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Discussion Starter #1
Hi,

Working through some performance calculations.

I have been looking at some motor curves...
HTML:
https://www.goldenmotor.com/eCar/HPM96-10000Curve.pdf
What is the probable starting torque for this motor?

Do BLDC motors generally have nearly constant torque from zero to the specified speed maximum torque?

Thanks
 

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The max torque at stall for 5-10 seconds would be of great interest, but these datasheets leave much to be desired.

The max starting torque is not known since they did not test, or did not report the test results of, the stalled motor torque.

However as you can see the Kt (torque/amp) constant can be found from the nearly linear current slope, so you can calculate the torque at different currents using that value (~0.2025 m-N/Amp)

The speed drops off with torque load in a nearly linear manner also, and in SI units the motor back-emf constant Kb has the same numerical value as the Kt. The motor speed constant Kv is the reciprocal of the Kb, and you can use that to get some idea of the speed at different voltages.
 

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Discussion Starter #3
Hi,

They published the maximum torque as 32nm at 3868rpm. If this is the maximum torque it suggests that torque will start to drop again at lower speeds or at best remain constant.

Is there a general characteristic for BLDC motors that I can refer to?
 

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I have been looking at some motor curves...
HTML:
https://www.goldenmotor.com/eCar/HPM96-10000Curve.pdf
I don't know why this was posted as code... it's just a link:
https://www.goldenmotor.com/eCar/HPM96-10000Curve.pdf

This style of presentation of motor performance is popular among motor manufacturers who do not have competent test facilities and don't care whether or not the user can determine how well the motor will work in an automotive applications.
 

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The max starting torque is not known since they did not test, or did not report the test results of, the stalled motor torque.
I assume that they didn't test it. The test "method" is to apply a brake to a free-spinning motor ("Upload point" in the test results), increase brake force until the motor is down to peak-torque speed ("Max torque point"), then quit ("End point" in the test results). They can't practically go slower, because they don't have a reasonable way to control the brake.
 

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Discussion Starter #6
Can I assume nearly constant torque from zero RPM up to RPM with maximum torque?


Can you point to some bldc motor manufacturers who publish better data?
 

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Can I assume nearly constant torque from zero RPM up to RPM with maximum torque?
Typically, the real limit on motor torque at low speed is a current limit; since that is typically a constant current up to the speed for peak torque, the corresponding torque is constant as well.

Can you point to some bldc motor manufacturers who publish better data?
A "brushless DC motor" is typically a three-phase synchronous motor with each phase just switched on and off; it's one form of electronically commutated motor. EVs typically use 3-phase AC motors: still three phases and the same stator designs, but driven sinusoidally rather than square-wave; either synchronous (which would be most like a "BLDC" motor) or asynchronous (induction). Major motor manufacturers targeting the EV market can publish good performance data, but they're for AC motors.

YASA is an example of an independent permanent magnet synchronous AC motor manufacturer; if you download one of their spec sheets, you'll see approximately constant torque up to a transition speed, and approximately constant power beyond that, with the transition speed determined by available operating voltage.

The most widely published performance chart for a production EV motor would be that for the Nissan Leaf permanent magnet synchronous AC motor, prior to 2018. Nissan provided a chart with efficiency, for the combination of the motor and the controller; the torque and power are limited by the controller programming (which in turn is determined to suit the limitations of the battery). The same characteristics of constant torque at low speed and constant power above that appear as with the YASA motors. For examples, just do an image search for "Nissan Leaf torque curve".
 

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Discussion Starter #9
right 'o,

I think the clouds are clearing.

Thanks chaps.

-So the real limit to torque at zero rpm is the current (obviously related to electrical resistance of motor and voltage) available from the controller. - With the proviso that nothing overheats.

- As the motor speed increases the current decreases (due to increasing back EMF of motor?)

-And data regarding this part of the motors characteristic is often lacking because cheapo manufacturers only publish data from brake tests which end when the motor stalls.

I imagine the YASA motors command astronomic prices?
 

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-So the real limit to torque at zero rpm is the current (obviously related to electrical resistance of motor and voltage) available from the controller. - With the proviso that nothing overheats.
Yes, and controller current capacity and motor heating are the important parts. Without those real-world limits, the zero speed current would be huge.

- As the motor speed increases the current decreases (due to increasing back EMF of motor?)
Once you're past the speed at which your current limit (set by the controller) is the limiting factor, then you run into the limit of your supply voltage... so yes, as more speed means more back EMF, less voltage is available to overcome winding resistance, and current drops. For many motors in production EVs this isn't a limitation until very high speed, because the controller is programmed to limit power to a lower level than this (to avoid overheating the motor, and the controller, and to protect the battery).

As an example, a pre-2014 Nissan Leaf is limited to 80 kW by the controller, which for most of the speed range corresponds to less current than the basic relationship of available voltage, speed, and resulting current would suggest.

-And data regarding this part of the motors characteristic is often lacking because cheapo manufacturers only publish data from brake tests which end when the motor stalls.
Right, for most of the speed range that you'll actually use.

I imagine the YASA motors command astronomic prices?
Yes, but to be fair...
  • it isn't just YASA - all of the high-voltage premium motors which are available are expensive, because they are specialty items; for example EV West sells occasional bits of equipment that they scavenge from somewhere, and a (presumably new) AM Racing motor built with an HVH 250-90 core (roughly the size of a typical mid-sized EV motor) is almost US$10K. For perspective, the transmission of a GM "two-mode" hybrid pickup or SUV had two of these motor cores in it... and that transmission couldn't have cost GM anything like US$20K.
  • YASA isn't all that special - they do have an unusual "pancake" format (large diameter but short axially) because of their axial-flux design, but you don't need to buy YASA to get high power over a wide speed range
  • I picked YASA because they publish good spec sheets which are readily available, but they're not the only ones with proper spec sheets. Continuing the earlier example, AM Racing doesn't even list their motors on their website (no specs, no prices), so while spec sheets exist they need to be scrounged from reseller's web sites (such as the power graphs posted by EV West).
For amusement, it would be interesting to see what a production EV motor would cost as an OEM replacement part - based on what I have seen of GM parts (for the Bolt and the discontinued Spark EV), it might be cheaper to buy one of these new than an aftermarket motor. Much of the cost is going through the distribution network, whether that is an auto manufacturer's dealers, or speciality EV component distributors and retailers.

Of course the practical challenge with an production EV is that they don't come packaged with easy-to-work with controllers intended for general-purpose applications. In the world of engines, a salvaged or bought-as-replacement engine with its controls can be very difficult to use outside of the original vehicle, too; the solution is a "crate motor" controls package, which is offered to make the same engine usable by those building race and off-road vehicles, as well as those putting new engines in old cars not subject to current emissions controls. It would be nice if EV manufacturers offered their electric motors with an easily usable controller as an electric "crate motor", but I've never heard of that being done. Maybe someday...
 
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