I know it is a very stale topic, but it is still relevant. I want to get the Kb and Kt (should be equal) for a motor (37d 30:1). If I use the calcs mentioned in the FAQ, I get two different answers that differ by a large percentage (more than double). I read this thread and it suggests that no load RPM might be somewhat variable (say 10%) and that the stall torque value might be completely unreliable.
I have been told that Kb=Kt. This is what I’ve read numerous places, but I don’t believe it. All proofs that state this start with the assumption power in = power out but they assume all power out is mechanical power to the load. Some power is IR loss in the windings.
How do I get the most accurate answer for the motor constant? In particular, extra friction from the gearbox (comment 2 above where Ben said “In a gearbox, torque is lost with every gear, but speed is not”) may be very relevant, but this should NOT affect the motor constants.
Suggestions:
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Based on the previous emails, each of the two versions of motor constant uses inaccurate raw data but the Kb calc might be better (speed spec is more reliable than torque).
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When calculating Kb, don’t simply follow the FAQ recommendations. Remember that the voltage is the nominal supply voltage less the IR drop in the winding. I is spec’d but R must be calculated.
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Using the motor only data and adjusting for the gearbox reduction (multiplying by the reduction ratio) should improve things a bit (data less affected by friction)
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Ideally, Kb can be measured by driving the motor to a known speed and measuring the open circuit voltage on the terminals then dividing the voltage by the speed (in rad/sec) to get the Kb.
So the big questions:
How to calc Kt and Kb reliably?
Is it safe to assume Kt=Kb?