Transformer Input Requirements

I picked up a 3kva transformer off a gentleman on this forum and am finally getting around to trying to use it. Right now I'm trying to figure out what I need to power the system through the transformer.

The motor I'm trying to power is 1.5hp, 550V, 3 phase, 1.8amp.
Transformer I have is 3kva, primary 570-600-630, secondary 240, Hammond K 3 9M.

I would be wiring the transformer in reverse and would like to know what I need in order to set it all up. There is a vfd close to me that I think is undersized (230V, 3phase, 4amp output) but priced cheaply and I only want to use it for the phase conversion aspect.

The other option is a RPC or static phase converter (or different vfd) but right now I just really need to make a decision and bite the bullet.

Is there a calculation for running the transformer in reverse so I know what input current I need for my desired result or should it be oversized in some fashion?

Thanks
Andy

You didn't say of the transformer is single or 3 phase. If its not 3 ph, then you can't get there from here. If it is three phase, then you are on thin ice. For one thing, you have to control the motor by switching the VFD, not at the motor, which means that the VFD must supply the current to energize the transfomer in addition to the motor.

A transformer can for all purposes be connected either way low voltage in, high voltage out; high voltage in, low voltage out. As for as the current ratios, the volt-amps in equals the volt-amps out. In your case, going back to the VFD, (1.8 amps * 550)/240 volts = 4.12 amps.

Your call

Tom

Sorry. The transformer is three phase. I plan on putting it after the vfd (or other phase converter). I take it from what you are saying that a vfd that small isn't going to cut it?

I'm trying to decide between purchasing a phase converter and rolling my own. If I build or buy an rpc and run it to a breaker panel I'm assuming a 15 amp breaker would be more then sufficient to energize the transformer and run the motor?

Thanks
Andy

Please don't put a transformer on the load side of the VFD. Wiring a transformer "backwards" causes enough issues for ordinary circuit breakers (crazy high magnetic inrush on backwards wired transformers). The results on the load side of a VFD are sure to be overcurrent faults within milliseconds of a start command.

So is a VFD a non-starter if I need to step up the voltage or is it the reversal of the transformer that is cause for concern?

millinCanuck said:

So is a VFD a non-starter if I need to step up the voltage or is it the reversal of the transformer that is cause for concern?

Click to expand...

Transformers are not directional. There is always a small loss in core magnetization and winding resistance so some have the windings adjusted to compensate, but in a simple power transformer they probably are not. They can have large starting surges. There was a discussion a while back and I did some tests. The conclusion was that when the current was switched off, the core would often remained magnetized in the last direction of current flow. It tends to fall back toward zero, but not nearly as close to zero as I previously thought. The result was that if the AC was switched on during the opposite half cycle, momentarily the winding would draw the same current as it would without an iron core and there would be a large inrush until the magnetic pattern was re-established. In other words, don't put a transformer between the VFD and motor. You can transform the power to the voltage you need and feed it to a high voltage VFD, but not the other way around.

You can put a transformer on the output of a lower voltage RPC. In fact, a three phase transformer on the output will probably improve the balance.

Bill

I think you are looking at your problem completely wrong. Why is that you want to keep the 550V 3ph motor? 1.5hp motors are readily available in various voltages and phase configurations and in the frame size you have unless it is some sort of antique with an oddball frame.

If you have only 240V 1ph available, then why not get a 240V motor? If you would do this, then you could run the VFD directly to the motor and everything would be fine and it would run at full power.

The other solution is to get a 550V-240V single phase transformer and then use a 550V VFD to create your 3ph.

No matter what you do, you do not have the components you will need to make this work correctly.

I would prefer to change the motor but I really want to maintain the lathe with the functionality it has. I picked up a Schaublin 102-vm from a government auction and posted about it at the beginning of the year. Unfortunately a death in the family and summer obligations caused me to not pursue it until now.

The 102-vm has a very non-standard frame size, is dual speed, 550V and 3phase. It is bolted to two independent arms so trying to find a replacement that will fit has proven very difficult. I've posted pictures of the motor previously in this sub-forum ( http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/schaublin-102-vm-power-301242/#post2518495 ) but I didn't want to add onto that thread as this is more of a question of what current I need to run this motor through the transformer and if that vfd would work.

Now I'm just tossing around the idea of building my own phase converter or just buying one. Unfortunately I seem to be in a used electrical wasteland (Ottawa Ontario) as I can't seem to find used or surplus stores that I can walk into and look around (there is one in Toronto that I was in previously but they've since shut down).

Thanks again for the feedback guys.
Andy

Do what 9100 says, go with an RPC. That will work and work well.

Tom