Curious, as you'll find me to be..

When you state 'R' are you speaking of resistance of the motor?

And would you explain that more?

P = E^2 / R

Sorry so long getting back to you. I haven't been on the board for a while.

This was a calculation for a dump load resistor, seeking to calculate the

normal-operating-temperature resistance of an electric heating element

from its power (P in watts) and voltage (E in RMS Volts) rating.

(The comments about calculators inspired me to run it thorugh in

a form that could be easily done by hand.)

P = E ^ 2 / R is "Power = Voltage Squared divided by Resistance."

This comes form combining

Ohm's Law (E = I * R, where I is current in amps) with the

formula for electrical power (P = E * I where P is power in watts).

I used ^ for the "exponentiation" operator, as is done in some

computer languages, rather than trying to get a superscript-2

into the text.

With this you should be able to understand and check the

computations yourself.

The idea was to estimate the resistance from the power rating

of a 120-volt heating element, in order to select one that would

provide the desired dump load if 70 volts was applied. That

comes out to about a 300 watt heating

element IF the resistance didn't change with temperature.

Since resistance of heating element wire does go up as it

get hotter, a 300 watt heater operating at 70 volts would

actually more current than intended, so I ballparked the

correction by going to the next lower typical value.