19-07-2020, 11:10 PM
Without a rewound field, nothing is electrically more efficient than the 3rd brush picking off a reduced voltage for the field. Lamps and resistors absorb precious watts, undesirable at full output. Very inexpensive multi LED dipslays are available with voltage range adjustable. Often used for monitoring oxygen sensors.
Cannot see the justification for a variable resistor.
Cannot see the justification for a variable resistor.