Multiple machines from one inverter

I've read (possibly here, certainly elsewhere) that it isn't possible to switch a single inverter between two motors. But I think the only explanation for this I have read is 'because the inverter must be directly connected to the motor', which seems to me to beg the question.

Can anyone explain in more detail? Or point me to an explanation?

Regards, Robin

According to this it ought to be possible

As long as the inverter has the capacity it can be done. What's the rating of the motors that you wish to run?

Brian

I have one inverter mounted on a saw and just wired a socket into it which I can plug my other saw. Works fine. ( I never run them at the same time)

Brian.

Hi Robin

I presume you only want to run one motor at a time. My manual states that one must not cut the power between the inverter and a motor while the inverter is delivering power, ie. don't put a switch between the inverter and the motor. To my mind, if you turn off the inverter at the mains, manually transfer and connect the 3 output wires to the second motor, and then turn the inverter on, then there should be no problem. So, connect a 3 pole, double throw, switch (with a high enough rating and good and firm switch contacts) to the 3 wires coming out of the inverter. Connect the both motors to this switch. DO NOT EVER EVER move that switch while the inverter is energised or if there is a voltage on any of the 3 wires, or if the motor is turning (inductive load?). Turn off the inverter, wait, then move the switch. I can't see why this would not work.

My concern is; does anything happen with the inverter when the switch contacts are moved when the inverter has just been powered off but 'is still warm'.

I've never done this (would like to also), so I stand to be corrected.

Steve

Edit : I just saw Donkeys post after posting this. Yeah, instead of a switch, manually swap the motors over using 3 pin plugs and a single socket connected to the inverter.

Edited By Steven Vine on 20/08/2015 08:55:37

As someone has posted above, my inverter output is wired to a three phase socket (red 4 pin)

so I can plug any machine in, one at a time obviously. As long as the inverter is capable of powering the largest

machine you should be fine. I normally power the inverter down before swapping plugs.

if you want to run several machines at once from one supply, then a rotary converter is the answer.

With the inverters I have, they are programmed for the specific motor rating. So in my case, the motors need to be the same, then I could do that. However all my lathes have different motors, so I have just gone and got separate vfd's for each machine. A lot more convenient and no more reprogramming or switching plugs etc.

Just turn on and use.

Neil L

Neil's posting beat me to it.

Unless the motors are exactly the same, you will loose the protection that the setttings give you for each motor, namely max current and ramp up/ramp down timings etc.

+1 on KWIL's post.

Some VFD's have a second set of parameters you can store, so you can have setups for two motors, just have to remember to switch the VFD parameters, when you change over.

I found some food for thought from these postings.

I have been running my Myford and Warco Mill/Drill from the one Newton Tesla CL750 box for several years by the crude method of powering down, unplugging the one machine and then plugging in the other machine. When buying the package of CL750 plus two motors from NT I made clear how I planned to use it and they seemed quite happy. The CL750 is pre-programmed and so KWIL's point whilst clearly valid seems not to apply (so far as I am aware the CL750 can't readily be programmed by the user).

Re: setting the max current for motors. Why do you want to limit the motor current? An industrial electrician told me that a motor draws the current that it needs. I would have thought that if the VFD is a match for the motor HP then you would not need to set the max current.

Re: the ramp up ramp down. On my 3 VFDs (if I remember correctly) the ramp is established by setting the time you want for the motor to reach full speed/frequency, and the time you want for the motor to come to a stop (soft start, soft stop). I would have thought it would be ok to use one ramp setting for 2 dissimilar motors.

Steve

Here's my solution and it works a treat:

One VFD to power both the drilling machine and shaper and another for the two milling machines, each in their own enclosure situated between respective machines.

Power from the VFDs is wired up to triple pole centre off selector switches housed in each enclosure, thus allowing me to easy select which machine to sent power to, BUT only after switching off at the wall socket, and most definately NEVER whilst a machine is running.

The VFDs are calibrated for the most powerful motor of the two.

Cannot, of course, run both machines at the same time, but wouldn't want to.

Once you've used machines with VFD power, you never look back!

Only the lathe has a dedicated VFD.

Geoff - Belt/pulley changing now history, just know knob twirling. Heaven

I have supplied multiple motors from one VFD in conveyor systems. but only one motor used at a time. selection was made using selector switch and contactors prior to powering up the inverter . Each motor had their own individual overload. The VFD was rated to take the largest motor.

The only problem I can see is that an inverter set up for a larger motor will underestimate the temperature rise in a small motor when operated at a given power level, so if you regularly run slow and hard you could easily overheat the smaller motor.

Neil

Thanks for all your replies, it sounds like what I'm thinking of is eminently doable. The two machines in question are a pillar drill (1 HP) and a mill (yet to buy, but probably 2HP) which would never be used at the same time. I'd be buying new motors for both, so I guess if I went to eg NT for a package and told what I want to do they'd sort me out. I was just scratching my head about this thing I'd read about it being necessary to 'hard wire' the inverter to the motor - couldn't see the logic in it, maybe because there isn't any. I can understand that the inverter wouln't be happy with the output being switched whilst working.

The drill ( JET) is possibly the most depressing machine I've ever bought - it clanks and rattles so badly I hardly like to use it for fear it'll shake itself to death. The quill and spindle seem good though, so I'm hoping I might rescue it by running it off a VFD and eliminating the intermediate pulley, which I suspect is a major contributor to the problem. But that's another topic...

Regards, Robin

It's entirely logical. It is normal to connect a motor to a 3-phase supply via contactors and thermal overloads. Doing the same with a VFD/motor combination is likely to result in the demise of the VFD if one of the intermediate devices opens when the inverter is running. So it is aimed at the technician who is used to wiring conventional 3-phase supplies. There may also be EMC advantages in that the complete wiring from VFD to motor can be shielded.

Andrew

It protects the motor in the event of a mechanical overload or an electrical fault; much the same as a conventional thermal overload, but rather quicker.

Andrew

Well mines been running in an industrial environment for close on 5 years.

As Allan my output 3ph 400V is fed in to various dockets hard wired in, machines plugged in and can use several machines all at the same time. This works exactly like a proper feed from electric supplier no need to faff about hard wiring direct to motors, mine all go through several contactors, switches and so on exactly the day the machines were made. Hence theyre called 'Plug and Play' inverters.

Do have a backup rotary convertor, simple job of disconnecting machine from hard wired in socket and inserting in to Rotary, nothing more.

Hi Andrew, thanks for that info - it makes sense of what I'd been told. I used the word 'logical' loosely perhaps - I'm a (retired) acadaemic physicist (therefore sometimes hopelessly impractical ) and was thinking about the in-principle 'electrical logic', which I couldn't understand, rather than the 'working practice logic' which you have explained. I haven't come across EMC? EM must be electromagnetic, C could be Contamination, Crossover, Crap.... Anyhow I know what you mean - I spent some time working in radiofrequency/microwave spectroscopy, so I understand how to make shielded and impedance matched connections, I did think of these things, they wouldn't be a problem.

Regards, Robin.

Shielded cable(s) Robin you should be ok.

EMC or EMI Electro Magnetic Interference.

To reiterate, if I programme in a 3 second soft start on my 'Plug and Play' inverters you hear the numerous contactors constantly tripping in and out until approx. 360V serves no purpose the startup is hard as in instant with a spindle start machine.

Clutch driven machines the motors permanently running until switched off or reversing direction after powering down = pain. Last machine used to take up to 20 secs to power down to standstill before you could power up in opposite direction, if not would carry on rotating in same direction regardless of intended direction.