3 phase inverter query?

I now have a good quality 2kW single to 3 phase inverter for sorting out a friends electrics.

He has 3 items he wishes to run on this inverter. Two have 750 Watt 3 phase motors and the third has a small Parvalux 3 phase motor rated at 110 Watts.

Now he doesn't want to do anything clever like soft start, feedback, and variable frequency. He wants to treat it as if he had 3 phase mains, full stop!

He queried me on running one of the 750 Watt machines at the same time as the 110 Watt machine. As far as I can see there should not be a problem, as he doesn't want any fancy implementations. The only thing I would do is to have a current limit on each phase.

So tell me that I am wrong! Please don't go on about the fancy things that can be done, he is quite adamant that he does not want anything except raw 3 phase at 50 cycles!

Andrew.

I have a Transwave static inverter, you really need to twiddle one of the knobs to get the required voltage when the machine motor is running. This may be a problem with two motors at the same time but worth a go.

The one thing that electronic inverters don't like is being switched, on the three phase side, so one does not normally have a stop/start switch or a reversing switch on a motor connected to the inverter.

Now, whether that applies to a modern 'good quality' inverter, or not, is open to discussion I would think.

Hello Chris,

Thanks for your input. I should have made clear that the inverter in question is of the electronic type. i.e. full wave rectification of the incoming 240 volt single phase supply, then smoothing and finally chopping into 3 phase output. So your experience with a static inverter isn't really applicable to this particular query.

Andrew.

Thanks for the point about not switching on the output of the 3 phase. I did wonder about this. I suspect that if the 110 Watt motor was switched on (on the 3 phase side) while one of the 750 watt motors was running. It should not be too much of a problem. But maybe totally wrong on that one!

Andrew.

Modern invertors monitor the current in the phases to apply feedback to the control electronics. Running two motors in parallel which may run at different speeds due to varying loading may confuse the feedback circuits. I would not try it unless the manufacturer says it is ok to run like that. You may just keep getting false fault reports that shut down the invertor at the worst possible time (is there a good time?).

Martin C

You would need to read the spec or ask the maker to know for sure, but most electronic inverters don't support sharing their outputs. A big part of their goodness is that they can be set to suit a particular motor and also, in operation, adjust automatically to get the best out of the motor. The poor thing is liable to get confused when asked to feed motors in parallel. Not only are the motors different in themselves, what the inverter 'sees' varies wildly depending on which motors happen to be running and what they happen to be doing moment to moment.

My guess is that you really need either a separate inverter for each motor, or a rotary convertor. Although rotaries are noisy and relatively inefficient they are much less upset by being shared.

Interesting to try it and see. Quite often for hobby use you can ignore 'best practice' and get away with it.

Dave

I bought a 'bare inverter' from Newton Tesla to power a small 3 phase motor for my Cowells lathe (I know, overkill, it's a long story). I can't remember the exact price but it was a few 10s of pounds

Edited By duncan webster on 31/10/2017 11:26:23

Interesting responses! I have had an email from a gentleman who has been running his 5HP inverter for 10 years. He uses it to run motors in parallel AND switches them on the 3 phase output! These are serious motors, one on a Schaublin mill a second on a Chipmaster and the third on a Startrite drill.

He simply treats the inverter output as genuine 3 phase and never has a problem. So in the real world, it would appear to be rather different than most model engineers appear to think.

Andrew.

Sorry to disagree, but there's much more to the real world than the one gent who emailed you! As we used to say in the trade what you're proposing is an "unbounded risk" - you don't know how big or small it is (yet).

As I suggested it's worth trying. I would. But, because no-one knows whether the inverter will work satisfactorily or not, I wouldn't put my money where my mouth is by offering a guarantee!

I am a cowardly lion though...

Dave

Andrew,

Just to be sure - t

his gentleman that emailed - did he specifically mention whether his inverter was static, rotary or solid state type?

Hello Dave,

It was indeed a solid state type. I have just been speaking to a local fellow, who does exactly the same thing with his electronic inverter. So that appears to two people who do this sort of sabotage to their inverters, without any apparent harm. Makes me think that maybe they are right!

If it were my inverter then I would not hesitate to follow their example. However I don't want to damage someone else's kit.

Andrew.

Providing the owner doesn't want to run all three motors at the same time and as you say, not interested in using all the feature available, how about one outlet from the inverter and THREE plugs,one for each machine, use any one machine as and when, type of thing. Providing all motors are say 4 pole and 1425 rpm or 2 pole and 2850rpm and set for highest current I can't see any problem.There appears to be a lot of mythology surrounding inverters, perhaps some one who has a lot of industrial experience might like to pass on their thoughts.John

Hello John,

My friend wants to run the 110watt motor together with one of the 750 watt motors. The 110 watt motor is powering a small grinding attachment which goes on the tool post of his lathe. I suppose they could be switched on simultaneously via the single phase "power in" to the inverter, but it isn't an ideal solution.

I too, suspect that a lot of myths are floating around concerning electronic inverters. Like you, I wish that someone that has a lot of practical experience would put us straight!

Andrew.

He should be fine a as long as he ensures the inverter is off before switching any load downstream of the inverter he should be OK. Most inverters don't like having no load attached, especially if the load is removed when they are running.

I suspect your 'lucky' user with a 5HP inverter benefits from the total load being much less than 5HP, especially if his Chipmaster is one fitted with a 1.5 rather than 3HP motor.

The caveat is that all the clever protection circuitry in the inverter will be useless as he won't be able to have it programmed with the characteristics of the motor it is driving.

Yes, but that's not what I think Andrew is after. His friend has a lathe with a tool post grinder, so there are at least two motors that the inverter has to run at the same time. It's the two or more off a single inverter that my reading suggests is controversial.

Having dipped into it a bit more:

• Of the small inverters I looked at (3) none say you can't run two motors off the same output, but all the words and diagrams only show one motor in circuit.
• All the small converters (2) found that do say they support multiple motors turned out to be rotary types.
• I found a vendor website that said you could run two similar motors off the same inverter (only in VxF) but their example was speed controlling a pair together, not independently. There were several 'ifs and buts' suggesting the arrangement isn't 100% reliable.
• Another vendor website explicitly says not to use their ordinary range of inverters on more than one motor. They do sell electronic converters for general purpose use but the blurb suddenly starts talking about Hospitals and Ships. The unit mentioned has a 100HP output.

It's all very confusing. My feeling is that :

• Most small inverters are intended to run a single motor only. Might work, might not in Andrew's application.
• Inverters able to run whole installations exist, but they tend to be hefty.
• Wouldn't be surprised to find that some small inverters are specified do what Andrew's after, but I wasn't able to find one.

Let's hope one of the experts will be along to shine light into this darkness!

Dave

Drives direct (perhaps not a supplier of the peak of quality to which Andrew is referring) clearly state to connect direct to the motor from their 'standard' range. Another make I had seen, emphasised that stopping and starting procedures should be via the signal inputs within the inverter.

Perhaps they have a higher quality range somewhere up their sleeve.

As I said in the second reply to this thread, whether that applies to more robust devices is open for discussion.

I think advice from the manufacturer or supplier (those that are expected to stand by their warranty/guarantee) should be obtained, in writing, before experimenting with the unknown.

Whatever is said on here needs corroborating evidence, not just hearsay.

Running multiple motors from a single VFD is common in industry, especially on pump stations and Fan type cooling towers.

Generally ALL manufacturers recommend:

All motors are started together.

VFD rating to be equal plus 20% to sum of all motors stalled rotor currents.

If motors are to be started and stopped individually on the VFD OUTPUT side ( VFD is not shut down during motor switch-over), then , for example, if two motors are used on one VFD:

The motors must be of the same rating.

At least one motor must always be running.

If of dissimilar ratings, only the SMALLER motor may be stopped and started on the VFD output side, while the other is running. Doing it the other way around results in a massive load-dump condition,with resulting high voltage fed back into the VFD.

If only ONE motor is left running, it can only be stopped by shutting the VFD down , NOT by disconnecting it ( via a contactor for example) on the VFD output side.

This all has to do with load dumping, and the VFD sensing the current demand and adjusting the PWM frequency of the output voltage, and hence the RMS value of the output 3 phase AC waveform. Suddenly disconnecting the load results in a massive jump in voltage, invariably popping the VFD outputs. Likewise, disconnected a high rating motor while a smaller rated one is still connected will result in large current flow for some hundreds of milliseconds, as the VFD adjusts its output - most often triggering and over-volt condition, and shutdown.

Also, it is mandatory that each motor is fitted with overload relays/sensors, as the VFD can no longer perform that function with multiple motors.

You may get away with flouting these recommendations, but probably not for long..

Joe

Thanks Joe,

A very interesting summary of the does and don'ts. It certainly puts some myths to rest.

Thanks again,

Andrew.

Sorry I'm a bit late! Ive got an inverter, 240v in 410v 3phase out which operates a 2kw hydraulic power pack on a guillotine. I have a 1/2hp pilot motor running continuously and the power pack comes in when I press the machine buttons which pulls the machine contactors in. Sounds terrible in theory but works, my reasoning is its just seeing an increase in load and the electronics is current limiting etc

Peter