# How to increase torque in a BLDC motor

I have a 350kv BLDC motor and a 150 amp ESC. I'm using them to run a boat prop. It spins with plenty of speed in the air but as soon as I put it in water the ESC shuts down. The battery has plenty of power, I am using an Arduino connected to the ESC instead of a remote controller, I was wondering if there is anything in the code that I could change to give it more torque instead of speed, or do I need a higher amp ESC? ////////////////////////////////////////

#include <Servo.h> //Using servo library to control ESC Servo esc;
//Creating a servo class with name as esc
void setup()
{
esc.attach(10); //Specify the esc signal pin,Here as D10
esc.writeMicroseconds(1000); //initialize the signal to 1000
Serial.begin(9600);
}
void loop()
{
int val; //Creating a variable val
val= analogRead(A0); //Read input from analog pin a0 and store in val
val= map(val, 0, 1023,1000,2000); //mapping val to minimum and maximum(Change if needed)
esc.writeMicroseconds(val); //using val as the signal to esc
}


Torque = current and 150A isn't enough, so...

Gear down to the prop.

Sanity check : a 5HP Seagull Silver Century drives its 11 inch prop at about 1000rpm, which you would reach with only 3V on your motor direct drive. If you're trying to run this thing at 12V that's about 4000rpm, and power required varies as the cube of speed (torque as the square of speed) so you'd need 16x the torque and 64x the power of that motor to drive the prop.

Go back to the physics; plot the power and torque required vs speed for the diameter and pitch of your prop. Equations are easily found.

Also understand the relationship between speed and voltage, and the implications of that for torque and current, for your motor.

Then decide on the correct gearing to match your motor to your propellor at the speed and power you want.

At 1:1 gearing you'll find you can only drive a tiny prop absurdly fast, producing very little thrust.

Well,

Either your ESC is not doing it's job well and Iq is not well controlled, either your need more current.

BLDC are rather simple beast, in your case you should be interested in back EMF and torque constant (the two are related with a sqrt(3) factor when in V/s and Nm/A).

The torque constant gives you the relation between torque and current, it's value is slightly negatively affected by temperature. So the hotter you run the lower the Kt.

Be aware, a BLDC has a typical torque/speed curve that should be followed. If not, continuous power dissipation will be an issue. There is also an absolute maximum torque defined by the core magnetic saturation and magnet strength.

Be careful, even if a motor can sustain higher than nominal current for short bursts the important part is to stay within the temperature tolerances of your insulation resin. If not cooled properly the enamel resin on the winding will fail, thus the entire motor will fail. Please make sure that your motor is appropriately cooled before stepping up current.

• Thank you, the motor is submerged so heat shouldn't be a problem. One thing I don't understand is, the ESC is rated at 150 amps, when I measure the current at full throttle it only draws about 7 amps. How are these things rated? Commented Apr 25, 2021 at 18:41