There is one more subtle point about operating universal motors on AC vs DC power. The comparisons discussed are for pure sine wave AC such as from a postwar transformer. If a modern power supply is used such as a ZW-L where at less than full voltage the waveform is chopped and full of harmonics these comparisons do not apply. A universal motor will be hotter when a chopped waveform is used than with a pure sine wave even though the engine runs better at low speed with the chopped waveform.
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