OT: Proximity Switch Telemecanique XS4P30MA230: Can this switch be wired in series?

OT: Can two of the Telemecanique XS4P30MA230 proximity switches (door interlocks on a drum mixer) be wired in series to control the Enable function on a TB Woods 50 HP, 480 volt variable frequency motor drive?

The drive has a 24 VDC control circuit. To enable it, you connect +24 vdc from the drive to a switch contact, then back to the EN terminal. I know that ONE proximity switch will work, but will two in series? Will the first switch close and send voltage to the second, allowing it to function if it sees a target? Will the additional current exceed the 200mA (DC) limit of the second switch? Will the added resistance be two much for the drive transistor detector circuit?

If this is not recommended by the manufacturer you should use relays for each circuit and then use the contacts from the relays as your control signal

Lancie1 said:

OT: Can two of the Telemecanique XS4P30MA230 proximity switches (door interlocks on a drum mixer) be wired in series to control the Enable function on a TB Woods 50 HP, 480 volt variable frequency motor drive?

The drive has a 24 VDC control circuit. To enable it, you connect +24 vdc from the drive to a switch contact, then back to the EN terminal. I know that ONE proximity switch will work, but will two in series? Will the first switch close and send voltage to the second, allowing it to function if it sees a target? Will the additional current exceed the 200mA (DC) limit of the second switch? Will the added resistance be two much for the drive transistor detector circuit?

Click to expand...

Lancie
I believe the switch you have is identical to one on a machine we have been having problems with. Had a timer relay to prevent reversing a motor instaneously - problem was the timer relay would not reset reliably pull it out and plug it back in and machine would start -- after the time delay of course.

Anyway the prox (only two conductor) was in series with whole control circuit - we wondered if prox was unable to handle the current draw by the motor start relays - what gave us this idea was the LED on the timer glowing dimly. What we did is have prox control a small relay (interposing) whose contact was in the control circuit. Problem has not reocurred since this change HOWEVER it has been only one day. Appears foolproof for now or until a better fool comes along.

Based on our experience I would have each prox control a relay then have relay contacts in the control circuit. Granted you have VFD (higher impedance and lower current) wheras on our machine the load on the prox switch was the two motor contactors (lower impedance and higher current).

Dan Bentler

I would probably use a safety monitoring relay in a situation like this. I have no idea about that specific device.

Thanks for the comments, guys. I may use two old-fashioned limit switches instead of the prox switches. I cannot add a relay without getting permission from the customer. It would require a power source. I think the prox switches might last longer in the corrosive environment, but if two in series will not work, that eliminates that type of switch.

I don't think there is a safety hazard to personnel if one of the doors is left unlatched, but it could spill material (urea fertilizer) out the door, which creates a clean-up problem.

Lancie1 said:

Thanks for the comments, guys. I may use two old-fashioned limit switches instead of the prox switches. I cannot add a relay without getting permission from the customer. It would require a power source. I think the prox switches might last longer in the corrosive environment, but if two in series will not work, that eliminates that type of switch.

I don't think there is a safety hazard to personnel if one of the doors is left unlatched, but it could spill material (urea fertilizer) out the door, which creates a clean-up problem.

Click to expand...

Lancie
Can you get prox switches with integral switch (ie 3 wire better yet 4)?
The mechanical switches will have a shaft thus leakage, plus the switch body will probably be aluminum. Dont think aluminum and urea are good mix.

The other thought I had was that the VFD power can be tapped with small transformer to provide power to the two proxes and relays. More cost of course.

I think your customer should think about 2 to 5 years down the road when the mechanical switches disentigrate from urea. A few more up front bucks will save many maintenance and down time bucks.

Dan Bentler

The difficulty with cascading proxes is that there is a small voltage drop across each prox. When you wire them in series, the drop is cumulative. The cumulative drop could result in the voltage falling below the detect threshold on the drive. If you can't verify that it will work before committing to the design, then go with the mechanical switches.

Thanks, Steve.

That is what I thought might happen. I had never seen proximity switches wired series, and your explanation provides the reason.

Now I need to talk to the boss about this. If he wants to make it long lasting using corrosion-resistant prox switches, we will have to add a control box with two relays, so each prox controls only 1 relay.

Now that I think about it, it could be done with 1 relay. 1st prox activates the relay coil. 2nd prox wired from +24 thru relay contacts back to the Woods drive. If either prox switch is not closed, drive doesn't go to enabled mode.

Also thanks to Dan. Your comments about the mechanical switches are on-target. My experience with urea is that it will attack iron, stell, and aluminum. I was looking at some switches with plastic enclosures. Even so, the shaft and roller head have metal parts. The roller will probably jam up all the time.

Anyone know of a limit switch with stainless steel parts in a NEMA 4X enclosure?