I am looking for loading a 15W BLDC motor with another BLDC machine, sort of like a master slave configuration. I'm not exactly sure how the slave BLDC machine windings should be connected. One idea was to short the windings together. Another is to connect resistors to the slave machine's terminals, operated as a generator, as loads. But in what configuration, star or delta? Can potentiometers be used? Any help will be greatly appreciated.

[OLD TEXT] I have a 15W Hurst brushless dc motor, whose rated torque is 0.07Nm, rated speed 2054 rpm (~215 rad/sec). I am looking for a simple method of exerting 0.07N.m load at rated speed, without having to purchase a magnetic brake (they're expensive). One idea I had was to attach an adjustable coupling of a specific weight to the shaft (radius ~ 0.004m). The coupling, having a certain mass and inertia, will exert a specific amount of load on the shaft at a certain angular speed (supposedly). Since the coupling is a hollow cylinder, the inertia can be given by

Inertia = mass \times radius^2 [kg.m^2]

Then the torque can be calculated as

Torque = speed \times mass \times radius^2 [N.m]

Edit: This formula for calculating torque is wrong as pointed out in comments

However, I suspect that its not that simple. The formula can be altogether wrong. With this formulas, for a coupling inner radius of 0.004 m, speed of 2100 rpm, I get around 20 kg of mass for the coupling, which is huge!!!

Can you please provide your inputs on this matter (Edit: How to best load the motor without a magnetic brake)?

• do you have 2 of these machines? or access to another? – JonRB May 3 '15 at 12:01
• You can't apply a constant torque to a rotating shaft just connecting a sort of fly-wheel: if the angular velocity is constant then torque is zero. That's probably why people use magnetic brakes, and definitely why your calculations leads to wrong results. Even the measurements units are wrong. And on top of all this question has nothing to do with electronic design, you're better off to the engineering stack. Just fix your physics or be prepared to be chewed up. – Vladimir Cravero May 3 '15 at 12:31
• I'm voting to close this question as off-topic because it's not about electronic design. – Vladimir Cravero May 3 '15 at 12:31
• @JonRB actually I do have an same rating BLDC, with slightly different internal parameters. – Adeel May 3 '15 at 13:21
• That's how you can make a dummy load for brushless motor -> circuitsathome.com/measurements/… If you want variable load add resistors or current sinks where I shorted the windings. – Oleg Mazurov May 3 '15 at 19:31