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Rockwell / Baldor 600-series 6" bench grinders -- replacing broken start relay

Joe Gwinn

Nov 22, 2009
Boston, MA area
I have an old Rockwell model 662e bench grinder that would not start, and just hummed, and emitted some smoke. Turned it off just in time.

The 662e is a clone of a Baldor 662e, and has a Baldor motor, so everything said here also applies to 6" Baldor grinders.

It turned out that the start relay failed, probably the contacts welded closed, so the start winding was continuously energized, rather than only applying power while the motor is starting up, a matter of a few seconds. The original relay is a Klixon 9660-057-170. The 9660 is the Klixon model number, the 57 is the mounting and connection style, and the 170 is the trip current.

The start relay coil is a few turns of heavy wire, and is connected in series with the motor run winding. When prime power is first applied, the rotor is stationary, and draws the locked-rotor current, which is about ten times the running current. This surge current is sufficient to activate the relay, which connects to start winding to the incoming line, so the motor starts to spin up. When it gets to something like a third of final RPM (3600 rpm in this case), the run current drops to the point that the relay deactivates, de-powering the start winding.

The start relay is not designed to be repaired, so replacement is in order. But nobody seems to make the direct replacement any more. I did buy a new old stock relay that would be a direct replacement, albeit with some redoing of connection details. Which relay didn't work - never activates. I bet that the moving part no longer moved, due to some corrosion over the decades sitting on a shelf.

Does anybody know what Baldor currently (or most recently) uses? Baldor no longer makes the 6" series, and has discarded essentially all the documentation.

Anyway, I did find a modern solution, a PTC (Positive Temperature Coefficient) Start Relay intended for small fridges, specifically a 3-pin model QP2-4R7, made in China and available from multiple sources. "Reyhoar Replacement Part 3-Pin QP2-4.7/ QP2-4R7 4.7 Ohm Refrigerator PTC Start/Starter Relay - Compatible with Whirlpool, Frgidaire, Kenmore and Most Mini Fridges and Coolers (2 Pcs) " Same unit for 1/3 and 1/5 HP compressors and fans, and grinders, where starting torque is low. Note that this unit is for 120 Vac operation only; a different unit is needed for 220 Vac.

Note that the number of pins is critical, and the 3-pin version allows one to wire into the grinder circuitry directly, without imitating a compressor hermetic header. Some way to mount the PTC unit will need to be invented. One could glue it to a metal bracket strewed to the grinder base.


For the record, one can get the same thing for $0.78 per unit on AliExpress.

This does work, with one perhaps slight limitation: If one runs the grinder for a while, and then turns the grinder off, it won't start for ten minutes or so, humming instead. It requires ten minutes of off time for the PTC slug to cool off. I don't know how annoying this will be in practice.
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