Download DC Motors

Devices and Applications
Ctec 201.
DC Motors and Relays
Supplement
Prepared by Mike Crompton. (Rev. 31 March 2009)
DC Motors
Converting electrical energy into rotary mechanical energy is a fair description of what a
DC Motor accomplishes. DC motors can be found performing all sorts of functions from
driving the CPU cooling fan to starting your car. They rely on two basic principles of
physics, ‘when current flows through a conductor a magnetic field forms around the
conductor’ and ‘like magnetic poles repel, unlike poles attract’.
The diagram at right represents the simplest
form of DC motor made up from:
 A magnet that provides a permanent
magnetic field called the ‘Field’.
 A loop of wire called the ‘Armature’
sitting between the poles of the magnet
in the middle of the Field. The Armature
can rotate through 360.
 Two semi circular metallic segments
called
the
‘Commutator’.
The
Commutator will rotate through 360 as
the Armature rotates. Each end of the
Armature is connected to one of the
segments.
 Two carbon blocks called ‘Brushes’ that
push against the Commutator and
transfer voltage and current to the
Armature.
 A DC supply voltage.
ROTATION
MAGNETIC
FIELD
N
S
MAGNET
(South Pole)
MAGNET
(North Pole)
+
Carbon
Brushes
Commutators
DC Supply
The operation of the motor is relatively simple. When the DC supply is connected to the
brushes, current flows from the supply, through one of the brushes, through the armature
loop and out of the second brush to the other side of the supply. In doing so a magnetic
field forms around the wire of the armature. Let us say that the direction of the current
makes the left hand side of the loop’s magnetic field form a North pole, and because
current is flowing in the opposite direction in the right hand side it becomes a South pole.
The North pole of the permanent magnet will now exert a powerful repelling force on the
North pole of the armature, and the South pole on the other side of the magnet will exert
an equally powerful repelling force on the South pole of the armature. The armature will
start to rotate due to this force. After rotating through 90 the repelling force is reduced,
but the attraction of opposite poles, as the N pole of the armature approaches the S pole
of the field and the S pole of the armature nears the N pole of the field, now comes into
play and the armature rotates a further 90. Without the commutator the armature would
now stop with opposite poles firmly attracting each other and preventing further rotation.
2
However the commutator has also rotated through 180 and the (left hand) segment that
was touching the positive brush is now touching the negative brush and vice versa. This
has reversed the polarity of the armature voltage and the direction of current flow. The
side of the loop that was a North pole is now a South pole (and vice versa). The poles are
now adjacent to the like poles of the permanent magnet again and the armature is forced
to rotate through a further 180. The commutator segments reverse the polarity once
again and the poles are back to their original configuration. This sequence of events
continues, and the armature rotates for as long as the power is applied.
In ‘real life motors’ there are multiple armature loops (windings) physically spaced at a
certain number of degrees apart (e.g. three windings would be spaced 120 apart). Each
winding has it’s own two segments on the commutator and they are at 180 opposites.
Multiple armature windings reduce ‘jerky’ rotation and increase speed and torque. Often
the permanent (field) magnet is replaced by an electro-magnet with it’s own field
winding. This prevents deterioration of the magnet strength due to handling and stray
fields, and allows for the field strength to be controlled and, if needed, to be varied at
will.
The carbon brushes are in spring-loaded holders that force them against the commutator.
Sometimes the brushes actually wear down to nothing and have to be replaced. If this
occurs a large deposit of carbon may also have to be cleaned off the commutator to
prevent segments from being shorted by the deposit. In some smaller motors the carbon
brushes have been replaced by a simple thin piece of spring wire resting on the
commutator segments. This eliminates the need for springs in the brush assembly, and the
problem of carbon being deposited on the segments. However it increases the possibility
of the wire brush overheating and burning out with increased current or extended use
under heavy load conditions.
Speed and direction are simply functions of the supply voltage and it’s polarity, higher
voltage = higher speed; reverse polarity = reverse direction. By applying a square wave
of 0V up to some maximum voltage, and changing the amount of time the wave is at
maximum voltage (i.e. On) compared to the amount of time the wave is at 0V (i.e. Off),
the speed can also be controlled. This is known as ‘Pulse width modulation’ and is
basically controlling the motor by digital means.
There will be a maximum voltage for each motor, above which the windings will
overheat and finally melt due to excess current flow. Remember good old Ohm’s Law
still applies, the resistance of the windings will be relatively low, so too high a voltage
will result in too high a current because I = V/R.
Stepper Motors.
A completely different type of DC motor is the ‘Stepper Motor’. As the name implies,
this type of motor does not rotate continuously but will rotate for only a small portion of
a complete turn (a step) each time it is energized. It’s construction is different from the
regular DC motor in that the rotating shaft has a series of magnets distributed around it
and the motor housing has a series of coils surrounding the shaft. Basically the opposite
3
configuration to the normal motor. If one of the coils is energized, it will attract (or repel)
the armature which will rotate to that position where it will stop and be held there as long
as the power is applied. This means the stepper motor is consuming power even when it
is stopped. By energizing the coils in a certain sequence the stepper motor can continue to
rotate one step at a time until the power is cut. This makes it ideal to be controlled
digitally, each bit will make it rotate one step, and counting the bits will tell how many
steps and ultimately how far it has rotated or traveled. However it needs ‘translator
software’ to ensure that the bits are fed to the coils in the correct sequence, and possibly a
driver to increase the power to the motor.
Stepper motors are described by their voltage, resolution (number of steps per revolution
or degrees per step from .72, 1.8, 3.6, 7.5, 15 and even 90 degrees) resistance and torque.
Torque is very high from stopped or low speeds compared to their normal DC motor
counterparts. They also have very easily controlled stop and start positions, unlike regular
motors where stop and start positions are somewhat random even with controlling
circuitry.
The disadvantage of stepper motors compared to normal DC motors is their high power
consumption, even when stopped and the need for specialized control circuitry and
software. DC motors would also require specialized control circuitry if positional or
rotational data is needed.
Relays
A relay is an electro-mechanical device used for remote switching, or for controlling
devices of varying voltage requirements with a single supply voltage. Often the
controlling voltage is much lower than the controlled device voltage(s).
Relays consist of a coil that becomes an electromagnet when current passes through it, an actuating
arm or lever and one or more sets of contacts. When
current passes through the coil it ‘energizes’ the
relay. The magnetic field created by the energized
coil attracts the actuating arm, which in turn opens
or closes sets of contacts. Any contacts that are
closed (touching) when the relay is de-energized
(off) are referred to as ‘Normally Closed’ or N.C.
contacts. Any contacts that are open (not touching)
when the relay is de-energized are referred to as
‘Normally Open’ or N.O. contacts. Naturally N.C.
contacts open and N.O. contacts close when the
relay is energized (on).
4
A considerable variety of relays are available. Differences in size, shape, actuating
voltages, AC or DC, number of contacts, current carrying capability of the contacts and
power handling are all variables. The function, however, is basically the same regardless
of the relay type.
1
The diagram at right represents a fairly
Pin
Allocation
typical relay that has 4 sets of contacts, 2
sets of normally closed and 2 sets of
1 & 16 = Coil
normally open. In this particular case there
4 = Common
are only 6 actual contacts instead of 8 (4 sets
4 & 6 = N/C
times 2 contacts per set) because there are 2
4 & 8 = N/O
13
= Common
‘common’ contacts’ (contacts #4 and # 13).
13
&
11 = N/C
In the de-energized condition contacts 4 &
13 & 9 = N/O
6, and 11& 13 are normally closed (N.C.).
16
Contacts 4 & 8, and 9 & 13 are normally
open (N.O.)
When energized these conditions reverse with 4 & 6 and 11 & 13 opening, while 4 & 8
and 9 & 13 close. The contacts will therefore control whatever device is connected to
them.
Pins 1 and 16 are the connections to the coil allowing it to energize the relay itself.
The relays used in the labs are usually 5V DC or 12V DC DIP relays. The diagram above
is of these two relays. The only difference being the activating voltage required by the
coils. DIP stands for Dual In-line Pack, which in turn describes the physical
characteristics. 2 (dual) sets of pins or legs that are ‘in line’ so they can plug into the
holes of a printed circuit board in the same way as a TTL Logic chip. Often a circuit
diagram is printed on the relay case. This is not to be confused with the pin locations.
When viewed from the top of the relay, pins 1 to 8 run from the bottom right to the top
right of the relay case and pins 9 to 16 start at the top left and run down to the bottom left.
One very important feature of any relay is the amount of current the contacts can handle
without welding themselves together or burning out completely. Either situation would
produce a catastrophic failure. Welded contacts cannot separate so the ‘controlled device’
would be continually on and may do severe damage. Burnt out contacts mean the
controlled device could never be turned on.
A typical example of an application could be the on/off control of a 115V AC motor
accomplished by 5V DC activating a suitable relay while at the same time indicating the
on or off condition of the motor with a green and red LED. The red ‘off’ LED (with it’s
current limiting resistor) would be connected through a set of N.C. contacts to a 5V DC
supply. The green ‘on’ LED and it’s resistor, would be connected to the same supply but
through a set of N.O. contacts. A second set of N.O. contacts would connect the motor to
the 115V AC supply. The relay coil would of course be connected through some type of
switch to the 5V DC supply. When the relay was de-activated the red LED would be on
via the 5V and the closed contacts. When activated the same contacts would open turning
the red LED off, at the same time the 2 sets of N.O. contacts would close turning on both
the green LED and the motor.
5
8
4
6
9
13
11