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View Full Version : Probably a stupid thought - VFDs, Transformer & switched load



m_c
15-09-2013, 07:50 PM
This is more than likely a stupid idea, but I'm trying to think of a reason why it wouldn't work.

If you were to connect a VFD's output to an isolating transformer (probably via a sine filter aswell), what is then stopping you from having switched loads on the secondary side of the transformer?
The transformer would mean the VFD outputs never suddenly go open circuit, but could the VFD handle the sudden surge current?

The reason I'm asking, is I've been looking at upgrading/replacing my rotary phase converter, and I'm intrigued by the newer digital options, especially since I found a picture of one that appears to include a bog standard VFD.
I'm thinking feed 240V single phase in (there are VFDs available in double KW figures that will convert 240V single in 240V three), then connect the output to a step-up transformer to get 415V.

Am I missing something?

EddyCurrent
15-09-2013, 10:16 PM
This would be problem
Using a power transformer at a frequency it wasn't designed for | EDN (http://www.edn.com/design/components-and-packaging/4369085/Using-a-power-transformer-at-a-frequency-it-wasn-t-designed-for)

m_c
15-09-2013, 10:43 PM
That would apply if I was changing the frequency, however for this application the frequency would be fixed at 50Hz when powered, although it would need to ramp up/down when powered on/off, but that could be set to a minimal amount.

EddyCurrent
15-09-2013, 11:08 PM
Transformer VS output filter in VFD and motor connection (http://www.acdrive-china.com/blog/transformer-vs-output-filter-in-vfd-and-motor-connection-569333.htm)

Some of the ideas posed in forums etc. have not been tested widely because in industry for example there would be no requirement as all the required voltage supplies and associated equipment are available.
I've used inverters with output chokes for long cable lengths but have not seen them used with a step up transformer, but that doesn't mean to say it's not done.

m_c
16-09-2013, 01:04 AM
You must of used similar google terms to me, as I've already seen that page.

Ideally I'd like to get my hands on one of the digital phase converters, but I really don't want to pay the price tag to get one!

BillTodd
16-09-2013, 07:27 PM
The vfd is not a three phase supply. It is designed to drive a motor only.

If you add in a transformer into the mix , it change the phase of voltages and currents that the vfd monitors and controls .

If you're looking for a simple answer to powering a 3ph machine from single phase then a rotary converter would probably be your best option.

Bill

EddyCurrent
16-09-2013, 08:34 PM
The vfd is not a three phase supply. It is designed to drive a motor only.

If you add in a transformer into the mix , it change the phase of voltages and currents that the vfd monitors and controls .

If you're looking for a simple answer to powering a 3ph machine from single phase then a rotary converter would probably be your best option.

Bill

This is even more true with a vector drive as it 'talks' to the motor as it were to fine tune it's parameters and estimate the rotational state of the rotor.

m_c
16-09-2013, 10:25 PM
I already have an RPC, it's just that I'm considering an upgrade, and I'm curious as to how certain companies are creating digital converters with what appears to be a bog standard VFD.

Jonathan
17-09-2013, 01:01 AM
Looks like you're question has been answered - it's not going to work, at least not directly.
A 'vector drive' (i.e. one that uses the field oriented control algorithm) senses a number of the phase currents and uses these to infer the position of rotor, it's field and the stator field. The controller then frequently modifies the currents to ensure the stator and rotor fields are always perpendicular, as this gets the most torque. It's hard to sense the motor phase currents accurately with a transformer in the way... There are plenty of other reasons not to do this, e.g. the transformer adds another time constant to the system.


I'm curious as to how certain companies are creating digital converters with what appears to be a bog standard VFD.

The basic power electronic circuit to convert from a fixed frequency supply to a single one is just a 3-phase bridge generally made from IGBTs. This circuit is used in all sorts of appliances, what differs is how it's controlled, so take a 'bog-standard' VFD, remove the control circuit and make your own, then (within reason) you can put whatever you want on the output. That's why for instance you can now get motor drives which can control induction motors and synchronous motors - the basic circuit inside them is the same, it's the control that differs.

bikepete
17-09-2013, 11:45 AM
Ignore this, didn't read the OP properly...