Motor speed

@ Roxanne and the rest of you,

A DC motor loses torque as you reduce it's speed, unless you have a special power supply to control it and a small tachometer/generator attached to the motor shaft to send motor speed information back to the power supply. The power supply then compares the motor shaft speed desired to the actual speed and increases or decreases the power being fed to the motor. This is especially helpful when the motor load is changing but the speed must be kept constant. Only enough power is fed to the motor to overcome friction, plus what it takes to keep the motor running at the correct speed, so it runs very cool with no load and progressively warmer as the load increases.

I once was involved in the design of a motor drive system that could accelerate a DC motor to 4,000 rpm in 1/2 revolution and then decelerate it to a stop, back on the "home" position where it started, and it needed to do this ten times a second. Sounds outrageous doesn't it? I had to have a special low inertia hollow field rotor motor built for this purpose that was liquid cooled because I was pushing almost 40 amps into the motor for 1/2 revolution, and then again in reverse polarity to decelerate and stop it. This motor drives an eccentric (crank shaft) to move a 40 lb assembly 0.040" down and back up with an equal 40 lb weight moving in the opposite direction and back the same distance that acts as a counterweight to keep vibration down.

These machines are part of the manufacturing line that builds the multi-layer ceramic circuit boards for the IC chips to attach to that are the CPU modules in computers and cell phones today (look for the light purple color ceramic). A similar motor control design drives the motors in the Apollo Lunar Rover, and I was a bit involved in those too.

Inductive type AC motors are frequency dependent. Special control systems change the frequency of the power going to the motor, but the power level is relatively constant to maintain torque. The frequency of the AC power and the number of poles in the motor determines the motor speed. Three phase motors are fed power by three inputs with the sine wave of each input delayed from the previous by 120 degrees or multiples of that. A three pole motor will rotate 360 degrees when the 3 phase power fed to it completes one complete sine wave. A 6 pole motor will complete 1/2 revolution with one complete sine wave. So the number of poles of the 3 phase induction motor is determined by the frequency of the AC power and the number of poles that the motor has in it's design. A stepping motor increments from pole to pole as you switch and change the polarity of the DC power being fed into it. Some, with the right control design can be used as a slow speed frequency controlled motor

Clear as mud, right? I've spent most of my working career in machine control systems design. Face it every day and it grows on you.
This is also why I play with trains and wood now.

@Jack,

You aren't going to be able to change the motor speed without changing the motor. Stepping the speed down through pulleys or gears is the only way to get the speed that you need if you go with that motor. It would be best and likely cheaper to just go with a motor that has more horsepower and is designed to run at the correct speed. You can't buy a controller for a small single phase induction motor, because the starting circuit in the motor will kick in at about 40% of speed and it will burn up if engaged for more than about 30 seconds.

Charley