When to use a Control Transformer in a Motor Starter?
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    Default When to use a Control Transformer in a Motor Starter?

    Hi all,

    I'm in the process of finalizing two combination motor starters to run my rotary phase converter and radial arm saw. I'm using an Allen Bradley NEMA 1 starter for the RAS, and an Allen Bradley NEMA 2 starter for the RPC. The NEMA 1 box has a 509 Allen Bradley contactor and the NEMA 2 box has an Allen Bradley 709. Both will be using Allen Bradley 800 series start/stop switches.

    Here's my question - in my parts bin I have an NOS Allen Bradley X-343858 Control Transformer. What are the guidelines for when to use these? I know they are supposed to stabilize the control circuit current - but on what type of start circuit is this actually necessary? Since I"ve got it - I was thinking that my RPC might be the one to put the transformer on, but these things are cheap - so I can get another for the RAS if it actually offers any advantage whatsoever. If there is no advantage for either piece of equipment, however, I'd happily leave it out and save it for a device that may need it down the line.

    Thanks!

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    The rule is if the control circuit wiring leaves the enclosure, then the voltage is 120 or less. If the control circuits are entirely within the enclosure, then line voltage control, i.e. 240, 480 or whatever is fine. This is the JIC.

    Tom

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    Thanks Tom,

    But what is the functional difference between having the relay/start control circuit voltage pulled from say one leg of the input legs to the contactor (assuming 120v control voltage on a 220v contactor) or 120v for the control circuit that's generated by a transformer?

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    Quote Originally Posted by Citxmech View Post
    Thanks Tom,

    But what is the functional difference between having the relay/start control circuit voltage pulled from say one leg of the input legs to the contactor (assuming 120v control voltage on a 220v contactor) or 120v for the control circuit that's generated by a transformer?
    One of the benefits of a control transformer is that it gives isolation from the mains and, properly connected will give you a manufactured neutral.

    I've built two RPC's in recent years using information from this site and I used control transformers on each.

    I looked up the Allen Bradley control transformer that you mentioned. It's .075 KVA. That should work nicely.

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    The neutral is not normally carried with the three power leads in three phase industrial applications. In all cases I have seen like this a transformer is used. In home use you can do pretty much as you want so long as it doesn't violate local or national codes.

    Tom

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    Before investing the labor to mount and hook up the transformer, be sure it will close your starters. The inrush on a contactor coil is much greater than the holding current and with a transformer that small, the internal resistance may be enough to drop the voltage below pull in. I would guess that it will work, but check it.

    Bill

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    Quote Originally Posted by Citxmech View Post
    Thanks Tom,

    But what is the functional difference between having the relay/start control circuit voltage pulled from say one leg of the input legs to the contactor (assuming 120v control voltage on a 220v contactor) or 120v for the control circuit that's generated by a transformer?
    One advantage is that one need not "assume" a specific line Voltage. The more valuable control transformers have a short ton of taps so sensitive and costly electronics don't have to change when different motors are used for "globalized" goods.

    You may not need that, but it is cheap enough, what with all the salvaged "pulls" from European and Asian CNC machinery out there.

    With a Telco pension? I don't even want 120 V controls.

    24 V rated - some actually run at even less - is common, safer, and gone pretty much standard on LOTS of goods - relays, contactors, solid-state relays and contactors, lighting, DC Drives, VFD, etc. as well.

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    The problem with 24 volt is the size of the transformer needed and fidelity problems with the controls.

    Tom

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    Quote Originally Posted by TDegenhart View Post
    The problem with 24 volt is the size of the transformer needed and fidelity problems with the controls.

    Tom
    Guess you could find a Divorce Lawyer? I like Crydom, but Potter & Brumfield do good work as well.



    Or are you on about signal to noise ratios?

    Tell it to the "vast majority", then. Use of 24 volt for controls has become common as housefly poop. See also HVAC and ignorant doorbells, "Class 2" wiring codes, and more.

    Safety munchkins like non-lethal, non fire-initiating voltages MUCH better than 120 V line voltages. So much so, that some applications you are no longer even allowed 120 V.

    Moreover, it mostly Just ******* Works, and for a VERY long time, already.

    Just try FINDING a major maker who does NOT have 24 V goods in their line of switches, relays, contactors, indicators....

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    Yeah, those 24 volters are sure fire resistant, too.

    enco-transformer-burnout-6-.jpg

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    Quote Originally Posted by Newman109 View Post
    Yeah, those 24 volters are sure fire resistant, too.

    enco-transformer-burnout-6-.jpg
    LOL! yah but.... "It's all relative..."

    YouTube

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    Thermite, you come from an entirely different world than industrial electricals. What you say for the low voltage stuff is great for the telecommunications field, but it caused havoc with the industrial area. Things are much better now, but in the 70's and 80's the industry wasn't prepared for the fidelity issue. Gold plated contacts, bifurcate contacts, reed switches, sealed contacts, problems with static electricity...the list goes on of problems and solutions.

    Silver is a great contact material for 120 and above, but below about 80 volts, it begins to have problems. Contact structures were not intentionally designed for contact scrub. It wasn't needed. Rhodium was for special applications. Everything you gleefully tell us backward farm kids the communications industry learned about decades ago, probably to solve the same problems.

    Yah, us electrical people just got off the turnip truck, but we are learning.

    Tom

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    Quote Originally Posted by Citxmech View Post

    But what is the functional difference between having the relay/start control circuit voltage pulled from say one leg of the input legs to the contactor (assuming 120v control voltage on a 220v contactor) or 120v for the control circuit that's generated by a transformer?
    120V control voltage? Do you have a neutral available for this? I wouldn't think of running a neutral wire along with my two 240V wires for 75 feet just to have 120V control voltage available. Otherwise you need a 220V to 120V transformer.

    You should be aware that your starters have replaceable coils. If you starter coils are 120V you are not forced to have everything at 120V. Change the coil to whatever you want. I use 24V because switches, contacts, and lamps fare better.

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    Quote Originally Posted by TDegenhart View Post
    Thermite, you come from an entirely different world than industrial electricals. What you say for the low voltage stuff is great for the telecommunications field, but it caused havoc with the industrial area.
    Bollocks. Only among those with heads up and locked. The knowledge base is OLD.

    The industries made that trek together, 1850's onward. Telco NEEDED power - and machinery - where power had not even existed before we needed it. Some submarine cables signaled @ 8,000 Volt levels, after all.

    Managing all that, and the power FOR it was among our own on-staff skillsets - mine among them.
    But never mind all that. You and I are just two more Old Farts, now.

    Tell it to Group Schneider, Siemens, ABB, Allen-Bradley, and all the other majors. Browse the distributor's catalogs.

    Simple sanity check, in other words.

    The reality is that 24 V (and under..) controls are here, "control" transformers for them are dirt-common, 120 V and above long in decline, and it has been that way for a VERY long time, already.

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    Sigh....

    Tom

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    OK - the comments about neutrals makes sense to me. Since my shop is basically an upgraded residential service (240v single phase, 600amp panel with a dedicated transformer) I guess I should be using transformers in every 2 or 3 phase 240v motor control box I have (e.g., pre- or post-RPC) since there would be no true neutral wires anywhere in my system. Is this correct?

    Regarding 24v controls - most of my equipment is pre-WWII, and some of it is pre-WWI! I'm pretty much setting everything up period-correct for somewhere between 1950-1970 (ie pre-OSHA. )

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    Quote Originally Posted by Citxmech View Post
    OK - the comments about neutrals makes sense to me. Since my shop is basically an upgraded residential service (240v single phase, 600amp panel with a dedicated transformer) I guess I should be using transformers in every 2 or 3 phase 240v motor control box I have (e.g., pre- or post-RPC) since there would be no true neutral wires anywhere in my system. Is this correct?

    Regarding 24v controls - most of my equipment is pre-WWII, and some of it is pre-WWI! I'm pretty much setting everything up period-correct for somewhere between 1950-1970 (ie pre-OSHA. )
    That could be one way. The cheap one, actually. New guts in the starters, same period-correct looking enclosures? A-B and others still have "reasonably" period-correct switches. Not hard to find a place to hide a control transformer. They needn't be huge.

    I chose to provide a re-derived "local Neutral" and local "independent ground" (PE) as well.

    (Delta-in, Wye-out EGS/Hevi-Duty "Drive Isolation" transformer).

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    If, as I stated, the starter is completely contained, then use line voltage control. Change the coils if necessary.

    Tom

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    Thanks everyone for your answers! I think that you have given me the info I need to block out the current flows and connections necessary for my installation.

    Thanks Thermite for the heads-up on the Delta/Wye transformer option!

    PS: Just to close the loop on my question (and illustrate the immense benefit of forums like this) after reviewing everyone's responses, going back to my picture and wiring diagram/picture stash, and revisiting my assumptions - I figured out where my major misunderstanding of how A-B mag starters work was.

    I thought each 509/709 contactor had it's own transformer built in, not just a single replaceable coil for closing the circuit. In many of the wiring diagrams I have collected, the separate transformer was included in the circuit drawing. It seemed reasonable to me that a 240v contactor would be designed with a 120v coil/control circuit that was able be fed by the provided 240v input voltage. It never occured to me that many of the pics I was looking at that didn't have transformers were contactors that had 240v coils and control circuits (all of my contactors are 120v coils). So in essence, I was asking why you'd want to put in a second transformer.

    Anyway, as usual, it's what you don't know you don't know that'll ruin your day - and thanks again to everyone's patient responses in helping clear this up.
    Last edited by Citxmech; 06-18-2019 at 12:14 PM.


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