VFD for multi-motor machine

I have a Brown and Sharpe 618 surface grinder and a number 1 universal grinder.

I'm trying to work out 3 phase power to these machines.

My original plan was to make a big rotary converter (I already have a 10 hp motor and the contactors and the caps). Of course, the 618 is wired for 440v and I'd have to reconfigure for 220. Don't know about the universal grinder, but I imagine it is 440.

Now- I may have an opportunity to get a 20hp VFD. I initially was thinking I could simply use it in place of the RPC- have it generate my 3 phase (it can put out 440) and keep the starting circuits intact on the machines.

But I've read that VFDs are not happy when they are putting out voltage and their load changes. They like to be hooked to a motor constantly.

Now, I could configure a drum switch to go between the 2 machines and bypass the mag starter circuitry. However, lets say I hooked up the main 2 motors (spindle and hyd pump) on the 618, there is a 3rd motor for the head raise/lower and that is used intermittently.

On the universal grinder, you would want to control the spindle and grinder motors separately. And I'd want to simply use the existing mag starter- so this would be bad.

It sounds like this is just a bad idea for these machines. What do you think?

I run my surface grinder from a single VFD. 2hp wheel motor, 1hp hydraulic pump, 0.3hp coolant pump. all switched independantly after the VFD. The 2hp Mill and another 2hp lathe will soon be added to this setup. The VFD is approx 24hp rating and I have a 4hp idler motor continuously connected across the output to ensure the VFD always sees an inductive load and to soak up any possible transients. I haven't had any problems in over a year of use and don't anticipate any.

good news

Thanks for replying. This is good news, and that is a great idea to add an idler to smooth things out a bit.

I did get the 20hp vfd I described (Yaksawa GPD 506/P5). Alas, it is the model rated for 440vac in and out. I think my 618 micromaster surface grinder is wired for 440. But I reckon I'll need a step-up transformer to power it from 220 single phase.

Cheers

The complete setup for mine includes a single phase 240/110 transformer on the input, wired as an autotransformer to give 365V from my 245V mains. This takes the VFD past its minimum acceptable input voltage. On the output, I have a 290/415V three phase isolating transformer to give the 415V that the machines need. The star-point/wye-point/neutral (pick your dialect) of the output transformer is earthed for safety. The voltages between the VFD and the output transformer are a long way away from ground due to the input transformer, but don't get out into the rest of the shop where they could cause worries.

Total cost of the setup was £10 for the VFD, £10 for the output transformer, £30 for the idler motor and the input transformer came out of our computer room at work for free when we rebuilt it.

Wiser heads than my will have to rule on the isolation Xformer between the VFD and the motor but I think it's recommended against.

Forrest Addy said:

Wiser heads than my will have to rule on the isolation Xformer between the VFD and the motor but I think it's recommended against.

Click to expand...

In this instance, it's required for two reasons:-

• As an isolating transformer so that the output neutral can be tied to ground for safety reasons (it would be close if it had a proper three phase input, but then I wouldn't need it).
• To raise the output voltage to the required level. The VFD could cope with that (it's rated at 650V) but I didn't have the right transformer to step up the input to the right level.

I don't have much invested in it, so I'm not out much if it eventually dies. But over the last year it's taken everything I can throw at it.

I think the main point of the argument is that there is no fundamental reason that a properly designed invertor can't accept disconnection of the load. They often really don't like leading power factor loads. But that's why line reactors are available as accessories.

This is also why the idler motor on my setup is on the VFD side. It protects the VFD from the parasitic capacitance of the transformer windings when unloaded. A three phase line reactor would be quieter, smaller and more efficient than the motor. But I haven't got one

Oops, double posted while editing

Mark, have you tried running your set-up without the idler motor? Typically VFD's don't like transformers on the output due to the low impedance, but in your case it may be working because of the idler. Interesting...

Lauffer. Interesting indeed. VFD feeding an idler motor and a step up transformer. All no-nos according to traditional wisdom. Thus the tension between the shade tree which I generally ridicule and the orthodox which I generally advocate.

It's a classic case where the rule breakers seem to fumble around and someting succeed in spite of dire prognostications. Not always but sometimes. Tkae tracotor pulling; one of the more useless of human activities consuming the substance of many for the delight of a few with noisy spectical and the promise expenssive parts turned into shrapnel. You'd this would be a mere HP race with a hilicopter turbine powered conglomeration being the winner every time but no. As usual there's a zillion tiny variables where the race goes to the combination best adapted to circumstances of the moment.

Same with electrical orthodoxy. Sometimes the damndest combinations win the trophy leaving egg on the face of the touts and experts. I hate it when it's my face as it often is. Here's to the shade tree guys I sneer at. You can't argue with success so long as you succeed and I don't embarrass myself. .

Actually, step-up transformers on VFD outputs are not that uncommon on larger drives (100's of kw to MW). The impedance of the transformer will not be lower than that of an appropriately sized induction motor. The idler motor is there specifically to prevent the VFD from seeing a leading power factor (bad news for power GTO-SCRs due to localised over current at turn on time). Being in parallel with the load, it also should help to absorb turn-on and turn-off transients.

One specific issue with having a transformer on the output is that the iron losses are considerably higher than normal due to the messy output waveform. The transformer does run quite warm as a result. But if I can (almost) hold my hand on it, I reckon that it's going to be within class F temperature rise limits with moderate load without forced cooling.

When I can free up the dual channel storage scope for it's current duty of holding up one end of a bench I need to see if I can generate some pictures of what is going on.

Bear in mind that I'm not completely shade tree. My degree was in electrical engineering and electronics and I served my apprenticeship with GEC Large Electrical Machines. I'd still be with them if they hadn't laid off over a thousand men at my plant the year I graduated (we were very dependant on British Steel and the National Coal Board for much of our output). So I take some of the urban myths about VFDs with a pinch of salt.

If youre running an idler whats the point of using a VFD?

If you are trying to use the VFD for its intended purpose of variable speed you may have issue with the transformer since it will be only rated to run at your native line frequency.