View Full Version : Control circuit help

04-22-2013, 02:09 PM
I'm just rigging up some contactors for mechanical fwd/rev for my lathe and I need some help designing a suitable circuit.
In the attached diagram I have a fwd-off-rev switch to fire each of the two contactors and I'm getting some NC auxilliaries to go on top to make an interlock (not shown yet). I also intend to make the switch low-voltage but it's shown switching mains voltage right now for simplicity.

Here's my problem. The drive has a run input which is +10V high active, and a +10V supply to feed it with. I plan to use a relay to invoke this AFTER either the FWD or REV contactors are pulled in which is easy enough. Where I am falling down is I want to also make it so the run is disconnected by moving the switch to middle (not connected) but BEFORE the relay drops out.

So simply, hitting FWD or REV brings in the relay then invokes RUN. moving the lever from FWD or REV disengages RUN then drops out the relay. How do I do it?


Paul Alciatore
04-22-2013, 02:38 PM
Sounds like you need either a five position lever switch OR a three position switch with time delays.

I would recommend the time delay approach because you could not operate it too fast. Forward - set forward relay - delay - start. Stop - delay to coast to stop - deactivate forward (or reverse) relay - Reverse - set reverse relay - delay - start. The built in delays would prevent powering in the opposite direction while still coasting in the original one. The Reverse command could be given before the coast is finished and it would just be delayed until the coast delay had expired.

The elegant way to do this today would be with a PIC chip but that would require programming. It could be done with time delay relays or delay circuits liked a 555 timer or even simple R/C networks. One point would be to require a "Stopped" relay to be deactivated before any run command would even start. It would be activated when either the forward or reverse condition exists and would have a time delay before deactivation to allow the coast to stop.

I will try to do a block diagram.

04-22-2013, 03:05 PM
Before going a timer or pic route, you may want to look into a Smart Relay, this may be slightly overkill but it will give you what you want and more, also it could provide a delay on the FWD to REV contactors to prevent instant reverse.
Also it may compensate if you are not using reversing contactors with mechanical AND electrical interlock?
The Smart relays are relabeled under several different brands, Siemens, Idec, Square D etc.
Ebay is a good source.

04-22-2013, 04:58 PM
I always get a bit nervous when I see a wound-field motor without a field loss relay... Lose the field supply and the motor can overspeed and do "interesting" things... A transformer before the field rectifier, with a *pair* of low voltage windings would monitor field current, one winding in series with the AC field current, the other connected to the field loss relay coil, so loss of field current will drop the armature current. Rate the transformer for a few volts x full field current, safe as can be.

Re getting a delay, how about a dc coil relay fed by a diode / bridge and resistor, with a fat capacitor across the coil? The time to charge the cap would delay the relay closing, the charge remaining in it would delay opening...

Just my ha'pennorth,
Dave H. (the other one)

04-22-2013, 05:07 PM
A rough outline of a fairly simple solution (forward section only shown):-


The R/C delay cause the relay to be activated a little after the switch is closed, and released a little after the switch is opened.

The 'forward' output goes high immediately the switch closes (via diode D1), but is held high by the relay contact after the switch opens. It only goes low after the relay drops out.

The 'run' output goes high via the relay contact a little after the switch closes, but goes low immediately the switch is opened. The diode D2 allows the 'run' signal from forward and reverse sections to be combined.

D3 prevents the C feeding current back out via R after the switch opens.

R and C values will depend on relay characteristics and the delay required.



04-22-2013, 05:08 PM
One thing I forgot to mention on the Smart Relay, is if you have a few spare I/O left over, it can be used to control any other functions you may need to control on the lathe.

Paul Alciatore
04-22-2013, 05:44 PM
Here is a block diagram for a circuit that should do what you want. It has the added advantage that you can move the lever switch from forward to reverse or vice verse, passing through stop and all the necessary delays should be activated in sequence. It is probably not OK to cycle the control back and forth quickly.

I did not fill out the delay blocks. They should be active types and I would suggest using 555 timer circuits for them.

I hope others will check it out and confirm that it is OK.

http://img.photobucket.com/albums/v55/EPAIII/LatheMotorControl1_zpsa0f79dda.jpg (http://smg.photobucket.com/user/EPAIII/media/LatheMotorControl1_zpsa0f79dda.jpg.html)

I see a mistake already and have corrected it.

04-22-2013, 06:24 PM
Some great stuff to work with here, sorry I am delayed in participating, I am now at work.

If I can point out a couple of things:

Paul: the fwd/rev is a three position switch with no 'stop' circuitry - it's basically a pair of microswitches pressed by a lever and I can't change the design as it's part of a mechanism on the Monarch.

Barrington: the 'run' connection must be closed after the relay, but opened before it.


04-22-2013, 07:09 PM
Barrington: the 'run' connection must be closed after the relay, but opened before it.

Pete.The relay in my diagram isn't the contactor, it's simply an additional component to provide delay.

If you look at the timing diagram, the 'forward' contactor output is activated before the run output, and released after it, as requested.



04-22-2013, 07:47 PM
It's not fancy, but you can get toggle switches that have a mechanical interlock that prevents switching from one selection to the other without stopping in the center ('Off') position. The little DC drive that I use with my Muley sawmill has a Fwd/Off/Rev switch like that.

04-22-2013, 09:12 PM
Barrington my apologies, I mis-understood your diagram, just checking in from work on my phone.

Paul Alciatore
04-22-2013, 10:14 PM

Paul: the fwd/rev is a three position switch with no 'stop' circuitry - it's basically a pair of microswitches pressed by a lever and I can't change the design as it's part of a mechanism on the Monarch.



I misunderstood that you want to work with existing controls.

OK, so what do you have for Start/Stop? More push buttons? A toggle or snap handle switch(s)? Or does the forward/reverse lever do it all. I can work with whatever.

J Tiers
04-22-2013, 10:18 PM
How MUCH delay do you want?

If you check the timing diagrams on the contactor aux switches, you should be able to determine the order of events. It is perfectly possible that an aux can be made to disconnect before the main, and that won't vary, since it is set with mechanical cams inside.. Most reasonable contactor companies show timing diagrams. Cheap chinese ones won't.

However, it won't be a long delay, it will just be "before" the main contactor elements open, probably a few milliseconds.

Don Young
04-22-2013, 10:27 PM
If you want delays in the order of seconds, there are small inexpensive solid state relays available in both 'delay-on-make' and 'delay-on-break' versions. They are commonly used on HVAC systems.

04-22-2013, 10:37 PM
You don't mention the make of drive, most of the SCR drives if a decent make have a pair of terminals that reset the drive so that in the case of instant reversal of the contactors, the drive is reset so that the the drive goes through normal acceleration rate, rather than being switched from a high rpm from one direction to the other.

04-23-2013, 12:16 AM
Drive is a Parker 512c. It does not offer any braking or reversing hence the circuitry. Rob's 514c does all that onboard but is four times the cost.

Delay I want only momentary, tenths of a second.

Barrington's circuit fulfils my request perfectly, though I have to study Paul's diagram to see what I can incorporate from that as he has gone 'above and beyond' what I asked for.

Braking is going to be mechanical I think as I don't want to risk my precious motor experimenting with resistor braking. I can let it coast down when there's no hurry or hit the pedal when threading to a shoulder etc.

Now to work on a field loss circuit. Minimum field volts is 50v or so, so I guess a simple circuit relay which drops out below 40v would do it.

Thanks a lot to all who have helped me on this.

04-23-2013, 04:51 AM
In retrospect I was thinking perhaps you might prefer to run from a 24vac supply?


- Same principle of using the switch signal and a delayed version, with the contactor energised when either is on, and a run signal when both are on.



04-23-2013, 11:18 AM
If your objective is to ensure the motor has stopped before reversing, there could be a simple solution.
The drive shows it has a At-Zero-Speed output, it would be simple logic to only allow/enable the opposite (FWD/REV) contactor when the drive is at zero?
Use the RUN input also to stop?

04-23-2013, 01:22 PM
BTW, have you seen this?
I assume yours is a 2 quadrant drive?

04-23-2013, 01:59 PM
No I hadn't Max, but it's pretty-much what I am aiming for. Thanks for the tip on the smart relays.

04-23-2013, 02:28 PM
The problem with reversing a DC motor by reversing the drive output like this is when you have a long run down when the armature is disconnected, this can be speeded up by placing a braking resistor across a set of contacts on the reverser placing the resistor across the armature, this requires double throw contacts though.