Another Direct Current motor question

[Perry Holbrook]

I'm thinking of converting my Jet mini lathe to variable speed. Several years ago I bought 3 dc motors with controller boards from Vega. I tried to use one when I converted a 6x48 sander to a wet sander for sanding the edges of glass. I got it to work but it didn't have enough power and stalled under heavy sanding pressure, so I switched to a standard motor.

The motors I have are listed as 1hp, treadmill duty motors. Vega was selling them for the purpose of converting lathes to variable speed, so it should work. But I have a question.

In general, without a motor curve or much other info, should I design the 2 pulleys so the motor runs at low, medium, or high speed to get the best motor performance during normal spindle speeds (600 to 1300 rpm)?

Thanks, Perry

[Tom Veatch]

Someone with a better understanding of DC motor performance can probably shoot this suggestion to pieces, but without a dynamometer, I would try to measure the rpm of the motor and it's current draw at several points across the speed range and multiply the two together. Depending how the motor efficiency varies with speed, the power output should vary as the product of rpm and amperage.

Plot your calculated points on a piece of graph paper, calculated points on the vertical axis, corresponding rpm on the horizontal, and sketch a smooth curve through the points. It's a pretty rough and dirty approximation, but if the drawn curve has an obvious peak, that should be the rpm at which the motor develops maximum power. Plan your pulley sizes accordingly.

[ian maybury]

This looks relevant: http://lancet.mit.edu/motors/

[Rod Sheridan]

Hi, motor torque is proportional to current, which has a fixed maximum value.

As the DC motors speed is reduced, so is its HP, so yes you should have several different drive ratios for maximum performance.

Also if you are running the motor at low speed, it may need a blower to force air through it for cooling.

Regards, Rod.