Torque and Speed Characteristics

Owned by Kevin Li
2021-08-15
3 min read

The figure below shows an example of the torque and speed characteristics of a BLDC motor.

Torque vs. Speed BLDC Motor Characteristics

There are two parameters used to define a BLDC motor.

  • Rated Torque: During continuous operations, the motor can be loaded up to the rated torque. In a BLDC motor, the torque remains constant for a speed range up to the rated speed. The motor can run up to the maximum speed, which can be up to 150% of the rated speed. At this point, the torque will begin to drop.

  • Peak Torque: Applications which have frequent starts, stops, and reversal of rotation with load on the motor demand more torque than the rated torque. This requirement comes for a brief period, especially when the motor starts from a standstill and during acceleration. During this period, extra torque is needed to overcome the inertia of the load and rotor itself. The motor can deliver a higher torque, maximum up to peak torque, so long as it follows the speed torque curve.

Selecting the proper type of motor for the given application and load characteristics crucial. Three parameters govern motor selection. They are:

  • Required peak torque

  • Required RMS torque

  • The operating speed range

Peak Torque

The peak torque required for an application can be calculated by taking the summation of the load torque, torque due to inertia, and the torque required to overcome friction. Other factors also contribute to overall peak torque requirements and can be compensated with a 20% safety margin as a rule of thumb.

The torque due to inertia is the torque required to accelerate the load from standstill or from a lower speed to a higher speed. This can be calculated by taking the product of load inertia, which includes rotor inertia, and load acceleration.

Note that JL+M is the sum of the load and rotor inertia, and a is the required acceleration. The mechanical system coupled to the motor shaft determines the load torque and the frictional torque.

RMS Torque

The RMS torque can be roughly translated to the average continuous torque required for the application. This value depends on many factors including:

  • Peak torque

  • Load torque

  • Torque due to inertia

  • Frictional torque

  • Acceleration and deacceleration

  • Run times

The RMS torque is quantified by the following equation:

Â