Re: DIY speed control of ~1hp PM DC motor?
- From: Mark Rand <randm@xxxxxxxxxxxxxxxxx>
- Date: Wed, 07 Jun 2006 23:40:11 +0100
On Wed, 07 Jun 2006 11:52:37 -0700, Grant Erwin <grant@xxxxxxxxxxxxxxxxxx>
wrote:
Back in 1996, an article appeared in the magazine "Home Shop Machinist". The<snip>
article contained a simple circuit for speed controlling a small permanent
magnet DC motor: http://www.tinyisland.com/images/temp/HSM-DC.jpg
<snip>
1. Rheostat R1 (a potentiometer wired as a variable resistor) should have
some resistance in series with it. Without significant resistance the SCR
could be destroyed by excessive gate current if power were applied when the line
voltage was at peak value and the speed control happened to be at the maximum
possible motor speed (minimum resistance). Under these particular conditions
the motor, diode D2, diode D3 (a diac) and the SCR gate are in series across the
peak valued full-wave rectified line voltage. I'd use a 470 ohm (2 watt)
resistor since that is large enough to limit gate current to a reasonable value
and small enough to have little influence on the motor speed control range.
OK. That's the background information. Now I have a question for the electronics
guys among us. What is the function of diodes D1 and D2 in this circuit? Also,
if one were to insert a 470 ohm 2 watt resistor as suggested, exactly where
should it be located?
Also, if I were going to implement this circuit using a stud-mount SCR, using a
4" electrical box, how should I implement a heat sink? Replace the steel box lid
with aluminum, and use that as a heat sink? Hate to have wires connected to the
box lid ..
Finally, there is an alternate circuit out there in the public domain. It is
Figure AN1003.16 in the app note on the littelfuse.com Web site:
http://www.littelfuse.com/data/en/Application_Notes/AN1003.pdf
Does anyone have any comments on why one circuit is better than the other?
Thanks,
Grant Erwin
.. just scrounged another dirt cheap electric lawn mower motor ..
Starting points Ignore if you know them already :-)
1) The SCR won't conduct current in any direction until there is a voltage in
the direction of its arrow (positive to the top in the drawing) AND there is a
current sent into it though the gate terminal on the side. After that it will
keep conducting until the current flowing through it falls to some fairly low
value. Then it resets. It will _only_ conduct in the direction of the arrow
whatever happens.
2) The DIAC D3 won't conduct any current until the voltage across it reaches
28 volts, then it'll conduct current until that current falls to some very low
value, Then it resets.
3) The Capacitor C1 stores electricity just like a gas bottle stores air.
4) The resistor R1 limits the flow of electricity like a leaky gate valve (it
can't turn all the way off :-)
5) The Diodes D1 and D2 only allow current to flow in the direction of the
arrow, just like non-return valves.
6) The permanent magnet electric motor will generate a voltage proportional to
its speed when it rotates (there's no real difference between a motor and a
generator). The Current it draws is proportional to the difference between the
voltage it is generating and the voltage the circuit is supplying it with and
the torque it generates is proportional to the current it draws. This means
that the motor will try to run at a speed that is proportional to the voltage
its fed and will slow down by a small amount from that perfect speed until it
draws enough current to generate the torque that the load requires.
7) B1 is producing a full-wave rectified sine wave. Ie. instead of going from
0 120 0 -120 0 it is going 0 120 0 120 0 60 times a second or whatever
voltage and frequency are relevant in its part of the world
Had enough egg sucking Grandma?
D1 is there to avoid the voltage spike that would otherwise occur when the SCR
turns off at its minimum holding current. i.e. when the voltage out of B1 gets
close enough to the back emf from the motor every half cycle the current
through the motor drops low enough that the SCR will not stay turned on. At
this point the SCR will go from conducting to non conducting very quickly.
Although the current is small at this point, there is probably enough
inductance in the motor that a bloody great voltage spike will ensue
(mechanical equivalent= shutting a large gate valve instantaneously with water
flowing through the pipe. the gate valve can get blown off the end of the pipe
(saw it happen once in a power station)). The diode allows the current to keep
flowing through the motor.
It's a bit like the condenser across the points in a spark ignition engine
except that if you used a condenser (capacitor) in this case, the rush of
current when the SCR turned on could destroy the SCR... You can't win.
D2 allows the motor speed to be controlled lower.... The SCR turns on when the
DIAC triggers and sends a pulse of current into its gate. This happens when
the voltage across C1 reaches about 30 volts. Since the voltage out of B1 is a
rectified sine wave, if the diode weren't there, C1 could be charged up and
then discharged through the R1 without ever reaching 30V when the R1 is set to
a high resistance. What the diode does is to stop this discharging so that if
the DIAC doesn't trigger the SCR on one cycle, it'll trigger it sooner on the
next cycle. This will cause the skipping that the article refers to, but the
alternative would be that the minimum controllable speed might be of the order
of 25% of maximum... not so useful.
The logical place for the 470 ohm 2 Watt resistor would be between D2 and R1.
Sorry about this post. I suspect it's a mixture of far too simple for some,
missing out major background for others and too much waffle for all. Still
it's worth every penny you paid for it :-)
Regards
Mark Rand
RTFM
.
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- DIY speed control of ~1hp PM DC motor?
- From: Grant Erwin
- DIY speed control of ~1hp PM DC motor?
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