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Reply to ""Universal" Motors: AC Vs. DC"

To operate their DC only engines Gilbert sold 3 products. #14 Electronic Rectiformer, rated at 150W, 1948. The #16 Electronic Rectiformer, 1950. The #15 Directronic Rectifier which is a 4A rated selenium disc full wave bridge rectifier with an included DPDT reversing switch.

In the older posts there is mention of VA ratings. Watts and VA are only equal in DC circuits and AC circuits with purely resistive loads. Universal motors present a load that is a combination of resistive and inductive so the current will be out of phase, lagging , the voltage. This results in Watts and VARS. Watts are real power and Vars are reactive power. Only real power provides work and heating in the circuit. VA is just the product of volts times amps ignoring the phase angle between them. In DC the phase angle is zero so it is a meaningful rating. In inductive AC circuits VA is always greater than the actual available wattage from the power supply. This is why universal motors usually run better but hotter on DC, the Vars are eliminated because the phase angle between voltage and current is zero providing more watts (real power) to the motor. This is a simplified explanation but it may help.

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