Is it correct to assume that a typical universal motor like they ones used in an AC drills for example, if supplied with rectified AC will increase efficiency, due to a reduction in iron losses? My thinking is the following:
Pe= Ke × f2 × B2 (Pe = power loss due E. currents)
TIA for any pertinent comments.
Gene
A universal motor is basically a DC motor anyway, so "rectifying" the AC is already done in the motor. Besides, why bother trying to improve the efficiency of something so seldom used anyway? Hardley worth the effort.
The universal motor is simply a DC series motor which will run on both AC and DC. It runs better on DC. The main culprit on AC is the inductance of the field and armature coils as this does cause a phase shift and a decrease in the back emf so the speed at a given torque will be less and performance does suffer. This effect will be much larger than a change in core losses.
Note- the area under the hysteresis loop depends on the peak flux and the mechanical construction. The loss change that you have referred to is due to frequency. Ditto for eddy currents. In the stator, you will get a reduction of these losses but not in the rotor which sees a varying flux at a frequency dependent on speed. A motor intended for DC only doesn't need a laminated stator or pole pieces but the rotor is laminated. --
Don Kelly @shawcross.ca remove the X to answer
I am not very familiar with universal motors. If they are actually to be used with dc, is there not a good chance that excessive current will flow under heavy load or stall? Do they have extra resistance built in because there will be no inductive reactance to limit current?
Bill
-- Ferme le Bush
They are basically series DC motors, with the advantages and disadvantages of the breed. Certainly the stall current and torque will be higher on DC but, as with any motor, AC or DC, operating above rated current is not recommended except for intermittent use. I would be more worried about no load conditions and overspeeding.
What will happen using AC would be that for a given torque, the speed would be lower than with DC and the maximum current and torque will be less.
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