Motor Torque calculations

Motor Torque calculations

Motor Torque calculations is very easy for anyone if you know the basic idea and formula related to it.

Motor Torque calculations

The 3 factors that determine the type of work   A motor can produce


But here we will know about how to calculate Torque for motors.

Work:- Work is defined as Force applied over a distance but, In the case of Flywheel wenches and motors , The work is called Torque.

Torque :- Torque is a measure of how much a force acting on an object causes that object to rotate.


Torque is a rotating force produced or available at a motor’s crankshaft.

Simply,Torque is a special type of work that produces rotations.

Torque occurs when a Force acts on a radius

Torque = F×r , typical unit for measurements of Torque is N-m or pound feet.

Why ,I am saying Torque is a type of work.When you will see the unit of both you will understand.

So,to calculate Torque we need two things

1.Force Applied

2.Distance of force from centre of rotation.

Power :- Rate of doing work. Unit of power is watts (J/s) or horse power.

P = E/t
P = F·v
P = V·I
P = F.s ( For linear Motion )
P = τ·ω ( For Rotary Motion )

For Rotary motion, we are finding power so that it is easy for you to understand.

Power = (Force × Distance ) / Time

Power = [Force × Distance (Per Revolution) ] / Time ( in min. )

Distance per revolution = 2π × Radius

Distance per revolution per time = 2 π × Radius × RPM

We know that Torque = Force × Perpendicular Distance ( Radius )

Now, Force = Torque / Radius

As we know Power = [ Force × Distance (Per Revolution ) ] / Time (in min. )

Now,putting the value of force and Distance

We get, Power = {Torque/Radius} × { 2 π × Radius × RPM }

Finally, Power ( in Watt ) = Torque × 2π × RPM. Or,

Power (in watt ) = (Torque × 2π × RPS )/60

Now,we get

Torque = (Power × 60)/(2π ×Rps)

Or, (Power )/(2π ×Rpm)

Must read :- Calculating brake horsepower

This is the basic formula for calculations of Torque but only problem you face is unit conversion.

Note :-For calculations of Torque if motor you need to know power ( in watt or horsepower)

Now, I am writting directly formula with unit.

Torque in Imperial units can be calculated as

Tinlb = Php 63025 / n                        


Tinlb = torque (in lbf)

Php =  horsepower delivered by the electric motor (hp)

n = revolution per minute (rpm)


Tftlb = Php 5252 / n                        


Tftlb = torque (lbf ft)

1 ft lbf = 1.356 Nm
Torque in SI units can be calculated as

TNm = PW 9.549 / n                       


TNm = torque (Nm)

PW =  power (watts)

n = revolution per minute (rpm)

DC Motor Torque Calculations

the total torque developed of a DC machine is,

This torque equation of DC motor can be further simplified as:

Gear motor Torque calculations

Gear box output :- Multiply the gear ratio by the input torque 

The torque unit is N.m (Newton · meters) or kilogram. meters (Kg · m).
Gear Reducer Torque = 9550 * Motor power / Rated Power motor’s input revolution*Ratio* gear reducer efficiency.
The above calculation is the torque equation.
Motor power = torque ÷ 9550 × Rated power‘s motor input speed /Ratio /service factor.

Here useful converted relationship to Output torque’s unit  :

100 N.m = 885.07457 9349 Ibs in

100N.m = 885.07458

100 N/m = 0.5710147 ib/in

Servo motor Torque calculations

Torque Required By a Servo Motor = (Torque Due to Force of Gravity on Links and Payload) + (Torque Due to Angular Acceleration of Links and Payload)

This torque due to angular acceleration is calculated using the following equation:

τ = Iα

where I is the rotational inertia (or moment of inertia), and α is the angular acceleration around an axis. 


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