Dumb question about BLDC motor voltage

Supposed I have a motor with the following ratings:

Power: 7 kW
Torque 14.8 Nm
Speed: 4500 rpm
Voltage 55.5 V (IP 54)
Current: 90 A (Duty S1 -EN 60034)
Ke: 11.2 V/1000rpm

Assuming that this motor would be driven by a controller that monitors DC bus voltage, phase current, employs FOC, and can reliably limit the current delivered to the motor at any time.

What would happen if I would decide to go with a much higher battery voltage around 120 V and an IGBT power stage. That power stage could easily handle the currents required to drive the motor at its 18 kW peak power level and would also be able to drive substantially more powerful motors if so desired later. OTOH, getting 18 kW out of 55ish Volt with MOSFETs is a challenge.

If I limit the current on my bench power supply and hook it up to a low resistance load, the voltage across the load will sag down to what would generate the desired current according to Ohms law. Would not the same happen (at least up to the speed where BEMF exceeds rated voltage) in a motor that is driven with a higher than rated voltage but with limited current?

Your motor will spin proportionately faster with the higher voltage. You can make more power. You will have more heat to get rid of and more mechanical stress, and more need for reduction.

18kw is not that big of a target though, certainly very easy to do with 55v on mosfets if the motor will draw the amps.

alpine44 said:

If I limit the current on my bench power supply and hook it up to a low resistance load, the voltage across the load will sag down to what would generate the desired current according to Ohms law. Would not the same happen (at least up to the speed where BEMF exceeds rated voltage) in a motor that is driven with a higher than rated voltage but with limited current?

Click to expand...

Sort of. But in a controller, usually the battery current is limited. With a higher pack voltage you can get a little more phase current for a given battery current at low rpm.

Most car sized EVs use IGBT inverters these days. Above about 120v they look more attractive. There are some good 120v MOSFETs out there these days but you need some head room in the ratings. TK72E12N1 for example.

The problem with IGBTs is the heat dissipation. They can handle it, but you may need to use water cooling on the controller.

alpine44 said:

Speed: 4500 rpm
<snip>
Ke: 11.2 V/1000rpm

Click to expand...

Inrunner or outrunner?

If the former, the speed may be limited by the ability of the rotor to hold together; depends on it's construction.

At 120v that's going to be over 10,000RPM at full speed, more than twice what it's rated for.


For specifics about what happens running a motor at different voltages, currents, etc., see the http://ebikes.ca/simulator ; read the entire page, and then experiment with different motors, controllers, and batteries, under the same conditions otherwise.


Regarding IGBT powerstage design, there's a number of threads discussing it (including actual builds) here on the forum. Probably not all of these are relevant, but:
https://endless-sphere.com/forums/search.php?keywords=igbt+power+stage&terms=all&author=&sc=1&sf=firstpost&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
and there's more that don't show up in that list, like Arlo1's stuff.

flat tire said:

Your motor will spin proportionately faster with the higher voltage. You can make more power. You will have more heat to get rid of and more mechanical stress, and more need for reduction.

18kw is not that big of a target though, certainly very easy to do with 55v on mosfets if the motor will draw the amps.

Click to expand...

18kW into the motor would require at least 330 A on the DC side. Depending on the PWM duty cycle that will be somewhere North of 450 A phase current. Even in a 24 FET inverter each FET in the active switch would have to carry well over 100 A. I think this is feasible but certainly not in the realm of "very easy to do".

Keep in mind that the maximum current ratings in the MOSFET data sheets are for very short duration pulses and if you read the fine print you'll see that the package has a much lower continuous rating (e.g. 75 A rms for a TO-247 casing).

Fast, clean switching, good current sharing, and very effective cooling would be essential to make an 18kW peak inverter work reliably. That's why you do not see any cheap Chinese controllers in this power range. They can cram 24 FETs in an Aluminum box but ignoring the requirements above, these tap out at 7kW peak or so.

If the 18kW inverter is 97% efficient 540 W would have to leave the inverter as heat. That's the output of a small space heater. A very optimistic 98% efficiency would still create 360 W of heat that need to be dealt with.

That's how it looks on paper. If you have squeezed 18kW out of a controller for about 10 seconds for every 60 seconds at about 5kw please send me a link to that controller. I am not being snarky here, sometimes paper and reality are two different things,

fechter said:

alpine44 said:

If I limit the current on my bench power supply and hook it up to a low resistance load, the voltage across the load will sag down to what would generate the desired current according to Ohms law. Would not the same happen (at least up to the speed where BEMF exceeds rated voltage) in a motor that is driven with a higher than rated voltage but with limited current?

Click to expand...

Sort of. But in a controller, usually the battery current is limited. With a higher pack voltage you can get a little more phase current for a given battery current at low rpm.

Most car sized EVs use IGBT inverters these days. Above about 120v they look more attractive. There are some good 120v MOSFETs out there these days but you need some head room in the ratings. TK72E12N1 for example.

The problem with IGBTs is the heat dissipation. They can handle it, but you may need to use water cooling on the controller.

Click to expand...

Heat dissipation is an issue regardless of switch type. See numbers in my previous post.
From what I can tell, the IGBT come in modules that are physically more suitable for shedding the heat. Albeit at a steep price.

amberwolf said:

Inrunner or outrunner?

Click to expand...

Axial flux with single rotor between two coil plates. Low resistance, low inductance, and low pole count (4 pairs). PMS-120 from Heinzmann as used in Brammo motocycle.

amberwolf said:

If the former, the speed may be limited by the ability of the rotor to hold together; depends on it's construction.

At 120v that's going to be over 10,000RPM at full speed, more than twice what it's rated for.

Click to expand...

I am not trying to go over 4500 rpm. That's why I was asking what voltage the motor "sees" internally when governed by current.

amberwolf said:

For specifics about what happens running a motor at different voltages, currents, etc., see the http://ebikes.ca/simulator ; read the entire page, and then experiment with different motors, controllers, and batteries, under the same conditions otherwise.

Click to expand...

I do not have enough data about the motor at this point to use the calculator but thanks for the link.

amberwolf said:

Regarding IGBT powerstage design, there's a number of threads discussing it (including actual builds) here on the forum. Probably not all of these are relevant, but:
https://endless-sphere.com/forums/search.php?keywords=igbt+power+stage&terms=all&author=&sc=1&sf=firstpost&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
and there's more that don't show up in that list, like Arlo1's stuff.

Click to expand...

While trying to find a suitable power stage for this motor I came across the threads you mentioned. At first, these projects seemed to be overkill for my needs. But after realizing that getting 18kW peak with small-package (like TO-247) MOSFETs is not trivial, I am tempted to "Go Big or Go Home". Hence the question of what will happen to my motor if I use a high voltage, high power stage but keep the current down to reduce power and rpm.

The higher DC voltage came with the consideration of going with IGBT modules. I planning to use 16s 2.6kWh packs that can source 450A rated current. Two parallel for MOSFETs and two in series for IGBTs. ~40 mile range in an ATV when using about 70% of maximum capacity (from 4.0 to 3.0 Volt per cell).

alpine44 said:

amberwolf said:

For specifics about what happens running a motor at different voltages, currents, etc., see the http://ebikes.ca/simulator ; read the entire page, and then experiment with different motors, controllers, and batteries, under the same conditions otherwise.

Click to expand...

I do not have enough data about the motor at this point to use the calculator but thanks for the link.

Click to expand...

You don't have to use the actual motor you have to get the idea of what happens with a system.

...axial flux with single rotor between two coil plates...

Click to expand...

This is the common medium-power configuration. It works.

I am very humbled to call LFP my friend. He has been a proponent of lower volts and higher amps. Zero motorcycles uses 28S (104V nominal?) and high amps. As a contrast, the now defunct Alta motorcycle company used the lowest amps possible (to get max range, and keep the cells cool for max life?), but the voltage was roughly 355V.

The fact that Zero continues to succeed, and Alta filed for bankruptcy is something that I would not use as a consideration. However...each has their benefits and drawbacks. That being said, attaining "X" top speed from "Y" voltage means that a low kV motor will have lower resistance. This is the only aspect where the physics cannot be disputed. If heat is an issue of consideration, low-kV at half throttle is better than high-kV at full throttle, due to the length of the copper wire-wrap on the stator teeth.

amberwolf said:

alpine44 said:

amberwolf said:

For specifics about what happens running a motor at different voltages, currents, etc., see the http://ebikes.ca/simulator ; read the entire page, and then experiment with different motors, controllers, and batteries, under the same conditions otherwise.

Click to expand...

I do not have enough data about the motor at this point to use the calculator but thanks for the link.

Click to expand...

You don't have to use the actual motor you have to get the idea of what happens with a system.

Click to expand...

I played around with the simulator but every result is fundamentally different than what the manufacturer of the motor published. The attached data are for a motor with different ratings and constants but close enough.
View attachment PMS 120 - 48 VDC - 6000 rpm -ID 833_a - WB 657 - EN.pdf

When pushed to max torque (solid blue line) the inverter is feeding a constant current up to about 4000 rpm. That is resulting in a flat torque curve. At higher rpm the BEMF limits the current flowing through the motor as evidenced in the decreasing torque.

I am only interested in the low rpm region with regards to my question of how the motor will react to a higher than rated DC bus voltage when the current is limited by the inverter control.

I know that with a higher input voltage the motor would reach higher rpm without field weakening but that is not my concern. I need the maximum grunt at low rpm without blowing anything up. The temptation of using IGBT modules is based on their current handling capabilities. I see the higher DC bus voltage more as a detriment in this application.

spinningmagnets said:

...axial flux with single rotor between two coil plates...

Click to expand...

This is the common medium-power configuration. It works.

I am very humbled to call LFP my friend. He has been a proponent of lower volts and higher amps. Zero motorcycles uses 28S (104V nominal?) and high amps. As a contrast, the now defunct Alta motorcycle company used the lowest amps possible (to get max range, and keep the cells cool for max life?), but the voltage was roughly 355V.

The fact that Zero continues to succeed, and Alta filed for bankruptcy is something that I would not use as a consideration. However...each has their benefits and drawbacks. That being said, attaining "X" top speed from "Y" voltage means that a low kV motor will have lower resistance. This is the only aspect where the physics cannot be disputed. If heat is an issue of consideration, low-kV at half throttle is better than high-kV at full throttle, due to the length of the copper wire-wrap on the stator teeth.

Click to expand...

For someone who just dabbled around with MOSFETs, even 104V nominal is "high". As you can see in my previous posts, the currents needed to extract maximum (peak) power from the motor at the rated 55.5V is giving me a bit of a headache.

Brammo used this motor in their Enertia motorcycle, apparently with a Sevcon controller. I have two issues with Sevcon. Firstly, their controllers are known as a bear to configure. And secondly, why would I throw my money at a manufacturer who apparently has zero empathy and support for hobbyists. We should vote with our money and discourage closed architectures and non-existing end user support.

China also does not seem to have anything to offer that tempts me to bet my money on. They are a good source for cheap, throwaway e-bike stuff but that's about it.

This leaves me with two choices. I could use the capabilities of the original Lebowski trough hole board and build a 24 FET power stage that with some luck (lacking expertise) can carry the required current without letting the magic smoke out. What works in my favor is that the two coil plates of the motor are paralleled in the external wiring and therefore can be driven by two different inverter stages. So instead of having to share the current between 4 FETs per switch I could have two separate 2 FET switches, one per coil side, controlled by the same signal. (Each phase has 2 switches and there are three phases). That would increase the effective current carrying capacity of each switch and the total output of the inverter.

The second choice is to bite the financial bullet and join the big-boys club around the Axiom VESC board that targets full-sized EVs with 650V 600A power stages. This might be overkill for the current project of electrifying a 250cc ATV but would leave the door open for bigger things like a side-by-side.

The remaining, and still unanswered, question is how the motor will react to a DC bus voltage more than twice its rating BUT with the inverter governing the current to not exceed the maximum permissible input power or rated rpm. Will it behave similar to a load on a constant current power supply, drooping the voltage down according to Ohm's law, or not?


Newsflash: I just read in the Axioms Hackaday log about a group that uses 650V 600A IGBT modules in a motorcycle with 70V DC bus. That's encouraging. I guess I should start saving the coin for 3 IGBT modules ($160 each) plus driver stages, plus Axiom board. In case my 24 FET DIY inverter flops

Have you considered the ASI BAC8000 controller? It can provide the power and current you are after.

alpine44 said:

For someone who just dabbled around with MOSFETs, even 104V nominal is "high". As you can see in my previous posts, the currents needed to extract maximum (peak) power from the motor at the rated 55.5V is giving me a bit of a headache.

Click to expand...

Whatever system voltage and controller you choose, the motor currents required to produce the torque you want are the same. And it looks like that motor current will be well over 500A.

alpine44 said:

The remaining, and still unanswered, question is how the motor will react to a DC bus voltage more than twice its rating BUT with the inverter governing the current to not exceed the maximum permissible input power or rated rpm. Will it behave similar to a load on a constant current power supply, drooping the voltage down according to Ohm's law, or not?

Click to expand...

If the motor currents are well regulated it will feel identical at lower speeds. The voltage step down is like a buck converter.

alpine44 said:

The remaining, and still unanswered, question is how the motor will react to a DC bus voltage more than twice its rating BUT with the inverter governing the current to not exceed the maximum permissible input power or rated rpm. Will it behave similar to a load on a constant current power supply, drooping the voltage down according to Ohm's law, or not?

Click to expand...

Correct. The controller will chop the DC bus voltage, which the motor inductance smoothes out, so the voltage the motor sees is lower. Motor current/torque is controlled by the phase voltage (simply I=V/R, as you say), either by the controller via the PWM to adhere to the current limit setting, or (at high speeds) by the back-EMF of the motor raising it's effective resistance.

So unfortunately your plan of running a higher DC bus for the same motor current/speed to gain low-speed torque won't work. It will result in lower battery current, but the phase current will be the same and as phase current is usually greater than battery current, the FETs or IGBTs are still going to have a hard time.

Your plan would work if you were suffering from torque dropping off at higher motor speeds. Or if you were going to spin the motor faster and gear it down more (potential problems with increased core losses not withstanding).

If your motor windings can be split it sounds like it may be possible to go for the easy solution of running two, inexpensive, controllers to share the load. This is out of my zone of direct experience but if I recall correctly it should be as simple as splitting the throttle & hall sensor signals to both controllers and connecting the phase to it's own half of the motor. It'd still be 3 phase (rather than 6), just paralelled.