potential relay question

[b7100]

I have an older phase converter made up of starting and running capacitors with a 15 hp balance moter that I run a Lathe (3hp), welder (325 amp) and air compressor. There is a relay that controlls the startup which I assume is a potential relay. The relay has burned out and I have to find a replacement. The relay is an AMF Potter Bomfield model MS4AY which is no longer availiable. Does anyone know of a source and how to determine what type of relay I need?

[Al_The_Man]

Is this the one with a 120vdc coil?
If so then the P&B PRD power relay series should work, just confirm the coil voltage.
DigiKey, Newark or Electro-sonic should have.
http://www.tycoelectronics.com/catal...ducts/en?q=prd
Al.

[b7100]

On the relay it says 3hp 240v 60hz.

[Al_The_Man]

Thats OK, the PRD should cover it, just as important, what is the coil voltage and AC or DC?
There is one system out there that uses a MS4AY with a 120vdc coil?
Al.

[b7100]

There is nothing on the coil or the converter that would indicate ac or dc. Is there some way of testing it to find out?

[Al_The_Man]

'Reverse engineering' may help find out where it is fed from, also measure the coil resistance, AC coils are very low compared to DC for the same voltage.
Al.

[Al_The_Man]

I pulled out an old 1989 P&B cat and it has the relay in there.
It looks as though it is an AC relay, two models 120v has 530ohm coil the 240v has 2100ohm.
Evidentially it is a motor start relay and operates off the BEMF of the start winding on a split phase motor.
So it depends on how they configured it in your set up as to whether the PRD will work.
Al.

[b7100]

It appears to be ac. 2.125 0hms. One leg of the 220 single phase feeds one side of the solenoid and one side of the switch. The other side of the solenoid goes to one side of a starting capacitor. The other side of the switch goes to the other side of the same starting capacitor.

[Al_The_Man]

If you google motor potential starting relay, the P&B, now Tyco have a series 38 that replace it, there is also many links to such relays as they are used in HVAC apps. and the like.
Al.

[b7100]

I found the 38 series on Tyco's web site. There are 4 different ones. They specify input voltage, mount direction, pickup voltage, and dropout voltage. How do I know which one or does it matter?

[Al_The_Man]

Its a job to know how they have used the relay, as it sounds like it is used in an application it may have not been designed for.
It may be they just used a relay to determine when the 3rd phase is generated and to switch out the start capacitor, if so, I do not seeing it being that critical?
Al.

[b7100]

OK. Your help has been greatly appreciated. I have made dozens of calls to find a solution to this problem. It was almost to the point where I was going to have to buy a new phase converter. If this works this has saved me many $. If there is anything I could ever do for you let me know.