Single phase to 3 phase convertor

[robgraham]

I've spent most of today attempting to get one of these to drive a wood turning lathe with a 3 ph motor and wonder if anyone here has any experience of these devices and can help me.

The problem I found was that if I wired the convertor straight to the motor it would work fine, but if I connected it though the lathe's supply terminals such that the power to the motor is switched by the lathe's on /off switches and contactor, the convertor appeared to be unwilling to work with the low current demand of the contactor coil (1200 ohms) - the output voltage of the convertor was low and unstable and the coil would buzz but not pull in.

Are these convertors like switching regulators in that they require a minimum load to actually work? I could just bypass the lathe switch wiring but there are two safety microswitches in the circuit and the on/off switches are a preferable size to those on the convertor, let alone a better position when using the machine.

Thanks Rob

[Tim S]

robgraham coughed up some electrons that declared:

Maybe. If so, could you add some dummy load to the converter's output, say 3

60-100W lamps between each phase and neutral?

Cheers

Tim

[cj]

Hi Another possibility may be to drive the motor from the inverter directly ,as you say this works . Then swap out the contactor coil for a 240v one and use the contactor to drive the converter.This should then allow the safety switches (usually emergency stop )to function as they will control the contactor. Unless of course the switch is the reversing type with 2 contactors.

HTH

CJ

[meow2222]

why not put the invertor between the switches and the motor insead of before the switches.

NT

[dom]

It's not clear which kind of converter you mean - I'm assuming you mean a VFD?

most important thing to understand is that the output of a VFD is

*nothing like* a normal sine wave. It's designed to drive a 3 phase motor and that's it - there should be a (very short) direct connection and you should not try to switch it in any way. In fact manufacturer data may well worn you *not* to disconnect the load.

(Remember a VFD fires full voltage narrow pulses from it's output - but varies the width or density of these pulses to produce the equivalent drive to sine waves. *Most importantly* remember that the output *will not show up* on a conventional voltmeter. Also remember it will probably be necessary to change the motor being driven from star to delta configuration)

However as VFD's are so commonly used as retrofits on machinery, the control inputs are usually designed to hook up easily to existing motor control.

I added an Omron VFD to a 3 phase Startrite bandsaw - the original incoming mains 3 phase wiring was disconnected from the contactor (and the wiring rerouted to carry incoming single phase direct to the VFD), the output of the contactor to the motor disconnected (and rerouted between motor and VFD) and finally using the manufacturers data the momentary switch part *only* of the contactor is used as an "on" signal, whilst the momentary off push button is daisy-chained with the safety interlocks so any one of them momentarily opening is the "off" signal to the VFD.

[Andy Burns]

Is there anything different about the output of these, such that it

*isn't* a simple case of swapping any two of the phases to reverse the motor?

I ask because my father has/had one driving his pugmill, the motor ran backwards, so he swapped the phases to the motor and it promptly blew what he described as a transistor in the VFD. Should he have swapped the phases on the input to the VFD instead of the output?

[robgraham]

Many thanks guys, Dom particularly. I'll now need to check the contactor and convertor to see if they have the same capabilities as Dom's system has.

The one thing I must admit I didn't try was to see if the contactor coil works of single phase 240v - it is currently wired between two of the incoming phases. I think it is reasonable for it to do so.

I didn't check the wiring of the motor but on the basis that the current incoming cable doesn't contain a neutral, I'm assuming the motor is star connected - anyway it ran when directly connected to the convertor, with the speed and direction being controllable.

Many thanks Rob

[dom]

No - swapping any pair of phases on output should be fine. After all - the motor coils are just coils.

[dom]

You probably want "3 wire control". My device had countless modes and options, but came pre-configured for 3 wire control, as I guess it's the most common requirement.

[Andy Dingley]

What do you mean by "converter"? A solid state inverter? Yes, these won't be happy driving a contactor coil alone. Your best bet is to talk to the inverter manufacturer and rig up some sort of "integrated" contactor switching (they'll have a standard circuit).

You can also run the contactor coils and switchgear from 240V single phase, or 50V, or whatever else you have handy. No reason at all why it has to be the motor supply. This assumes that the inverter is happy if left powered up, merely with its load being switched in and out.

Personally I prefer rotary converters, recycled from big old 3 phase motors.

[robgraham]

manufacturers of the convertor and they pointed out that trying to operate the system in the way I was doing - ie just firing 3 phase from the convertor into the 3 phase leads of the lathe would in all likelihood damage the convertor. If I had got the contactor to close, the motor would not have been ramped up by the electronics and the instant load would have quite possibly killed the black box!

So yes I now have to look at operating the thing in Remote mode.

Thanks all

Rob

[Andrew Mawson]

unstable

Most static 3 phase converters (rather than inverters) generate an 'artificial leg' that relies on the motor load to actually produce the voltage by transformer action as it starts to spin on two phases. If you happen to have used this phase for the control electrics then things don't start to happen - as you note.

Rotary converters usually are simply static one with a pony motor added to avoid the problems noted above and also allow the use of a wider range (particularly smaller) motors without over volting one phase (typically the small suds pump motor of a lathe)

AWEM

[Tim..]

You should not interupt the convertor's output before the motor, as you are attempting with the lathe's inbuilt controls. As you say they must see a balanced load across the 3 phases "to work properly".

Re-wire the lathe controls as remote's for the convertor on/off - the instruction book will tell you how.

Tim..

[robgraham]

Tim - I didn't actually mention a balanced load, but I can see that that might be a requirement. In fact I'll put that as a definite question as I did have thoughts of using one of these to vary the speed of a single speed induction motor - is that not going to work ?

Rob