How much can I "overpower" my brushless motor?

AlexLTDLX

AlexLTDLX

Administrator
Staff member
I started doing research again the hard way (googling, reading technical papers, etc) when I realized - We have a forum! So here's my question for you all - as you probably know, I'm switching over to an LTO battery pack. That should give me more power to deliver to my ESC/motor combination. Right now, I'm just under 14 kW going down track according to my ESC datalogs. The measured power going into the ESC is a bit more, but I'm going to go by what the ESC is telling me rather than my cheap chineseum meters.

My motor is rated for a 15 kW peak (9 kW continuous - but I'm only concerned about peak because I'm interested primarily in drag strip performance). How can I tell when my motor has no more power to offer? What should I be looking for in my datalogs? With this new pack, fully charged at "resting" voltage, I should see about 73 volts. But my ESC is rated at a max of 70 volts (or 68, according to the people who made it; I need clarification on that). Either way, voltage drop under power should be significantly less than I was see with my LiPo pack (67.2 fully charged, and about 52 going down track (on the meters); 50 if you go by the ESC data). I was thinking of starting our next dyno session at either 64 or 66 volts, with the idea that it probably won't drop further than about 54 volts.

The question - how do I know if I can keep turning up the voltage? I know at some point the motor will get hot, but I'd like to know what to look for in the data logs. I'm open to all ideas and suggestion (except, maybe, the classic, "keep going until it catches fire, explodes or melts, and then turn it down a couple of volts...")

Thoughts?
 
What is the motor RPM at 6lbs of boost?

Do you have a temp sensor on the motor?

When they say peak is at 15kW, do they give a time limit it can stay at peak?

Most of the motors I work on are high torque, we don't push our motors too hard because we need longevity. This is a bit of a different animal for me, still very new to brushless motors.
 
Last edited:
Here's the eRPM log from my two runs (to calculate motor rpm from eRPM, simply divide by 3):
Screenshot (49).png
Screenshot (59).png

I don't have motor temps, but I run ice water through the cooling jacket. It seemed to be just fine, but then again, it was only running for 12 seconds at the longest. What I do have is ESC temps:
Screenshot (48).png
Screenshot (58).png

I think the time rating for peak is 30 seconds - but I'll see if I can verify that. Which gives me some hope of pushing it harder.
 
30 Seconds, wow!!!!

The way I look at it, it comes down to heat and time. So motor temps would be crucial. We can probably extrapolate from the data, and the limits to get us in the ballpark, but anytime we push stuff its on borrowed time.

I do not think there is an equation where we can say that this is the cut off line for power.
 
Last edited:
Right now I am going through a crash course on brushless motors. So, I have a huge gap of understanding that I need to close before I can be very helpful.
 
I think we all are going through the same crash course - lol. I got a reply back from Jeremy at APD (the makers of my ESC). This is what he said:

Hi Alex,

Good to hear from you. To answer your question, we're basically looking to see if the motor's duty cycle matches the input duty cycle. If there is a difference, then the ESC is limiting for some reason. Increasing the timing advance can give you an increase in power up to a point.

Kind regards,
Jeremy

I sent him a follow-up, but basically my motor's duty cycle is less than 100% when I'm feeding it 100% duty cycle. So I'm not sure what the limiting factor is - voltage/available current is the only thing I can think of. Any thoughts?
 
I like this channel : https://www.youtube.com/channel/UCrKM0T_jQkCApz0nSH8Iynw
verdict: since we are only in "low voltage areas" we are not going to jump through insulations ... critical is more current flow/HEAT produced ... affecting insulations, magnets and bearings!

( I found many of his explanations very interesting as well ... e.g. lower KV != more torque on same can size! )
 
Last edited:
cmoalem said:
I like this channel : https://www.youtube.com/channel/UCrKM0T_jQkCApz0nSH8Iynw
verdict: since we are only in "low voltage areas" we are not going to jump through insulations ... critical is more current flow/HEAT produced ... affecting insulations, magnets and bearings!

( I found many of his explanations very interesting as well ... e.g. lower KV != more torque on same can size! )
Click to expand...
Yep, he is the guy I have been watching as well. Very informative!!!!
 
That's an awesome video - thanks! And of course, my biggest takeaway - I can overpower this thing. Efficiency will go down a bit, but if it gets us closer to the magic 750 hp number, then that's awesome. I'm going to revisit some of his videos too. Now back to the garage - I've got busbars to finish!
 
The other hard limiting factor will be the ESC ... 16s will cap at 70V ... (if the motor holds up between 50 and 70V)
 
That's true. If not enough voltage, I guess I'll try to fry to TP Power motor by adding another cell to each pack and throwing down the APD's UHV ESC (good to 85 volts) - fully charged, those LTO cells would then be sitting at 78 volts. But I think we can get into the 9's with the hardware I have, and then this system will be "done" - since I don't have the safety stuff (in particular, a full roll cage - I have a 6 point roll bar, which is intrusive enough) to run 9s anyway. Then we'll build a "big" system - one that can support well over 1,000 hp. I think I'll target 1,500 hp at first, but I'll have to see what the best combination of motor, ESC, and blower are for that level.
 
When pushing for limit, please remeber about wires and connectors. Maybe motor will not run hot with water jacket cooling but other pieces might not like it. I don't know what are Your wire gauges for motor and ESC but just don't forget: For every system when components have different limits of operation, trying to achieve highest one will remind You about all lower ones that You forgot about.
 
Last edited:
Good point. But I think I've got that covered. I've started assembling the first pack (there are 2 to get to 70 volts). 6 more cells and the balance board go in the right hand side. Note the Anderson Power Pole connector and 50' of 2 gauge cable. The dollar is for size reference. I live by voltage drop calculators.
Pack1UnderConstruction.jpg
 
BTW, Thomas at Castle Creations (he's one of their tech guys) and I spoke today. He went through my datalogs and basically came to the conclusion that I need to hit it with more voltage. We'll see soon enough how this turns out! Just assembling the packs and making the control box (that holds the arduino and timers to power the thing on with one switch) is all that's left before our next tests.

Thomas is a genius, esp. when it comes to brushless motors. He's forgotten more than I know about these things. I've asked him to join the forum. Hopefully he does.
 
From an electrical efficiency point of view .. Voltage is good .. Current is bad.
You will be better off running the highest voltage possible with the smallest current than having a small voltage and a huge current.

The motor itself is not limited to a voltage but it's really limited by the power it can dissipate ( how hot it gets before it melts down)

Running a LOW KV motor ( lower than you have) with a High voltage (than you have) will result in greater efficiency than the opposite due to less resistive losses for the complete system not just the motor. But specifically talking about the motor there will be less resistive heating so the motor wont get as hot and therefore you can run the motor at a greater total power for the same heating (real limit)

However in your case you're limited by the ECS max voltage so you can't increase the Voltage beyond that.

On motor meltdown .. i believe it's water cooled why don't you add ice to the water since your only concerned about the drag strip. and this could cool the motor and esc.
 
Those are very good points. And there's something that's been confusing me since our first drag test about the datalogs, directly related to your post, but I *think* I'm starting to figure it out. Specifically, that the more voltage I feed the system, not only do I get more boost (rpm), but the current AND power goes down. I'm going to start a thread tomorrow on the topic, since I want to use the datalogs that are on my other computer in the office, and it's too late tonight to do it (have to work in the morning).

Oh, BTW, I am using ice in the water - an old habit. When I had the Whipple on the car with the A/W intercooler, I'd melt literally 60-80 lbs of ice on a test n' tune night at the track. One pass would melt 10-15 pounds in 10 seconds. You can hear the ice knocking around the tank as I stage in the interior shots of this video:

But I was thinking - as I just bought a CO2 cartridge bike tire inflator (to use both as an air duster and, well, carry with me on my bike, on the rare occasions I get to ride it) and it gets really cold for obvious reasons when you use it. What if I just sprayed the stuff straight in the ESC and in the motor while making a pass? Though I'd be inclined to think simply getting the motor and ESC near freezing beforehand would be more effective.

Hey - since the batteries are now in the trunk, why don't I use the old ice tank? I had it custom made by an RV tank company out of thick HDPE to fit perfectly as my console. Of course, the 2000 gph bilge pump I used would need to go. Just a *bit* to much flow for this. Heck, one fill up of ice would last an entire racing night now... Here's an oldie but a goodie - testing that bilge pump, complete with getting called names by my daughter in the process:

The pump I'm currently using is a little deal I had kicking around. Literally a bit larger than 1 cubic inch in size. I should look for a happy medium... esp. since my coolant lines are no longer 1" but 8mm ID.
 
AlexLTDLX said:
Good point. But I think I've got that covered. I've started assembling the first pack (there are 2 to get to 70 volts). 6 more cells and the balance board go in the right hand side. Note the Anderson Power Pole connector and 50' of 2 gauge cable. The dollar is for size reference. I live by voltage drop calculators.
View attachment 44
Click to expand...
What kind of battery cells are these and where can I get them? Ty
 
You must log in or register to reply here.
Top