Slip Speed Ratio

One important aspect of power production using AC motors as alternator is slip. This is the relationship between the speed of current revolving around the stator windings (1800 rpm) for any AC motor here in the states and the actual speed of the rotor. I have always used the terms "synchronous or asynchronous motors", as their rotors revolve at synchronous speed (1800 rpm) and accordingly have little or no slip. Let us say for the sake of argument that you have an AC motor with a rated rotor speed of 1725. To have such a motor produce any usable power you must drive it beyond it's slip speed ratio. Here is how this is computed. First you subtract the rotor speed from synchronous speed (1800 - 1725) = 75, we now divide synchronous speed by 75 to get our ratio (75 / 1800) = 24, this is our slip speed ratio. To get the rpm where you will see actual power being produced you will multiply the slip speed ratio by 100 (24 X 100) = 2400. Now to reach the upper gross deadband (that speed required to accommodate the motor's rated load in watts) we add 2% and come up with a rotor speed of (2400 + 2%) = 2448 rpm. If done in this manner you will also see a sinusoidal sine wave of 60 cycles + or - 2% depending on load considerations.

It should now be obvious why asynchronous motors are most often used in such applications as they will produce usable power at a much lower rpm, about (1836 or so). By the end of 1999 there will be a paper made available from the University of Texas at Austin depicting the above technique in detail. I have been helping a graduate student there to produce single and three phase AC power simultaneously, using capacitors to self excite a three phase ac motor of the same rpm rating we have just discussed. Living in the sun belt he is using solar powered super heated steam to drive a steam engine for the primary driver.

Offered by Jay.