Potential relay for replacing a centrifugal start switch???

challenger · Sep 14, 2017

I have an air compressor with a fairly fried start switch. The contacts are quite pitted etc and I no longer trust it. I'd like to know if I could reliably replace it with a potential relay.
This is a 5 hp, 23amp Blador L1410T single phase motor running 240VAC.
Thanks for any advice in selecting the proper unit.

Tony Quiring · Sep 14, 2017

No...Use the switch to control a relay as the coil draws milli - amps and the existing switch can be cleaned up and be fine.

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challenger · Sep 14, 2017

How would you suggest I clean the contacts? I realize the current would be minimal so maybe removing the original plating would be OK?

Galaxy S4, Slimkat
If I wasn't married I'd quit fishing

challenger · Sep 14, 2017

What should I size the relay at? It will see start up current all the time so???
Thanks

Galaxy S4, Slimkat
If I wasn't married I'd quit fishing

SAF · Sep 14, 2017

1Φ mortor starting switch

You might want to consider a solid state start switch designed for the purpose, or replace the mechanical one with a new one. Installing a remote start relay away from the winding location can be a challenge as well.

Have a look at this post for details.

http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/repair-old-peerless-motor-pics-309683-post2628390/#post2628390

SAF Ω

Tony Quiring · Sep 14, 2017

To clean the contacts you need real fine abrasive.

Simply cut strips of PAPER and drag it between the contacts as they will be closed and have pressure so pull open to get strip in then pull it through with the contacts closed.

This burnishing process will not remove plating.

Coil voltage same as motor.

Current rating 10 to 15 amps.

Use a 2 pole 10 amp relay and parallel the contacts to get 20 amps.

Be sure coil and contacts rated for ac volts.

Connect hot side of existing switch to common of relay and the NO side of relay to wire you remove from switched side.

Connect coil to where wire wss removed and return or opposite polarity of line.

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rons · Sep 15, 2017

challenger said:
How would you suggest I clean the contacts? I realize the current would be minimal so maybe removing the original plating would be OK?

Galaxy S4, Slimkat
If I wasn't married I'd quit fishing

No, don't remove the plating with abrasives or with a file. Use a brass bristle brush or use a stainless steel brush with a gentle touch. Remove the plating and points will wear faster.

A replacement points kit is an option if you mean the starter contacts. A centrifugal start switch? You mean inside the motor?

challenger · Sep 15, 2017

rons said:
No, don't remove the plating with abrasives or with a file. Use a brass bristle brush or use a stainless steel brush with a gentle touch. Remove the plating and points will wear faster.

A replacement points kit is an option if you mean the starter contacts. A centrifugal start switch? You mean inside the motor?

Yes the centrifugal switch inside the motor is what I am concerned about.
The plating is already gone on these contacts judging by the degree of pitting.
Perhaps I am best off just buying the proper OA replacement.
I don't understand using a 20A rated relay contact(s) to take the load of the start winding current. Wouldn't the inrush current be well beyond 20A. I know we aren't talking about the run windings but it is still the initial current upon start that I am not sure about. I am not questioning the recommendation to use such a relay but I am questioning my understanding of it IYKWIM.
Thanks

Tony Quiring · Sep 15, 2017

If the motor is rated at 23 amps then you have 2 sets of windings being run and start that both are connected when start.

Safe to assume run is larger load but just splitting them a, d rounding up you can just guess 12 amps per winding.

20 amps is much greater than this so it should work.

However the plan is not the best as replacing contacts if available would likely be best option.

Larger relay would add reliability to the solution.

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challenger · Sep 15, 2017

Thanks. I now plan to order the switch.
I have another question about this same motor.
The start winding look a bit crispy. The motor tripped the circuit breaker a few weeks ago and has been down since central then as I figure out the best solution. It turned out that the start capacitor was blown. It was the result of a couple of possible problems all of which are going to be fixed or are fixed. The motor ran fine during a short test run so I'm going to bring it to a motor shop and have them meg the stator. If it megs OK I'm going to have them dip and bake it. I will take it apart but I'm wondering if I should change the bearings? They seem fine but the motor will be apart so it would be easier now than later. Normally I'd let it go since they seem OK but I'm afraid if one did go bad it would result in the motor becoming trash. Any thoughts on this type of PM?

Galaxy S4, Slimkat
If I wasn't married I'd quit fishing

SAF · Sep 15, 2017

I noticed that the start switch links I gave earlier were dead.

Here's a replacement start switch in 2 or single contact versions, match what you have.

25 Bearing Stationary Switches On TORQ Corporation

From the Baldor performance page, your motor start current is 140A at full load. L141T - Baldor.com

So your start winding circuit need to handle about half of that to be reliable. 70A start winding current needs a switch with 3/8" silver contacts.

I would suggest you look at your compressor unloader system and make sure it's functioning.
With a tripped breaker, blown start caps, burnt start switch and crispy start winding, suggests that its having a real hard time getting up to speed.

I would replace both caps and switch and insure that the start current is not excessive under the load starting. Otherwise it's likely to repeat the process with this or a replacement motor.

SAF Ω

challenger · Sep 16, 2017

SAF said:
I noticed that the start switch links I gave earlier were dead.

Here's a replacement start switch in 2 or single contact versions, match what you have.

25 Bearing Stationary Switches On TORQ Corporation

From the Baldor performance page, your motor start current is 140A at full load. L141T - Baldor.com

So your start winding circuit need to handle about half of that to be reliable. 70A start winding current needs a switch with 3/8" silver contacts.

I would suggest you look at your compressor unloader system and make sure it's functioning.
With a tripped breaker, blown start caps, burnt start switch and crispy start winding, suggests that its having a real hard time getting up to speed.

I would replace both caps and switch and insure that the start current is not excessive under the load starting. Otherwise it's likely to repeat the process with this or a replacement motor.

SAF Ω

Thanks. I think you are correct. I discovered this a few days ago and ordered an inline check valve that has a port for an unloader in the pressure switch. This head/motor are not on a tank. The pump feeds a remote tank located 8' away. When I set this up I got some bad advice and was told that the unloaders that are built into the heads would unload the system but this turns out to not be the case. I'll install the check valve and the pressure switch unloader which should fix the problem.
Funny thing is I'd have never noticed this as the pump always started without any indication that the motor was being taxed more than it should.
I also noticed the links were dead. I ordered an OE stationary switch too. Unfortunately it was before seeing this post and I paid over 40.00 for it. Who knows. With shipping NG the one from Tork may have been as much.
I appreciate the help and information.

Galaxy S4, Slimkat
If I wasn't married I'd quit fishing

SAF · Sep 16, 2017

Pointers,

Install your check valve closer to the pump than the tank, so that is doesn't have to bleed down the entire length of your discharge line. But putting it too close to the pump discharge where it gets very hot,from the heat of compression, will cause the sealing disc to crystallize and leak.

The best would be to make an aftercooler piping arrangement to cool down some of the superheat of compression, before the check valve and pressure switch.

If your pump has a centrifugal unloader valve assembly built in, I would use that instead of the pressure switch type.
A problem with a switch type unloader valve can be: If the power is removed before the switch opens, you will be starting under full pressure again, the next time power is restored. The mechenical type does not suffer this problem, it's always open at zero speed.

SAF Ω

challenger · Sep 17, 2017

Thanks. No mechanical unloader so I need to use an unloader on the pressure switch. I can mount it in a location the will be remote enough to keep it from getting hot.

Galaxy S4, Slimkat
If I wasn't married I'd quit fishing

6PTsocket · Sep 21, 2017

SAF said:
You might want to consider a solid state start switch designed for the purpose, or replace the mechanical one with a new one. Installing a remote start relay away from the winding location can be a challenge as well.

Have a look at this post for details.

http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/repair-old-peerless-motor-pics-309683-post2628390/#post2628390

SAF Ω

We had a Leeson motor come in with one of those solid state relays. I went to the relay manufacturer's site and there was all the info. needed for replacing any centrifugal switch for 120 or 240. You only needed the current rating off the motor plate to size it properly. It is a current control relay. Two wires go in series with the motor. The resistance is so low that it is not a problem. The other two wires connect to the wires that went to the old centrifugal switch. It is solid state. The "contacts" start out closed. As soon as the motor start current drops to just above the rating on the plate, the solid state "contacts" open, disconnecting the start cap and winding. It is simple and there is no more centrifugal switch needed. I know a number of companies make them but I am sorry, I don't remember the brand that was on the Leeson. I would definitely go that way. You never have to open a motor to clean another contact. That is the way to go. The mechanical switch uses the fly weights to estimate 80% of full speed when the switch should open. The solid state relay does it by monitoring the drop in start current as the motor reaches full speed.

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6PTsocket · Sep 21, 2017

challenger said:
Yes the centrifugal switch inside the motor is what I am concerned about.
The plating is already gone on these contacts judging by the degree of pitting.
Perhaps I am best off just buying the proper OA replacement.
I don't understand using a 20A rated relay contact(s) to take the load of the start winding current. Wouldn't the inrush current be well beyond 20A. I know we aren't talking about the run windings but it is still the initial current upon start that I am not sure about. I am not questioning the recommendation to use such a relay but I am questioning my understanding of it IYKWIM.
Thanks

That is a poor solution. You are using an external relay to take the load off a set of contacts that, if replaced, should last for years, just like the original set. The springs, weights and contacts are a crude set up, anyway. A solid state relay totally does away with the centrifugal switch. The motor never needs to be opened for switch service, again. They are sold as direct replacements for centrifugal switches. You only need the current rating off the motor plate to correctly size it.

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JST · Sep 21, 2017

I see folks referring to "plating" on the switch contacts. Any heavy current switch of that type should not have mere "plating" on it, it should have a solid contact of a suitable material. If it were only plated, the plating would be burned off in a few starts.

Some dual usage switches DO have contacts that ARE plated, but those are plated for use as low level "dry" contacts. If used for higher currents, the plating burns off and the "dry contact" capability is lost. That does not apply to motor start contacts.

challenger · Sep 21, 2017

Thanks all. The stator is being rewound. It meggerred OK but the shop guy said it looked really bad and it did.
I already got a new switch so I'm replacing that and the bearing and I'm going to put it back together and hopefully it will still be running well after I stop.

Galaxy S4, Slimkat
If I wasn't married I'd quit fishing

Potential relay for replacing a centrifugal start switch???

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