# How does the power usage and voltage of a universal motor change when running on DC?

I have a 2400w universal motor from an AC powered garden blower (230V @50Hz here in Australia) and am wondering what voltage and power requirements it would require from a DC source to give roughly the same amount of mechanical power and speed?

Also the armature brushes and the field winding's are wired in parallel series, could I gain some additional efficiency out of the motor by powering them from different voltage/current sources?

Reason I'm asking is that these motors are mass produced ridiculously cheap (the entire blower is only \$35 at Bunnings - a tool store chain here in Australia) whereas any other motors of a similar power rating I have found are usually at least a factor of 10 more expensive and I plan on using multiple motors in my project but will be running from batteries so would like to know if there is room for optimising on the cheap.

• 2400 W 230 V DC. Beware though that this ridiculous cheapness comes from inadequate cooling. Your blower will not run continuously nor will the motor. Requirements on external cooling (the blower itself) comes to mind too. Commented Dec 30, 2016 at 15:43
• That is an interesting question. I think I will get out my Variac and Kill-A-Watt, find a rectifier and run a test. Back later.
– user80875
Commented Dec 30, 2016 at 15:56
• As @winny said when you supply 230V DC your motor will consume 2400W and run at rated speed and torque. Cooling will be a factor since these motors are probably not continuous operation rated. I see no real benefit to separating windings/brushes. Keep in mind each motor draws roughly 10A, which may be a problem if distance between batteries and motors is longer than present cable. Commented Dec 30, 2016 at 17:17
• A commutator that works fine on AC may arc over and self destruct when run on continuous DC. A DC arc, once struck, can track across the commutator segments. The effect is worse the higher the DC voltage. The regular drop to 0V of an AC supply prevents the arc tracking across the commutator segments.Half or full wave rectified AC should still be OK, since it drops to 0V every 1/2 cycle. Commented May 20, 2019 at 0:21

I tested a Shop Vac with a bridge rectifier and no DC filtering. The nameplate is 120 V, 60 Hz, 7.4 A. Connected directly to AC, I measured 119 V, 5.5 A, 618 W, 655 VA, 0.94 pF using a Kill-A-Watt. With the rectifier inserted, I measured virtually the same thing on the K-A-W and 105.4 VDC on my TEKDMM 155.

When I pushed the AC voltage up to make the DC voltage 119 V, the K-A-W measured 133.5 VAC, 5.6 A, 660 W 715 VA and 0.94 pf.

In both cases, the K-A-W readings drifted around quite a bit.

I have seen textbook representations of AC vs DC universal motor characteristics. It seems to me that there was a bit more difference, but the information was probably based on pure DC rather than unfiltered rectified AC.

Note that the armature and field windings are connected in series in a universal motor, not parallel. They could be separately powered. You would need to figure out the appropriate voltages and currents for each. That would change the speed vs. torque characteristics. Depending on the type of load etc., there might be a performance advantage, but probably no efficiency advantage.

The price difference likely has more to do with market competition than anything else. For consumer products, motors are designed to do just what the product requires and no more. Identical motors are built in very large quantities. In a blower or vacuum, the air flow used for product function also cools the motor, so the motor is designed with that in mind. Typical duty cycle is also considered.

Here is a comparison of 60 Hz AC and DC torque-speed curves for a universal motor. It is copied from Fitzgerald, Kingsley, Umans Electric Machinery 4th ed. I have added a theoretical load curve for a fan showing that the operating speed and torque should be expected to increase by about 6% and 12% respectively. The operating point is the intersection of the motor curve and the load curve. The power should therefore be expected to increase by about 19%. The efficiency would probably not increase very much.

• Universal motors can be used with the windings in series or parallel, or in rare cases with them powered by separate supplies. That's part of what makes them "universal". A motor whose windings are in series will behave very differently from one whose windings are in parallel, though it's important to note that some series-wound motors, if disconnected from their mechanical load so they're driving anything, may run faster and faster until they destroy their bearings or otherwise self-destruct. Commented Dec 30, 2016 at 21:31
• @Supercat: Universal motors are series connected commutator motors that can be operated on either AC or DC power. That is not possible with shunt connected motors. I doubt the existence of DC motors than can be either series or shunt connected.
– user80875
Commented Dec 30, 2016 at 22:05
• Great work on the test bench thanks Charles - as for the semantics of whether a universal motor is still just that when the windings are in parallel or if the arrangement goes by another name I don't know... But I can tell you for certain that what I'm looking at in front of me is from every description I have read a universal motor but the field and armature windings are in parallel (or shunted as you guys say). Or could this wiring put it in a different name/category? Commented Dec 30, 2016 at 22:17
• If the motor has a commutator and operates on AC, the armature and field must be connected in series. Is it possible that there is a rectifier hidden in the blower someplace?
– user80875
Commented Dec 30, 2016 at 22:41
• @CharlesCowie: Why can't shunt-connected motors run on DC? The articles I've read suggest that they can, and that they have excellent speed-regulation characteristics. Motor direction would be independent of the polarity of the applied voltage. Commented Dec 30, 2016 at 23:11