Possible Replacement for Steveco 90-66 Potential Relay

In researching further, I found a potential relay with 50 amp contacts that may be suitable for use in rotary phase converters, especially larger ones. A data sheet for the device, called a Universal Motor Starting Relay, can be found at the URL below. It is available from a variety of sources including amazon.com where I bought it for $13.50 including shipping.

http://www.icmcontrols.com/ProdImages/ig_LII332.pdf

The device has the same connection regime as other commonly used potential relays like the Steveco 90-66. However, it is a bit more sophisticated than the typical potential relay because it has extra circuitry that implements a "safety timeout". I haven't seen an explicit description of what this does but my assumption is that it will open the contacts after a period of time even if the drop out voltage has not been reached.

There has recently been another thread regarding a replacement for the Steveco 90-66 in an application where that potential relay needs to be modified to separate the relay contact terminals from the coil terminals. With that in mind, I opened up the UMSR-50 to see if it could be modified similarly. What I found looks promising but I have not actually tried it.

The first and second photos show some of the internals with annotation indicating which terminals are which.





The third photo shows the single-sided printed circuit board that contains circuitry to control the relay coil. I've shown on this photo where a trace could be cut (using a Dremel moto-tool, Exacto knife or something similar) to separate the contacts from the coil. After doing this, it would be necessary to drill a hole through the case near the T4 terminals and connect a wire from T4 to the isolated end of resistor R1 (on the corner of the PCB). After making this modification, the coil terminals would be T4 and T5 while the relay contact terminals would be T1 and T2. Given the voltages involved, the cut in the trace should probably be at least 1/4" and a wider cut would not hurt.



An alternate method of modifying it would be to de-solder the lower end of resistor R1 and drill a hole through the PCB a safe distance away from the pad, making the connection to T4 from the free end of R1. Here again, a 1/4" or larger separation would be advisable.

I plan to install the (unmodified) UMSR-50 in my RPC some time in the next few days to see how it works as a direct replacement. I'll report back when I have results.

Will be watching for the report. Lots of circuitry in that unit.

I had the opportunity today to test the UMSR-50 in my 5HP RPC. After moving the three wires from the existing potential relay to the same-numbered terminals on the UMSR-50, I powered it on and the RPC sprang to life as expected.

I also went ahead and modified the UMSR-50 to separate the contacts from the coil/sensing terminals. The photos below show how I did that. The first photo shows a small hole drilled in the PCB after removing the resistor R1. I placed the hole near the copper dot so that I could bridge solder between it and the resistor lead thus anchoring the resistor to the board.



The second photo show the hole I drilled through the case near the T4 terminals. I soldered a short section of #20 wire to one of the terminal tabs.



The third photo shows the added white wire connected to the end of R1 that was formerly connected to T2.



I haven't actually tested this in a situation where the contacts needed to be separated from the coil. Rather, I merely connected T2 to T4 externally for the test in my RPC.