Connection between VMot and Control PWR on MDC2230

Hello all,

I am using an MDC2230, with a relay breaking the 24V power to Vmot. But maintaining 24V on the Control Power input, to keep logic power on.

The relay could weld closed after lots of use because it's breaking high currents. So I want to know for sure that it has opened - if it welds shut it could lead to loss of safety. Unfortunately no force-guided relays exist for 100A (that could check the relay), so my intent was to put a (~5/10k) pull-down resistor across Vmot & ground; and then monitor the voltage at Vmot. So if the relay welds shut, then Vmot stays high.

Now there's a fly in the ointment: in initial testing, VMot appears to be linked to Control Power: test was 24V into Control Power, and 5k resistor pulling Vmot to ground, with the relay closed, I still measure 22V at Vmot.
I wonder - is there a way of breaking this link so I can retain the functionality that I want? Or can I run Control power at 12V, while still running Vmot at 24V. (so the linked through voltage will be 12V not 24V)?

Thank you for your attention - and any help with this would be hugely appreciated.

Frank

Do remember that you should have a pre-charge resistor across the contacts. With that resistor the output side will remain high, but there will be a voltage drop across the resistor. I use a pair of voltage dividers, one on B+ side, one on the output side, on my HDC2450 controllers and compare them to check for fused contacts.

Hi LROBBINS,

Thanks for this - yes I forgot to add that detail. I've got a 1k precharge resistor across the contacts, so when the contactor is correctly open the Vmot would read 20V ish (with 5K pulldown). Then I was going to compare that with a similarly divided voltage from the main 24V supply (which is a large 130Ah battery so fluctuates) - let's say at a nominal 22V.

I've realised that using a pull-down resistor on VMot also reduces the effectiveness of the pre-charge resistor. Without a pulldown the precharge means that the capacitors rise to the line voltage so there's no spark&large inrush current, whereas with a pulldown there will be 4 volts' worth of spark and inrush into the capacitors when the contact closes.

I'm interested at your solution, but can't quite understand it, sorry for slowness - by "B+" do you mean Control Power (battery+)? And "output", do you mean the motor output? Will the motor output directly link through the VMot without a signal?

Thanks again,

Frank

By B+ I mean the main wire from the battery to the input side of the contactor. By output I mean the wire from the contactor to the Roboteq. Both voltage dividers are Vin - 47k - 4.7k - GND with the taps between the two resistors monitored by analog pins of an Arduino Nano. Hence, they present very little drain for the pre-charge.

Hi LROBBINS,
Thanks for this - sounds very similar to what I was thinking of doing, comparing the Vmot to the supply rail into the relay; but with effective 52K "pulldown" through the voltage divider. The 52k should have negligible effect on precharge - good.
And so you're measuring 2% extra resistance of the precharge resistor (1k compared to 47k) between sides of the relay. That sounds sensible.
Thanks again for your help.
All the best,
Frank

Let me add something. In general it's a bad idea to open the contactor when there is significant current flow - the contacts will not last very long, for some contactors as few as a few hundred cycles. In my system I'm using a TE Connectivity (Tyco) V23132-B2002-A200, but to avoid the coil current when the system is idle I do disactivate it - but my script opens or closes it only when there's almost no current flowing even though it has a life of tens of thousands of cycles at rated current. Because I'm actively controlling it through a Roboteq Digout, the script also has its own subroutine equivalent to the Roboteq's "no MOSFET fail" action. (BTW, I did clear this way of doing things with TE applications engineering before setting things up this way.)

You might also want to study two application notes from Tyco: "Coil Suppression Can Reduce Relay Life" and "The application of relay coil suppression with DC relays". The usual practice of putting a reversed-bias diode across a relay coil is not a good thing to do for DC relays as it delays armature movement on release, and can even cause armature bounce. They suggest other ways of protecting your switching circuitry.