I think you may well need one for each machine ? You would need a heavy duty supply ie 32A Noel.

I think that in effect they are just VFDs, except maybe not variable frequency!

A friend is looking at something similar that makes 380v from 220-240v input, but can go up to 60hz, not just the 50hz as the input voltage. So it replicates connecting to a 3phase power supply 380v 60hz. The one he is looking at is not intended to be variable on the 380v output. He has a Hardinge small milling machine that has a constant torque 2 speed motor.

Is there any more information on the units in your link? My simple search came up with not much. Be nice to look at a user manual for one.

Thanks Neil

Neil

Apart from Phase Perfect and similar devices which are built as industrial 3 phase utility replacement devices with corresponding costs, all these boxes are based on VFD internals so stepping up to 60 hz should be no problem. Certainly the (expensive) ones from Drives Direct will do the deed.

Have to accept that 220 to 380 /420 volt boxes inevitably loose voltage and overload current headroom when compared to normal same input and output voltage devices. But the 50 to 60 hz change will have no noticeable effect.

Clive

Edited By Clive Foster on 13/01/2022 13:12:27

I am using a modified Teco from drives direct sold as a 'digital phase converter' It works perfectly well and has no problems with DOL starting any and all of my three phase machines. before that, I used a very second-hand, unmodified DDanfoss VFD fed from a step-up transformer, which also did the job perfectly well until one of it's current transformers suffered a failure.

Modern VFDs which have IGBT output stages and current sensing to protect from inadvertent overloads cope with this sort of use with no problems whatsoever. The urban myths about needing a separate one for each motor and all the rest of it are just that:- urban myths. They date from the '80s and are no longer relevant.

not done it yet

We seem to be getting at cross purposes here.

There is a vast difference between simply starting a naked motor with only a plain shaft to drive and starting one under load.

If you wish to use one of these VFD boxes as a transparent substitute for mains utility three phase it needs to be able to direct online start a motor under the same maximum load as it can manage when fed proper three phase. My experience suggests that the four times derating is prudent to be sure things will always work.

There can be no dispute that under the right conditions a considerably more powerful motor can be handled but defining right conditions given the myriads of potential applications and motor designs is flat out impossible. Possibly the best that could be managed in the real world is for the supplier to define maximum naked shaft motor and maximum fully loaded motor powers. Not going to happen for all sorts of reasons. If its even practical.

My experience with my 10 HP Plug'n Play box is that the 3 hp motors on my Hydrovane 502 compressor and Smart & Brown 1024 lathe are capable of making the box work very hard. Almost certainly pushing it close to the single motor start up limits defined by the overload protection programming.

Under sub-zero conditions the Hydrovane motor does the rurrm-rooom-rummm sound speed hunt as it struggles to pull through the middle of its speed range. Accompanied by nicely flickering shop lights. I know from experience with a similar compressor running with the motor connected in delta off a name brand VFD that the rurrm-rooom-rummm is right on the edge of overload trip for a properly set-up vector drive VFD without torque boost enabled. That one overloaded out maybe one time in three under sub zero conditions. Enabling torque boost was a complete cure. Not such an issue for the bigger Plug'n Play box, which will have considerably more built in overload margin than the matching 3 HP VFD had, but clear indication things are working hard.

If I attempt to start the 1024 with the reeves drive set to maximum direct drive speed contactor chatter due to volt drop in the line feeding the controls accompanied by workshop light flashing is almost certain. My machine has 220 volt contactors running direct off one phase so clearly there is a good deal of voltage drop. Run up is noticeably slower even allowing for the chatter. Something I don't do unless an emergency stop has bollixed plans.

The one time I inadvertently started the 2 hp two speed motor on the P&W lathe with the clutch engaged the box really didn't like it. Realising my mistake immediately I de-clutched about half way through run up and the motor spun up as normal. Two speed motors are notoriously inefficient during start up so its likely that if I'd just left the clutch in the box would have hit its overload limits.

VFD overload protection integrates current, voltage and time to, ideally, protect the box from letting the magic smoke out if a short circuit, or near short circuit, occurs either at start up or when the motor is running. There is no way of knowing what the programming of any specific box is but when using in this manner we have to be sure that the start up surge won't exceed the programmed limits. In the real world 3 times derating seems to be OK with the Teco boxes used by Drives Direct. If, as I suspect, the boxes supplied by JFK are made for lighter duty applications the overload settings will almost certainly be relatively lower so 4 times derate is prudent if you expect to start a motor on full load.

Obviously if you know you aren't going to draw heavy single motor starting loads then a smaller box will do. But folk like us aren't really in a position to know what the real loads will be and how they interact with the programming of the box we have bought. Easy to underspecify. I'm on 3.3 derate and can clearly get quite close to the edge without being silly.

Clive

A misunderstanding of how VFDs work I think.

A 1HP VFD is designed to run a 1HP motor. The customer doesn't have to allow for surges when looking at ratings because VFD electronics provide a soft start.

The built-in soft-start is one reason why 3-phase to 3-phase VFDs are almost always used these days to drive stop-start 3-phase motors such as fans, lifts and machine tools. By only rationing the motor with just enough power to start it turning and then ramping up relatively slowly, the VFD avoids the massive surge caused when a motor is walloped with power by an ordinary switch or contactor. Managing the surge current is a considerable cost saving, and it means the VFD and associated wiring don't need to heavy duty.

Digital Phase Converters aren't the same as Variable Frequency Drives, Digital Phase Converters are single frequency, outputting whatever the input is, 50 or 60Hz. Their wiring is similar to the Idler Motor method of creating a third-phase, but instead of a noisy, wasteful thicko motor the third phase is generated electronically.

The first two phases are provided directly by Line and Neutral. The single-phase input is also rectified to charge a large capacitor with high-voltage DC, which is then converted electronically to imitate the third phase. In effect the phase of the Line Input is delayed by 'n' degrees to create the third phase. Roughly like this:

Two of the phases are directly connected to the motor, so there's nothing to get hot or go wrong with them. The third phase is more complicated, because it needs a hefty DC power supply, and beefy electronics to switch the output in time with Line frequency, but later.

The electronic phase shifting would be smarter than my diagram implies. Probably a microcontroller monitors all three phases and actively manages the phase shift for best results on every cycle. As microcontrollers are a bit slow, it may be phase shifting is done by something fast like a field programmable gate array, which would push the price up.

Advantages:

• Should be able to handle high-power up to several tens of HP
• The output is plain 3-phase that can power several machines in parallel as if a real 3-phase supply were available.
• Quiet
• No complicated manual or set-ups

Disadvantages:

• Costly – one phase needs high-power switching and smart electronics
• No soft-start or other energy saving features
• No speed control, jog, reverse, or other VFD goodies.

Whether Digital Phase Converters end up in a home workshop or not depends on cost. In the recent past, they've been expensive, but I see a few units costing less than £1000 are on the web now. It's usual for early adopter electronics prices to start sky high and then plummet as soon as the item becomes ordinary. Maybe these things about to become consumer items. VFDs were Unaffordium not that long ago and now look at them.

Dave

Colin I think that unit would be suitable for your purposes. If as some are stating about heavy load start up, which I doubt you will have then simply fit a soft start unit to that piece of kit.

We fitted soft start units to all heavy compressors at Nestle. The Ingersol Rand Hydrovane compressors had backup Circuit breakers at 150 Amp, so very large units. They were even fitted to all the Air con unit compressors.

Even if you had say a large gearbox in mesh you could unload just for start up purposes

Steve.

SillyOldDuffer

Nice diagram and explanation of a proper digital phase converter. Thats the sort of thing Phase Perfect in the USA make. By far the best solution technically but very much not cheap, inevitably so. Guess they'd be starting at £4,000 + for a 10 hp box if they came to the UK.

Robert

No data, no brand name, is indeed a very big red flag when it comes to power electronics. Drives Direct seem to know what they are doing and produce boxes that work, even if there are various technical breaches of regulations involved.

I take your point about VFD boxes not being designed for long cable runs and the potential for radiated interference. Mine sits right next door to about £800 worth of box containing hefty chokes and other smoothing devices which nicely deals with those issues. Probably also provides a bit more back isolation protection for the VFD from any motor and switching issues.

That said I've always been a bit sceptical as to whether significant high frequency transients can routinely survive being hooked up to a running motor with its considerable mass of copper and iron along with all the magnetic effects associated with being, effectively, a rotating transformer.

Sitting directly driving an open line now is …..

Hello I'm your aerial today, what can I radiate for you! ….

Basic checks suggest that my smoothing box deals well with any potential radiation issues.

Steviegtr

Putting a soft start box in line to help reduce current demands when starting up under load is an interesting idea. Not sure if its viable as an economy soft start box is very similar in price to a simple VFD of the same power rating. Individual VFD boxes are likely to work out cheaper overall unless you only have one nasty load. The economy soft start boxes only work on one phase so there is a risk that the phase imbalance will trigger the overload programming.

Bottom line is that the only affordable source for the electronics needed to make the sort of device we are discussing are VFD boxes. Although not specifically designed for the job some such capability is inherent which can be exploited even if its officially mis-use. So it just boils down to finding something that will tolerate this mis-use to do the job we want at a viable price.

My experience is that a sufficiently husky VFD box with appropriate smoothing can work very well and reliably when used as a substitute for line power. It may not be strictly to code but it is safe and non-interfering. The key is sufficiently husky. Trying to pare down to you exact needs "today" is risky. Ultimately we are using a device out of its specification so it pays to be gentle.

Clive

Bound to happen sooner or later, like those monkeys eventually typing Shakespeare…

Dave

It's unusual but I have to disagree with you a bit Andrew,
VFDs are intended to be permanently connected to a motor and control start-up currents when they are commanded to start the motor. Even if set to maximum speed at turn-on there is some ramp up. Some also increase the current limit during ramp up at maximum rate. If you turn the VFD on and then switch a load onto it the only protection is the over-current. This is a fault condition and exceeding it will trip the drive off-line. Thus it has to be set high and may also be part of the 4x oversize recommendation. Of course setting the current limit high means the motor is no longer protected against over-load.

Starting a smaller motor first has also been mentioned on this thread and even running an un-loaded motor permanently connected is suggested by some sources. Not very efficient.
I'm not making this up I've had a serious look at and experimented with using VFDs as 3 phase converters. Just the amount of filtering required to tame the emissions when running off load was a show stopper.

Robert G8RPI.