# Electric Motor Torque Types

To turn over the load and keep it moving, an electric motor needs to generate a certain amount of torque. There are four distinct types of torque generated by a motor when it is coupled to a load.

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Torque can be broken down into four categories: locked rotor torque (LRT), pull-up torque (PUT), breakdown torque (BDT), and full-load torque (FLT) (FLT). View Figure 1.

## Locked Rotor Torque

Locked rotor torque (LRT) is the torque a motor produces when its rotor is stationary and full power is applied to the motor. See Figure 2.

The locked rotor torque may get the load spinning, but if the motor doesn’t have enough PUT, it won’t be able to reach its rated speed. When the motor reaches its rated speed, the full-load torque keeps the load spinning. The term “accelerating torque,” which is synonymous with “PUT,” is also used. figure 2. locked rotor torque lrt is the torque a motor produces when its rotor is stationary and full power is applied to the motor.

## Breakdown Torque

Breakdown torque (BDT) is the maximum torque a motor can provide without an abrupt reduction in motor speed. See Figure 4.

Torque is increased by operating a motor under a greater load on its shaft. The motor stalls as the load increases beyond a certain point. The torque at which something breaks occurs here.

Figure 4. Breakdown torque (BDT) is the maximum torque a motor can provide without an abrupt reduction in motor speed.

Full-load torque (FLT) is the torque required to produce the rated power at the full speed of the motor. See Figure 5.

With the help of a horsepower-to-torque conversion chart, you can determine the amount of torque a motor generates at full power and speed (full-load torque).

Figure 5. Full-load torque (FLT) is the torque required to produce the rated power at full speed of the motor.

To calculate motor FLT, the following formula is applied:

Where

T = torque (in lb-ft)

HP = horsepower 5252 = constant

RPM = revolutions per minute

What is the FLT of a 30 HP motor operating at 1725 rpm?

Solution