"Pure" 240 volt loads, like a motor or heater, do not use a neutral connection. You have 2 hots and a ground. In this case, you only needed 10.2 wg, but you have 10/3 wg. Just tape off the neutral wire.
When you see a 4 wire hookup, like a stove or dryer, it is because those devices used 120/240. That is , the big loads ( heaters) are 240, but smaller items like a motor or clock , and control circuits, are set up at 120, so you have to have the 2 hots PLUS the neutral.