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Direct Current Motors
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They are very popular because:
- They are usually very fast, spinning at severalthousand
revolutions per minute (rpm);
- They are simple to operate.- Their starting torque is large,
which is the main
reason for using them in several tractionapplications;
- In a special form, they can be used with either ana.c. or d.c.
supply.
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Theory of Operation
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DC Armature
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Wound-Field DC Motors
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Permanent Magnet Motor
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PM DC-Motor Model
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• If we apply the law of conservation of angular momentum we
have
• The motor torque is given by
• and the load in this case is the rotor inertia. The only
torquegenerated by the load is the friction (or damping) torque
expressedas
• Applying Kirchof's voltage law on the electrical system
we have
• Neglecting the inductance La
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Control of DC Motors
• Switch ON-OFF
• Speed:
- Analog- Pulse With Modulation (PWM)
- Continuous Control
• Direction
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Switch
• Diod Switch
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• BJT
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• BJT Switching Characteristics
When the transistor is saturated, it acts as a closed switch.
When a transistor is in the cutoff
region, it acts as an open switch. When it is in the active
region, it acts as a current ( i B )
controlled current (i C) amplifier.
Realistically, transistor switching is not instantaneous. The
turn-on time t ON of the transistor is
the sum of the delay time t D and the rise time
t R. Similarly, the turn-off time t OFF is the
sum of thestorage time t S and the fall time
t F . The turn-on and turn-off time of a
transistor limits the
maximum switching frequency. Typical switching frequency for a
power BJT is between 2 and 20
kHz.
BJTs can switch at a higher frequency than thyristors but can
handle less power. Power BJTs can
handle currents up to several hundred amperes and
V CE up to about 1 kV.
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• MOSFET
When operating in the enhancement mode, a MOSFET behaves very
similar to a BJT. Instead
of base current, the MOSFET behavior is determined by the gate
voltage. When carefully
controlling the gate voltage of a MOSFET, the transistor can be
made to operate as a voltage
controlled switch that operates between the cutoff (point A) and
the Ohmic (point B)
region.
One advantage of a MOSFET device is that the MOSFET has
significantly larger inputimpedance as compared to BJT. This
simplifies the circuit that is needed to drive the
MOSFET since the magnitude of the gate current is not a factor.
This also implies that a
MOSFET is much more efficient than BJTs as well as it can be
switching at a much higher
frequency. Typical MOSFET switching frequency is between 20 and
200 kHz, which is an
order of magnitude higher than BJTs. Power MOSFETs can carry
drain currents up to several
hundreds of amperes and V DS up to around 500 V.
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Control of Direction
Reversing the PM Motor• To reverse the rotation direction of the
PM motor, the polarity of the applied
voltage must be reversed. One way to accomplish this is to
have a motor-driver
amp capable of outputting a positive and negative voltage.
• When the drive voltage is positive with respect to
ground, the motor turns
clockwise (CW). When the drive voltage is negative with respect
to ground, the
voltage polarity at the motor terminals reverses, and the motor
rotates
counterclockwise (CCW). The LM12 power op-amp is capable of
providing positiveand negative output voltages.
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Relays
• In many applications, the drive amplifier cannot output both
positive and
negative voltages, in which case a switching circuit must be
added to
reverse the motor. One approach is to use a double-pole relay .
When the
relay contacts are up the positive voltage is connected to
terminal A of themotor, and terminal B is connected to the negative
voltage. When the
relay contacts are down, the positive voltage is connected to
terminal B,
and terminal A goes to the negative voltage, thus effectively
reversing the
polarity.
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H-Bridge• Forward-reverse switching can also be done with
solid-state devices using four
FETs.
• When Q1 and Q4 are on, the current I1,4 causes the motor to
turn clockwise. When
Q2 and Q3 are on, the current I3,2 flows in the opposite
direction and causes themotor to turn counterclockwise. The
entire
• switching operation can be performed by a single IC, such as
the Allegro A3952 .
• This IC contains four separate driver transistors that are
controlled by
• internal logic to operate in pairs . The A3952 controls a
motor-supply voltage of up
to 50 V with up to 2 A of output current.
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L293 H-bridge chip
DC M D i S d C l
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DC Motor Drive Speed Control
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DC Motor Analog Drive Speed Control
Using a single power transistor: • The circuit could be
either:
1- the common emitter (CE) configuration, which gives current
and voltage gain, or
2- the common collector (CC) configuration, which gives only
current gain.
• When the base voltage (V B ) is increased (beyond
the forward-bias voltage), the transistor
begins to turn on and let the collector current
(I C ) flow. The collector current is
30 –100
times greater than the base current, depending on the gain of
the transistor.
• Once the transistor starts to conduct, I C
increases with V B
more or less linearly.
• Note that all of I C goes through the motor, providing
the drive current
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Heating Problem with BJT• Power transistors are physically
bigger than signal transistors and are designed to carry large
currents. In control systems, they are used to provide the drive
current for motors and other
electromechanical devices.
• When a transistor has a large current and voltage at the same
time, the resulting power (V C IC )
must be dissipated in the form of heat. A typical power
transistor is designed to operate up to200°C (360°F) above ambient
temperature. However, its power capacity is derated
proportionally for temperatures above 25°C .
• The power transistor case has a flat metal surface to provide
a thermal escape path for the
heat. Therefore, to operate at anywhere near the rated power,
the transistor must be
mounted firmly to the chassis or a metal heat sink—a piece of
metal with cooling fins to
dissipate the heat into the air • Many times the case
itself is the collector terminal. If the collector must be kept
electrically
insulated from the mounting chassis, then a special mica
insulator is used, together with a
thermally conducting white grease.
( )
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Power IC Drive (LM12)
• The power IC driver is a single-package DC amplifier with
a
relatively high current output. An example is the LM12
(National Semiconductor)• The high-power operational amplifier
can supply up to 13 A
with a maximum voltage of ±30 V. As in any op-amp circuit,
feedback resistors are added to adjust the gain to any
desired
value.
Darlington Power Transistor Drive
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Darlington Power Transistor Drive
• The Darlington configuration consists of two CC amplifiers
connected in such a way
that the first transistor directly drives the second. Although
the voltage gain is only
1 (maximum), the current gain can be very high. The transistor
shown in the Figure
is a TIP 120, which has a current gain of 1000 and a maximum
output current of 5 A. The motor must be placed in the emitter
path of the output transistor. A
separate small-signal amplifier, probably an op-amp, would be
needed to provide
any voltage gain required.
P MOSFET
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Power MOSFET
• Notice the output current (ID ) is 0 A when the input
voltage
(V GS ) is in the 0-5-V range but then climbs to 12 A
when V GS
rises to 13 V.• Using a power MOSFET, the motor is in series
with the drain,
which means the FET will provide both voltage and current
gain.
•
The gate voltage is supplied from an op-amp circuit that
isdesigned to interface the controller with the FET.
DC Motor Control Using Pulse Width Modulation
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DC Motor Control Using Pulse-Width Modulation
• Pulse-width modulation is an entirely different approach
to
controlling the torque and speed of a DC motor. Power is
supplied to the motor in a square wavelike signal of
constant
magnitude but varying pulse width or duty cycle.
• Duty cycle refers to the percentage of time the pulse is
high
(per cycle).
PWM Control Circuits
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PWM Control Circuits
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DC DC C t
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DC-DC Converters• The purpose of a DC-DC converter is to supply
a regulated DC output voltage to a variable-load
resistance from a fluctuating DC input voltage. In many cases
the DC input voltage is obtained by
rectifying a line voltage that is changing in magnitude.
•DC-DC converters are commonly used in applications requiring
regulated DC power, such ascomputers, medical instrumentation,
communication devices, television receivers, and battery
chargers . DC-DC converters are also used to provide a regulated
variable DC voltage for DC motor
speed control applications.
• The output voltage in DC-DC converters is generally controlled
using a switching concept, as
illustrated by the basic DC-DC converter.
•
Early DC-DC converters were known as choppers with
silicon-controlled rectifiers (SCRs) used as theswitching
mechanisms.
• Modern DC-DC converters classified as switch mode power
supplies (SMPS) employ insulated gate
bipolar transistors (IGBTs) and metal oxide silicon field effect
transistors (MOSFETs).
• The switch mode power supply has several functions :
1. Step down an unregulated DC input voltage to produce a
regulated DC output voltage using a buck
or step-down converter.2. Step up an unregulated DC input
voltage to produce a regulated DC output voltage using a boost
or
step-up converter.
3. Step down and then step up an unregulated DC input voltage to
produce a regulated DC output
voltage using a buck –boost converter.
4. Invert the DC input voltage using a Cúk converter.
5. Produce multiple DC outputs using a combination of SMPS
topologies.
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• DC-DC conver
• DC-DC conver
DC-DC converter voltage waveforms.
Pulse width modulation concept.
Ch
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Choppers
• Choppers are DC-DC converters that are used
for transferring electrical energy from a DCsource into another
DC source, which may be
a passive load. These converters are widely
used in regulated switching power supplies
and DC motor drive applications.
• Choppers are one-quadrant, two-quadrant,
and four-quadrant
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• Step-down (buck) converter and step-up (boost) converters
are basic one-quadrant converter.
A step-down converter produces an average output voltage,
which is lower than the DC input voltage
• A step-up converter produces output voltage always greater
than the input voltage.
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Step-down buck converter.
Step-up boost converter.
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• The two-quadrant chopper, which, in fact, is a current
reversible converter, is the combination of the two basic
topologies. It has the ability to operate in two quadrants
of
the (v – i ) plane. Therefore, input and
output voltages arepositive; however, input and output currents can
be positive
or negative. These converters are also named current
reversible choppers.
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• In four-quadrant choppers, not only can the output current be
positive
and negative, but the output voltage also can be positive and
negative.
These choppers are full-bridge DC-DC converters. The main
advantage of
these converters is that the average of the output voltage can
be
controlled in magnitude as well as in polarity. A four-quadrant
chopper is acombination of two quadrant choppers in order to
achieve negative
average output voltage and/or negative average output
current.
DC Motor Control for Larger Motors
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DC Motor Control for Larger Motors
• For larger motors—say, 20 A or more—the
hardware needed to supply pure DC becomes
bulky and expensive.
• An alternative solution is to drive the DC
motor with rectified AC, where no attempt is
made to smooth the waveform.
• A device that is frequently used in this
application to provide both rectification and
some measure of control is the silicon-controlled rectifier
(SCR).
Basic SCR motor control circuit
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Basic SCR motor control circuit • the power source is
single-phase AC and that the DC motor is connected in series
with the SCR.
• The gate of the SCR is driven by a trigger circuit that
provides one pulse for each
cycle of the AC.• The free-wheeling diode (D) across the motor
provides an escape path for the
energy stored in the motor windings when the SCR switches
off.
Half – wave rectifier
Full-wave Rectifier
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Full wave Rectifier• SCR1 is triggered during the positive
half of the AC cycle, and SCR2 is triggered
during the negative half cycle. The result? The motor receives
two power pulses
per cycle.
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• In this case, four diodes are used for the full-wave
rectifier, and a single SCR controls the delay of each
half cycle.
h i i d ib d h f i d h i h
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• The SCR circuits described thus far are triggered somewhere in
themiddle of the AC positive half of the AC cycle. The resulting
abruptvoltage rise generates high-frequency harmonics known as
electrical“noise,” which can cause interference with other
circuits, such as withradio and TV. A solution to this problem is
called zero-voltage
switching.• With zero-voltage switching, the SCR is triggered on
only at the very
beginning of the cycle, when the voltage is zero anyway;
consequently,there is no quick voltage change. If less than full
power is desired,then, for example, only three out of four cycles
would be triggered on(or some other ratio). Zero-voltage switching
requires a more
sophisticated trigger circuit than the phase-shift circuit
discussed sofar.
• Electric motors have a large starting current that is many
times morethan the running current. For smaller motors, this may
not present aproblem; for larger motors (over the range of 1-2 hp),
however, specialreduced voltage-starting circuits are used.
• A reduced voltage-starting circuit will limit the
armature current tosome acceptable value when the motor starts. One
way to do this is tohave a resistor in series with the armature.
After the motor comes upto speed, a relay is used to bypass the
resistor, allowing the full linevoltage to the motor.
Single- Phase Full Converters
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g
Single Phase Dual Converter
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Single-Phase Dual Converter
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Braking the DC Motor
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Braking the DC Motor
• Dynamic braking, uses the fact that a spinning motor becomes a
generator
when the power is removed.
•
when the armature windings are switched to a resistor as the
motor iscoasting down, the “generated” current from the motor
delivers power to the
resistor, which dissipates the power as heat.
• The power to heat the resistor has to come from somewhere, and
in this case
it is coming from the mechanical inertia of the spinning motor
shaft.
• Plugging braking is by reversing the polarity of the armature
windings
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gg g gand thereby causing the motor to apply a reversing torque
to the load.
• The problem is that when the voltage is reversed, it becomes
the same
polarity as the CEMF, so they add. The sudden large voltage will
cause a
large in-rush of current, which could damage the armature. To
prevent this
problem, a series resistor is put in the reversing voltage
circuit . Also, when
the motor finally comes to a stop, the reversing voltage should
be
switched off so that the motor doesn’t start to run
backwards.
• This switching could be accomplished with a centrifugal
switch.
BRUSHLESS DC MOTORS
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BRUSHLESS DC MOTORS• The brushless DC motor (BLDC) operates
without brushes by taking
advantage of modern electronic switching techniques.
• In three-phase BLDC, the armature (called the rotor) is a
permanent magnet,
and it is surrounded by three field coils. Each field coil can
be switched on and
off independently. When a coil is on, such as coil A, the north
pole of the rotor
magnet is attracted to that coil. By switching the coils on and
off in sequence
(A, B, C), the rotor is “dragged” around clockwise—that is,
the field has rotated
electronically.
• The three-phase BLDC has three optical slotted couplers and a
rotating
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shutter (Hall-effect sensors can also be used for this
application). These
position sensors control the field windings. When the shutter is
open for
sensor P1 , field coil A is energized. When the rotor
actually gets to field
coil A, sensor P1 is turned off and P2 is turned on,
energizing field coil B and
pulling the rotor on around to coil B, and so on. In this
manner, the rotor ismade to rotate with no electrical connection
between the rotor and the
field housing. These signals are passed directly on to
solid-state switches
that drive the motor coils.
• A more sophisticated motor-control system would provide for
the motor
to reverse direction (by reversing the sequencing) and would
control thespeed by using PWM techniques.
hibi ll d l f
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• BLDC motors exhibit excellent speed control. In fact, some
models come with a built-in tachometer that feeds back to
the control unit, allowing a speed regulation of 0%
(perfect).
When used in a variable-speed motion-control system, BLDCscan
vary their speed in the range of 100:1.
• However, unlike a brushed DC motor, the BLDC has a
minimum operating speed (around 300 rpm) below which the
individual power pulses can be felt (called cogging).
• Besides being more reliable, modern BLDC motors have
performance advantages over brushed DC motors and even
AC induction motors. Specifically, BLDC motors have higher
power efficiency (they use less power for the same
horsepower) and are smaller and lighter than other types of
motors with the same horsepower.
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Selecting a Motor
Expensive and
complicated drives
Maintenance free, longlifetime, no sparking,
high speeds, clean
rooms, quiet, run cool
B r u s h l e s s
M o t o r s
Maintenance required, noclean rooms, sparking of
brushes causes EMI and
danger in explosive
environments
Inexpensive, moderate
speed, good high end
torque, simple drives
B r u s h e d
D C
Noisy and resonant, poor
high speed torque, not for
hot environments, not for
variable loads
Inexpensive, can be run
open loop, good low-end
torque, clean
rooms S t e p p e r
Robotics
Pick and place
Very high torque
applications
Velocity controlHigh speed control
Positioning
Micro movement
DisadvantagesAdvantages Applications
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DC Servomoters
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i i d l i f db k l f d
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Position and velocity feedback control of a dc motor
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Phase-locked control
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Main components of a PWM drive system
for a dc motor
