Transcript DC Motor Drives

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
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
Vcc
Control
Signal
Relay
Relay
Control
Signal
M
Control
Signal
Relay
Relay
Control
Signal
H Bridge example circuit
Vcc
Control Signal/
ULN2003 output
Control Signal/
ULN2003 output
Relay
Relay
M
Free
Wheeling
diode
Vcc
Free
Wheeling
diode
MosFet Driver circuit – Totem Pole
Driver
Vg
microcontroller
ULN2003
R1
Vs
R
MosFet Driver circuit – Totem Pole
Driver with optoisolator
Vg
R1
microcontroller
Optoisolator
Vs
R
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 is possible
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?