Thread: Interface 3ph rotary switch with low voltage

I would like to interface a rotary switch (top left of schematic) on a 3 phase lathe with either 12 or 24 volts. I want the output of the switch to be low voltage to for use with a VFD. The original switch allows one to pick a direction and speed(s). I am looking for some pointers as to where I should start in terms of hooking this up to 24v. Any idea on what pins I should run the DC voltage to?

rotary_switch_3ph_2.jpg rotary_switch_3ph.jpg rotary_switch_3ph_3.jpg

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Once you've removed all the 3 phase wiring it should be easy to find terminals to do what you want. If I had to guess I'd start with the three terminals on the left have side. Middle common. top FWD, bottom REV or Vice-Versa, but thats just a WAG. I'd be tempted to remove it and replace with a couple of spst swtiches.

What VSD and motor did you decide to go with? I ask because I wouldn't be rushing to remove the pole switching until you found you were happy with the results of a one speed motor.

Stuart

Originally Posted by Stustoys

Once you've removed all the 3 phase wiring it should be easy to find terminals to do what you want. If I had to guess I'd start with the three terminals on the left have side. Middle common. top FWD, bottom REV or Vice-Versa, but thats just a WAG. I'd be tempted to remove it and replace with a couple of spst swtiches.

What VSD and motor did you decide to go with? I ask because I wouldn't be rushing to remove the pole switching until you found you were happy with the results of a one speed motor.

Stuart

Thanks Stuart for the tip. I was planning to do something similar. Before leaping in I wanted to check for someone that might have traveled this road..

As for the VFD and motor, I went with a 4kw HY VFD, and a 2.2kw WEG W21 3ph 4 pole motor (marketed as "VFD ready"). The reason being was that my 5hp Phase Changer was not up to the task of running the existing motor in the higher speed setting. It bogged the RPC and sat on 7 (out of 8) on the scale of load. That is just running the motor with no load. I figured either the RPC or the motor was toast. Without wanting to spend endless time on it I grabbed a VFD and WEG motor and was going to convert it over.

I intend on marking everything before unscrewing even a single connection. I want to be able to get it back together should the VFD setup under-perform. I am not sure why it would though? Big VFD and more powerful motor than stock?

Originally Posted by variant22

I would like to interface a rotary switch (top left of schematic) on a 3 phase lathe with either 12 or 24 volts. I want the output of the switch to be low voltage to for use with a VFD. The original switch allows one to pick a direction and speed(s). I am looking for some pointers as to where I should start in terms of hooking this up to 24v. Any idea on what pins I should run the DC voltage to?

rotary_switch_3ph_2.jpg rotary_switch_3ph.jpg rotary_switch_3ph_3.jpg

I perfectly understand your aim. You want to keep that fwd/stop/rev switch, because
- its exactly there where it always was, your hand automatically reaches there in an emergency
- you like the feel of the long stick and its positive "click" feedback feel (as opposed to say a whimpy small rocker switch)
- it appears to be less work reusing the existing switch, rather than beginning from scratch
- you may like to retain the original look and feel of old iron


I was in the same situation before. These rotary switches are made out of several modules, usually one for each set of changeover contacts. They come more or less easily apart (watch out not to loose springs, cams or contacts....). To control a vfd, you do not need the whole stack of 6 modules. One is enough. Remove all others. You will also figure out, how to modify the indent mechanism to have only 3 positions for fwd-stop-rev, not like now 5 positions including fast and slow speed for a 2-speed motor. It can be some work to this properly, but it is far better than just reusing exactly what is there.

On thing to consider is, that the contacts of these switches are made for 400V AC and a sizable motor current of several Amperes. You want to use them at 12 or 24V DC, just as signals for the VFD, with very low current of a few milli-Amperes. The material these contacts are made of may (or may not) not be sufficiently self cleaning at such low DC voltages and currents. They may oxidize after a while and cause problems later. For this reason alone, you may now or later want to consider the option of using a completely new rotary switch.... My converted rotary switch still works fine at controlling the VFD after 12 years, I never had any contact problems.

Ok If you wanted to get a little more excited, you maybe able to use the multi speed inputs on the VSD so speed "1" is set to say 35Hz and speed "2" is set to 75Hz*, so it would appear to work as factory. I'm not sure you'd be able to use the speed pot then though, I havent tried on a HY. (I 'd try it on the Teco using jog speeds and that disabled the speed pot)

Maybe for later on?

Stuart

*you'd want to have a look at BobL's tests as a starting point to pick the Hz and then a pulley to suit in the end I guess

I always though it was high DC currents that are likely yo cause problems like electrolysis when using mains type switch gear?

While we are talking using old 3P switch gear to control VFDs one thing I have wanted to do several times is use the momentary ON and OFF switches from 3P gear to switch a VFD on/off but the VFD usually requires a Make (ON) break (off) switch. One thing I thought of using was a Passive bistable latching relay as an intermediate component. Has anyone tried thay?

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No liability is accepted by UBeaut or the Wood Working Forum's administrators
or moderators for advice offered by members posting replies
or asking questions regarding electrical work.
We strongly advise contacting a Licensed Tradeperson for all electrical work.

WARNING

Information supplied within posts is not to be considered as detailed formal instructions to complete a task.
Members following such information do so at their own risk

Originally Posted by BobL

While we are talking using old 3P switch gear to control VFDs one thing I have wanted to do several times is use the momentary ON and OFF switches from 3P gear to switch a VFD on/off but the VFD usually requires a Make (ON) break (off) switch. One thing I thought of using was a Passive bistable latching relay as an intermediate component. Has anyone tried thay?

Can't you just use a (I think its called double pole switching) switch. Using one NC pole to make the stop circuit and the two NO poles to make the fwd and stop circuits(no I haven't tried it.)

Stuart

Originally Posted by Stustoys

Can't you just use a (I think its called double pole switching) switch. Using one NC pole to make the stop circuit and the two NO poles to make the fwd and stop circuits(no I haven't tried it.)

Stuart

I can't visualise this - I need to see it in a circuit diagram or something . . . . . .

Sorry Bob, I misread what you wanted to do. So what I was talking about in my last post is rubbish

All the 3ph momentary switches I recall seeing have both NO and NC... so I'm not seeing your problem any more.. or at least I'm not understanding you.

Stuart

Originally Posted by Stustoys

Sorry Bob, I misread what you wanted to do. So what I was talking about in my last post is rubbish

All the 3ph momentary switches I recall seeing have both NO and NC... so I'm not seeing your problem any more.. or at least I'm not understanding you.

Stuart

OK - Sorry I should have made it clearer. I want to use the original ON and OFF switches to switch the motor on via the VFD.
BUT the original contractor/mains switch can't be used because the VFD has to be hardwired direct to the motor.

A VFD uses a simple on/off switch to start/stop the VFD - I call this a MAKE and BREAK switch.

3P machines typically use
To turn on - a momentary NO switch is closed and this causes the contactor/main switch to trigger ON and latch (stay ON).
To turn off - a (NC) switch is momentarily opened that cuts the power to the main switch which de-latches (stay OFF)
This is no good to a VFD

What I was thinking was to
1) use the original momentary NO switch to supply a 12 or 24V pulse to a Passive bistable latching relay (which is connected to the motor on/of witch on the VFD). The pulse would turn the relay ON and it would stay ON and so should the motor
2) To turn off, the normal 3P NC "off switch" would have to be converted to a NO switch and used to to supplies another 24V pulse to turn the relay off, which in turn switches the VFD motor switch OFF and it stays off

I kind get nervous when A needs B which needs C etc to control machinery because the more things there are in a loop the more problems there may be - I would need to set up a test circuit and trigger it quite a few times to feel comfortable with this.

These bistable pulse driven relays are quite cheap (a few $) and there are nice little pre-boarded versions available that have all the contacts set up for wiring.
eg http://www.ebay.com/itm/Passive-Bistable-Latching-DPDT-8-Amp-Power-Relay-Module-24V-Version-RT424F24-/131340206513?pt=LH_DefaultDomain_0&hash=item1e947c81b1

As usual I may be overly complicating the situation or missing something. If anyone can suggest alternatives I'm all ears.

Did a similar conversion on my lathe, the original switch controls forward, stop and reverse, the switch (another rotary) that also changed speed to high-low (was a 2 speed motor) was also retained.

Just used a multimeter to find the right connection points on the rotary switches (NC and NO etc)

Can take some pictures tonight if your interested....

Originally Posted by BobL

O

As usual I may be overly complicating the situation or missing something......

You are overcomplicating. Forget bistable relays and such stuff, it would only cause you grief.

My previous answer was assuming that you want at any cost keep the old switch for the looks and feel. Should that not be an issue, you best toss that old switch away, and along it anything else that did previously attach to your old motor.

All you need to fully control your VFD/Motor are a small $2.50 toggle switch to control forward - stop - reverse, plus a $5 potentiometer to control motor rpm. The control voltages for both the switch and the potentiometer are only 12V DC. Alternatively you also could use three momentary pushbuttons instead of a rocker switch. Or a small 3 position rotary switch. What ever you like best.

Originally Posted by cba_melbourne

You are overcomplicating. Forget bistable relays and such stuff, it would only cause you grief.

I thought so, but I'd be interested to know what the grief would be?

My previous answer was assuming that you want at any cost keep the old switch for the looks and feel. Should that not be an issue, you best toss that old switch away, and along it anything else that did previously attach to your old motor. All you need to fully control your VFD/Motor are a small $2.50 toggle switch to control forward - stop - reverse, plus a $5 potentiometer to control motor rpm.

That's what I have been using and not a problem in my shed.

As far as the thicknesser is concerned all the old wiring and switches have been thrown away. The Thicknesser is potentially a very dangerous machine and its not going to be used in a regular workshop environment but a timber yard so what I'm really chasing is switchgear that is larger and more rugged than a $2.50 switch, and separate ON and OFF switches.

Separate switches make real sense in these environments. One problem with a single ON/OFF switch is that even if it's clearly labelled, in the confusion of an emergency, there is a chance that if nothing seems to be happening that users will push a switch multiple times.

This means that if someone hits the single ON/OFF switch to turn it off and nothing seems to be happening (i.e. big machine takes time to slow down) the user might think he has not turned it off and inadvertently turn it back on. That's why I want separate ON and OFF switches.

Originally Posted by BobL

3P machines typically use
To turn on - a momentary NO switch is closed and this causes the contactor/main switch to trigger ON and latch (stay ON).
To turn off - a (NC) switch is momentarily opened that cuts the power to the main switch which de-latches (stay OFF)

Isn't that exactly what "3 wire control" on the VSD does?

Stuart

p.s. oh wait what VSD are you talking about. maybe you dont have 3 wire. In which case you just need a 2 pole contactor to do the latching for you. (or 2 2 pole contactors if you want to get really excited)

p.p.s I dont like circuits that have to be "made" to switch machines off.