Current Limiting for DC Motor

Thread Starter

altusXe

Joined Oct 22, 2021
4
I have a 390 DC Brushed motor, which actually came with a RC Car. I was trying to add an ESC to the the motor to control it through PWM Values. I had connected it to a 20A rated ESC. But, when I tried to calibrate it for the max speed, the ESC burned up which means the motor drew more than 20A current. But, the problem is I am not getting high Current rated ESC in my region and some are taking more than a month to arrive. I thought of using a smaller motor but then I don't want to mess with the gearing of the RC car and don't want to spend much time on it. So, I would like to know if there is any way I can limit the current drawn by the motor with the ESC rated 20A so that it won't burn up the ESC. Any current limiting circuit or any other way I can do that??
 

Thread Starter

altusXe

Joined Oct 22, 2021
4
Is your motor the '390' described here?
It lists the stall current (the maximum the motor will draw) as 19.88A, so it shouldn't blow up a 20A ESC.
https://nfpshop.com/product/28mm-24v-dc-22500rpm-high-speed-390-dc-motor-used-for-electric-tool
yes, that's the motor I have. I can't think of anything else why it would burn up the ESC though. Wiring should be fine because the other servo motor was also being powered through this ESC and it worked fine. Any suggestions as to why it might've burned up?

oh sorry. But the voltage of the battery that came with the RC car was 7.4V and also the one I used during the calibration was also 7.4V but, the rated voltage here is 24V. So, I think this is not the DC motor I have.
 

Thread Starter

altusXe

Joined Oct 22, 2021
4
Can you find a specification for the motor that you have?
I have tried a lot to find the specification of the motor but didn't find any. Just some mechanical specs have been uploaded on the sites. So, just settled on trying to find a way to limit the current to the motor to less than 20A, the ESC I have.
 

shortbus

Joined Sep 30, 2009
8,963
My first thought is are you using the correct type of ESC? There are two types for small motors like that one for brushed motors and the more common one for BLDC motor. Usually a brushed motor type is sold as a speed controller, not an ESC.

Quote, "Different types of speed controls are required for brushed DC motors and brushless DC motors. A brushed motor can have its speed controlled by varying the voltage on its armature."
From - https://en.wikipedia.org/wiki/Electronic_speed_control
 

Thread Starter

altusXe

Joined Oct 22, 2021
4
Y
My first thought is are you using the correct type of ESC? There are two types for small motors like that one for brushed motors and the more common one for BLDC motor. Usually a brushed motor type is sold as a speed controller, not an ESC.

Quote, "Different types of speed controls are required for brushed DC motors and brushless DC motors. A brushed motor can have its speed controlled by varying the voltage on its armature."
From - https://en.wikipedia.org/wiki/Electronic_speed_control
Yes, I am using the correct type of ESC for a brushed motor.
 

Alec_t

Joined Sep 17, 2013
12,315
the ESC burned up which means the motor drew more than 20A current.
Not necessarily. Many products (ESCs included) sold online have advertised ratings which are exaggerated or downright inaccurate. You may have been unlucky with your ESC and had one with a true current capability less than 20A.
 

ElectricSpidey

Joined Dec 2, 2017
1,970
You should always derate, if a motor draws nearly 20 amps at stall, then you should use a controller rated higher than 20 amps.

EDIT:
Here are the specs I found for the 7.2 volt version.

Specifications:
Item name: Brushed Motor
Suitable for: 1/16 RC off-road, monster truck and flat racing Car
Color: Silver
Material: Metal
Rated voltage: 7.2V
Operating voltage range: 4.5V-9.6V
Rotation: CCW
NO load:
Speed: 20000rpm
Current: 0.7Amp
Stall:
Torque: 1500g.cm
Current: 57Amp <<<<<
Maximum efficiency:
EFF: 60.76%
Speed: 18000rpm
Current: 6.33Amp
Torque: 150g.cm
Output: 27.692W
Diameter: 27.5mm
Length: 46.5mm
Shaft length: 11mm
Shaft diameter: 2.3mm
Weight: 113g/ 3.98oz

The specs were found at rcmoment.com
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
23,923
The definitive way to measure the maximum current draw of the motor is to feed with a very low DC source and lock the rotor while measuring the current, test at a few positions of the rotor, and use the highest current and applied voltage to calculate.
 

Audioguru again

Joined Oct 21, 2019
3,846
When the motor needs 57A (!) to begin running, it might not start running if you limit its current to only 20A.

I put larger propellers on some of my RC airplanes. If I suddenly give full throttle to start, the ESC stops with an over-current warning. So I start with a slower throttle increase and it works fine.
 
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