I'm building a robot and want to use a small BLDC Motor. I want a motor to deliver 1 - 1.1 Nm torque (Before gearing).

If i use a BLDC motor with a Nominal torque (max continuous torque) of 560 mNm, with a stall torque of 7480 mNm. How do i know how long the motor can deliver 1100 mNm before overheating?

Some specs: Nominal Voltage: 36 V Kt= 109 mNm/A Kv= 88 RPM/V Stall torque: 7480 mNm Stall Current: 69 A Terminal Resistance Phase to phase : 0.522 Ohm.

Thermal Resistance Housing - Ambient: 1.91 K/W Thermal Resistance Winding - housing: 2.6 K/W Thermal time constant winding:46 s thermal time constant housing:283 s

Max Winding Temperature: +125°C

I think the answer has mostly to do with cooling of the motor. How would i go about calculating the time i can overload this motor to 1100 mNm, so i know this motor is suitable for my application?

What do the thermal time constants mean?

Thank you guys very much!


1 Answer 1


You consult with the manufacturer. They may have information on short term overload performance, or performance at <100% duty cycles.

If they don't, or if they won't stand by their motor under specified overload conditions, then you can't rely on the motor.

However in non safety critical applications such as hobbyist or experimental conditions, you can estimate a range of conditions under which it'll probably work.

Loosely, you can use it for short enough bursts that it won't exceed its rated temperature - 1.1Nm at a 50% duty cycle gives the same mean torque load as 0.55Nm continuous operation, and therefore, for short enough bursts, should be safe - leaving the question, what does "short enough" mean?

That's where the thermal time constants come in. They have the same meaning as the time constant (= RC) in an RC network, allowing you to calculate the rate at which the voltage (or temperature here) rises to its final value.

One simple way of using this is to calculate the half-life or time taken to reach half the final value, which is 0.693* the time constant, or 32 seconds for the (46s) winding time constant. After 32 seconds at twice the rated power, it will reach half the final temperature, which should be within the temperature rating.

Of course it needs to cool before repeating the operation, or subsequent 32 second bursts may exceed the rated temperature.

Modeling that properly would require simulation, including heat transfer to the case (whose temperature rises more slowly, with a much longer time constant) and cooling terms according to airflow past the windings (if it's not a sealed motor) or over the case if it is. You can use R-C networks and an electrical simulator like the built-in one to approximate the thermal model.

Or experiment on a motor.

But (without having done the simulation), if your application allows running the motor less than 50% of the time, in bursts less than about 15 seconds with cooling periods of 30 seconds, I think it'll probably work.


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