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I'm trying to get the torque constant of a bldc. Based on this: how to read datasheet I just divide the torque at any given point with the current. I tried out that method with different values and I get different constants. What do I do wrong? How can I calculate the constant if this method is wrong? datasheet

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It looks like that data sheet is for the current drawn from the 48V supply. The torque constant of a motor applies to the ratio between the shaft torque and the current applied to the motor itself. It's fairly close to the motor torque over a wide range of shaft speeds (in both directions -- torque vs. current is close to the same when the motor's acting like a generator as when it's acting like a motor).

The current drawn by the motor driver of a BLDC is different, because the driver (if it's decent) is a switched amplifier, so the current drawn is just the total power (motor shaft power plus any losses) divided by the supply voltage.

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  • \$\begingroup\$ Okay, it makes sense now. To be honest I need the constant of a similar bldc hub motor for a simulation but I found similar datasheets like above or uumotor.com/webstore/resources/image/18/97/5.jpg Could you recommend something to find similar motors with the constant I would need? I only found Chinese datasheets so far with ~45 Nm and 25+ km/h top speed. Or can I estimate the torque constant somehow from the given datasheets? \$\endgroup\$ – H. Csaba Apr 16 at 20:00
  • \$\begingroup\$ well, it seems that the datasheet in my comment has a torque - current graph that looks okay, only checked the voltage column, for future reference correct me if I'm wrong \$\endgroup\$ – H. Csaba Apr 16 at 20:16
  • \$\begingroup\$ Yes, that graph looks pretty sensible. In general, any time you buy a system (such as an electric bike kit) and break it down for its components you're just rolling the dice as far as getting the same thing each time, or being able to trust any engineering data. \$\endgroup\$ – TimWescott Apr 16 at 20:20

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