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Motor braking power

Old 02-27-2018, 11:34 PM
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Default Motor braking power

This question is simply for personal curiosity. Using 2 identical esc's having identical programming. Does a 3.5t motor have the same, weaker, or stronger brakes then a 25.5t motor?
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Old 02-28-2018, 12:42 AM
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esc determines braking power as in amps drawn. but a 3.5T will take long in micro seconds to brake due to turn # over a 25,5T which will be instantly ..so a 25.5T will look like it has more brakes due to its slower Kv rating .
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Old 02-28-2018, 05:15 AM
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If they had exactly the same gearing the 3.5 would have stronger brakes at higher speed and the 25.5 at lower speed.

However if you geared them for a similar top speed the 3.5 would have stronger brakes everywhere.
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Old 02-28-2018, 06:39 AM
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Originally Posted by faqcya View Post
This question is simply for personal curiosity. Using 2 identical esc's having identical programming. Does a 3.5t motor have the same, weaker, or stronger brakes then a 25.5t motor?
The 3.5T will have much stronger braking than the 25.5T.

This is directly related to the torque of the motor. At the same voltage, a lower-turn motor has more torque (and more RPM!) than a higher-turn motor.

Last edited by howardcano; 02-28-2018 at 04:00 PM.
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Old 02-28-2018, 06:40 AM
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Originally Posted by the rc guy View Post
esc determines braking power as in amps drawn. but a 3.5T will take long in micro seconds to brake due to turn # over a 25,5T which will be instantly ..so a 25.5T will look like it has more brakes due to its slower Kv rating .
Funny I have a tc3 rsx with a 2.5

And tc4 with a rsx and a 17.5 both equal settings just the 17.5 has 10*

And braking is 10000x harder and responsive on the 2.5

Not everything on the interweb is accurate or true.

Oh and my tc6 mmp Tekin 5.5 brakes harder than my sprint2 Tekin rs pro/lrp 5.5
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Old 02-28-2018, 06:46 AM
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makes sense
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Old 02-28-2018, 09:08 AM
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how the brake frecuency (PWM) can be adjusted o n the ESC to get more brake?
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Old 02-28-2018, 01:57 PM
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Originally Posted by Byronf1 View Post
how the brake frecuency (PWM) can be adjusted o n the ESC to get more brake?
Depends on the ESC. Many can't. Check your manual.
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Old 02-28-2018, 02:12 PM
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If the motors were out of the car and spinning at the same RPM, the 25.5 turn would provide more braking force.

If both motors are in a car and geared for the same top speed, the 3.5 would have a much larger mechanical advantage due to the gear reduction would provide more braking force at the same speed.
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Old 02-28-2018, 02:22 PM
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These are interesting responses and I appreciate everyone taking the time to answer.
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Old 02-28-2018, 03:54 PM
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Originally Posted by Krio View Post
If the motors were out of the car and spinning at the same RPM, the 25.5 turn would provide more braking force.
That would only be true if the ESC and wiring resistance were a significant fraction of the motor's resistance. Otherwise, if the motors have the same stator and rotor design, the rotors have the same strength, and the stators are both at 100% fill, the braking torque would be the same.

Originally Posted by Krio View Post
If both motors are in a car and geared for the same top speed, the 3.5 would have a much larger mechanical advantage due to the gear reduction would provide more braking force at the same speed.
Correct. I have edited my previous post to match your observation.

Last edited by howardcano; 02-28-2018 at 04:36 PM.
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Old 02-28-2018, 05:21 PM
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Originally Posted by howardcano View Post
That would only be true if the ESC and wiring resistance were a significant fraction of the motor's resistance. Otherwise, if the motors have the same stator and rotor design, the rotors have the same strength, and the stators are both at 100% fill, the braking torque would be the same.



Correct. I have edited my previous post to match your observation.
You are assuming the voltage created in the motor is the same between a 25.5 turn motor and a 3.5 turn motor at the same rpm. This isn't the case. Let's say the 25.5 motor is rated as 1,500kv and the 3.5 is 9,000. This rating also tells you (roughly, not accounting for losses/inefficiencies) the voltage created by the motor when you spin it. At 3,000 rpm, the 25.5 is creating 2 volts. The 3.5 would only be creating 0.33 volts.
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Old 03-01-2018, 03:30 AM
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Originally Posted by Krio View Post
You are assuming the voltage created in the motor is the same between a 25.5 turn motor and a 3.5 turn motor at the same rpm. This isn't the case. Let's say the 25.5 motor is rated as 1,500kv and the 3.5 is 9,000. This rating also tells you (roughly, not accounting for losses/inefficiencies) the voltage created by the motor when you spin it. At 3,000 rpm, the 25.5 is creating 2 volts. The 3.5 would only be creating 0.33 volts.
I did not assume that.

The back-EMF of the motor is proportional to the magnetic field strength, enclosed flux (rotor and stator size), speed, and the number of turns.

At the same speed, stator and rotor sizes, and rotor field strength, the back-EMF of the 3.5T is (3.5/25.5) = 0.137 times that of the 25.5T.

The resistance of the motor is proportional to the square of the number of turns (if the stator is completely filled with wire).

The 3.5 has (3.5^2/25.5^2) = 0.0188 times the resistance of the 25.5T.

Torque is proportional to the current, the magnetic field strength, enclosed flux (rotor and stator size), and the number of turns.

At the same speed, stator and rotor sizes, and rotor field strength, current through the 3.5T is V/R = (0.137/0.0188) = (25.5/3.5) =7.29 times that of the 25.5T.

Finally, the torque of the 3.5T = 7.29*(3.5/25.5)=1 times the torque of the 25.5. The torque is the same.
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Last edited by howardcano; 03-02-2018 at 04:56 AM. Reason: Italicized general rules for motor parameters. Corrected "1/(3.5^2/25.5^2)" to "(3.5^2/25.5^2)"
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Old 03-01-2018, 09:13 AM
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Originally Posted by howardcano View Post
I did not assume that.

The back-EMF of the motor is proportional to the magnetic field strength, enclosed flux (rotor and stator size), speed, and the number of turns.

At the same speed, stator and rotor sizes, and rotor field strength, the back-EMF of the 3.5T is (3.5/25.5) = 0.137 times that of the 25.5T.

The resistance of the motor is proportional to the square of the number of turns (if the stator is completely filled with wire).

The 3.5 has 1/(3.5^2/25.5^2) = 0.0188 times the resistance of the 25.5T.

Torque is proportional to the current, the magnetic field strength, enclosed flux (rotor and stator size), and the number of turns.

At the same speed, stator and rotor sizes, and rotor field strength, current through the 3.5T is V/R = (0.137/0.0188) = (25.5/3.5) =7.29 times that of the 25.5T.

Finally, the torque of the 3.5T = 7.29*(3.5/25.5)=1 times the torque of the 25.5. The torque is the same.
Sorry for my assumptions and glad to see you provide calculations. Hard to know off the bat what the technical background is based on a quick answer on a forum.

I won't disagree on the calculations for an ideal motor as, yes, they should be the same. My answer was based on including inefficiencies is all in addition to the data I have come across through work. I didn't mean to imply that the 25.5 turn has 10 times the braking force due to the voltage difference in my previous post, but I wasn't sure where to start with my reasoning.

I just realized that the 25.5 in question is probably ROAR spec'd to a certain wire diameter, so my assumption that the 25.5 turn would have more fill was incorrect. In the end, I meant that the 25.5 will have maybe 5% more braking power at the same rpm if the wire size was optimal for each motor.
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Old 03-02-2018, 05:04 AM
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Originally Posted by Krio View Post
I won't disagree on the calculations for an ideal motor as, yes, they should be the same. My answer was based on including inefficiencies is all in addition to the data I have come across through work.
Certainly getting two motors to be identical except for the number of turns is difficult at best, and all of the inefficiencies and other real-world problems will almost always skew the results compared to ideal models. That's what makes things so interesting!

Speaking of calculations, we both missed my error in the calculation for resistance. Somehow a "1/" snuck in there! I've fixed it.
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