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Improved
Speed
Controller
for
Universal
Motors
By JOHN CLARKE
This new speed controller can be used with power tools and
appliances rated up to 5A. Use it to control the speed of circular
saws, routers, jig saws, electric drills, hobby lathes, lawn edgers
and other appliances with universal “brush type” motors.
14 Silicon Chip
www.siliconchip.com.au
T
his circuit is essentially a re- smoothness which you can obtain. that in mind if you want to use an
vised version of the Speed Con- Some power tools and appliances electric drill as a power screwdriver
with this control. You can do it for
troller published in September don’t run smoothly at very low speeds
short periods but if you want to do it
& November 1992. It has been a par- when run from this type of control
ticularly popular project and readers circuit and frequently display a be- for long periods you run the risk of
have come up with countless appli- haviour known as “cogging” whereby severely over-heating the drill motor.
it runs in short bursts.
cations for it.
In the middle range of the speed conIn fact, you might think that since
So the practical minimum speed
trol, the circuit gives good speed regthere are now so
ulation. This means
many battery-powthat if the motor is
WARNING!
ered and 240VAC
loaded down, the
power tools with
applied voltage is
(1) This Speed Controller circuit operates directly from the 240VAC
inbuilt speed conautomatically inmains supply and is potentially lethal. Do not build it unless you
trols that the uses
creased to compenknow exactly what you are doing.
for a circuit such
sate.
DO NOT TOUCH ANY PART OF THE CIRCUIT WHILE IT IS
as this would have
At the maximum
declined. In fact,
end of the speed
PLUGGED INTO A MAINS OUTLET and do not operate the cirthis circuit is more
control range, this
cuit outside its earthed metal case.
popular than ever
circuit will not give
and people are confull speed operation
(2) This circuit is not suitable for use with induction motors or shaded
stantly coming up
when in variable
pole motors used in fans – see “What motors can be controlled”.
with new applicamode. This is betions for it.
cause at the maxiApart from drills
mum speed setting
(3) This circuit must only be used with universal “brush type” (series
and circular saws,
it is essentially just
wound) motors with nameplate ratings up to 5A.
the speed control
a high power diis particularly useode and this means
(4) Power tools with inbuilt fans must not be operated at low speeds
ful for controlling
that it is feeding
for long periods otherwise they may overheat and suffer damage.
routers and jig
half-wave rectified
saws (when cutting
240VAC to the moplastics, for exam(5) This circuit must not be used to control the power to lamps or elec- tor or about 170V
ple), hobby lathes
RMS.
tric radiators. To do so would contravene the regulations of
(which use sewing
So if do you want
the NSW Energy Authority and affiliated state energy authorities.
machine motors),
full speed from the
food mixers (where
the in-built speed
control has failed) and lawn edgers
(where full speed operation tends to
frequently break the Nylon line). The
speed controller also offers much improved operation of sewing machines,
in comparison to the conventional
resistive pedal controller.
The new 5A Speed Controller is
housed in a diecast box which is much
more rugged than the previous project
which used a plastic case. The diecast
case also improves heat dissipation
from the Triac.
The controls on the front panel
comprise a full/control switch and a
knob to set the speed. Power comes
in via a 3-core mains lead and mains
plug while the outlet is a flush-mount
mains GPO (General Purpose Outlet)
socket on the front panel.
Speed control range
This speed controller will enable
you to set the appliance operating
speed over a wide range down to a
very slow rate. In fact, the minimum
speed will mainly depend on the
www.siliconchip.com.au
for any appliance motor depends on
freedom from “cogging”.
There is another factor which limits
the minimum speed that an appliance
can be run at and that is the fact that
most universal motors have an inbuilt fan for cooling. Below certain
speeds that fan is largely ineffective
and so there is no cooling at all. Bear
A
10k
5W
SPEED
4.7F
630V
2k
D1
SCR
A
D2
G
K
1k
MOTOR
N
Fig.1: this circuit demonstrates the
basic principle of motor speed control.
The SCR feeds half-wave rectified AC
to the motor and its trigger point is
made sooner or later in each positive
half-cycle to vary the power.
motor, set the speed
switch to “full”.
Basic circuit operation
The circuit is very similar in principle to simple SCR speed controls developed years ago except that we are
using a Triac. So to explain the circuit
principle, have a look at Fig.1 which
is just about the simplest speed control you could get. The SCR device
conducts in one direction only and
feeds half-wave rectified AC to the
motor.
Essentially, the SCR is a rectifier
diode which only conducts when it
receives a trigger voltage at its gate.
Once it starts conducting, from anode
(A) to cathode (K), it stays conducting
until the load current drops to zero or
the circuit voltage is reversed.
Because the SCR is a switching
device, it can be used as a very efficient power controller, varying large
amounts of current while itself dissipating very little power.
The circuit of Fig.1 controls the AC
power to the motor by triggering the
SCR into conduction late or early in
October 2002 15
So how does the circuit give speed
regulation?
The answer is that the circuit monA2
150k
itors the back-EMF from the motor.
TRIAC1
1W
BTA41-600P
Back-EMF can be defined as the
D2
SBS1
G
1N4004
2N4992
A1
voltage developed by a motor which
SPEED
VR1
opposes the supply voltage. The higher
10k
LIN
the speed of the motor, the higher the
GPO
47nF
1k
back-EMF. This circuit monitors the
240VAC
VR2
back-EMF in the following way.
INPUT
5k
A
N
SET MIN
Notice that one side of the motor is
SPEED
connected directly to the SCR’s cathD3
ode while the other side is connected
D1
R250H
E
to the cathode of diode D1 and to the
1N4004
6A 600V
mains Neutral wire. This means that
the gate-to-cathode voltage applied
N
to the SCR is the difference between
the wiper voltage from VR1 and the
E
(CASE)
back-EMF generated by the motor (disregard the voltage drop across diode
Fig.2: this was the circuit of our speed control published in
September 1992. It uses a silicon bilateral switch and 47nF
D2).
capacitor to give strong gate pulses for reliable triggering.
Actually, in so-called universal
motors (AC/DC series motors with
each positive half-cycle of the 240VAC half cycles?
commutators as used in most power
waveform; being a diode, the SCR does
The answer is that we could but then tools and appliances), there are two
not conduct at all during the negative
a fundamental advantage of this basic back-EMFs generated. The first is
AC half cycles.
circuit would be lost. The advantage a function of motor speed and the
If the SCR is turned on early in each is speed regulation. A circuit with
remanent magnetism (remanence)
AC half cycle, the power fed to the mogood speed regulation will maintain of the field coils. It is generated durtor will be relatively high. Conversely, a selected motor speed regardless of
ing the time when the SCR is not
if the SCR is turned on late in each AC variations in the load. If you are using a conducting; ie, during the negative
half cycle the power fed to the motor speed controller with an electric drill, half cycles of the AC waveform and
will be relatively low and hence the
you don’t want the motor to bog down during the first portion of the positive
motor will run slowly.
when you start to drill into the heavy
half cycles before the SCR conducts.
The trigger voltage for the SCR stuff, do you?
The second back-EMF is generated
comes from VR1, a
during the time
2kΩ potentiometer
when the SCR is
connected in series
conducting and
What motors can be controlled?
with a 10kΩ resistor
since the there will
Virtually all power tools and small appliances use so-called “universal”
and diode D1. This
now be current
motors. These are “series wound” motors with a commutator and brush2kΩ potentiometer
flowing in the field
is fed with half-wave
coils (and also in
es. The “series wound” term refers to the fact that the motor armature
rectified AC which is
the armature). This
and field windings are connected in series and this allows the motor to
partly smoothed by
back-EMF will be
be run from AC or DC, ie, “universal”.
the 4.7µF capacitor
higher than the first.
across it. The resultWe are only conInduction motors must not be used with this speed controller. So how
ing ramp voltage
cerned with the
from the wiper is fed
back-EMF generdo you make sure that your appliance is a universal motor and not an
to the gate of the SCR
ated while the SCR
induction motor?
via diode D2.
is not conducting
since it is this voltIn many power tools you can easily determine that the motor has brushSpeed regulaage which deteres and a commutator – you can see sparking from the brushes and that
tion
mines how late or
settles the matter. But if you can’t see the brushes, you can also get a
early in each posiNow you might
tive half cycle that
clue from the nameplate or the instruction booklet.
ask: why just use
the SCR begins conan SCR and allow
duction. Hence, the
conduction on only
So how do you identify an induction motor? Most induction motors used
motor applies negapositive AC half cyin domestic appliances (eg, washing machines, fridges, water blasters,
tive feedback to the
cles? Why not use a
swimming
pool
pumps)
will
be
2-pole
or
4-pole
and
always
operate
at
gate of the SCR.
Triac which can be
This negative
a fixed speed. Typically this is 2850 RPM for a 2-pole unit or 1440 RPM
triggered into confeedback enables
duction on both posfor a 4-pole unit. Bench grinders typically use 2-pole induction motors.
the circuit to give
itive and negative
A
F1
10A FUSE
16 Silicon Chip
www.siliconchip.com.au
10A FUSE
A
F1
TRIAC1
BTA41-600P
100k
1W
Fig.3: this is the
revised speed
control circuit
which now
uses a sensitive
gate SCR as the
trigger source
for the high
power Triac.
D2
1N4004
SPEED
VR1
10k
LIN
SCR1
C103B,
MCR100
A
A
G
47k
240VAC
INPUT
VR2
10k
G
A1
100
K
K
A2
GPO
1 FULL
A
S1
2.2k
2
1nF
N
CONTROL
1k
MIN
SET
E
(CASE)
47nF
A
K
D1
1N4004
K
D3
R250H
6A 600V
A
N
E
1N4004
(CASE)
A
MCR100
K
MAINS CORD
(CLAMPED TO
CASE WITH
CORD GRIP
GROMMET)
Now the basic circuit presented in
Fig.1 will actually work. In fact, it was
*USE 7.5A MAINS
RATED WIRE
VR2
R250H
1k
A
100
47k
2.2k
1nF
47nF
D1
N
PC BOARD
MOUNTED IN
CASE WITH 6mm
NYLON SPACERS,
NYLON SCREWS
AND NUTS
CS
D3
K
RELLORTNOC ROTOM
TRIAC1
(MCR100
MOUNTED
UNDER
BOARD)
12001101
NEUTRAL
(*BLUE)
D2
100k 1W
1
2
G
Fig.4: Use this diagram when
assembling the PC board and
completing the wiring inside the
case.
*GRN/YELLOW
SCR1
A1
A2
NOTE: IF USING
C103B FOR SCR1,
FLAT SIDE FACES
BOTTOM OF PCB
A
A WS
A
the basis of most speed control circuits
used about 30 years ago.
However, it has a number of drawbacks. First, the power dissipation
through the 10kΩ resistor is about 2.4
watts which means that it gets rather
hot. Second, even though the current
through the 10kΩ resistor and VR1
is relatively high, it won’t be enough
for reliable triggering of higher power
Better circuit
10A
ACTIVE
(*BROWN)
A
K
K
tive half cycle and hence more power
will be applied to the motor. This
will tend to correct the drop in motor
speed.
It’s not perfect but it’s a lot better
than having no speed regulation at all.
FUSE F1
BTA41-600P
G
G
A
5A UNIVERSAL MOTOR CONTROLLER
good speed regulation. Say a particular
motor speed is set by VR1 and then the
motor speed tends to drop because of
an increase in loading.
This reduces the motor back-EMF
and therefore increases the voltage
at the gate the SCR. More correctly,
it means that the ramp voltage at the
SCR gate will exceed the voltage at
the SCR cathode earlier in the posi-
C103B
N
*GRN/YELLOW
*
*
*GRN/YELLOW
*
VR1
EARTH
POT
LOCATING
PIN
*BLUE
Fig.5 (below): the mounting
details for the Triac. It is an
isolated tab device and does not
need an insulating washer.
1
E
2
*BROWN
*BROWN
A
N
S1
(CASE LID)
*BROWN
MAINS
OUTLET
FIT HEATSHRINK SLEEVING OVER ALL SWITCH CONNECTIONS
www.siliconchip.com.au
PC BOARD
CASE
TRIAC
6mm
TRIAC MOUNTING DETAIL
October 2002 17
Scope 1: This waveform shows the Speed
Controller set for maximum output when
driving a resistive load. Note that the waveform
is essentially a half-wave rectified sinewave
with an RMS value of 170V. The early part of
each positive half-cycle has been chopped out,
due to the fact that the trigger circuit does not
fire the Triac until about 2 milliseconds after the
start of the cycle.
SCRs. And third, the circuit is not
particularly good at very low speed
settings.
Now fast-forward to September 1992
and have a look at the speed control
circuit of Fig.2. You can see the similarities between it and Fig.1. Instead
of an SCR, we have used a Triac and
instead of feeding the gate directly
from VR1 as in Fig.1, a trigger circuit
consisting of a silicon bilateral switch
(SBS1) and a 47nF (.047µF) capacitor
has been used.
While the Triac is capable of conducting on both positive and negative
half cycles of the 240VAC 50Hz waveform, this circuit only enables it to
trigger on positive half cycles, because
of the rectifier action of diode D1.
A silicon bilateral switch is a voltage
breakover device; ie, at voltages below
its breakover point it is essentially
open circuit but once the breakover
voltage is reached, it becomes a low
value of negative resistance.
Don’t worry too much about the
Scope2: The same waveform now superimposed
on the 240VAC 50Hz input waveform (in blue).
Notice there is some small voltage loss across
the Triac. The “flat-topping” of both waveforms
is commonplace in areas where there are lots of
fluorescent or gas discharge lights and/or PCs,
which clip the peaks of the mains waveform.
This is because capacitive-filter supplies take
their power from the peaks of the AC waveform.
“negative resistance” bit. All you
have to remember is that it is used in
conjunction with the 47nF capacitor.
This charges up from VR1 via diode D2
until it reaches the break-over voltage
of about 8V. At this point it dumps
the capacitor’s charge into the Triac’s
gate to trigger it into conduction and
the cycle repeats for the next positive
half cycle of the mains AC waveform.
The energy stored in the capacitor is
quite enough to trigger even insensitive Triacs, hence we are able to use a
high power 40A device in this circuit.
In this circuit, the motor back-EMF
acts to reduce the charging voltage to
the 47nF capacitor rather than reducing the SCR gate voltage as in Fig.1. But
although the circuit arrangement is a
little different, the speed regulation is
just as good. The circuit efficiency is
improved too, with only 200mW being
dissipated in the 150kΩ resistor which
feeds VR1. This resistor has a rating of
1W to ensure that it has an adequate
voltage rating.
Scope3: This waveform s
Controller set for maximu
driving an electric drill (
motor). Notice that there
beginning of each positiv
waveform, caused by the
This caused the gross err
measurement of 1.497kH
The functions of the three diodes
in the circuit need to be explained.
Diode D1 is there to reduce the power
dissipation of the series resistor string
and to ensure half-wave operation
of the circuit. D2 is there to protect
the gate of the Triac when it is in the
conducting state – terminal A1 can be
above the potential of the gate.
Diode D3 has been included as a
flyback diode to quench the large inductive spike generated by the motor
at the end of each positive half cycle.
While the voltage spike does not cause
any damage to the circuit, it does have
the effect of disrupting the back-EMF
monitoring system described above.
VR2, the 5kΩ trimpot in series with
VR1, is there to provide a minimum
setting for the circuit.
One question we have not answered
so far is why we specified a Triac
instead of an equivalently rated SCR.
The reason is quite simple. We did it
to avoid the need for parts stockists
to have to order in another device.
The completed PC board,
reproduced same size.
At right is the underside
of the board showing the
mounting of the triac.
Its tab is isolated so no
insulating washer is
required – but give it a
good smear of heatsink
compound to help it
keep its cool!
18 Silicon Chip
www.siliconchip.com.au
shows the Speed
um output when
(ie, with a universal
is some hash at the
ve swing of the
e motor’s commutator.
ror in the frequency
Hz – should be 50Hz.
Scope4: Now set for a lower speed from the
electric drill, the Triac is on for a shorter time
and the RMS value is reduced to 115V. Again,
the “judder” in the waveform is caused by
commutator hash. Again, this hash also caused
the slight frequency measurement error.
We have specified a 600V 40A device
so that it can withstand the “locked
rotor” current of any power tool with
a nameplate rating of up to 5A.
Note that a “locked rotor” condition
may well blow the 10A fuse but the
40A Triac should not be damaged. Yes,
we have done this test!
Another reason for using the 600V
40A Triac is that it is an isolated tab
device. This means that it can be
attached directly to the metal case
without any need for a mica washer
or other means of insulation.
Latest circuit version
The circuit of our new 5A Speed
Controller is shown in Fig.3. As already mentioned, this is a revised
version of the design we featured in
the September & November 1992 issues. Our new circuit replaces the now
hard-to-get SBS with SCR1, a C103
sensitive gate SCR and this provides
the same capacitor dump function
as the SBS.
The 47kΩ and 2.2kΩ resistors form
a voltage divider between the anode
and cathode of the SCR with the divided voltage applied to the gate. The
SCR conducts when the gate voltage
reaches 0.6V and is triggered by a mere
200uA of gate current.
Because of the resistive divider, the
voltage across SCR1 must rise to some
13.4V before the gate reaches the 0.6V
sufficient to trigger the SCR. When
the SCR fires, the charge on the 47nF
capacitor is dumped into the gate of
www.siliconchip.com.au
Scope5: Now set for a very low speed from the
electric drill, this scope waveform demonstrates
motor “cogging”. As you can see, the Triac is no
longer firing reliably on each positive halfcycle. The frequency measurement is also
meaningless. This speed setting is too low to be
useful as the drill will not run smoothly.
Triac1 via the 100Ω resistor to fire the
Triac. The 1nF capacitor between anode and gate of SCR1 is there is provide
more reliable triggering when there is
lot of commutator hash from the motor
being controlled.
Note that the revised circuit uses
slightly different component values
compared to Fig.2. Apart from the
resistors and capacitors associated
with the SCR, the original 150kΩ 1W
resistor is changed to 100kΩ while
the trimpot VR2 is now 10kΩ instead
of 5kΩ.
By the way, the 100kΩ resistor is
a 1W type, not for disippation but to
Parts List – 5A Universal Motor Speed Controller
1 PC board coded 10110021, 79
x 38mm
1 diecast box 120 x 66 x 38mm
(Altronics H-0453 or
equivalent)
1 panel label 119 x 65mm
1 mains flush-mount socket (Jaycar PS-4090 or equivalent)
1 SPDT 250V 6A rocker switch
(S1; Altronics S-3215)
1 10kΩ linear 24mm
potentiometer (VR1)
1 knob for potentiometer
1 7.5A three-core mains cord and
moulded 3-pin plug
1 10A M205 fast blow fuse
2 M205 PC board mount fuse
clips
1 cord-grip grommet to suit mains
cord
2 crimp eyelets or solder lugs for
earth connection
4 6mm Nylon spacers
2 M3 x 10mm csk screws
1 M3 x 10mm screw
2 3mm star washers
4 M3 x 15mm Nylon screws and
nuts
4 stick-on rubber feet
1 200mm length of blue 7.5A
250VAC wire
1 200mm length of brown 7.5A
250VAC wire
5 100mm long cable ties
8 PC stakes
Semiconductors
1 BTA41-600P Triac (TRIAC1)
1 C103B sensitive gate SCR
(SCR1)
1 R250H 6A 400V diode (D3)
2 1N4004 1A 400V diodes (D1,D2)
Capacitors
1 47nF (.047µF) 63V MKT
polyester
1 1nF (.001µF) 63V MKT polyester
Resistors (0.25W 1%)
1 100kΩ 1W 5%
1 47k
1 2.2kΩ
1 1kΩ
1 100Ω
1 10kΩ horizontal trimpot
code 103 (VR2)
October 2002 19
These two views inside the case show exactly how the wiring should be done. When we say exactly, we mean it: don’t
take chances or shortcuts with 240VAC. Ensure that any hookup wire you use is 250VAC-rated.
ensure that it has an adequate voltage
rating.
As mentioned above, switch S1 provides full power operation, bypassing
the Triac so that the motor gets the
full 240VAC applied to it. Note that
the switch must be a changeover type
to select either Active or the Triac A1
output rather than just using a single
switch across the Triac. In the latter
case, there would be a short circuit
(which would blow the fuse) when
diode D3 and the Triac conducts on
negative half-cycles of the 240VAC
mains.
Construction
All the components of the 5A Speed
Controller are mounted on a PC board
coded 10101021 and measuring 79 x
38mm. It is housed in a diecast box
measuring 120 x 66 x 38mm.
Begin construction by checking
the PC board against the pattern in
Fig.7. There should not be any shorts
or breaks between tracks. If there are,
repair these as necessary. Use the
diagram of Fig.4 as a guide when assembling the PC board and completing
the wiring inside the case.
Start assembly by inserting the PC
stakes at the external wiring connection points on the PC board. Then
insert the resistors, using the table
below as a guide to the values. When
inserting the diodes, take care with
their orientation.
Note that in a kit you may be supplied with a C103B or MCR100 for
SCR1. If so, not that the pinouts for
the C103B are reversed to those of the
MCR100, as shown on the circuit of
Fig.3. Make sure you insert SCR1 into
the PC board correctly, otherwise the
circuit won’t work.
The capacitors can be installed next.
Use the table below to check the values. VR1 can also be installed at this
stage. Fuse F1 is mounted in fuse clips
which attach to the PC board as shown
on Fig.4. Clip the fuse into the clips
first, insert them into the PC board and
solder in position.
The Triac is mounted on the underside of the PC board with its leads
protruding up through the holes in the
PC board. Bend the leads so that the
copper side of the PC board is 6mm
away from the back of the Triac body,
as shown in Fig.5.
Insert the PC board into the case
and mark out the mounting hole po-
Resistor Colour Codes
No. Value
❐ 1 100kΩ
❐ 1 47kΩ
❐ 1 2.2kΩ
❐ 1
1kΩ
❐ 1 100Ω
4-Band Code (1%)
brown black yellow brown
yellow violet orange brown
red red red brown
brown black red brown
brown black brown brown
20 Silicon Chip
5-Band Code (1%)
N/A
yellow violet black red brown
red red black brown brown
brown black black brown brown
brown black black black brown
sition for the standoffs and for Triac1.
Remove the PC board and drill out
the holes. You will also need holes in
the end of the case for the cord-grip
grommet and the earth lug screw. The
cord-grip grommet hole is elongated
but must be a tight fit to properly grip
the mains cord.
The hole for Triac1 must be deburred with a larger drill before it is
secured in place.
Attach the PC board to the case
with Nylon standoffs and Nylon M3
x 15mm screws. Nylon screws are
essential here, to avoid the possibility
of arcing from the PC board tracks to
the mounting screws.
Use metal screws for the Triac and
earth connections. Secure Triac 1 to
the case with a metal M3 x 10 screw
and nut after applying a smear of
heatsink compound on the mating
surfaces. Note that the specified Triac
is an insulated tab device and does not
require an insulating washer.
Attach the mains cord wires to the
PC board and lock the cord in place
with the grommet. Mark out and drill
the front panel for the mains outlet,
speed control pot (VR1), the earth
screw and the switch.
You can use the front panel label as
a guide to the positions. The cutting
template for the mains socket is shown
Capacitor Codes
Value
47nF
1nF
Old
Value
.047µF
.001uF
IEC
Code
47n
1n
EIA
Code
473
102
www.siliconchip.com.au
Note that all of the circuit is connect- adjust VR2 and then try again. You
ed to the 240VAC mains supply and
may then want to try other power
is potentially lethal. Do not touch any tools to get a compromise setting for
part of the circuit when it is plugged
the trimpot.
into a mains outlet. Always remove the
IMPORTANT: Do not operate the
plug from the mains before touching circuit with the lid off the case. SC
the circuit. In particular
this applies to adjustA
ment of trimpot VR2.
SW A
After testing the conA
troller, then you need to
adjust trimpot VR2. Plug
SC
in your favourite power
N
tool and note how it runs
at the minimum setting
MOTOR CONTROLLER
of VR1. If it could run
N
slower, disconnect the
circuit from the power, Fig.7: actual size artwork for the PC board.
10110021
in Fig.6.Note that it is important to
drill a small hole for the locking tab on
the potentiometer to prevent it rotating
inside the case.
Attach the front panel label and
secure the mains socket. Attach the
pot and switch.
Wiring must be done using 7.5A
250VAC-rated wire. Earth connections
are soldered or crimped to the solder
lug using green/yellow mains wire.
The lugs are secured to the case using
a metal screw, nut and star washer.
Tie the wires with cable ties to prevent them breaking from their terminations.
Finally, attach rubber feet to the base
of the case.
MAX
240V - 5A
MOTOR SPEED
CONTROLLER
Small hole 4.5mm
in diameter
MAX
240V - 5A
MOTOR SPEED
CONTROLLER
CONTROLLED
SILICON
CHIP
www.siliconchip.com.au
For universal-type motors
up to 5A nameplate rating
Do NOT use on induction
or shaded-pole motors
14mm
10.9mm
Semicircular part
33mm in diameter
16.5mm
(CASE LID)
Fig.6: these are the panel cutout details for flush-mount AC socket.
CONTROLLED
SILICON
CHIP
www.siliconchip.com.au
For universal-type motors
up to 5A nameplate rating
Do NOT use on induction
or shaded-pole motors
Fig.8: actual size artwork for the front panel label.
“Universal” motors:
the inside story
This photo shows the construction of a typical double-insulated jig-saw which uses a
universal motor with brushes
and a commutator. The plastic case provides the “double
insulation” construction and
it also provides the alignment
for the motor bearings, brushes
and gears. Note the integral fan
on the motor shaft to provide
cooling.
This jig-saw also has speed
control built into the trigger
switch. This motor is actually
wound for operation from 180V
so it gives a wide (no load)
speed control range from 500
to 2900 RPM.
www.siliconchip.com.au
October 2002 21
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