# Motor dummy load resistor calculator

1. ## Motor dummy load resistor calculator

I am developing a test rig for my motor controller.

Below is the image which shows a motor is connected back to back to act as a load with load resistor

left side is the controller that will run the motor of 400W.

Right side i am connecting another motor with a coupling in between and resistor R1, R2, R3 at the end as dummy load.

Voltage generated on right side motor is 24V (back emf)

what vale of resistor R1, R2, R3 should I use to act as load of 400W at 100% pwm by the controller ?.

I saw a resistor of 200W 50ohm.

using three in parallel resistor values will be 16.5ohm

using I = V/R, 24V/16.5ohm = 1.4545A

power = 24V X 1.4545A = 34.9W (am I generating only 34.9W of load?).

Will this resistor value will work ? as 200W X 3 = 600W.

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2. ## Re: Motor dummy load resistor calculator

What type of motor is the motor which is electrically loaded?

Is it a BLDC or separately excited synchronous motor?

If it is an induction motor, it will not self generate back EMF.

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3. ## Re: Motor dummy load resistor calculator

Both the motors are BLDC.

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4. ## Re: Motor dummy load resistor calculator

If the 24 Vac you quoted was measured line to line, the resistors should be connected in delta.

Also, the equation for the delivered power is

P = I x V x sqroot(3)

5. ## Re: Motor dummy load resistor calculator

While not necessarily related, or closely, I have observed
that resistor loads do not act the same "as the driver sees
it", as windings.

I once undertook to defeat the traction control "throttle
relaxer" on one of my cars (on the cusp of transition from
cable throttle, to drive-by-wire, this model had a cable
throttle with a variable "slack adder" gizmo, motorized).

Anyway, to keep the thing from setting a dash light I tried
to unplug and plug in various "dummy loads". No resistor
would satisfy the key-on diagnostic, but a junk box motor
from some cast off battery drill or other made it happy.
Probably looking for a dI/dt signature or something. Point
being, the electronics sure could tell the difference.

And so too might your much higher power drivers here;
particularly, while the BLDC may be 24W time averaged
it will be some triangular waveform, while the resistor
load will be square and will limit the current that can be
taken from the driver if you (say) "overstay your welcome"
and drive the windings*armature into saturation with a
poorly chosen switching frequency or whatnot.

It's my belief that you would be better served by proper
valued power inductors (or maybe a power transformer
bunch, to whose outputs you could add burden resistors
but keep the inductive qualities that emulate the real deal).

I think resistors stand a chance of hiding quicksand.

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