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View Full Version : On Topic: Another "Sourcing" Question; Control Transformers?



Fasttrack
08-04-2012, 09:39 PM
Anyone have any leads on control transformers in the Midwest? I've found a few places selling them but I was surprised at the cost. I'd like to buy one for less than $50... used is fine. The used ones I've seen are quite a bit more than that and the new ones cost more than I paid for the entire machine!

The transformer in my Kearney Trecker 2D blew up a while back. It's in the upper left hand corner of the picture, only now there is black, sticky, burned varnish/goop all over the wiring diagram and transformer and it reeks of burnt windings. The original is a type S1, 0.25 KVA transformer with 220/440 v primary to 120 v secondary rated at 2.27 amps. It was made by Sorgel.

http://i108.photobucket.com/albums/n22/fasttrack237/Kearney%20Trecker%202D/P1010054.jpg

I'm still not sure what happened since I wasn't there, but I suspect it may have been a condensation issue. The breaker was on but it sat for several weeks during the spring without being touched (this was when it was on my BIL's farm and I had moved to Indiana). I got a call from my BIL one afternoon asking why the milling machine wasn't working. Long story short, all these machines sweat like crazy in his shop during the spring time (the shop floor is 5' in the ground, so the concrete and big chunks of cast iron tend to sweat a little even during humid summer days) and I suspect that water somehow shorted out the transformer.

DFMiller
08-04-2012, 10:02 PM
This does not quite meet your price point but much better than the Grainger price.
http://www.triadautomation.com/product/Acme-TB-81213-Control-Transformer-250VA-220230240-440460480--120115110-Secondary-Single-Phase-1247.cfm

Dave

Duffy
08-04-2012, 11:38 PM
If you can find someone in the HVAC service business, they should be able to help. I would expect these to be common on the controls for rooftop units that serve freezer and refrigerator cases in supermarkets.

J. R. Williams
08-04-2012, 11:51 PM
Why not bring in a 120 volt line and forget the control transformer? Then take your time to locate a surplus transformer.

macona
08-05-2012, 02:07 AM
Here you go, $23.50 in indiana:

http://www.ebay.com/itm/GS-HEVI-DUTY-25KVA-STEP-DOWN-INDUSTRIAL-CONTROL-TRANSFORMER-T250-SMT-180-/290713107700?pt=BI_Circuit_Breakers_Transformers&hash=item43afd9fcf4#ht_2511wt_1127

rdfeil
08-05-2012, 02:22 AM
+1 on JR's suggestion. Just bring in a 120 volt source from a wall outlet. Just remove the transformer and hook up a simple line cord. There are a couple of things to check when you do this. First make sure there is a fuse in the control circuit, If there is then you are good there, if not then add a 2 amp fuse. Next, and very important, check the connections on the 120 volt side of the transformer. It is very common for the unfused line (think neutral) to be grounded to the frame of the control panel. This gives the control circuit a ground reference for safety. If this is the case then disconnect the ground from the control circuit and hook the neutral to the control circuit at that point and hook the line cord ground to the frame ground. This will keep the safety ground and not place a ground on the neutral wiring of the shop. Plug in the cord and start milling :D.

lakeside53
08-05-2012, 02:39 AM
I'd just get a transfomer like Macona found. Bringing a separate 120v ac supply to a three phase machine is not a good idea - you now have two points of disconnect.

Hopefuldave
08-05-2012, 08:42 AM
I'd just get a transformer like Macona found. Bringing a separate 120v ac supply to a three phase machine is not a good idea - you now have two points of disconnect.


In addition to the "two disconnects" problem, a transformer isolates the 120V control circuit from ground, so there's a lot less chance of electric shock should coolant find its way into a switch somewhere - a big plus to my mind! Definitely use a transformer, for your own safety!

My old Holbrook lathe had all the original contactor coils and control switches running at 415V (ouch!) so as part of the update to fit a VFD and get 415v 3-phase for the motor (from 240V single-phase) I fitted 24V-coil contactors and a 70VA control transformer, and 24V indicator lamps in place of the originals (which were 6V lamps powered from *extra windings on the contactor solenoids*!)

I also found a way (a relay flip-flop circuit from a 1920's book on electrical engineering!) to make the coolant push-button "toggle" the pump's own VFD power, rather than having to power off the machine to turn the coolant back off... Can provide the wiring (needs a pair of 2- or 3-pole changeover relays) should anyone have a similar application!

Dave H. (the other one)

J Tiers
08-05-2012, 10:12 AM
I'd just get a transfomer like Macona found. Bringing a separate 120v ac supply to a three phase machine is not a good idea - you now have two points of disconnect.

AND, two OTHER issues....... one a serious safety issue

1) the 120V can fail/go off on its own, which will drop out the controls in the middle of a cut maybe.

2) if the main power goes down, the 120 may stay up... then you no longer have the safety feature of the controls being "off". The machine will "look like" it is "off", but it is actually "on". When main power comes up again, the machine will start automatically.

With both from one source, the contactors will drop out and the machine will not re-start automatically.
.
.
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Sola (hevi-Duty) and Hammond Power Solutions (HPS) both make good control transformers.

BTW, control transformers are made slightly different from "ordinary" transformers.... they are intended to power relays and solenoids that have a higher inrush current, and are generally "lower impedance" for their nominal size. So a 250VA control transformer application *may* need a larger unit in nominal power if an "ordinary" type is used to replace it. Not always, but.....

Fasttrack
08-05-2012, 10:30 AM
Darn! I looked on ebay last night and couldn't find anything I was looking for. Now that listing Macona posted is no good - someone must've snagged it before I saw it.

I thought about running a separate 120 v line to the control box but decided that was a safety concern. I may still do that as a temporary "fix" until I find a transformer, but I don't really like the idea. As JT, Lakeside and Hopefuldave pointed out, there are some legitimate concerns there.

Thanks JT for the tip on control transformers. I wondered what made "control transformers" different from "power transformers". Good to know!

Fasttrack
08-05-2012, 10:31 AM
Here you go, $23.50 in indiana:

http://www.ebay.com/itm/GS-HEVI-DUTY-25KVA-STEP-DOWN-INDUSTRIAL-CONTROL-TRANSFORMER-T250-SMT-180-/290713107700?pt=BI_Circuit_Breakers_Transformers&hash=item43afd9fcf4#ht_2511wt_1127

What did you use as search parameters to find that? All I can turn up on eBay are voltage converters that are meant to plug into a European outlet and give you 110 with an American outlet on the other side.


Oh ... never mind. Looks like I must've been searching for "220 v to 110 v" or something like. When I search "control transformer" I get all kinds of hits.

Thanks!

MaxHeadRoom
08-05-2012, 11:01 AM
In addition to the "two disconnects" problem, a transformer isolates the 120V control circuit from ground, so there's a lot less chance of electric shock should coolant find its way into a switch somewhere - a big plus to my mind! Definitely use a transformer, for your own safety!
Dave H. (the other one)

In N.A. anyway, it is optional whether to re-reference the 120v control to ground, but the general practice is to ground one side of the secondary and set up a 'local' neutral to re-reference to ground.
Max.

lazlo
08-05-2012, 12:38 PM
In addition to the "two disconnects" problem, a transformer isolates the 120V control circuit from ground, so there's a lot less chance of electric shock should coolant find its way into a switch somewhere - a big plus to my mind! Definitely use a transformer, for your own safety!

Like Dave says, that's the whole point of a control transformer -- to isolate the operator from a short!

Tom: Ebay. There are dozens of new control transformers for not much more than the price of shipping.

Fasttrack
08-05-2012, 12:49 PM
Thanks guys!

I think I'll still buy one, but the pressure is off ... you're going to laugh, but I actually found a 0.25 KVA transformer on the other side of the milling machine. There is a storage cabinet built into the cast iron column and I never thought to look in there. Opened it up and I found two oil cans and a transformer. It has a 120 volt plug on it and I think it was used to power the DRO at one point. It was wired up for 480 to 120, which means it was an addition from two owners ago. The fellow I bought it from ran it on 208/220 and had the DRO plugged into a wall outlet.

Anyway, I've got a couple of questions:

Is there any reason why I shouldn't remove the sheet metal enclosure around the transformer? The size of the laminated core + windings is identical to the size of the old Sorgel one, but this one has a "type I indoor enclosure" to conceal the connections. Since this is going in an enclosure, I don't see any issue with stripping the sheet metal off of it.

Also, this is UL listed as a power transformer, not a control transformer. It is not an autotransformer, so there is still isolation, but not sure about the impedance. JT, you mentioned that I may need a larger power transformer to match the characteristics of the same nominally sized control transformer. Anyway to check/test this power transformer?

EDIT for Transformer Details:
Dongan brand
Insulation class A, 55*C temp rise
Cat. No. 35-1020
250 VA

Hmm ... catalog says its rated for 2 amps output at 120 volts. The Sorgel was rated at 2.27 amps at 110 volts. Just checked the control box again and now I've got even more questions...

I measured the resistance of the coils. Printed on the relay coils is "110V 60 Hz", but when I measured their resistance I got between 57 and 62 ohms for each one of them. I disconnected all leads to the coil to make sure I wasn't measuring the sum of parallel resistances in some kind of circuit I wasn't aware of. I also checked my crappy HF meter against a known resistor (which was verified to be 49.2 ohms using a calibrated Keithley bench meter). Am I crazy or does 60 ohms seem really low? 2 amps in a relay coil?! I also noted a small amount of heat damage to one tap on two coils. Incidentally, these two coils control the relays for the table feed and the spindle, so they spend the most amount of time in the "on" position.

Also, there is an Allen Bradley timer in my control box. What the heck is the timer for? I sure wish I could read the wiring diagram...

J Tiers
08-05-2012, 01:59 PM
AC coils have low resistance, and also low impedance when the power is first applied.... after pull-in it is high, the coil now has a 'core".

it is not unusual to have a 10:1 difference..... which accounts for the 'control transformer" issue.

Try it... if it works, fine... if you can vary the voltage and it works at the lowest mains voltage, that's even better, and is the best test. Not every machine needs monster power, the 250VA may just be one the mfgr had as their smallest standard unit.

Fasttrack
08-05-2012, 02:03 PM
AC coils have low resistance, and also low impedance when the power is first applied.... after pull-in it is high, the coil now has a 'core".

D'uh. I feel pretty silly. :o

Thanks for the info!

MaxHeadRoom
08-05-2012, 02:06 PM
Like Dave says, that's the whole point of a control transformer -- to isolate the operator from a short!



Not according to current practice of every CNC and control cabinet I have come across, last time I saw isolated control was back in '72.
Household 120v is also grounded for safety reasons!!
(REF: NFPA79).
Max.