Siemens 1200 Micro - 0-10 VDC - Ratiotrol VEL/VEH 150

[ZestyMozzarella]

I have a conundrum with this current circuit. I have a Siemens 1215C AC/DC/Relay and an old drive - Ratiotrol (now Boston Gear company I believe), VEL/VEH 150 model. It has a potentiometer for speed I decided to hijack because I want HMI controls for it.

My 1215C has two analog outputs. I put them across a 500Ohm resistor pack and put a 0-10VDC signal on the COM and Wiper of the old POT location. It scales just fine 0-10VDC like the POT provided. Issue being...the drive and motor runs at 7FPM when it's a 25FPM drive maximum. Since I hijacked the location, the 0-10VDC signal I can see full scale on my meter coming from the PLC feeding the same area the POT did. I figured some kind of noise or issue was present in the signal, so...

I put a signal isolator (random old one as well) in the line. This improved the situation. Still 0-10VDC scaling IN and OUT of the isolator. Now the drive scales from 0-12 FPM but not the full range...annoying.

Theoretically in my mind, I can see the same 0-10VDC the old POT provided yet the full drive scale doesn't appear and I cannot figure out why that would be....shouldn't be any different. The only thing I can think of is perhaps the older drives need a higher current source than what the PLC or the Isolator can provide for the full range? This old Ratiotrol drive feeds a 180VDC Arm, 7.7 Amps, 1.5 HP, 1750 RPM DC motor.

Does anyone have any idea why this might be? My isolator is powered on 24 VDC but the supply is a 5A and not being pushed to max, I was thinking I could up it to a 10A but I don't think that matters here. The only thing I can think of is some kind of loop issue perhaps. These old drives have poor grounding and are hard to find schematics on. I'm thinking I should just upgrade the drive to a KB Electronics or GSD8-120-10N Automation Direct drive of sorts with an analog control module.

I'm open to ideas of getting this working sooner than sourcing new equipment though if anyone has any thoughts? Thanks!

 

[ZestyMozzarella]

Hmm...now that I think more about it...I don't think there is anything I can do in a digital age except to upgrade it. The more I think about it, the old circuit was always 10VDC static on that 5K Speed POT. The resistance across that POT was the key factor being varied...I'm now varying the 0-10VDC signal with no way to control resistance...not the same control method for that circuit. Just a different standard for back then compared to modern now...rough heh.

Certainly can comment if anyone has thoughts but I think upgrading to a modern KB Electronics drive might be my most viable option.

 

[BryanG]

There is a thing called a Digital Potentiometer that could help, there is a video about one here:
https://www.youtube.com/watch?v=c8eTzigpJu8
Not sure how you interface it to a PLC as it is a 5V device, probably need some resistors and zener diodes.

 

[ZestyMozzarella]

Noice! I was just sitting around with a co-worker today like, "I wonder if a digital POT is a thing?" Appreciate the share, that is pretty entertaining. Born from a need to jerry rig no doubt!

I think we're chasing some drive upgrades but I may pick some of these up just to have on hand. Lord knows I got enough old equipment laying around the joint just asking for one in the near future

 

[Steve Bailey]

Are there three wires connected to the pot or only two? Two wires suggests that your assumption is correct and the resistance across the pot is key. Three wires suggests you're connecting the analog signal from the PLC to the wrong place on the drive. In the three-wire case, the analog signal should connect to where the pot's wiper was connected and the DC- of the analog module should be connected to the DC- on the drive.

 

[ZestyMozzarella]

Yes, three leg POT. I hooked the PLC up like the picture shown across a 500 Ohm resistor. On the other side of that resistor, I got the COM going to pin 1 and the Positive going to Pin 3 being the wiper position. I can drive the 0-10VDC there and see it on my meter but even out of a signal transmitter on the end to the field device, I only get 12.5FPM out of a 25FPM drive. I can only assume the current is not sourced correctly because the POT being specifically 5K must matter to balance the old analog circuit? Uncertain actually but that is my guestimate.

 

[Steve Bailey]

It looks to me like you should see +10 VDC on terminal 4 of the drive with terminal 1 being DC-. I would try leaving the pot in place but disconnecting the wiper from terminal 2. Replace the wiper with the AO+ wire from the PLC and connect the AO- wire from the PLC to terminal 1.
In my opinion, you would only need the 500 ohm resistor if you had a 4-20 mA analog output, but you said you had a 0-10 VDC signal. Also dropping a 4-20 mA signal across a 500 ohm resistor gives you a 2 - 10 volt range instead of 0 - 10 volts.

 

[Puddle]

"I wonder if a digital POT is a thing?"

Motorised potentiometer! Tiny motor coupled to a rotary pot. Two buttons to increase and decrease reading by spinning the motor forwards and backwards.
Not quite the same, but works on an analog circuit.

I was on a drives course a few months ago and they brought up that some VFDs have this as a built in feature so you can use two DI to control the speed. Many heads were shaken about such an antiquated control method and I had to sit there and tell people that we still have a production line using one for the master speed. I can replace it in five minutes with two lines of ladder, but it's not broken yet.