Size is relative. This is a generic instruction booklet. There's a chart that shows what size idler motor is recommended for the load. Basically, since were only using two legs of the three-phase you have only 2/3's of the load capacity. You would need to multiply the load by 1.5 and pick the nearest larger HP motor available as the idler. For example, for a 7-1/2 HP load you would need 7.5 * 1.5 = 11.25 HP idler. Since that size does not exist, go to the nearest larger motor available which would be 15 HP.
The link following is a good reference. http://www.sprecherschuh.com/downloa...chart_v206.pdf
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