BLDC Motor Performance at high altitude?

Question

How does a BLDC motor performance change at high altitude where the air density is really low? For example, say I have a given BLDC motor with given torque-RPM performance curves. Now, I assume these curves have been generated based on thermal limits at STP conditions. If I want to operate this motor at 50,000 feet, how would I account for the different temperature and low density on the performance curves at these conditions?

EDIT: My (limited) understanding of how torque-RPM curves are generated comes from this datasheet: https://www.logic-control.com/datasheets/8/Catalog/Intro/011-Understanding%20Torque%20Curves.pdf

This told me that the upper bound of the continuous torque curves comes from testing to thermal limits, which led to my understanding that if the thermal environment changed, these curves themselves would change. From some of the answers, it seems like this is not the case.

With regards to the purpose of the motor, it is to drive a propeller load with a known torque and RPM demand. Thus, I was mostly curious about whether I can trust a BLDC motor's torque-RPM curve from a manufacturer, given that they probably test at STP. And if so, what corrections do I make.

Answer 1

Altitude brings with it two characteristics

1. reduction in ambient temperature (assuming some part is exposed).
2. reduction in pressure.

With decreased temperature comes increase magnetic strength. Take Samarium-cobalt, there is usually a +-5% change centred around 25C (-55 -> 105), however there is usually a +-5% manufacturing tolerance on the magnet strength. At first glance this increase in magnet strength (increase in Kt) is beneficial? However, is it of practical use? Just because the ambient is at -55C or -30C does not mean the rotor is going to be at that temperature.

Case 1. unit is continually operated from takeoff until altitude. The decrease in ambient will help with the cooling of the stator (copper and iron losses) and in turn keep the rotor cooler, but it had already been operating for an extended period of time and depending on your operating duty and loading, might not be significantly cooler.

Case 2. unit was un-powered during accent and then was power at altitude, with suitable soaking time. Yes the magents would be at their coldest and your system would benefit from the higher torque per current. However... usage will still still warm the stator and rotor up until it again stabilises like Case 1. It will take some time, a time associated with the thermal mass and thermal resistance.

A product I have worked on was driving against a mechanical drive chain. This presented a higher drag at cold temperature. At -55C Qualification testing the overall performance was lower than nominal - motor-drive was performing better but the mechanics were not. At +90C ambient the overall performance was lower than nominal - motor-drive was performing worse than nominal while the mechanics were slightly better. All within spec but a noted overall characteristic.

Pressure.

Pressure brings with it two characteristics.

A reduction in pressure means a reduction in air density and thus less air to push against to maintain lift.

A turboprop has operated upto 20km ( https://en.wikipedia.org/wiki/Grob_Strato_2C ) and when you consider you are looking into characteristics up to 50,000ft (15km) and a "propeller load", this is something to consider.

At altitude the voltage breakdown characteristics also reduces. Pachen's curve. This means you could suffer from partial discharge or corona effects, if you operate at "high voltage" where high in this case is 1000V. This will drive improvements in winding insulation to ensure they do not break down. You have not stated the operating voltage thus this might not be a concern for you.

Answer 2

I've sometimes discussed motors for space applications with high-end motor vendors. While I can't say this is conclusive, different torque curves for ground testing vs space were never brought up by the vendor.

However, one thing that did come up is lubrication. As mentioned by several comments and answers, your motors are at risk of running far hotter than on the ground due to reduced convection cooling. However, you also have a risk at the cold end. Will your motor ever be powered off for a significant period of time? (I'm guessing this is a balloon payload?) If powered off, the motor could cool until the lubricant thickens too much, causing the motor to bind when you turn it on. Your motor should list its minimum operating temperature.

Answer 3

The biggest difference will be due to lower heat dissipation. As copper gets hot, its resistivity goes up, causing your motor to be less efficient.

If you know the "sea level" parameters for the motor, you could probably get close by derating the duty and/or max current with info from here: Altitude Corrections

Answer 4

There are two effects that change the motor performance-

1. Winding temperature - a low ambient will noticeably reduce the resistance of the windings, which brings the torque-speed curve up - so long as the reduction in air density doesn't reduce the cooling enough to offset that reduction in temperature.
   2. Magnet strength. Most magnets have a negative temperature coefficient on Br, so they will get stronger at lower temperatures. Some magnets, especially samarium alloys that contain some gadolinium, can have very low TCs however. Stronger magnets won't change the torque speed curve much, the Kt value (torque generated per amp) improves so the current at a particular load reduces, but the Ke value also goes up, so the applied voltage rises, so apart from the resistive loss in the windings, the power consumed changes little. Maximum speed achievable on a given supply will fall though.