC motor speed.

The most practical method to control The most practical method to control voltage is via the PWM method.voltage is via the PWM method.

General Overview – General Overview – DirectionDirection

Basic conceptBasic conceptThe only way to control direction is change The only way to control direction is change

the ‘polarity’ of the DC motor.the ‘polarity’ of the DC motor.The ‘polarity’ of the DC motor will The ‘polarity’ of the DC motor will

determine the direction of current flow determine the direction of current flow hence will determine the direction of hence will determine the direction of rotation of the DC motor.rotation of the DC motor.

General Overview - StartGeneral Overview - Start

Starting a DC motorStarting a DC motor The most easiest way to start a DC motor is by The most easiest way to start a DC motor is by

switching in the power supply.switching in the power supply. However, it would introduce high current spikes However, it would introduce high current spikes

into the motor circuit due to the motor inertia, into the motor circuit due to the motor inertia, Inductive characteristics and also start with load Inductive characteristics and also start with load condition.condition. Consequences: ‘fried’ controller, mosfets, stripboards, Consequences: ‘fried’ controller, mosfets, stripboards,

break down of insulation and also ‘fried’ circuits!break down of insulation and also ‘fried’ circuits!

General Overview - StartGeneral Overview - Start

SolutionSolutionBy reducing the supply voltage during By reducing the supply voltage during

starting (ramp start).starting (ramp start). Introducing high resistance during starting.Introducing high resistance during starting.All this can be achieved by using a All this can be achieved by using a

controller – atmel, PIC, OOPic, etc etc.controller – atmel, PIC, OOPic, etc etc.

General Overview - StopGeneral Overview - Stop

Stopping a DC motorStopping a DC motorSimplest method is by switching off the Simplest method is by switching off the

power supply.power supply.However, if the motor has inertia, However, if the motor has inertia,

instantaneous stop is not possible.instantaneous stop is not possible.

General Overview - StopGeneral Overview - Stop

SolutionSolutionSudden reversal of polaritySudden reversal of polarityRegenerative braking / Electronic BrakingRegenerative braking / Electronic BrakingThese solution would require the use of a These solution would require the use of a

controller.controller.

General Overview - General Overview - PositionalPositional

Is positional possible?Is positional possible? Yes – Servo motor / Stepper Motor.Yes – Servo motor / Stepper Motor. Servo motor – position is based upon the pulse Servo motor – position is based upon the pulse

width of PWM signal going into the servo.width of PWM signal going into the servo. Stepper motor – position is possible by counting Stepper motor – position is possible by counting

the number of pulse sequence given to the the number of pulse sequence given to the stepper.stepper.

All this is possible by incorporating controller.All this is possible by incorporating controller.

H bridge DC Motor Drive H bridge DC Motor Drive CircuitCircuit

Shaped like the letter ‘H’Shaped like the letter ‘H’Advantage – ability to control direction, Advantage – ability to control direction,

speed, start and stop.speed, start and stop.Disadvantage will be dependent upon the Disadvantage will be dependent upon the

component used.component used.

H Bridge ComponentsH Bridge Components

RelayRelay Mechanical switchMechanical switch Very robustVery robust Types of relay – 5V, 9V, 12V.. Etc etcTypes of relay – 5V, 9V, 12V.. Etc etc Relay types will be dependent upon supply Relay types will be dependent upon supply

voltage and the voltage at which pulse is given voltage and the voltage at which pulse is given to the relay contacts.to the relay contacts.

Speed control is not possible.Speed control is not possible. Safety – interface to relay must be protected, Safety – interface to relay must be protected,

free wheeling diode must be installed to protect free wheeling diode must be installed to protect from the ‘inductive’ kick.from the ‘inductive’ kick.

H Bridge ComponentsH Bridge Components

MosfetMosfetSemiconductor relaySemiconductor relayCapable of withstanding high current flowCapable of withstanding high current flowEg: IRF, IRFZ seriesEg: IRF, IRFZ seriesFor optimized H bridge configuration, must For optimized H bridge configuration, must

have ‘p’ and ‘n’ channel mosfets.have ‘p’ and ‘n’ channel mosfets.Disadvantage, gate is sensitive to high Disadvantage, gate is sensitive to high

currents.currents.

H Bridge Example CircuitsH Bridge Example Circuits

H Bridge Example CircuitsH Bridge Example Circuits

H Bridge Example CircuitsH Bridge Example Circuits

H Bridge Example circuitsH Bridge Example circuits

H Bridge Example circuitsH Bridge Example circuits

H Bridge Example CircuitsH Bridge Example Circuits

H Bridge Example circuitsH Bridge Example circuits

H Bridge Example circuitH Bridge Example circuit

M

Vcc

Control Signal

Control Signal

Control Signal

Control Signal

Relay Relay

Relay Relay

H Bridge example circuitH Bridge example circuit

M

Vcc

Control Signal/ULN2003 output

Free Wheeling

diode Free Wheeling

diode

Control Signal/ULN2003 output

Vcc

Relay Relay

MosFet Driver circuit – Totem MosFet Driver circuit – Totem Pole DriverPole Driver

ULN

2003microcontroller

Vg Vs

R1 R

MosFet Driver circuit – Totem MosFet Driver circuit – Totem Pole Driver with optoisolatorPole Driver with optoisolator

microcontroller

Vg Vs

R1 R

Optoisolator

Speed Control IssuesSpeed Control Issues

All relay driverAll relay driverSpeed control is not possibleSpeed control is not possible

All relay driver with MosfetAll relay driver with MosfetSpeed control is possibleSpeed control is possible

Semiconductor based driverSemiconductor based driverSpeed control is possibleSpeed control is possible

Speed Control IssuesSpeed Control Issues Speed control can be achieved by giving a PWM Speed control can be achieved by giving a PWM

pulse train to the semiconductor swtich.pulse train to the semiconductor swtich. This will determine the duration at which the This will determine the duration at which the

motor receives voltage.motor receives voltage. If the switching is done fast enough, the average If the switching is done fast enough, the average

voltage received by the motor may be controlled.voltage received by the motor may be controlled. Therefore, the average voltage received by the Therefore, the average voltage received by the

motor will be dependent upon the ‘duty ratio’ motor will be dependent upon the ‘duty ratio’ (time ratio at which the switch is ‘on’) of the (time ratio at which the switch is ‘on’) of the switch.switch.

Protection IssuesProtection Issues

MotorMotor If the motor only turns in one direction, free If the motor only turns in one direction, free

wheeling diode is recommended to discharge wheeling diode is recommended to discharge any remaining voltage stored in the motor when any remaining voltage stored in the motor when the motor is not rotating.the motor is not rotating.

MosfetMosfet A free wheeling diode is also recommended to A free wheeling diode is also recommended to

discharge the internal capacitance in the Mosfet discharge the internal capacitance in the Mosfet during turn off and turn on.during turn off and turn on.

Protection IssuesProtection Issues

FuseFuse It is also recommended to put a fuse to generally It is also recommended to put a fuse to generally

protect the circuit in the event of a current surge. protect the circuit in the event of a current surge. Remember, fuse is cheap, mosfets and relays Remember, fuse is cheap, mosfets and relays are expensive!.are expensive!.

ConnectionsConnections All connections to the motor must be ‘wired’, DO All connections to the motor must be ‘wired’, DO

NOT use the copper stripboard as the NOT use the copper stripboard as the connectors to motors. Copper stripboard do not connectors to motors. Copper stripboard do not have current capacity.have current capacity.

Interfacing IssuesInterfacing IssuesMicroC via ULN 2003 vs MicroC via MicroC via ULN 2003 vs MicroC via

Optoisolators?Optoisolators? Using ULN2003 is simple but it has a disadvantage Using ULN2003 is simple but it has a disadvantage

of a common ground for both MicroC and Motor of a common ground for both MicroC and Motor circuit.circuit. In the event of a current surge during start, stop, direction In the event of a current surge during start, stop, direction

change, excess current will go to ground and could ‘fry’ the change, excess current will go to ground and could ‘fry’ the microcontroller.microcontroller.

Solution is by using an optoisolator (4n25) to separate the Solution is by using an optoisolator (4n25) to separate the ground for both MicroC and Motor circuit.ground for both MicroC and Motor circuit.

Cost wise, optoisolator would cost about RM1 each while a Cost wise, optoisolator would cost about RM1 each while a MicroC would cost a lot!!.MicroC would cost a lot!!.

Thank you :o)Thank you :o)

Questions?Questions?