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'[EE] Dimmer control for synchronous motor'
2012\10\02@203000 by IVP

face picon face
Way back when, I wrote

> Sent: Monday, April 02, 2012 4:35 PM
> Subject: Re: [EE]:: Ceiling fan speed control using simple
> TRIAC phase control.

> The latest Silicon Chip has a single / 3-phase induction motor
> controller using the STGIPS20K60 and a dsPIC

I eagerly awaited the kits. And waited. And waited

As of today, 6 months on, the latest news is

- Hi Joe,
- There were few issues with the first batch so they are being
- reviewed, I have no set date for the new units
- I am unsure about what the issues were. All I got told for the guys
- at Head Office was it would not be sent to stores due to a minor fault.
- I have been told however that the new batch is being tested by
- Silicon Chip in Aus at this very moment.

So, having now a distinctly uneasy feeling about this SC project,
I'm looking at an alternative. Maybe it'll be temporary, maybe not
if something like a dimmer circuit works. The SC project does have
the advantage of being speed control via variable mains frequency
and that's why I'd prefer it, particularly for the slower speed range,
despite the initial cost. But for now I really need/want to be doing

A quick Google brought this up

My immediate need is for a 230V 120W synchronous scrollsaw
motor. Is this a reasonable circuit to try out ? I have all the parts
to hand



2012\10\02@233244 by Bob Blick

It is worth a try, it may work well enough within the range you need.

But keep watch of the motor temperature, you probably will not be able
to get anywhere near continuous use without overheating.

Friendly regards,


On Tue, Oct 2, 2012, at 05:29 PM, IVP wrote:
{Quote hidden}

> -

2012\10\02@235715 by IVP

face picon face
> It is worth a try, it may work well enough within the range you need


> But keep watch of the motor temperature, you probably will not be
> able to get anywhere near continuous use without overheating

Yes, although the motor has a fan blade on its shaft I do have forced
cooling planned

Another option I may try is to copy what my lathe, drill press and
bandsaw do, and that's have pulley 'gear boxes'. So for example I
can reduce blade speed without changing the motor speed, with the
option of motor speed control too. eg 1:5 gearbox reduction and
only 1:2 motor speed reduction -> 1:10 overall. But all that entails
rebuilding the scrollsaw, which is probably OTT


2012\10\03@011849 by Perry Curling-Hope

picon face
This does work.

I have applied a similar circuit, replacing the control potentiometer with
a PWM output generated by a PIC.
The speed is monitored and tracks a digital setpoint on a display.

This is a power controller, not a 'speed' control as such, and the power is
reduced by phase shifting the triac firing point in relation to the zero
crossing point until the synchronous motor cannot hold lock and starts
This produces a humming sound and excess heat, which becomes progressively
more pronounced as speed is further reduced.

Crude indeed, but acceptable if the control range required is sufficiently

The only 'proper' way to vary the speed of a synchronous motor is with a VFD

On Wed, Oct 3, 2012 at 5:57 AM, IVP <> wrote:

{Quote hidden}


2012\10\03@015222 by IVP

face picon face
> This is a power controller, not a 'speed' control as such

Yes, unfortunately the simplicity of the design does mean that
it's just reducing the amount of energy available and the load
will affect the speed. I'm hoping that loaded and unloaded
speeds won't be too problematic. Cutting fine detail in thick
wood for example


2012\10\03@105257 by RussellMc

face picon face
You can buy low power VFDs at what may be acceptable prices - probably 2nd
hand on "trademe".
Nothing there at a good price at present.

$150 if nobody else bids -  2.2. kW motor and drive


2012\10\03@181336 by IVP

face picon face

> $150 if nobody else bids -  2.2. kW motor and drive

Sounds a bargain, but I wonder if the speed control is mechanical,
like a continuously variable transmission, rather than electronic. The
housing suggests CVT


As you'd expect, tools in that price range are not comparable
to those around $1000+, like the Excalibur. You can put up
with a lightweight drill but a saw with no 'substance' is like
an under-powered soldering iron. Although mine was given
to me and is of the lesser variety, I've beefed it up (heavier
plate, bolted to a concrete block, good blades and adaptors
etc) so it feels and behaves more like an expensive one, in
some respects better

AFAIK all scrollsaws have dimmer speed controls BTW.
Some are OK, some aren't. The fiasco with the Silicon Chip
VFD is very exasperating, I really was looking forward to
having one. I've got a big DC motor that my eye wanders
to every now and then, would make a good scrollsaw drive
and speed control would be a snip


2012\10\03@204414 by IVP

face picon face
Are MT1 and MT2 of a triac interchangeable ?

I see circuits with either MT1 or (more often) MT2 to phase. I
don't use triacs a lot, but do recall making a heater control (an
immersion element) that didn't work with MT1 to phase but did
with MT2 to phase. Gate control was DC from a CMOS gate,
a 4017 I think. Are the positions of MT1 and MT2 less important
if gate control is AC, through a diac for example ?

When I've used low-power optotriacs like MOC302x to drive
triacs, I've never had a diac. Gate current is AC via a resistor
from phase through the opto's triac

In this document

Thyristors & Triacs - Ten Golden Rules for Success In Your Application

MT2 is to phase, as it is here

with note that filters should be referrred to MT1, likening it to the
'cathode' of an SCR, implying that MT1 is to neutral

In the previous version of his dimmer

the author has MT2 to phase, but in the revision of my original
post MT1 and MT2 are not identified

I'm asking for general knowledge and so that I can make a
small controller PCB



2012\10\03@205240 by Brent Brown

picon face
On 4 Oct 2012 at 13:44, IVP wrote:

> Are MT1 and MT2 of a triac interchangeable ?

>From memory there are 4 "quadrants" of TRIAC triggering...
MT1 +ve, MT2-ve - Gate +ve
MT1 -ve, MT2+ve - Gate +ve
MT1 +ve, MT2-ve - Gate -ve
MT1 -ve, MT2+ve - Gate -ve

All are viable, some are more sensitive than others, eg. lower gate current required
-- Brent Brown, Electronic Design Solutions
9 Titoki Place, Pukete,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069

2012\10\03@212507 by Bob Blick


On Wed, Oct 3, 2012, at 05:44 PM, IVP wrote:
> Are MT1 and MT2 of a triac interchangeable ?

No, because gate is referenced to MT1. Required gate current is
different depending on polarity, both absolute and relative, but is
lowest when gate polarity is the same as MT2.

Best regards,


{Quote hidden}

-- - Access all of your messages and folders
                         wherever you are

2012\10\04@025748 by Perry Curling-Hope

picon face
Regarding MT1 and MT2

Being a bidirectional device, it does not matter whether 'Phase' (Live) or
'Return' (Neutral) is conected to either, or on which side the 'Load' is

It does matter to which terminal the gate signal to 'fire' the triac is
referenced, being MT1 as Bob has pointed out.

A diac is only appliedf it's characteristic breakover voltage at which it
starts conducting is being used to derive the triac firing point at some
interval after the phase crosses zero.
This is usually established with an RC network which causes a phase delay
dependant upon a potentiometer resistance, as in a budget designed 'Dimmer

When controlling the triac via an optocoupler such as the MOC20xx, which is
a 'random phase' device, having no internal ZCD, the zero crossing point of
the mains phase will need to be established by an external zero crossing
detector, which will then serve as a timing reference from which a phase
delay may be generated.

Optocouplers such as the MOC30xx are essentially ON/OFF devices, the triac
beng fired at ZC if the LED in the opto is on (100% phase switched) or
never fired if the opto is off.
'Control' is effected by some or other sensor such as a thermocouple being
used to alter the ON time to OFF time ratio, as in the case of a heater


On Thu, Oct 4, 2012 at 3:25 AM, Bob Blick <> wrote:

{Quote hidden}


2012\10\04@173733 by IVP

face picon face
Perry, Bob, Brent, thanks

While I'm in the mood I might have a rummage through the
new-fangled triacs as well


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