Thread: How to choose the proper magnetic starter for 20 HP RPC

Folks,

For reasons of both cost as well as buying me time to surf the "DIY home electrical engineer" learning curve, I intend to install a bare bones, 20 HP RPC to power a Gorton tracer mill. The mill unit consists of a 2HP spindle motor (230V 3 ph) plus an outboard hydraulic power unit ("HPU") with its own 3 HP motor. The mill has a large fuse box mounted on the side. Being of 1962 vintage it is an antique. The mill originally had a flood coolant tank and pump inside the pedestal. So, three (3) separate motors were wired into, and protected by, the fuse box mounted on the side of the mill. I hope to assimilate this original wiring into my 3 phase RPC distribution panel.

Our location is somewhat rural. The power fails with distressing regularity. Several members here have admonished me of the dangers inherent to "locked rotor condition" when an RPC and load motor(s) loses power supply voltage, spin down, and then the power comes back on.

So, as soon as I can afford to do so, a magnetic starter will be the first "frill" I intend to acquire.

In connection with magnetic starters I have a few questions on which I hope y'all will shine some answers.

First, "derating". If I understand how the circuit goes together, the magnetic starter will be between the single phase supply and the three phase motor. I'm told that you can use a 3 phase magnetic starter on single phase but that you need to derate it.

The second point of confusion: "Coil voltage". Some starters have a 120V coil. Others have 240V coils. I assume that the "coil" and the "coil voltage" or what create the electromagnetic force which clamps the starter contactor(s) into the "on" position. When power is lost, electromagnetic contact is lost, and the thing disengages. Yes?

I would like to know whether it makes any difference which coil voltage to use and/or what the considerations are for choosing one voltage over the other. If I understand how they work, I would tend to prefer a 240V coil as this could be energized by the same conductor that feeds the RPC. Or am I in error?

I have also seen references to "reversing" and "non-reversing" starters? I would appreciate an explanation of what those terms mean. All I'm trying to do is protect the RPC and the motor(s) it powers. The simpler the solution the better I will like it.

One additional consideration: I have two motors for the project. Both are Dayton motors. But one is a 20HP and the other, a 25HP motor. If I go to the expense of buying a magnetic starter, and since I will apparently have to derate it for single phase, I would prefer to buy one that would work with EITHER motor in case I ever need to swap the 25HP unit in and the 20HP motor out.

It sounds like a size 4 starter might be better?

These things are prohibitively expensive new and not exactly cheap used on ebay. Do any of you have a ball park description of brand, model, size, specifications (reversing, non-reversing, etc.) likely price-point, and/or suggestions on where to shop for one?

I think that covers things for now. I'm hard at work running 200 amps to a new series of disconnects and associated 200 amp sub-panel. Once the single phase panel is installed I will proceed to install the 20HP Dayton motor, the output of which will feed into a Square D 3phase breaker panel. Once that's done I will no doubt have a boatload of new questions about the 3phase distribution panel as well as questions on subsequent improvements to the system.

Although it will be a separate and future topic I also hope to tap the collective wisdom about whether I will be able to incorporate the Gorton mill's old electricals into the new system. There is a large and ancient paper schematic folded up inside the cabinet.

In summary: What I hope to learn is: What brand, size, rating, specification, price, and possible source of supply will cover my bases here? I hope to end up with a high quality, reliable, and safe 3 phase system that will have more future expandability than I am likely to ever need.

Thanks!

Vernon

First, "derating". If I understand how the circuit goes together, the magnetic starter will be between the single phase supply and the three phase motor. I'm told that you can use a 3 phase magnetic starter on single phase but that you need to derate it.

Usually a tag on the unit, claiming HP ratings at various voltages, single and three phases.

One member mentioned the IEC units with integral overcurrent, on those, you will need to feed one of the lines back through the "spare" set of contacts. THANK YOU, whoever mentioned that. NEMA stuff is more straight forward.

The second point of confusion: "Coil voltage". Some starters have a 120V coil. Others have 240V coils. I assume that the "coil" and the "coil voltage" or what create the electromagnetic force which clamps the starter contactor(s) into the "on" position. When power is lost, electromagnetic contact is lost, and the thing disengages. Yes?

Yes, you are correct. If/when a voltage mismatch between supply and coils are encountered, that's where a "machine tool transformer" comes in handy.

I would like to know whether it makes any difference which coil voltage to use and/or what the considerations are for choosing one voltage over the other. If I understand how they work, I would tend to prefer a 240V coil as this could be energized by the same conductor that feeds the RPC. Or am I in error?

24 volts is safer, I recall the world record for lowest voltage electrocution is 36 volts or so.

I have also seen references to "reversing" and "non-reversing" starters? I would appreciate an explanation of what those terms mean. All I'm trying to do is protect the RPC and the motor(s) it powers. The simpler the solution the better I will like it.

I know of "reversing, mechanical interlock".... two contactors, with a linkage to prevent both engaging. Used for hoists, elevators, conveyors, etc. Most likely overkill for a mill.

One additional consideration: I have two motors for the project. Both are Dayton motors. But one is a 20HP and the other, a 25HP motor. If I go to the expense of buying a magnetic starter, and since I will apparently have to derate it for single phase, I would prefer to buy one that would work with EITHER motor in case I ever need to swap the 25HP unit in and the 20HP motor out.

You really aren't starting under load, right? Not sure if those two sizes are bracketed to one size starter.

These things are prohibitively expensive new and not exactly cheap used on ebay. Do any of you have a ball park description of brand, model, size, specifications (reversing, non-reversing, etc.) likely price-point, and/or suggestions on where to shop for one?

Motor shops, industrial surplus, industrial electricians, Dumpsters, etc.
Hang out at the entrance to the scrapyard, see what comes in? Scrappers have NO idea what they have, and if you offer more than the scrapyard, it's yours. Don't drool, you will give yourself away.
This came out of an employer's Dumpster.

Place a Wanted ad in the Tools section of craigslist. Call around, know exactly what you want, have a wish list. Some huge (obsolete) NEMA contactor with one bad set of contacts (out of three sets) would be adequate for running the motor, since you are supplying single phase. Don't quibble about the coil voltage, have them toss in a machine tool transformer, if needed.

You find the right guy, he might toss in some freebies, like this enclosure. My guy does.

Okay, others can answer the rest.

Steve

One last thing:

Originally Posted by S_W_Bausch

24 volts is safer, I recall the world record for lowest voltage electrocution is 36 volts or so.

Actually I think the record is probably lower, but may not exactly "count". I read that somewhere in the military records (might have been US Navy) there is an incident where a trainee set out to measure the resistance of himself in Ohms. ge did this holding the probes in his hands. Then he stuck some kind of probe into himself on each hand...then applied the ohm meter again, and killed his dumb self. Many ohm meters use either (1) or (2) 1.5 volt dry cell batteries.

Bill

A few general comments. Terminology: a contactor is the power switching unit; a starter is a combination of a contactor and an overload relay. An overload relay is a thermal analog of a motor, the heaters are chosen to match as close as possible the the heating of a motor. A starter can only protect one motor because of the overload relay. For what is needed here, only a contactor is needed. Most surplus switching units are starters. They can be used by removing the overload relay part and just using the contactor. A point of caution here, this applies to US or NEMA starters. Some European or Japanese starters have the OL as an integral part and are difficult to separate.

Next, Hp vs current ratings and how to handle the coil issue. Also proper wiring for the coil circuit.

More later, my wife says it is time for me to leave.

Tom

Continuing, the coil of American NEMA starters are readily replaceable. There are coils available for just about any conceivable voltage. The problem is finding one at a reasonable price. With so many contactors and starters on Ebay, it may be just as well to buy a unit with the coil you want. As to the best coil to use, that largely depends on the application. If the pushbuttons and other control devices are inside the same inclosure with the contactor, use line voltage, in your case 240v. Control lines that leave the enclosure should be 120 volts or less. This is a JIC standard. As to lower than 120 volts, the limiting factor is the coil current as most control circuit components are limited to 10 amperes.

The coil should be set up with a holding interlock (this is the expression for a auxiliary contact that is part of the contactor, usually a small switch mounted on the side of the contactor and operates when the coil is energized) so that when the coil power is shut off for any reason, that the contactor remain off until the system is restarted. The way this is done is to have one side of the coil power go through a normally closed pushbutton (called the STOP pushbutton), that is, one that when not operated is closed electrically. From the NC contact power is connected to a normally open pushbutton (the START pushbutton) then to the coil. The other side of the coil is connected to the other side of the coil power circuit. Wired in parallel with NO pushbutton is a NO contact (the holding interlock) from the contactor. The action is this: when power is applied to the coil circuit. the two NO contacts prevent power from going to the coil. When the NO pushbutton is pressed, the circuit is completed and the coil pulls in the contactor. At the same time, the holding interlock closes and bypasses the START PB. So long as the STOP PB is not pressed and power is supplied, the contactor will remain energized. Pressing the STOP PB or loosing power will cause the contactor to open and in turn the holding interlock will open. When power is resupplied, the START PB must be pressed to restart the system.

As far as Hp and current ratings are concerned, the contactor is a current rated device that must withstand the starting current of the motor and be able to extinguish the arc on opening. On the label there will be two sets of ratings: one is the hp the starter is rated for a various voltages and a current rating. The current rating is that amount of current the contactor can handle continuously without overheating. The single phase current rating is on a per pole basis, meaning that all three poles of a three pole contactor can carry that current simultaneously. Now the question is, do you intend to just power this mill which has a load of some five hp, or do you want to wire the RPC to handle up to 25 hp of three phase load? A 25 hp 1800 rpm motor at 230 volts draws 62 amps. Converting to single phase, this is a line current of 107 amps which means a size 4 contactor. If the RPC is just for the mill, I would look for a smaller idler motor.

A reversing starter/contactor is a combination of two contactors coupled together mechanically and or electrically so that only one of the contactors can close at a time. The common way to reverse the rotation of a three phase motor is to reverse two of the three leads. That is what a reversing starter does. These would be used on the actual machine motor, but not as a safety contactor for the RPC.

You will as see reference to combination starters. These are enclosed starters that have a disconnecting means such as a circuit breaker or disconnect switch, fused or non-fused, in the enclosure, wired to the starter and having a main power switch handle on the outside of the box.

Tom

Something to consider is that a pony started 20 or 25 HP idler servicing a 5HP load will not see 20 or 25 HP loads. Free 10HP with 440 coil here if you need it and feel like accommodating the coil. There is no enclosure or PB station.

I would recommend a Nema 4 size contactor for the 25HP. I was in the same boat, needed an RPC that was relaible and safe. I bought a used Nema 3 size contactor for my 30HP converter at first. I had to dress the contacts every few months of use as the load was too much. When I started working the CNC's run off it the Nema 3 contacts wouldn't last a week before problems with brownouts and such would be popping up from burnt contacts again. I found a Nema 4.5 for a good price. No more issues.

You want a plain contactor for motor starting duty. There are lots of lighting contactors and I'm not sure these will work for you.

Well, folks. Thanks for educating me about these. The contactor issue is still pretty far "out there". I have now started in earnest with an expansion of the 200 amp single phase service. Working in the attic in the Texas Gulf coast heat and humidity is pretty tough duty. However, I've nearly finished installing the run of 2" EMT from the source panel to the sub-panel. Tomorrow I should have it ready to pull the cable. During the heat of the afternoon I broke and went to a local Home Depot to buy 100' of 3/0 copper THHN cable. At $3.16 a foot it was actually cheaper than I feared it would be. I had some old stuff on hand but some of the insulation looked dicey. I wanted to use it... but I couldn't get my courage up to do so.

The new box will have the full 200 amps. I will run 125 amps out the front wall to the front porch and finally have adequate power for a Miller Syncrowave 300 welder I bought several years ago. Between the new single phase panel and the RPC 3 phase distribution system, I will be in high cotton, electrically speaking.

Vernon

Here is another way to start a pony started RPC. You get the pony running and then hit the idler with power via one of these before depowering the pony. This one is three pole, but a two pole would do for that chore. This one is fused, so you buy the right size slow blow fuses.

Square D D322N 240V 60A General Purpose Safety Switch NEW | eBay

Quote by John Oder:
"Here is another way to start a pony started RPC. You get the pony running and then hit the idler with power via one of these before depowering the pony. This one is three pole, but a two pole would do for that chore. This one is fused, so you buy the right size slow blow fuses."

John,

Are you saying that this fused safety switch would serve in stead of a magnetic starter?

My concern about "locked rotor" condition causing a fire/explosion in the RPC is the only reason I was interested in a magnetic starter. Several people have indicated that a breaker will not necessarily protect against this. Hence, I don't understand why a fused disconnect would, either. But if it does, I'm in!

As a matter of fact, my Miller Syncrowave 300 welder has one of these mounted on the side. The welder is powered by single phase. It is a 3 phase fused safety switch. They simply did not hook up the third leg.

Thanks!

V

Originally Posted by johnoder

Here is another way to start a pony started RPC. You get the pony running and then hit the idler with power via one of these before depowering the pony. This one is three pole, but a two pole would do for that chore. This one is fused, so you buy the right size slow blow fuses.

Square D D322N 240V 60A General Purpose Safety Switch NEW | eBay

John, I am not quite sure I follow what you are suggesting, but if this disconnect is on the line side of the rpc and the rpc is going to be used at full rating, then two issues. The first is Vernon's concern that if power is interrupted to the building, that the rpc will not automatically restart when power is restored. This could only work if the rpc could automatically restart by itself and would not work with a pony.

Secondly, the thermal rating is not sufficient for a 25 hp unit. I mentioned in my post that if the rpc is only used for a 5 hp load then one need not use the high amperage equipment, and if this is the case, why use a 25 hp unit to supply a 5 hp load. To supply single phase power to the 25 hp rpc, then,

25 hp @230 v = 63 amp 3 phase
63 amp 3 phase = 109 amp single phase (1ph amp = 3ph amp * sqrt(3))
size 4 contactor thermal rating is 135 amps

Use the size 4 and be safe.

Tom

Simple systems have a greater likelihood of success when there isn't a supervisor growling "I don't care, this needs to be done, and if YOU can't do it....."

Nothing like piecework to screw up well-intentioned systems.

Many fossils and antiquities have been broken up by the finders, to increase the piece count, therefore the compensation.

Tom,

If I understood you, to protect my RPC against a locked rotor, all I need is a contactor, and not a motor starter. Yes?

If so, it appears that advice will save me about $125 to $150 dollars.

If I've identified the right critter, nema size 4 contactors are readily available on ebay (used) for around $100 or slightly more.

Is this an example of what I need?

Square D Size Sz 4 Class 8502 Motor Contactor 240V | eBay

All I'm trying to do is prevent the RPC from re-starting underload after a power loss.

Thanks!

Vernon

That will do it. Two pole also.

Tom

Folks,

I'm pleased to report that I got the 3/0 cable pulled through a 25' run of conduit and now have 200 amps to the front room of the house .. er.. machine shop. I still have to make connections through the front wall from the 200 amp disconnect to the 30 space 200 amp box. But that will be easy. I'm through melting tallow in the attic... at least for THIS project.

I'm thinking about building a bench just to the side of the box. I will make it high enough and big enough to store a shop vac and garbage can beneath it and will place the 20hp idler motor on top of it. It will be just a couple of feet from the panel. The only reason I don't put it directly under the panel is the "refrigerator rule".

My genuine appreciation to all.

Vernon

Here's an even better deal than the SqD
GE CR305F0**ASH CONTACTOR 115-120V COIL SIZE SZ 4 135A 135 AMP A CR305F0ASH | eBay
The only issue is that you will have to buy a pair of terminal lugs for the load side.

Tom