Motor Starter Relays Current Type Fractional HP Bench Grinder

I bought a couple of used Dayton bench grinders recently, and found some Klixon current-type motor starter relays being used instead of centrifugal switches for switching between Start and Run windings in these small split-phase 120VAC induction motors.

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I mention these relays because they would be a suitable replacement for damaged mechanical components or switch contacts related to centrifugal switches.

These relays are the types used with refrigeration and air conditioning compressors, and also completely suitable for other split-phase motors. They have a heavy duty winding that the motor current passes thru, not a separate coil like common relays. When the power is turned on, the relay is is pulled to make contact to the Start winding. The starting amperage of the motor is initially high, but it drops as the rotor approaches Run speed, the relay drops out, connecting the line voltage to the Run winding.

There are charts of different models of the SR start relays (in PDFs at the link above), but I didn't find any type of selector guide for choosing what the appropriate amp ratings would be for various HP ratings. The models of Klixon SRs I was looking at were applicable for motors up to

15A (3CR series), and the next larger series of SRs were for up to 25A, so these ratings would cover nearly all single phase split-phase motors (capacitor start or non-capacitor start) used in a home shop environment.

One particular aspect of the SRs is that they are gravity-dependent, and therefore need to be installed with the proper end pointed up. Aside from that, they are highly reliable starting devices, rated for about

1 million cycles.

The specific ratings used for a 1/3 HP bench grinder motor use an 11A pull-in (Start) spec and a 6A drop-out (Run) spec. Of course, larger motors would use higher rated specs for pull-in and drop-out currents, and the ratings vary in tenths-of-an-ampere (11.5A/6.3A, etc).

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Klixon used to be a Texas Instruments product line. They had software on their website to tell you which part you needed, based on a motor's specifications and the application.

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Michael A. Terrell

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