Since a universal motor can run on AC or DC, is there any advantage of using DC over AC? If so, what chracteristics of the motor improves, if any. What about noise levels?
Electronic – ny advantage in running a universal motor on DC power
dc motormotor
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Brushed universal motors are largely independent of AC frequency, and as you have heard, will also run on DC. Their maximum speed is normally way above any synchronous speed referenced to line frequency. In their construction, the phase angle between the rotor and stator fields is set by geometry, not line phase. This is also why they run at high speeds - they just don't care about the line frequency. This is ideal for appliances such as vacuum cleaners and tools such as routers and planers, which don't need to maintain constant speed under varying loads. On the down side, they are typically not very efficient at low speeds, but for an application where they can run fast, that's not a problem.
As long as you get the voltage right, you'll be fine.
Variable speed controllers will also work well.
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.
Best Answer
I was told in the 1970s that the Universial motor runs better due to less sparking at the brushes .I tried this on a vacuum cleaner motor and a sewing machine motor and found that it was true .Bridge rectifiers suitable for mains were starting to become cheap .A Si bridge rectifier on 230VAC mains wastes about 1% so it is plausible that you are better off with DC .I did not use a filter cap because I was concerned about peak charging currents and did not want to give the motor 315VDC .The electro caps in my junk box in 1976 were not big enough being more suited for valve radios.These days you could do a simple boost PFC stage giving the motor fairly pure DC which can be bucked down for speed control .When the barefoot universial motor is stalled on its AC mains supply the resistance and the reactance of the windings add vectorialy to limit the current .When the stalled motor is given DC it is just the resistance that is limiting current .This increased stall current issue makes the motor less idiot proof on DC unless some other form of current limiting is employed .The universial motors that I have mucked around with are two pole .This means that operation below about 3000rpm from a theory viewpoint wont be as smooth .I found that controlled DC gave much better low speed operation than the cheap orthodox triac controlled AC .