Inverter Wiring

This seems stupid question to ask... but I'm paranoid about blowing up my inverter... (IMO CUB).

I have two 9-core+earth (shield) cables, but I need 10 wires...

The 10V circuit for the analogue input/output and the 24V circuit for other control inputs are electrically isolated. My assumption is that I can tie 0V on the 10V circuit and CM (0V) together without ill effects. Is this correct?

Neil

Certainly there will be some way to save one wire so your 9 core cable can be used.

Without seeing the manual I would not like to say whether its best to use the screen as the ground connection for the 24V or the 10V supplies. My gut feeling is to use three dedicated wires for the pot connections as the 24V lines sound like digital inputs.

Do you have a link to a PDF of the manual?

Ian P

Have a look here

**LINK**

Edited By Robbo on 11/10/2013 22:58:23

.

This might be the manual, but needs Neil to confirm.

... 170 pages

MichaelG.

Edit: You beat me to it Robbo

Edited By Michael Gilligan on 11/10/2013 23:00:05

I don't think they would isolate them without a reason. I suspect that reason is to prevent interference between the digital and the more sensitive analogue signals. The usual recommendation is to use a separate 3-core screened cable for the analogue potentiometer.

Russell.

If you use the screen as an earth then I don't think it is a screen anymore. Something about circulating currents. I seem to remember you only fasten the screen at one end for that reason.

HTH

Cheers Michael.

A separate screened cable for the potentiometer is the bullet proof ideal, (even better if it was 3 core screened cable for the three pot connections with the screen connected only at the input end) but we are splitting hairs here.

Quite rightly, the manufacturers advice is to cover the worst industrial environments. In reality even in an (electrically) noisy location three unscreened wires would most likely be trouble free for the speed potentiometer as long as they were not 50 feet long!

In a home workshop connecting the pot through a few feet of multicore cable is a not going to give any problems, the other wires in Neil's cable are only carrying low level DC signals so will not be radiating anything worthwhile.

If the CUB is anything like the manual linked to, then the 24V signals effectively have a noise immunity of nearly 20 Volts! so even if the screen was used as a ground and had a few millivolts of noise its not going to make any difference.

Ian P

Hay Neil 10 wires?

I have done a quick calculation of the numbers of cables you will need to control your device and come out with 7 asuming you need forward, reverse, jog, and 3 for pot.

even if you incoperate a big red emergency stop you will need a Max of 9

if the other 3 cables are for the motor connection then I cannot stress strongly enough do not do this.

The motor connection must be a seperate dedicated cable.

Regards Richard

Hi All,

Yes, that's the right manual. Finally found my paper one after losing it for a month. It si NOT the best document to read as a PDF...

Yes, separately screened pot is recommended, but it's running OK without one. For other signals they recommend twisted pair, but as I'm running less than a metre.

Rick> a big red emergency stop you will need a Max of 9

Close... but the pendant I made is a PITA (It keeps falling off the bench & dangling like a...pendant?). Instead I've converted the original control box to have a soft start and stop plus a big red E-stop (which will also be linked to optional limit switches). There's your 9. But then add one more for a 0-10V signal to drive a voltmeter reading power output!

I have some 'massively multicore' from an old parallel cable, but it's even thinner than the 9-core serial cable I'm using at the moment and I fear that a random tug will break or pull a wire out of the connectors. I have 9core+earth of 0.75mm cable from a 12V boat wiring loom I ripped out it will make a fine job if it doesn't glitch.

Somewhere I have about forty feet of ~20-core individually shielded cable that was always going to be for when I had a stage PA... can I find it?

As consensus is I wont blow it up with the two negatives joined, I'll have a go and see how it works.

Thanks all,

Neil

My comments are interspersed in Neils text above

Ian P

If this multi-wire cable is to be connected to a remote control box here's a neat circuit diagram posted by David Thomas 6 some years ago:

Diagram

Points to note:

(1) The switch sensing is critical. In the diagram the Start/Reverse/Jog functions are controlled by 'Normally Open' (Push-to-Make) switches wheres the Stop function requires a 'Normally Closed' (Push-to-Break) switch. As it happens both my VFDs (Telemecanique and Allen Bradley) conform to this pattern but you should check the particular requirements of each control line per your manual before adopting this pattern.

(2) I'd echo Rick Parry's caution about using a single cable carrying both 'power' and 'signal' lines. Apart from all else, light gauge signal wire will suffice for the signal lines whereas you'll need heavier gauge mains wire for the power connections to the motor.

Joe

Edit — argh, too late.

Edited By joegib on 12/10/2013 11:27:00

Hi Joe,

The cub allows you to invert the sense of switches by adding '1000' to the function code (but not FWD/REV for some reason). My new box will require me to reverse the sense of the 'hold' function for example. the options are manifold, to be honest - lots of ways depending how you want it to work. You could do everything with all push to make switches, but as my E-stop button is push to break.

Unlike David's circuit, mine will allow two switches a small 'soft stop' switch that will decelerate the lathe over a programmed interval (default 6 seconds) and an e-stop that will let it 'coast to stop' (i.e. instantly cut the power resulting in a prompt halt.

One nice idea I have had is that when the (switched) speed pot is set to zero, it will automatically go into jog mode.

Fear not - the power lines are a completely separate heavy duty 4-core.

Neil

Edited By Stub Mandrel on 12/10/2013 12:02:05

Gentlemen,

I do apologise if I'm about to teach granny to suck eggs but please remember that any EM Stop has to be wired through a latched relay so that when it is operated it kills the motor relay that then needs a proper reset, this is to stop you reseting the EM stop and allow the motors to start unexspectedly.

Martin P

Hi Martin,

With an inverter it works differently - the controls are all on a completely separate system, an e-stop in the supply to the inverter or the inverter output to the motor would be liable to damage it (the instructions say you can switch the inverter on and off with a magnetic contactor, but only once an hour).

There are various ways to wire in the e-stop, but if I fit it as in the diagram referenced above it causes a 'soft' stop. That is why I want to fit a separate 'coast to stop' that acts like a normal e-stop but through software. the latching to prevent a restart when it is released is built into the inverter.

Equally the inverter, by default, will not restart the motor at power on automatically. It IS possible to override this (for things like pump motors in remote l;ocations that might suffer interruptions to the supply). The manual has dire warnings about using this facility.

Neil

Neil,

Sorry I should have been more specific, I didnt say where to put the EM Stop, I work for an Italian company and we make very large industrial washing machines and Automation, robotic bottle washing equipment, for hospitals, when I say large you could put a small car inside, we use Siemens inverters to drive pumps, extraction fans etc. After the inverter we have the motor contactor which, if the EM is pressed, drops out to stop the pump, the inverter is also stopped should the inverter fail to stop the contactor cannot run the motor. Machine building regulations doesnt allow EM stops to be connected through PLC software stops because of the possibilities of glitches that can happen with PLC's etc.

Martin P

I think it will help if you re-examine what is meant above. If your inverter can do a controlled interval stop it is using DC injection braking (do check the manual on this however).

For an e-stop, the DC injection braking is much preferable to killing the power, either before or after the inverter, as killing the power means the motor will coast down under its own momentum, which can take rather a long time, especially on larger machines or if you have something hefty in the chuck.

Some inverters can have more than one stop condition, with different ramp times on the DC injection, for example one slowish on a small stop button, and one harsh stop on an e-stop circuit.

Beware using braking if your lathe has a threaded spindle, it can have entertaining results.

Hi,

Thanks for these thoughts. Inverters are still relatively new in model engineer's workshops and there doesn't yet appear to be a clear consensus on how they should be used. It's clear that suppliers expect users just to use a supplied pendant with a single stop switch wired to invoke the 'soft stop' without DC braking. The manual makes it clear that the inverter should be fed through a circuit breaker of some kind (mine is via an RCCD on the distribution box) and an isolater (13A plug into a switched socket).

Careful; reading of the manual does not appear to prohibit an e-stop (they say magnetic contactor, but that just means "big switch operated by a relay" on the secondary circuit (motor side) but it does say: "ensure the motor and inverter are both completely turned off before you turn the MC on or off". In other words anything on this side of the inverter is 'emergency only'. I interpret that as "you can put one here if regulations require it, but don't use it".

If such an e-stop was fitted it would have to be a second one, ideally mounted directly on the motor, not the lathe, which sounds like a recipe for confusion to me.

A contactor (on off switch) before the inverter should only be used once an hour max. I assume this limit only applies when the inverter/motor combination is running.

The inverter as supplied (without a brake resistor fitted) does two types of stop.

1. A soft halt that ramps the motor down over a defined period.
2. A coast to stop.

Most users (including those using the circuit above) will only be using (1) above but without a braking resistor and probably on the default period of 6 seconds. Without a resistor, reducing the period too much will fault the inverter due to current overload.

The relatively modest inertia of my lathe means that (2) is much faster than six seconds, and is therefore preferable for an e-stop or limit switch. It is in any case what was the situation with a 'normal' motor.

I could set 1 to a much shorter period, but to do this effectively would require adding a large and expensive external braking resistor, would probably not be much different from 2 and would mean I lose the soft stop, which I prefer.

So I'm actually adding an additional and quicker stop to the default setup by doing this, not taking anything away.

I'm not using a PLC, it's all under 'direct idiot user control'. I don't pretend that the inverter itself couldn't throw a wobbly, but I have to assume that, if nothing else, it has been designed to fail safe, so once stopped it can only be started by pressing start (unless programmed otherwise).

The e-stop switch is NC so any break in the control wires will cause a stop and prevent a restart.

Finally, the control panel will have a small green button for start, a small red one for stop and a great big red mushroom e-stop, so I don't imagine it causing any confusion for someone unfamiliar with the machine.

Neil

P.S. my lathe has a flanged fitting for chucks but yes I can see how fast stops are an issues for lathes with screwed chucks.

hello there, I've 3 inverters all pre owned. I've made my own remote control boxes and all three have their screened power lines (Sy cable i think) to the motor separate from the control wires. The screen is earthed at one end only to avoid current loops.The supply from the mains in each case is via a large double pole (10 amp) relay in which is wired the large RED stop stop button. To run the inverter I press the green start button, the relay energises. To power the motor I have a latching on/off switch togerther with a reversing switch, a potentiometer for speed control and a small switch for jog, handy when setting up. For the control circuit I've used ex computer screened lead. Once switched on I don't need to switch the inverters on and off, I leave them running until its time to pack up for the day, just use the motor local control as and when type of thing.I hope that might be useful.Ted