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'[EE]:Speed control of Synchronous motors'
2003\10\14@071153 by Simon Davies

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Hi Everyone,

I have a small home project that involves speed controlling a 650W 220V ac
synchronous motor that runs at 1450rpm.

The motor drives a lathe and it would be really useful if I could adjust the
speed of the motor such that:
a) I am not constantly changing the drive belts
b) the speed can be slowed down considerably for more delicate operations
such as cutting threads etc.

I know that controllers and motors are available but they are 'serious'
prices and seem to be intended for industrial use and appear to be 'DC'
types - i.e. sliprings and brushes.

I had thought about taking the 220vac, rectifying it and then feeding it
through a chopper circuit with a frequency varying between say 35Hz and 85Hz
controlled by a panel switch.

So the questions:
Has anyone any experience/circuits/suggestions about this form of speed
control?
Any comments or suggestions about my proposed methods - especially with
respect to the capacitor start mechanisms that are fitted.

Thanks in advance for your help.

Regards,

Simon



Simon Davies

Senior Business Consultant

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2003\10\14@072849 by Ian McLean

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No answers, but a question for you.  Where might be the best place to get my
hands on a synchronous AC motor ?  The spec's you are stating are pretty
much exactly what I am after.  This motor is to drive a syncronous spark gap
for a 1.8kW tesla coil generator.

If you find more info on controlling these motors, esp. via a micro, I would
be very interested.

Rgs
Ian.

{Original Message removed}

2003\10\14@074137 by Jinx

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part 1 454 bytes content-type:text/plain; (decoded 7bit)

A couple of listers helped me with this circuit a couple of years
ago, which I really really appreciate. Unfortunately the customer
disappeared (ggrrrrrrrrrrrr), so it never got built, even though I
got all the parts. Sigh. It was intended for a 65W Brevel motor

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part 2 7062 bytes content-type:image/gif; (decode)

2003\10\14@075346 by Thomas C. Sefranek

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> -----Original Message-----
> From: pic microcontroller discussion list
> [.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU]On Behalf Of Simon Davies
> Sent: Tuesday, October 14, 2003 7:01 AM
> To: PICLISTspamKILLspamMITVMA.MIT.EDU
> Subject: [EE]:Speed control of Synchronous motors
>
>
> Hi Everyone,
>
> I have a small home project that involves speed controlling a 650W 220V ac
> synchronous motor that runs at 1450rpm.

Building a variable frequency POWER source of 650 watts is a NON-trivial
adventure.
I too have a lathe, 3 phase 220 powered, an even worse situation.
I have decided to replace the motor with a DC motor,
and use PWM to drive it at variable speeds.

Good Luck, let us know what you end up with.
{Quote hidden}

 |  __O    Thomas C. Sefranek   EraseMEWA1RHPspam_OUTspamTakeThisOuTARRL.NET
 |_-\<,_   Amateur Radio Operator: WA1RHP
 (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz

hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

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2003\10\14@081108 by Richard Stevens
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Simon,
       This one is 1ph in, variable 3ph out, complete with motor.
cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=2564089363&category=400
03
Thought you might be interested,

Richard.

> {Original Message removed}

2003\10\14@081318 by Simon Davies

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Mine came firmly attached to the lathe ;)

However I would check out anywhere where washing machines and the like are
dumped/re-cycled since these used(?) to be synchronous I believe.

Simon

{Original Message removed}

2003\10\14@082809 by Jinx

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> However I would check out anywhere where washing machines
> and the like are dumped/re-cycled since these used(?) to be
> synchronous I believe.
>
> Simon

My lathe originally had a 1/4HP washing machine motor (which
you can pick up for NZ$10-20), now I've a 2/3HP motor from an
industrial tumble dryer, which cost NZ$25

I'd agree with Thomas re speed control. A DC motor would be
preferable to build a controller for. Even considering the expense
of a DC motor (presumably 12V - you should be able to get a
scrap winch from a car/trucker wrecker, although they tend to get
pounced on pretty smartly) vs the cheap AC synchronous motor

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2003\10\14@083638 by Simon Davies

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Thanks Jinx,

Just a couple of questions
- any suggestions on a value for the photo transistor collectors
- ditto for the R in series with the motor
- where are the photo-diodes driven from?

I prefer to continue with the synchronous motor rather than going down the
DC route especially a 12V version which is going to be rather high current
rated - I will end up building a chunky PSU instead!

Simon


{Original Message removed}

2003\10\14@085747 by Mauricio Jancic

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#include<pic.h>         // So no one get's mad.... :)

I've worked for a company that makes exactly what you are needing. The
real world AC motor speed controllers rectify the AC to a 310 VDC BUS
and then do something like this:

----------------310V
               |
               |
PWM1------FET
               |
               |--------------Phase R
               |
PWM1'-----FET
               |
               |
---------------- 0V

And the same for the rest two phases. Now you will need 6 PWM outputs.
The thing is to generate a sine wave with the PWM output, the high semi
cycle with the PWM1, and the low with the PWM2....

Oh, you must be very carefull not to make both FETs to be conducting at
the same time :)

Best regards


Mauricio Jancic
Janso Desarrollos
Microchip Consultant
(54) - 11 - 4542 - 3519

{Original Message removed}

2003\10\14@095114 by Ken Walker

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picon face
Synchronous motors are controlled by frequency, that's why there called
Synchronous, they sync to the mains frequency and are not effected by
voltage fluctuations. Put a Synchronous motor on a 50v 50hz supply and then
on a 240v 50Hz supply and it will run at the same speed. Do that with a
squirrel cage or induction motor and it'll be slow and fast.

So to control the speed, feed it with a variable frequency and keep the
amplitude constant.


Ken

{Original Message removed}

2003\10\14@124125 by Gaston Gagnon

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Simon Davies wrote:
{Quote hidden}

You might be interested in this OSMC group. As the acronym says, the
group is dedicated to Open Source Motor Controller (Big currents).
At first they have designed a controller for DC motors for which the
schematics is available.

They are now beginning the design of Open Source Brushless Controller.
At this moment they are mostly discussing the brawn circuit. The
processor will most likely be a DSP from Motorola.

http://groups.yahoo.com/group/osmc

Hope this is of interest.
Gaston

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2003\10\14@124125 by Denny Esterline

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I just went down this path a couple weeks ago. Mine was for a small
milling machine I was building.

Ultimately I scrapped the idea, the major stumbling block was the
motor itself. Single phase synchronous motors have a designed
frequency (usually 50 -60 hertz). Operating them more than a few
percent outside this range produces very undesirable results. - gets
hot -

I'm not an expert on the technical details, but as I understand it,
the inductance and capacitance inside a single phase motor generates a
second phase that's shifted. When the freq. is off spec, the shifted
phase is no longer where it needs to be and the motor fights itself.

However, three-phase motors don't have this problem. As I understand
it, that's because they don't generate the shifted phase(s)
internally.

So three phase motors are readily controllable. There are many
industrial control systems for this purpose, many in the sub $200 USD
price range.

If you want to "roll your own" head on over to the Fairchild web site
and read about their "Smart Power Modules" (SPM) some of the
documentation is inconsistent and calls them "Intelligent Power
Modules" (IPM). They also have a -bunch- of app notes on the concept
in general to get you started.

BTW, they sent me a couple samples of 1Kw SPMs for free (normally like
$40 ea)


If you want to go DC, look at a treadmill motor from BG Micro. It's
rated 110V DC and although there's no horsepower spec (or amp draw at
load) it's like 15 pounds, eight inches long and four inches diamater-
so I'm guessing it's in the ball park.

-Denny



{Original Message removed}

2003\10\14@131024 by gtyler

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One thing about that circuit: If none of the opto's are driven then it turns
into a pile of charcoal! So, what happens before the Pic starts?
Also, are you guys not talking about induction Motors. A Syncronous motor
runs at exactly the syncronous speed with no slip and are very rare. Washing
machine, tumble dryer motors are induction motors but are not syncronous.A
syncronous 50Hz 4 pole motor will run at exactly 1500 rpm, if the nameplate
says 1450 rpm then it is probably not syncronous, but a normal induction
motor.
If you run the motor at speeds below the rated frequency you have to reduce
the voltage in poportion to the frequency as the windings are inductors and
as the frequency drops the current increases. Running at speeds above the
syncronous will not be a problem exept that the torque will drop if you
don't boost the voltage.

Regards,
   George

----- Original Message -----
From: "Simon Davies" <sdaspamspam_OUTINFITEL.COM>
To: <@spam@PICLISTKILLspamspamMITVMA.MIT.EDU>
Sent: Tuesday, October 14, 2003 2:36 PM
Subject: Re: [EE]:Speed control of Synchronous motors


{Quote hidden}

> {Original Message removed}

2003\10\14@171606 by Jinx

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> Just a couple of questions
> - any suggestions on a value for the photo transistor collectors
> - ditto for the R in series with the motor
> - where are the photo-diodes driven from?

I can only provide general pointers. I just don't have the experience
to speak with any authority

The BU508AF has quite a low hfe, typically < 5, so (depending on
the motor wattage ?) the resistor must be capable of passing enough
current to turn the BU508 hard on. The opto must be rated for that
current too, for when it grounds the base

The SCR gate resistors depends on the gate current needed. For
the BT151-800R this is around 10mA

The H-bridge was to be controlled with PIC PWM. Provision was
to be made for the PIC to be on before the HV section so as to
get control of the optos

However, I really hate bangs and smoke and have to admit being
more than a little reticent to actually make this circuit. Although I
was talking with people obviously more learned than myself, there
was some disagreement between them. In future I think I'll stick to
DC motors and just turn AC motors on/off. Variable speed
synchronous drive seems like a hell of a lot of work and just not
worth it for a one-off

This is a quote about the series resistor and other parts

======================================================

You need a series R for sure. Measure the DCR or the motor winding
using a DVM and set R equal to that for starters. You will be able to
reduce it to 1/10 to 1/100 of the motor's DCR later. The filter inductor is
not needed imho. However you could use a capacitor instead of it to
pseudo-resonate the motor at the working frequency. This only works
with very small motors and large capacitors rated for mains ac and high
ac current. So imho short out the choke and remove it. R will dissipate
a lot of heat when large. Plan on 20W ceramic body for start. Even
better, read on:

If you have a drive glitch the bridge will self destruct. You will have
drive glitches. The zeners in the thyristor gates are superfluous. They
will never conduct either way. You are using too many parts imho. The
firing scenario of the thyristors is dubious imho. You are driving a coil
so when the opposite NPN opens, the Cathode voltage will drop, but there
will be no current on the thyristor. Only its gate current will flow
through the coil. Later the coil begins to suck current and the thyristor
will fire suddenly. I don't know what this will do to your waveform, but
it will not help it to be a better sinus, for sure. Meanwhile the gate
resistor on the other thyristor has almost 320V at 30mA across it, which
is 10W or so. This for half the time (so 5W). The resistor will need to be
15W probably. The drive circuit alone will likely burn about 15-20W
(efficiency 65-70% ?). A transformer would outperform it imho. When the
NPN turns off the motor L will generate a terrific kickback and continue
to run current through the thyristor and the opposite thyristor's reverse
diode. This current will continue to flow until the L and the mechanical
motor load and eddy currents and Murphy decide it is time for it to stop.
If this is a little bit later than when you open the NPN under it for the
next half cycle, you lose (everything).

Me, I'd have used two (2 pcs) MOSFETs for this in a scheme like so:

        +320V  +-------------(K D1 A)---+--(R2)--<AC1
               |                        |
             |-                         |
             |<+                        |
        G1---|-+                        |
               |                        |
        GND1---+---(M)---(R1)-------------------<AC2
               |                        |
             |-                         |
             |<+                        |
        G2---|-+                        |
               |                        |
        -320V  +-------------(A D2 K)---+

There are two electrolytic caps missing in the scheme between +320V and
AC2 and AC2 and -300V respectively, each rated about 33 to 47 uF 350Vdc.
I'd suggest BUT11 or other such MOSFETs (abt. 3A 800V I think). Make sure
that you get the kind with reverse diodes inside. Drive circuits are as
you please, but must be push pull, like digital optocouplers with CMOS
compatible outputs (pushpull). They need not supply a lot of current
because switching is in voltage (MOSFET) and slow (coil). I've had
success using two small audio transformers with high insulation rating and
a resistive snubber at the gate for this. Try modem transformers. This
removes the need for resistors in funny places in the gate circuits,
zeners, etc., and drives the beasts rail to rail with very little power.

You want to drive this with a 'magic sine' type of signal. Bear in mind
that the motor will kick back the moment you turn off the drive in either
direction every cycle. Using a rather large capacitor in the snubber
across the motor and reducing its series R will help (try 10uF 400Vac and
100R 5W). Your drive would look like:

      +----+        +----+
      |    |        |    |
G1:----+    +--------+    +---- ...


             +----+        +----+
             |    |        |    |
G2:-----------+    +--------+    +---- ...

Where the total Ton will determine the actual motor voltage. The Vef of a
square wave is 1.0 Vpk and is NOT 0.707 Vpk as in a sine wave. If you will
drive the duty cycle higher than that you will burn the motor. So Ton
(total) should be 0.707 or 2 x 0.350 roughly.

Think hard about fan cooling the motor. Its own fan will not be enough
imho. Serious servos and steppers use forced cooling.

hope this helps,

PS: If the motor is tightly dimensioned and hot then it may hit the Curie
temperature or core saturation or both. If this happens there will be a
temporary dead short as a load to your circuit. You need to make sure that
this does not 'detonate' the thing. Fuses are not enough, but a flameproof
R1 helps a lot imho. You can add another in the ac power lead to lengthen
the life of the rectifiers etc. This is R2 in my schematic.
The optos were to be driven with PIC PWM within a limited frequency
range. Mention was made at the time of design, ISTR, about making
provision for the PIC to turn on before the high voltage section so it
could set the state of the optos

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2003\10\15@043404 by Simon Davies

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Gentlemen,

Thank you all for the input, you have certainly given me a lot of food for
thought!

Denny wrote:
-

>Ultimately I scrapped the idea, the major stumbling block was the
>motor itself. Single phase synchronous motors have a designed
>frequency (usually 50 -60 hertz). Operating them more than a few
>percent outside this range produces very undesirable results. - gets
>hot -

Aha, I suspected that something like this might happen...

>I'm not an expert on the technical details, but as I understand it,
>the inductance and capacitance inside a single phase motor generates a
>second phase that's shifted. When the freq. is off spec, the shifted
>phase is no longer where it needs to be and the motor fights itself.

OK.
>However, three-phase motors don't have this problem. As I understand
>it, that's because they don't generate the shifted phase(s)
>internally.

Ok, but I don't have 3ph at my house and it will be cheaper to buy the full
professional controller etc than to get my Dutch electricity supplier to
install 3ph!

>If you want to "roll your own" head on over to the Fairchild web site
>and read about their "Smart Power Modules" (SPM) some of the
>documentation is inconsistent and calls them "Intelligent Power
>Modules" (IPM). They also have a -bunch- of app notes on the concept
>in general to get you started.

>BTW, they sent me a couple samples of 1Kw SPMs for free (normally like
>$40 ea)

Thanks for the references

{Quote hidden}

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2003\10\15@043406 by Simon Davies

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Thanks again Jinx, I am fumbling in the dark as well!

This was very interesting, especially the comments about using FETs which
was my original thoughts.


>However, I really hate bangs and smoke and have to admit being
>more than a little reticent to actually make this circuit. Although I
>was talking with people obviously more learned than myself, there
>was some disagreement between them. In future I think I'll stick to
>DC motors and just turn AC motors on/off. Variable speed
>synchronous drive seems like a hell of a lot of work and just not
>worth it for a one-off

Agreed, especially since the object of the exercise is to make my lathe more
useable not to go into a major development cycle for a one-off!

Perhaps I will just go and look at the contents of the piggy bank and go
down the DC motor route.

Simon

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2003\10\15@044234 by hael Rigby-Jones

picon face
> -----Original Message-----
> From: Simon Davies [KILLspamsdaKILLspamspamINFITEL.COM]
> Sent: 15 October 2003 09:34
> To: RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU
> Subject: Re: [EE]:Speed control of Synchronous motors
>
> Ok, but I don't have 3ph at my house and it will be cheaper
> to buy the full
> professional controller etc than to get my Dutch electricity
> supplier to
> install 3ph!

The idea is to make your controller produce the three phases from your
rectified and smoothed single phase supply, ( using an H bridge
configuration with another pair of devices for the extra phase )  This gives
the ability to control speed without the problems ascociated with induction
motors.

However, I certainly share the feelings about working on projects that are
liable to go bang in a big way if/when something goes wrong!

Regards

Mike




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2003\10\15@071310 by dr. Imre Bartfai

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Hello,

I guess - as another people marked - it is an asynchronous motor. To solve
your problem, find a VFD on e-bay. I just did it and have one perfectly
fits for your problem, current bid is $189. I never would manufacture such
a beast for this price!

Regards,
Imre


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On Wed, 15 Oct 2003, Simon Davies wrote:

{Quote hidden}

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2003\10\15@113829 by Simon Davies

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Ah yes...next time I will read what was 'actually' written and not what I
expected to be written!

Thanks for your comments.

{Original Message removed}

2003\10\15@114453 by Denny Esterline

picon face
No. You misunderstand, you don't need three phase power, just a three
phase motor. The controllers rectify and filter the single phase line
current, then generate three phase with a modified H-bridge design.

-Denny

{Original Message removed}

2003\10\15@165918 by Peter L. Peres

picon face
www.st.com makes a lot of controller chips for motors and it is likely you
will find what you need there. Including basic theory slides on motor
speed control.

Peter

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2003\10\15@170547 by Peter L. Peres

picon face
> Building a variable frequency POWER source of 650 watts is a NON-trivial
> adventure. I too have a lathe, 3 phase 220 powered, an even worse
> situation. I have decided to replace the motor with a DC motor, and use
> PWM to drive it at variable speeds.

Do you plan to use a proper servo (expensive) or one of the new washing
machine drum motors (brushed, 500+Watts sometimes over 1.5kW) ? I am
toying with the washng machine motor idea myself but not for a lathe.

Peter

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