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.

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

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

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

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

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

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

Hi Andrew,

Whilst I might be a beginner with machining…I understand electronics. Your reasoning is sound, the only thing that might create problems and I stress might is the initial inrush current generated/demanded on the supply when you start another machine/motor.

Otherwise, using the inventor as you suggest …i.e as a plug and play device is only limited by its capability i.e power that it can deliver in watts.

Whilst the loading you are suggesting is tight, it falls just below the inventor you suggest is capable of handling, so no reason for concern other than as I said before on initial inrush demand when the motors start

Gary

The potential problems in switching motor configurations on the output of a VFD are very little to do with current limits. They're to do with what happens when you disconnect a running motor and try to instaneously change the current in an inductor, and what happens to the energy stored in the motor.

Modern IGBTs are pretty robust and will withstand short term current overloads, they will not withstand C-E over-voltage.

Andrew