# Strange VSD?

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### #1 Guest__*

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Posted 16 April 2005 - 06:50 PM

Anyone seen one of these ?

It is a 30year old italian speed controller

The motor is standard 3 phase 6 wire 415Vac (for delta connection)

### #2 marke

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Posted 16 April 2005 - 08:13 PM

This would be a voltage controlled system using SCRs to control the voltage to the motor.
This requires that the motor has a high resistance rotor and is usually only found in integral fan motors.

Do not apply this method of controll to a standard indcution motor. The excess slip at low speed will cause the motor to overheat.

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### #3 jraef

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Posted 16 April 2005 - 08:21 PM

I doubt that this is a variable frequency drive, it is probably a solid state soft starter. Although your internal motor wiring is not complete on your sketch, that would be how it would appear to be connected on the outside. It is a way of saving cost / size of the soft starter by applying it to only a portion of the motor windings. The soft starter can be sized as 58% of the motor FLA instead of 100%. If it were a variable voltage controller you may be able to get a small amount of speed control, but you would be doing so by decreasing torque while increasing current, so it would not be suitable for most applications.
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### #4 marke

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Posted 16 April 2005 - 08:27 PM

Hi Jeff

Did you pick up the circuit error?
The circuit can not actually work as drawn as the windings go back to the same phase!! Just a small slip.
Interested in your comments about the 58% for soft starters in six wire connection. If you do a junction temperature analysis, you find that because the rating is determined largely by the starting current, and the conduction angle during start is about half, you have a higher current for a shorter period of time, the shape factor is quite diferent from a three wire profile and you have to rate the six wire starter at very close to 66% rather than 58%.
Sorry, a little off topic but worth a note.
Best regards,

### #5 Guest__*

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Posted 17 April 2005 - 09:39 PM

Hi

The drawing is actually correct, i am guessing that the phase swops somehow in the speed controller

It is definately not a soft start

The unit controls speed up to nameplate rpm

I am not sure as to your suggestion about a high resistance rotor as this is a standard squirrel cage motor with 6 leads ie it can be connect star / delta for normal use on a star delta starter.....

Like i said earlier it is around 30 years old now and of Italian manufacture

### #6 marke

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Posted 17 April 2005 - 09:47 PM

The plot thickens!!

For it to work on standard cage motors, it must be varying the frequency as well as the voltage. In that case, why would it connect back on to input phases?? I can not see any logic here at all.
I could certainly understand a variable voltage fixed frequency with a high resistance rotor, but with a standard A or B rotor, you would be in big trouble with straight variable voltage control.

You have my curiosity aroused!!

### #7 jraef

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Posted 17 April 2005 - 09:58 PM

It is possible that if it is an inverter of some sort (30 years ago I doubt it was PWM!), it is possible that the phase relationship is shifted inside of the drive, although if the terminals are marked this way, it sure is confusing. And if it were VVVF control, how would it work to have one frequency reference on one end of a winding, and a different one at the other end? Hmmm... I'll have to think about that one.
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### #8 jraef

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Posted 17 April 2005 - 10:06 PM

How low can the speed be turned down?

If it is strictly a voltage controller, I don't think it needs to be a high resistance motor Marke, as long as the application could handle a sever reduction in torque and the low end of the control range is limited. I have seen unloaded Star-Delta motors run for hours in Star unloaded, but of course they were only putting out 1/3 of their nameplate torque and so could barely keep themselves rotating. Motor cooling would be problematic however.
"He's not dead, he's just pinin' for the fjords!"

### #9 Guest__*

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Posted 18 April 2005 - 08:11 PM

So far i have been told that to run the motor at full speed without the Speed Controller you connect as follows

Phase 1 - U1 & U2
Phase 2 - V1 & V2
Phase 3 - W1 & W2

Baffles the *beep* out of me!!!!!!!!!!!!!!!!!!!!!

I may have some pictures next week

### #10 marke

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Posted 19 April 2005 - 05:19 AM

All I can say is, "that can not be a normal motor!!"

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### #11 Guest__*

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Posted 19 April 2005 - 06:33 AM

Speeed Range is

30 - 1400 rpm

motor has forced draft cooling fitted

### #12 jraef

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Posted 19 April 2005 - 03:43 PM

"Curiouser and curiouser", as Lewis Carroll once wrote...
"He's not dead, he's just pinin' for the fjords!"

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