Motor drive of shim rods

2 Hot Drive of Shim Rods. The motor drives for the shim rods have been described in Section 2.4.5. Here it is necessary only to recapitulate those features directly relevant to control of rod motions. 

The shim-rod motors are three-phase 220-volt 1/6-hp squirrel-cage motors, of 1800 rpm rating. Start, stop, and reverse are the only control actions called for. Reversal is accomplished. by phase, interchange. This simple control picture is, however, slightly complicated by two considerations the desire to have a finer speed control than is obtained by simple on-off switching, and concern as to the possible safety implications of transition from three-phase to single-phase operation by loss (open circuit) of one power wire. 

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