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| Dave J.... |
 Posted: Tue Apr 07, 2009 9:04 am |
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Guest
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I wonder if anyone here could provide some hints on the subject of the
reactive components used in (commercial examples of) motor speed
controllers.
The methodology I'm thinking of is that of using a phase controlled triac
as per a light dimmer. While this works quite well for medium speeds it
obviously falls down as the speed is reduced below a certain level and the
motor's rotation becomes jerky.
I know I cannot hope for smooth rotation all the way to zero but I have
experimented with commercial units and they maintain it to a much slower
rate than my attempt.
The only difference in design (beyond that the trigger circuit in mine is
an experimental unit pointlessly capable of 100 percent variation) is the
capacitors and inductors in the production unit. I know I could try a
verbatim copy of their circuit but, even though I may end up with
identical values, I consider it desirable to understand the function of
the components before creating my own equivalent.
Thanks for any hints, I'm puzzled as to how the circuit can maintain the
smoothness despite obviously reducing the available power. It is certainly
triac switched, so can only be 'on/off'.
Thanks,
Dave J. |
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| Dave J.... |
| Posted: Wed Apr 15, 2009 8:10 am |
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In MsgID<grfmft$ja2$1 at (no spam) news.datemas.de> on Tue, 07 Apr 2009 15:04:13 +0100,
in sci.electronics, 'Dave J.' wrote:
Apols for following up on self.
[quote:2bf09cc8e6]I wonder if anyone here could provide some hints on the subject of the
reactive components used in (commercial examples of) motor speed
controllers.
[/quote:2bf09cc8e6]
[a phase controlled triac, equiv to a light dimmer circuit, controlling
the rotational velocity of an AC motor]
[quote:2bf09cc8e6]The only difference in design (beyond that the trigger circuit in mine is
an experimental unit pointlessly capable of 100 percent variation) is the
capacitors and inductors in the production unit. I know I could try a
verbatim copy of their circuit but, even though I may end up with
identical values, I consider it desirable to understand the function of
the components before creating my own equivalent.
[/quote:2bf09cc8e6]
Really no one here who can give me a clue or two?
I've done the obligatory google searching but I seem to be using the wrong
terms as all that comes back is either designs for phased controllers that
have no reactive 'extras' (a simpler but effectively identical version of
my experiment) or A level stuff on the physics of impedance.
All I'm after is a hint on how the capacitors and coils reduce the
'jerkiness' caused by the triac (with no more than simple light dimmer
style triggering) at low power settings.
I'm not completely ignorant, I understand how reactive components have
impedance and I'm comfortable enough designing tuned circuits and hp/lp
filters at RF. I just cannot get my head around the way that the coils and
capacitors in the commercial equivalent of the motor-speed controller I've
built seem to allow it to drop the rotational speed to a much lower rate
before the motion becomes unacceptably staccato.
I can see that there is a technique that works, because of the empirical
demonstration of the unit I studied allowing quite a low minimum speed
before jerkiness intercedes. I just cannot see how.
Dave J. |
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| John Jarman... |
| Posted: Tue Jun 02, 2009 3:54 pm |
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Guest
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On Wed, 15 Apr 2009 14:10:16 +0100, Dave J. wrote:
[quote:f3b6f66063]In MsgID<grfmft$ja2$1 at (no spam) news.datemas.de> on Tue, 07 Apr 2009 15:04:13
+0100, in sci.electronics, 'Dave J.' wrote:
Apols for following up on self.
I wonder if anyone here could provide some hints on the subject of the
reactive components used in (commercial examples of) motor speed
controllers.
[a phase controlled triac, equiv to a light dimmer circuit, controlling
the rotational velocity of an AC motor]
The only difference in design (beyond that the trigger circuit in mine
is an experimental unit pointlessly capable of 100 percent variation) is
the capacitors and inductors in the production unit. I know I could try
a verbatim copy of their circuit but, even though I may end up with
identical values, I consider it desirable to understand the function of
the components before creating my own equivalent.
Really no one here who can give me a clue or two?
I've done the obligatory google searching but I seem to be using the
wrong terms as all that comes back is either designs for phased
controllers that have no reactive 'extras' (a simpler but effectively
identical version of my experiment) or A level stuff on the physics of
impedance.
All I'm after is a hint on how the capacitors and coils reduce the
'jerkiness' caused by the triac (with no more than simple light dimmer
style triggering) at low power settings.
I'm not completely ignorant, I understand how reactive components have
impedance and I'm comfortable enough designing tuned circuits and hp/lp
filters at RF. I just cannot get my head around the way that the coils
and capacitors in the commercial equivalent of the motor-speed
controller I've built seem to allow it to drop the rotational speed to a
much lower rate before the motion becomes unacceptably staccato.
I can see that there is a technique that works, because of the empirical
demonstration of the unit I studied allowing quite a low minimum speed
before jerkiness intercedes. I just cannot see how.
Dave J.
[/quote:f3b6f66063]
SCR soft-starts and motor controls, as far as I know, use regular phase
control for controlling the motor voltage.
The capacitors and coils in your commercial unit are almost certainly for
RFI filtering, as the very sharp current delta that occurs when the SCR
switches on causes a lot RFI to be radiated back into the mains supply.
In other words, these components probably don't affect motor performance.
The 'jerkiness' sounds to me like a problem with the triac occasionally
not switching off at the AC zero-crossing. Motors are very inductive, and
as such there is probably quite a lot of reactance in the system. This
reactance only has to create enough imaginary power to maintain current
above 0 through the zero-volt crossing, and the triac will fail to turn
off for a whole half-cycle. If this occurs frequently enough, the motor
will be 'jerky' due to the changes in average voltage, and the effect
will only be visible at low speeds due to the motor's low rotational
inertia failing to smooth things out.
The solution is to include a snubber across the triac - usually some R-C
contraption.
Of course I could be wrong and the jerkiness has another cause. The only
way to be sure is to 'scope up the outputs of each controller and look at
the waveforms.
Hope this helps,
John |
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