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Old 02-15-2010, 01:47 PM   #1
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Default Motor Heat - vs - Battery Voltage?

Motor Heat - vs - Battery Voltage?

A few weeks back while I had the Novak Sentry Data Logger setup in my 13.5 WGT (1s), I got my motor very hot, 204 F. Oops

Equipment list: Tekin RS ESC, Thunder Power 5000mah 40c 1s, Novak Ballistic 13.5, 16ga wire from the battery to ESC and motor.

After reviewing the Novak Sentry logs, here is what I saw. At 4m10s into the 6m race, my battery voltage was constant at 3.6V (0% throttle) to 3.2V (100% throttle). The voltage was on a nice gradual discharge at this point. Once my motor hit 200+ F at 4m 40s, the voltage dropped down to an insanely staggering 2.0V at 100% throttle. The car slowed significantly and I stopped.

I'm fairly certain the battery did not dump, as I was only able to return 3000mah of charging back into the battery pack post race, and the voltage was at 3.5V when I started charging the pack. I have continued to use the same battery and motor to race since this time without issue.

What this data showed me is that high temperatures on the motor significantly increase the load presented to the battery. The question is why?

At the high temperatures, is the ESC not reading the motor timing sensor correctly thus causing the ESC to do something strange, or is the increased motor heat causing the high load?
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Old 02-15-2010, 03:59 PM   #2
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Originally Posted by kn7671 View Post
Motor Heat - vs - Battery Voltage?

A few weeks back while I had the Novak Sentry Data Logger setup in my 13.5 WGT (1s), I got my motor very hot, 204 F. Oops

Equipment list: Tekin RS ESC, Thunder Power 5000mah 40c 1s, Novak Ballistic 13.5, 16ga wire from the battery to ESC and motor.

After reviewing the Novak Sentry logs, here is what I saw. At 4m10s into the 6m race, my battery voltage was constant at 3.6V (0% throttle) to 3.2V (100% throttle). The voltage was on a nice gradual discharge at this point. Once my motor hit 200+ F at 4m 40s, the voltage dropped down to an insanely staggering 2.0V at 100% throttle. The car slowed significantly and I stopped.

I'm fairly certain the battery did not dump, as I was only able to return 3000mah of charging back into the battery pack post race, and the voltage was at 3.5V when I started charging the pack. I have continued to use the same battery and motor to race since this time without issue.

What this data showed me is that high temperatures on the motor significantly increase the load presented to the battery. The question is why?

At the high temperatures, is the ESC not reading the motor timing sensor correctly thus causing the ESC to do something strange, or is the increased motor heat causing the high load?
Yes.

As wires heat up it adds resistance to the poles.
The 21.5 motors drop off the most around 145 degrees.
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Old 02-15-2010, 04:28 PM   #3
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Yes.

As wires heat up it adds resistance to the poles.
The 21.5 motors drop off the most around 145 degrees.
Wait a minute, something doesn't make sense here. The resistance of the motor windings should increase with temperature, which means the motor voltage should increase as well... but he's seeing a DROP in voltage at high temps...
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Old 02-15-2010, 04:35 PM   #4
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What is motor voltage?

I think it makes sense Kelly. The 2nd poster is correct that temp affects resistance and the increased temps caused an increase in current draw. It would be interesting to have had the current sensor in when this happened.
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Old 02-15-2010, 09:09 PM   #5
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What is motor voltage?

I think it makes sense Kelly. The 2nd poster is correct that temp affects resistance and the increased temps caused an increase in current draw. It would be interesting to have had the current sensor in when this happened.
Hmm. I recall that the resistance of most metals increases with temperature. Therefore, a motor's current draw should DECREASE as it gets hotter, no? In other words, the load presented to the battery should decrease as a motor gets hotter (assuming its windings are pure metal.) So the fact that his battery voltage dropped to 2V all of a sudden suggests the battery was probably dumping, because the load on it was decreasing at that time. The fact that the battery came back to life is good fortune in my opinion

Am I wrong here?

Farzad
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Old 02-15-2010, 10:16 PM   #6
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I gotta admit I'm no expert on any of this. Just arm chair engineering. :
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Old 02-15-2010, 11:29 PM   #7
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It looks like you had a huge current drain all of a sudden which probably rules out high gearing (coupled with the fact that you ran with no problems later, I suppose you kept the same gearing). My guess is then that something might have caused a momentary overload on the motor perhaps some rubbish stuck in the gears which could possibly lock the transmission and stop the motor briefly which equates to shorting out the ESC output. This would cause a higher than normal current drain which in turn explains the battery momentary depletion.
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Old 02-15-2010, 11:47 PM   #8
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So if I too may armchair some more here

If the motor didn't melt to cause a short, and neither did the esc, then itwas likely the battery losing it's ability to pruduce voltage. Maybe a lipo expert can tell if this could happen without damaging the cell. I just don't think the hot motor was the cause of the big drop in batt voltage.
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Old 02-16-2010, 07:26 AM   #9
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Did you happen to hit something? Stuffing it into the wall at full throttle will bring the motor to a halt very quickly. I run 1/12 mod and when I do this, it will cause my esc to brown out and take about 2 seconds to reset before I get power back (its a longgggg 2 seconds to wait btw)
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Old 02-16-2010, 09:36 AM   #10
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Did you happen to hit something? Stuffing it into the wall at full throttle will bring the motor to a halt very quickly. I run 1/12 mod and when I do this, it will cause my esc to brown out and take about 2 seconds to reset before I get power back (its a longgggg 2 seconds to wait btw)
Nope - was still driving it around the track racing for the lead when it suddenly started slowing down. I know the reason it got hot was too much and too aggressive timing on the Tekin RS.

My question is what is the relation between motor heat and battery voltage, and if there is a relation. If not, than I have to speculate that a hot motor caused some strange readings at the sensor and caused the ESC to apply power incorrectly.

Someone stated above that in Oval Racing the motors start to drop off about 145F. Most manufacturers tell you to keep the motors at 160F or less, so why does oval start to drop off at 145F when the motor manufacturers tell you a higher max temp?

What I would also like to establish is at what temperture (130F, 145F, 160F, etc..) will the amperage draw start to increase at a rate of deminishing return.
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Old 02-16-2010, 10:53 AM   #11
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Originally Posted by zangonli View Post
Hmm. I recall that the resistance of most metals increases with temperature. Therefore, a motor's current draw should DECREASE as it gets hotter, no? In other words, the load presented to the battery should decrease as a motor gets hotter (assuming its windings are pure metal.) So the fact that his battery voltage dropped to 2V all of a sudden suggests the battery was probably dumping, because the load on it was decreasing at that time. The fact that the battery came back to life is good fortune in my opinion

Am I wrong here?

Farzad
As the temp in the motor windings increases so does the resistance. You are right about that. From that you are thinking that based on Ohm's Law if the resistance increases then the current will decrease if voltage remains the same, but this is not correct in this situation.

In this situation the current draw of the motor is the constant (e.g. 40A at 100% throttle). As the resistance increases in the motor the voltage required to produce that current also increases, effectively increasing the load on the battery, and causing a larger voltage drop. Eventually the battery can't keep up and "dumps", no longer providing much voltage at all, current drops off, and the run is over.

Another scenario that could have happened is that the rotor demagnetized from the excessive heat. Once this happens the current draw will spike and would cause the high voltage drop in the battery even though the car would seem like it doesn't have any power.

Last edited by simplechamp; 02-16-2010 at 11:03 AM.
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Old 02-16-2010, 05:30 PM   #12
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As the temp in the motor windings increases so does the resistance. You are right about that. From that you are thinking that based on Ohm's Law if the resistance increases then the current will decrease if voltage remains the same, but this is not correct in this situation.

In this situation the current draw of the motor is the constant (e.g. 40A at 100% throttle). As the resistance increases in the motor the voltage required to produce that current also increases, effectively increasing the load on the battery, and causing a larger voltage drop. Eventually the battery can't keep up and "dumps", no longer providing much voltage at all, current drops off, and the run is over.

Another scenario that could have happened is that the rotor demagnetized from the excessive heat. Once this happens the current draw will spike and would cause the high voltage drop in the battery even though the car would seem like it doesn't have any power.


That is interesting because it's not how I understood a bldc motor to work in the rc car scenario ... I thought that the esc drives current into the motor (with timed voltage pulses from the battery) to achieve a certain rotation speed (per throttle position), but that it doesn't "regulate" the current from the battery (until it reaches it's max limit.) Perhaps one of the motor guys can clarify, but I understand that for a given throttle setting, the esc tries to achieve a constant motor rotation speed. This however will be produced at different current draws depending on the mechanical load on the motor, e.g. the gearing, whether the car is accelerating or is at the end of a straighaway, etc.

All I am saying is that as a running motor gets hotter, it should decrease the electrical load it presents to the esc/battery. I think it will then be running less efficiently, meaning it can provide less torque (for the same rotational speed.) I think Kelly's perceived drop in batt voltage doesn't make sense unless the battery saw a huge load; this however would not be due to the motor heating up..

So, I think your other explanation may be right (motor inductance drops due to heat, therefore motor cannot develop the needed torque to reach the desired speed, and hence the esc pushes all of the current it can into the motor, which then causes the battery output to drop. Perhaps some of the motor guys can confirm this behavior.)

Or as niznai or another posted above pointed out, there might have a been a stalled rotor situation due to a rock or a temp esc failure due to crash or bad sensor reading from the hot motor ... I guess all I will say now is hope it doesn't happen again!
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Old 02-16-2010, 06:18 PM   #13
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That is interesting because it's not how I understood a bldc motor to work in the rc car scenario ... I thought that the esc drives current into the motor (with timed voltage pulses from the battery) to achieve a certain rotation speed (per throttle position), but that it doesn't "regulate" the current from the battery (until it reaches it's max limit.) Perhaps one of the motor guys can clarify, but I understand that for a given throttle setting, the esc tries to achieve a constant motor rotation speed. This however will be produced at different current draws depending on the mechanical load on the motor, e.g. the gearing, whether the car is accelerating or is at the end of a straighaway, etc.

All I am saying is that as a running motor gets hotter, it should decrease the electrical load it presents to the esc/battery. I think it will then be running less efficiently, meaning it can provide less torque (for the same rotational speed.) I think Kelly's perceived drop in batt voltage doesn't make sense unless the battery saw a huge load; this however would not be due to the motor heating up..

So, I think your other explanation may be right (motor inductance drops due to heat, therefore motor cannot develop the needed torque to reach the desired speed, and hence the esc pushes all of the current it can into the motor, which then causes the battery output to drop. Perhaps some of the motor guys can confirm this behavior.)

Or as niznai or another posted above pointed out, there might have a been a stalled rotor situation due to a rock or a temp esc failure due to crash or bad sensor reading from the hot motor ... I guess all I will say now is hope it doesn't happen again!
I don't think the ESC adjust current based on load as it doesn't get feedback from the motors current state of load, only a sensor position reading. At 100% throttle the ESC simply knows to allows 100% of the battery voltage to flow through to the motor. In essence, our ESC's are are " open-loop" systems, meaning it does exactly as told with zero feedback. A "closed-loop" ESC system would take readings from several things simultaneously and dynamically adjust based on those conditions.

What I think happened is that the motors resistance effectively increased due to the high heat, most likely reducing the rotor's magnetic strength.
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Old 02-16-2010, 06:25 PM   #14
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Found a research paper:

How Temperature Affects a Magnet's Strength

http://www.usc.edu/CSSF/History/2005/Projects/J1502.pdf

and a website with Magnetic Strength and Heat Related info:

http://www.sciencebuddies.org/scienc...hys_p025.shtml

Another Link:

http://www.kjmagnetics.com/neomaginfo.asp

Demagnetization
Rare Earth magnets have a high resistance to demagnetization, unlike most other types of magnets. They will not lose their magnetization around other magnets or if dropped. They will however, begin to lose strength if they are heated above their maximum operating temperature, which is 176F (80C) for standard N grades. They will completely lose their magnetization if heated above their Curie temperature, which is 590F (310C) for standard N grades. Some of our magnets are of high temperature material, which can withstand higher temperatures without losing strength.

Last edited by kn7671; 02-16-2010 at 07:10 PM.
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Old 02-16-2010, 07:33 PM   #15
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simplechamp is correct. remember that voltage is applied and current is draw. what is actually happen is that current draw from the motor increase as heat increases in the wire. this is because the motor still wants to produce the same power output. because the internal resistance of the motor increase then the current draw of the motor increase. since current draw from the battery increase, then the voltage potential of the battery drops. remember a battery is just an internal circuit so ohm's law still applied.

short story, motor heat increases, current draw increase, battery voltage decrease. it is simple as that.

the thing that you want to find kelly is at the current draw delta is as the motor temp increases. I would assume at the start this is pretty linear or close to linear, but at a point on a motor, this slow will increase exponentially. I can only assume that from the motor companys that the 145 mark is the point. the hard part about this is that you would need to take 1 brand of motor and 3-5 of those motors and take readings on those motors and you will probably find a cluster around a small temp range. but you will also find out lyers. these data point will need to just be dropped.
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