Electric Motor Dyno - Can I use a DC motor as a brake?

Thread Starter

bd0876

Joined Oct 4, 2022
2
Need to build an electric motor dynamometer for a college project. I have a rough idea of a design.
The motor being tested (1/3hp) will be coupled to a DC resistance motor (1hp). A photoelectric sensor will be used to measure the speed of the rotating shafts and an arm attached on the resistance motor will be attached 1 foot from the center of the shafts. A load cell will take the torque measurements from this arm. The resistance motor will be mounted by its shaft to pillow blocks so that it can rotate freely.

I have watched a few youtube videos of a similar set-up, but this is all a little over my head. I will have more specific questions as the build starts, but right now I wondering if anybody here has any experience with this type of set-up? What should I look for in a resistance motor for this design?

My main concern is that the resistance motor will not be able to provide resistance throughout the RPM range of the test motor (0-1825 RPM).
 

MaxHeadRoom

Joined Jul 18, 2013
25,979
Not sure exactly what you are doing, ?
I am familiar with resistance motors that simply move the magnets of the generator used in exercise bikes, all this motor does, is move the magnets in and out of the stator in order to increase or decrease the 'resistance' felt by the user.
Essentially using it as a variable brake.
Show the YouTube vid that shows what you intend to do.
 

Thread Starter

bd0876

Joined Oct 4, 2022
2
Not sure exactly what you are doing, ?
I am familiar with resistance motors that simply move the magnets of the generator used in exercise bikes, all this motor does, is move the magnets in and out of the stator in order to increase or decrease the 'resistance' felt by the user.
Essentially using it as a variable brake.
Show the YouTube vid that shows what you intend to do.
Here's a video showing what I was thinking of building:
I like the idea of the exercise bike motor, that might be a better option as long as the resistance can remain constant through the RPM range, so I can get accurate torque measurements.
 

Sensacell

Joined Jun 19, 2012
3,064
A DC motor used as a brake relies on the induction of electrical current caused by rotation of the permanent magnets or field coils. A variable elecrical load then dissipates the energy from the motor. The concept requires relative motion to induce currents and therefor create retarding torque, what this means is zero speed = zero torque. You will find it very hard to produce torque at low RPM. This might not be problematic if your RPM range doesn't go to zero?
 

WBahn

Joined Mar 31, 2012
27,392
It wouldn't seem that getting accurate torque measurements is the problem -- you said that you wanted an arm mounted on the brake motor's frame to the load cell. So if the torque produced by the brake motor varies, that information will be captured in the load cell data.

But it seems like what you might want to do is have a control system in which you set the torque you want and then the controller take that information and the load signal from the load cell and drives the brake motor such to create the desired torque.
 

GetDeviceInfo

Joined Jun 7, 2009
2,118
It will work fine. Your brake motor needs to be of a permanent magnet design, dc or ac, with appropriate controller. The torque developed in the brake motor is directly attributed to current, so once you’ve scaled the relationship with the torque arm, the torque arm is no longer required, and you can calc torque directly from reading the brake current.
 

MaxHeadRoom

Joined Jul 18, 2013
25,979
If you use a PM BLDC variety, you can use a suitable 3ph rectifier on the output in order to just shunt the 2 conductor DC in order to dynamically brake the motor.
 
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