Any Electrical Engineers around?

[RYH64E]

Can someone help with a motor speed control problem? I've had a couple of new machines installed at work, each machine has an inverter speed controlled 1.5kw 3 phase motor and a few heating elements. There's a 3 phase supply (2.5mm) to a 32amp isolator, followed by 2 off sockets, one for each machine. The problem is that when the motor on the second machine is switched on the motor on the first machine slows down considerably. The speed of machine 1 can be increased back to where it should be by adjusting the inverter control, but then the machine runs far too fast if machine 2 is switched off. There's no voltage drop on either supply, and the supply is easily sufficient for the load, so I'm guessing there's some frequency related feedback from one machine to the other?

The electrician who did the install left scratching his head promising to do some research, I'm not expecting him back any time soon, and the machine manufacturer haven't been much help, so I thought I'd ask on here... Is there some kind of filter or isolator I can add to each machine to prevent any frequency feedback, if that is the problem? Any ideas?

[anonymous-user]

What is setting the "speed target" on the inverters? Assume a passive potentiometer into an analogue input?

[RYH64E]

Some kind of touch screen, there are menus for set up, temperature, product counters, motor speed etc, I'm guessing it's a digital input.

If I set machine 1 to a speed of say '30' (30 what I don't know, it's an arbitrary value), when the motor on machine 2 is switched on the speed display on machine one remains at '30' but it's actual speed drops, resetting the required speed to '36' brings the speed back to where it was, but stopping the motor on machine 2 cause machine 1 to speed up to an actual '36', which is far too fast. Switching the heating elements on on machine 2 has no effect on machine 1, just the speed controlled motor.

[martinnitram]

A 3 phase supply running a 32a isolalator running 2 x socket outlets.
Each machine has motors and heating elements
Each motor effects the other, but heating elements (single phase) dont effect the motors speed.

Im not an electrical engineer just a regular sparky, but there is something there to do with the single phase elements (with a neutral) and the motors running delta (assuming) causing this, could be miles off the mark.

Have you tried putting the other machine on a different phase ?

You sure supply is large enough?

Not really an answer but asks a few more questions.

Cheers

[anonymous-user]

Ok, so we can assume that the target speed is not moving when the 2nd motor is switched on.

That just leaves us with the speed feed back getting noisy and hence the inverter speed control loop believing that motor 1 is suddenly going faster than it is.

Do the motors have speed sensors fitted? I assume not, and the inverter is just using the phase voltage waveform to calculate the speed?

If you get switching noise from the other motor on the 1st motor, it is possible that this could be seen as "extra" speed on motor 1, and hence the inverter slows it down a bit


What are the phase cables between the inverters and the motors, and how are their shielded / earthed etc?

Do the inverters have a "DC link" that is exposed? ie could you connect both inverters DC link negative terminals together to help limit ground shift noise etc?

[RYH64E]

martinnitram said:

Have you tried putting the other machine on a different phase ?

Tried that, made no difference.

martinnitram said:

You sure supply is large enough?

The electrician who did the install thinks the supply is large enough, there's no sign of any voltage drop, and I can get both motors to run much faster than I need, it's the lack of control that's the problem.

[RYH64E]

Max_Torque said:

That just leaves us with the speed feed back getting noisy and hence the inverter speed control loop believing that motor 1 is suddenly going faster than it is.

That was my conclusion.

Max_Torque said:

Do the motors have speed sensors fitted? I assume not, and the inverter is just using the phase voltage waveform to calculate the speed?

If you get switching noise from the other motor on the 1st motor, it is possible that this could be seen as "extra" speed on motor 1, and hence the inverter slows it down a bit


What are the phase cables between the inverters and the motors, and how are their shielded / earthed etc?

Do the inverters have a "DC link" that is exposed? ie could you connect both inverters DC link negative terminals together to help limit ground shift noise etc?

Unfortunately I can't answer any of the above, my background is Mechanical Engineering not electrical, and that was a long time ago. If I had to guess I'd say there was some earth leakage but I've no evidence for that. I was hoping for some form of plug in filter/isolator that would prevent the noise from one machine feeding back to the other.

[anonymous-user]

The motors, are they single phase or 3 phase?

How many wires run from the inverters to the motors. Generally, the "Power" wires are big and have nice large screw terminals, and speed sensor wires are likely to be small and in a seperate loom to the power wires.

[anonymous-user]

PS. A picture is worth a thousand words...... ;-)

[RYH64E]

3 phase motors.

I'm not at work today, will get some photos on Monday.

Thanks for your help, much appreciated.

[AER]

Is everything correctly earthed?

[RYH64E]

AER said:

Is everything correctly earthed?

Good question. As supplied it wasn't, but this was corrected during the installation, so yes, I believe everything is correctly earthed.

[spikeyhead]

Pictures?

[RYH64E]

spikeyhead said:

Pictures?

Sorry, forgot.

Will have to work out how to post pictures, never done it before.

[spikeyhead]

RYH64E said:

spikeyhead said:

Pictures?

Sorry, forgot.

Will have to work out how to post pictures, never done it before.

Hope this helps,

[RYH64E]

Hope this works.

[spikeyhead]

http://www.ebay.co.uk/sch/i.html?_trksid=p3984.m57...

I'd buy a few of a suitable size and clip them in place. That could remove a chunk of interference.

Maplin also sell a range if you have a local store

http://www.maplin.co.uk/search?text=ferrite+clip&a...

[rossw46]

Is the supply into the controller 3 phase ?

[RYH64E]

spikeyhead said:

http://www.ebay.co.uk/sch/i.html?_trksid=p3984.m57...

I'd buy a few of a suitable size and clip them in place. That could remove a chunk of interference.

Maplin also sell a range if you have a local store

http://www.maplin.co.uk/search?text=ferrite+clip&a...

Thanks, that sounds like a good idea. Should they go anywhere particular, or just all over?

[spikeyhead]

RYH64E said:

Thanks, that sounds like a good idea. Should they go anywhere particular, or just all over?

Try them either end of each cable. it may be something completely different, but this is going to cost pennies and will eliminate the most likely issue.