Greetings:

I was just wondering how full load current ratings were calculated for brushless AC Servo Motors (Permanent Magnet Rotors). The motors have nameplate ratings in kW. I have used the following formula from 3 phase power systems. i.e.. Power (kw)=3DSqrt(3)*VL*IL*Cos(theta).. where the dc bus of the power block (Pulse width modulated) is 300v dc (600 V peak to peak via the gating IGBT transistors). Therefore if you assume a "Normal" sinewave output (PWM) the RMS value would be approximately 212 V (300Vpeak/(sqrt2)). By plugging these values into the above equation.. once again assuming a unity power factor (Cos(theta)=3D1) you can solve for IL. I usually multiply it by 125 % and get an answer that is relatively close to data sheets supplied by the manufacturer (Jenglish data sheets..hehe); However, you know what happens when you ASS UME. One thing that I do know is that these motors can dissipate great amounts of heat; Therefore is there some type of rating / class factor that I am missing? If it matters, the motors are manufactured by Sanyo Denki.

I would appreciate some feedback so that I can calculate these motors more properly.

Thanks In Advance:

Stephen Bennett
Kawasaki Robotics (USA), Inc.

 

First, the motor's power factor is not 1 or unity as I think you're assuming. I've seen 0.8 to 0.85 for induction motors. I'm also confused by why you say the 300Vdc bus gives 600V via transistor gating? If the bus is 300V end-to-end then the max is 300V across motor phases, not 600V i.e.
one phase pulled HIGH and one LOW via transistors = 300V. However, you correctly calculated VL = 212 RMS from 300Vdc bus anyway.

The difference between the nameplate power (output/shaft) and input power is just the total losses. I think the rule-of-thumb is still 85%
efficiency. So, Pin should be approx. 1.2x Pout. Using this for Pin and solving for IL using the formula for 3-phase complex power should give
reasonably close number. If not, it could be because the actual pwm drive used for motor test gives an RMS VL value much different than what you think. Or losses, power factor much different. Or combination of all.