# What is Starting Torque?

I'm having a hard time finding an answer from the internet. What does starting torque mean?

There are two different aspects to start-up torque. One is associated with the motor and the other with the system which you are trying to drive.

The start-up torque of a mechanical system is the minimum torque required to get it moving from a standstill. the start-up torque of a motor is the maximum torque which it is capable of producing from a standstill. If the start-up torque of your motor is not greater than the start-up torque of your mechanical system, then the motor will not be able to get the system moving.

The image below shows the torque curve for a typical AC motor. The start-up torque, although not marked, is the torque value at the far left of the plot (at zero speed). If your mechanical system has a higher start-up torque than this, then this motor will not be able to get it running even if the torque required to keep it running is lower than the full load torque. Is the torque required to make something that is not turning start turning.

You can think of starting torque as an instantaneous value (the torque just before the motor starts moving). You can also thing as the torque function from that moment until the motor reaches the desired speed.

The torque provided by a motor is

$T_{motor}(0) = T_{static friction}\\T_{motor}(t>0)=T_{dynamic\ friction}+J\dot{\omega} +K\omega^2$

where:

$\omega$ is the angular speed

$\dot{\omega}$ is the angular acceleration

$J$ is the polar moment of inertia of the motor rotor plus load (as seen from the motor)

$K$ is constant (in a simple model) that represents the viscous forces

• Well that sounds logical. But what does it mean practically: is it related to the minimum angular speed of a motor? – user1791 May 28 '15 at 21:39
• No, the starting torque is required when there is zero speed. In most systems as the system speeds up, less and less torque will be required. – Ethan48 May 28 '15 at 22:00
• @mandrill may I get the reference, please? – Fennekin Apr 12 '16 at 15:27

"Starting torque" and "locked-rotor torque" are the same thing. So if you run across the later term, you can treat it as a synonym for "starting torque." The NEMA MG-1 definition of locked-rotor torque is:

"the minimum torque which [a motor] will develop at rest for all angular positions of the rotor, with rated voltage applied at rated frequency."

The starting torque is the torque required by the starting motor to rotate from idle position during the engine cranking.This torque must be high so that the starting motor should rotate.

• More detail would improve this answer, such as mentioning mass, torque & rotational inertia. – Fred Aug 24 '17 at 3:02

At slip=1 and speed of rotor N=0 rpm ,motor produces a torque called starting torque

• This answer is very brief. More detail would improve it – Fred Apr 29 '17 at 8:52