Universal motor question

[Dick Linn]

So called "universal" motors can be powered from AC or DC. Let's say I have a motor rated for 120 volt (RMS), 60 Hz. AC operation. Would it need 120 volts DC for equal performance? A bit less perhaps?

The application here is running an angle grinder, such as used by welders. Sometimes it is more convenient to power the grinder off the output of a stick welder than to run an extension cord. These machines typically have somewhere around 70 volts maximum. We did note that the grinder ran noticeably slower on the welder AC setting vs. the DC.

[Frank Neuperger]

The rms power of 120VDC is the same as 120VAC (rms)and as such performance should be very similar at 120VDC. I am guessing that there is same very small correction (less performance) for inductive reactive effects of the AC case that may put AC performance slightly lower but this is just gut feelingas it has been ~25 years since I did that motors lab. .

70 VDC will have significantly less power but you can' t just use scaling by ~(70/120)^2 everywhere on the rmp vs power curve because the motor is not a constant resistance load. i.e. back emf needs to be considered. Without actually running the spreadsheet referenced below I am still estimating that you will have ~1/2 or less mechanical power output near the center of the old power curves.. Lets see.... stall current and hence the torque curve be scaled by 70/120, no load rpm will be scaled by 70/120. OK if you look at peak power for 70V and 120V the 70V case will be scaled by (70/120)^2 . The peak power for 70V will also occur at an rpm scaled by 70/120. At other rpm points on the performance curves the scale factor will be different.

Use that motors specs spreadsheet that has been around CD for years to model your universal motor as a DC motor and then you can compare the curves.

Hard to rationalize the AC vc DC setting performance on welder as I don't know if there is a scale change associated with it. You should put a Volt meter on welder output to rationalize this.

Frank
Team 39