SCR speed control oddity ?

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Hi it to all my fans...


I picked up a nicely priced "Maximate"  jig saw from K-Mart today.

Rated at 450 watts, it has the usual AC/DC brush motor inside -  but no
internal trigger speed control.

So, I tried my trusty SCR drill speed controller to see if I could do it
externally.

No go.

The motor emits a weird chirping noise and draws a fairly large current -
but goes no where.

My 4 amp variac controls the speed just fine.

Anyone know what's up ??

Before I pull the darn thing apart to find out.



......  Phil



 



Re: SCR speed control oddity ?
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I had a similar case once with a cordless drill. Took the SCR control
switch  of an older one and replaced the simple switch. Worked fine.
Maybe a filter Cap in the jig saw messes with the external SCR?

Tony


Re: SCR speed control oddity ?

"TonyS"
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** The motor will run with as little as 15 volts DC applied -  of either
polarity - in the same direction.

Tested with an audio gen shows very little parallel C -   maybe 10nF.

Reads 32.1 ohms on a DMM, either polarity.

Weird.



.......   Phil






Re: SCR speed control oddity ?
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I am already running out of ideas...
Good luck with pulling it apart:-) Maybe you will find some diodes inside?

Re: SCR speed control oddity ?
What you call an SCR is really a Triac or Dual Thrysistor. An SCR is a
1/2 wave device, while the Triac is a full wave device, which is
equivalent of having two SCR devices back-to-back.

The triac devices uses the technique of changing the pulse width or duty
cycle of the AC cycles, and thus gives the effect to an incandescent
lamp as if the voltage is changing. When the duty cycle is being
changed, the sine-wave of the AC is being chopped. They do this using a
phase shift feedback control to the gate of the triac device.  In actual
fact if the average power to the lamp that is being changed rather than
the voltage. Also, non brush motors such as inductive or synchronous
motors cannot be used on a triac device. These are frequency dependent
for their RPM.

The motor being a reactive device will ignore the effect of the triac
device. Because the AC is no longer a proper sine-wave for the motor, it
starts to act as if there was some DC being applied. Being an AC motor,
it will start to pull too much current and possibly overheat. There are
motors that are designed to work with triac devices.

The variac is a true variable voltage transformer. Its output will
remain to be a proper sine-wave through its total control of its output.
This way, true AC devices will work properly with it as long as they can
work at the different voltages without being damaged.

--

Jerry G.


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Re: SCR speed control oddity ?

:What you call an SCR is really a Triac or Dual Thrysistor. An SCR is a
:1/2 wave device, while the Triac is a full wave device, which is
:equivalent of having two SCR devices back-to-back.

You would be aware, no doubt, that some speed controllers use a bridge rectifier
AND a thyristor (SCR) for full wave control. It doesn't necessarily use a triac.

eg. http://home.maine.rr.com/randylinscott/aug99.htm

Re: SCR speed control oddity ?
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rectifier
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triac.

Interesting - I wonder why? It surely can't be cheaper that way can it?
Is it easier to make the output symmetrical perhaps?

BTW, that circuit has a DC output. It's possible to control an AC load
using an SCR and a bridge, but not this way. It'd also make some nice
smoke if you cranked the pot to the top - 100 ohms is way too small,
there's no need for half an amp of gate drive. That pot is already
dissipating 1 watt, way too much. In short, that circuit is stuffed.

Clifford Heath.

Re: SCR speed control oddity ?

"Clifford Heath"

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**  You really are a full on wanker.

The SCR will fire long before the gate current reaches 0.5 amps -  after
which the two 100 ohms resistors have only a small voltage imposed.




......   Phil




Re: SCR speed control oddity ?
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Ok, yes, but the 10K pot still has 10mA passing through it, and 1W is
to much for most pots.

And the circuit still doesn't put out a voltage that suits a motor
that doesn't like phase control (I've experienced this too with a
couple of motors I have).

Re: SCR speed control oddity ?

"Clifford Heath"
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**  Nope.

  The situation is non simple



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**  Yawn  -   so chalk is not cheese.

  You are still an unmitigated wanker.




........  Phil






Re: SCR speed control oddity ?

:> You would be aware, no doubt, that some speed controllers use a bridge
rectifier
:> AND a thyristor (SCR) for full wave control. It doesn't necessarily use a
triac.
:
:Interesting - I wonder why? It surely can't be cheaper that way can it?
:Is it easier to make the output symmetrical perhaps?

The SCR was invented back in the early 1960's but the triac came on the scene
long afterwards. The SCR was seen as a convenient, if not ideal, device for a
compact speed control system for that time.
:
:BTW, that circuit has a DC output. It's possible to control an AC load
:using an SCR and a bridge, but not this way.

The circuit was one example only of a typical SCR motor speed control. Almost
all small electrical appliances use a "Universal Motor" which can run on either
AC or DC
http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId02%nQXGrrlPbFqM

The first SCR universal motor speed controller I came across was in a GE
Thyristor Applications booklet published in 1967 and it used an SCR connected in
series with the motor and the AC supply (with the appropriate gate triggering
arrangement). It didn't bother with full wave control and this is possibly why
some motors don't function very well with this type of controller.

 It'd also make some nice
:smoke if you cranked the pot to the top - 100 ohms is way too small,
:there's no need for half an amp of gate drive. That pot is already
:dissipating 1 watt, way too much. In short, that circuit is stuffed.

Do some research....

Re: SCR speed control oddity ?
That is also very true. Most of the ones that I have seen work with a triac.

--

JANA
_____


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Re: SCR speed control oddity ?

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ROTFLMAO



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