# Does a universal motor run better on flat DC as opposed to pulsed DC?

Is there any advantage in running a universal motor on DC power?

I want to know, does it improve the motor characteristics if you supply the motor flat DC instead of pulsed (fully rectified) DC?

And what happens when you want to speed-control (e.g. PWM) these two methods, is there a significant advantage in those cases?

To conclude: what's the shape of voltage and thus switching-type that you can best use to speed-control a universal motor to maximize motor characteristics (less heat/noise/friction/wear, more efficiency)?

Thanks.

• From a system point of view, and assuming a single-phase supply, filtering the mains to give smooth DC requires a large electrolytic reservoir capacitor (at the mains frequency x2) and that may well determine how long your system lasts before service is required. An average life of a few thousands of hours at high temperature is not atypical for such a component. – Spehro Pefhany Oct 15 '17 at 23:43
• While capacitor issues aren't to be overlooked, in fact quite a lot of industrial motors get their power by way of rectifying the mains to a DC capacitor bank and then synthesizing 3-phase AC from that at a variable frequency - while many of these have 3 phase input which helps a little, there are derating tables for single phase input. Hopefully these electronics aren't in the hottest part of the machinery, have a cooling fan, etc - point being that this is a routinely solved problem. But in fixed installations, while universal motors are mostly used in hand-portable equipment. – Chris Stratton Oct 15 '17 at 23:58
• Universal motors aren't really well suited to controls applications as their output torque is proportional to $I^2$ – sstobbe Oct 16 '17 at 1:53