Conveyor System Control

[Wooly]

Hi all,

Not sure if anybody can help, or even if I'm in the right place.

A bit of background info first, I've been working as a maintenance electrician for 10years now, and have just been asked by a friend to help with a new installation. Having not really designed anything from scratch before, I was wondering if I could get any advice.

The system consists of 1x30kW motor, 1x15kW motor and 1x3kW magnet. Both motors to be inverter driven, with the magnet DOL. I was curious as to whether I would need a main contactor directly after the isolator, as well as one after each inverter, and one for the magnet control. I have many ideas, but am not really sure how viable they are.

Thanks in advance for any feedback.

Wooly

 

[ashrow]

wooly

after 10 years you should know how things work and how the stages of set up are
Good luck

 

[Wooly]

Thanks for the reply, but I'm just a bit unsure on the design side. Just wondered if anyone could give me any pointers. If you'd rather not, then thanks anyway.

 

[grantr37]

what's the system for? that's quite the magnet you got there

 

[Wooly]

Filtering recyclable waste. Thats just the info I've been given. Have you any advice? Was going to have main isolator, split out to 3 seperate breakers, then to the inverters, then through contactors. Was going to have 24Vdc control circuit, using a Pilz relay for estop control of the contactors, and drive enable signals? Sound along the right lines?

 

[grantr37]

thought that it would be for grabbing cans from recycling or something like that.... make sure that your emergency stop set up is good, a good number of Emergency stop buttons and put overload breakers at each motor incase one gets jammed by rubbish to stop it blowing the drive inverter or its windings...

- - - Updated - - -

just to add that all 3 motors should be stopped by 1 of them overloading/jamming

 

[Wooly]

Thanks for that advice.

So, would that be putting the overloads in series with the estop circuit?

One other thing would be whether the estop should drop the signal to drive enable, and the contactor coils simultaneously, or if I would need some kind of delay.

 

[Knobhead]

You do not go from the inverter through a contactor. You’ll damage the inverter!

Draw out what you propose (electrical and product flow) and lets have a look at it.

 

[stef]

Thanks for that advice.

So, would that be putting the overloads in series with the estop circuit?

no, emergency stop chain has to be separate. Link the overloads into the control start/stop circuit.

One other thing would be whether the estop should drop the signal to drive enable, and the contactor coils simultaneously, or if I would need some kind of delay.

No, estop has to stop straight away. For this purpose you will need a contactor before the inverter, not after. This would, as Tony said, wreck the inverter. Needless to say that you need to have a two channel safety relay in the emergency stop chain.

an auxilliary contact of the safety relay provides the enable signal.

 

[Knobhead]

E-Stops are a bit of a nightmare with inverter drives.
I redesigned a centrifuge control system. It had always been a standing joke that if the E-Stop was pressed it would take the best part of five minutes to stop. I altered the system so the E-Stop didn't kill the power to the centrifuge but put it in to maximum ramp down. I brought it to a halt in 10 seconds. Even saved on the lecky bill, on ramp down the regenerative braking pumped 200KW back in to the mains.

Don't know about the legal side of it (that's NBP's field), but it was a bloody site safer that the official method!

 

[Wooly]

Thanks for all the feedback guys.

I meant to say contactors first, don't know why I said after.

I was going to have a seperate 24Vdc power supply to control the start/stop and control circuits, putting the inverter 24V control through relays for the drive enable and FWD commands.

The PILZ unit will be dual channel, and will drop off control voltage, and coil voltage to all contactors. I was thinking of having the drive enable signal going through an auxilliary contact on the preceding contactor, so when the contactor drops out, so will the drive enable. The FWD command would then be operated through a relay in the stop/start circuit.

Does this sound ok?

 

[paulr57]

Operate the magnet with a DOL starter, adequate overload protection and E.stop cct.

Feed each inverter with adequate overcurrent protection - see manual.

Feed each motor DIRECTLY from each inverter. Do not use contactors, or BANG!

The safety relay controls the inverter drive voltages and magnet E.stop cct.

That's it.

You MUST learn all about the inverters, current/torque ramps, acceleration/deceleration, etc and DC injection to brake freewheeling motors.

At these power levels MAKE SURE you know what you're doing.

Good luck!

 

[paulr57]

Yes, I've come across this problem many times in the manufacturing industry.

Most common way round this is a DC operated clutch type brake which stops the motor
freewheeling. Alternatively, some inverters inject a DC voltage into the motor to act
as a brake.

 

[Wooly]

So, would you have a DOL starter and not have another contactor with stop/start control on the main panel door?

Having to make the panel as well, I was going to have: estop reset, control on/off, and three sets of Start/stop buttons on the main door, all with indicator lights.

One other thing, would it really be wise to estop the magnet, as it may be potentially more dangerous to turn off than leave on?

Also, given the conveyor sizes and weight of material on these, I would imagine that there will be little, if no run on, so would some sort of braking really be necessary?

This whole thing is giving me a headache!

 

[Robotstar5]

You could fit positive guided safety contactors and monitor the N/C of each through the reset line on the safety relay.

 

[paulr57]

Yes Wooly.

Your panel sounds OK. What I meant is a DOL starter circuit mounted on the front panel for the magnet, and stop start relay circuits for the inverter controls. I don't know what the magnet does so if it has to stay on, exclude it from the E.stop circuit.

Of course, if driving a large mass there should be little motor run-on. This can only be ascertained during running.

In summary you need a panel-mounted DOL start/stop plus overload,
Two stop/start relay circuits with E.stop loop
Appropriate overcurrent protection for each inverter.
Some study time with the inverter manuals!

The manuals usually contain some good diagrams illustrating FWD/REV and ESTOP
circuitry.

 

[Knobhead]

Switch the magnet supply on the AC side of the rectifier this saves having to break a DC arc. The large magnets I used to work on we couldn’t switch the AC. We used 125A AC 4 pole contactors with two contacts in series to safely break the DC. The contactors were rated at 660V AC they used to burn up switching 180V DC. Also fit a reverse bias diode across the magnet feed to prevent high voltage spikes from the collapsing field.

 

[Bobster]

Switch the magnet supply on the AC side of the rectifier this saves having to break a DC arc. The large magnets I used to work on we couldn’t switch the AC. We used 125A AC 4 pole contactors with two contacts in series to safely break the DC. The contactors were rated at 660V AC they used to burn up switching 180V DC. Also fit a reverse bias diode across the magnet feed to prevent high voltage spikes from the collapsing field.

I can't see him understanding all of that Tony if he's asking us how to set a conveyor belt system up.

 

[Robotstar5]

The only thing I remember about switching magnets was they draw a big arc when the contacts open! had to switch the DC as it was powered by a motor-generator set.

 

[Wooly]

Thanks for that really useful information RoB2, really helpful.

Robotstar5 and tony, I hadn't even thought about the magnet really. Is it really a pain to control? I feel very embarrassed to have to ask, but with magnets I haven't a clue.

 

[Bobster]

No worries .

I'm sorry but I feel like your out of your depth, if you've been in industry a long as you have, you should have delt with setting up simple systems like this up before.


If you give me a couple of days I'll throw a quick drawing together on how I'd do it.

 

[Knobhead]

You a mind reader RoB? I still don't know what's going where!

And Wooly, 10 years in engineering? This should have been done on the back of a smoke packet in 5 minutes!

 

[Wooly]

Fair comments, I suppose.

I was only trying to get things right in ny head.

After 10 years of doing the same monotonous maintenance work, and no design or installation work, I'm sorry if I can't switch into that mode instantly.

Maybe I was a bit quick to ask for help on the conveyor side, but that was mainly for the estop cct, and the more I've looked into it, the more I seemed to have overanalysed it and over complicated it, mainly due to worries about safety.

Think what you like about my ability, I'm just very rusty after years of not using my brain, and I know I am more than capable of understanding what's going on.

Except, not when it comes to magnet control.

I know I sound like a dimwit, but I rather ask a stupid question and be happy that I'm on the right lines, than not ask and be cocky and do it all wrong.

Thanks for all comments so far, good or bad.

 

[Knobhead]

As RS said and I was hinting at, when you break the magnet circuit you have to contend with the power of the colapsing magetic field. An easy way around this is a diode across the output to the magnet to divert the power. It's fitted +ve to +ve so normally it doesn't conduct. when the circuit is broken the polarity reverses and diverts through the diode. Hence it's name, reverse bias diode. The diode should be rated aprox 2 X the normal running current.