Criteria for Motor Classification

Torques in Electrical Induction Motors

thanks lyn!! The article gave quite a good explanation of various torques but my query was how r motors defined as 'high torque' or 'low torque' motors? what values define 'high torque' n low torque'?

Torque multiplied by angular velocity equals shaft power.

There is no such standard. Various mfrs might have their own divisions based on the total range of their products.

So, if i've a particular motor and i know the speed and torque of the motor, how do i know whether it falls in the 'high torque' or 'low torque' category? There would be some criteria specifying the ranges for these categories. I've looked around but was unable to find one. So thought of asking you experts. Would appreciate if someone can just guide me in the right direction.

Thanks!

The best way would be to telephone the motor manufacturer, and ask.

That's because there is no such standard.

But NEMA classifies its motors as follows:

NEMA design A

• maximum 5% slip
• high to medium starting current
• normal locked rotor torque
• normal breakdown torque
• suited for a broad variety of applications - as fans and pumps

NEMA design B

• maximum 5% slip
• low starting current
• high locked rotor torque
• normal breakdown torque
• suited for a broad variety of applications, normal starting torque - common in HVAC application with fans, blowers and pumps

NEMA design C

• maximum 5% slip
• low starting current
• high locked rotor torque
• normal breakdown torque
• suited for equipment with high inertia starts - as positive displacement pumps

NEMA design D

• maximum 5-13% slip
• low starting current
• very high locked rotor torque
• suited for equipment with very high inertia starts - as cranes, hoists etc.

This means that must be something that defines the highlighted 'normal / high' torque.

Then NEMA is the correct place to ask the question.

Look on the nameplate. The NEMA class has more information that high/low torque - that's starting torque, by the way. As stated, your original post led us all to think you were referring to running torque.

In addition to this NEMA has code letters(A to .?) for different values of motor's starting kVA/hp. In 1960s BS had normal and double cage rotors only in addition to wound rotor.

NEMA motor code letters for motors vary from A(starting kVA/rated hp)=3.14 to V=24.4,giving a wide choice of motors for various load characteristics.

NEMA motor code letter varies from A=3.14(starting kVA/rated hp) to V=24.4,giving a wide choice of selection of motors for different load characteristics.

Read NEMA MG-1 standard for details.

I am not an electrical engineer, but I have seen NEMA motor curves for A, B, C, and D motors and there are differences with the torque/speed curves. As well as differences between different types of motors.

...and torque is what is presented to the motor by the load, not the other way round.

The classification of High torque / low Torque in electrical induction motors is only related to the initial starting torque.

if you want to search, then search for "high initial or starting torque motors".

Asynchronous Motors do not not have high starting torque, but can be specially made (Wound Rotor ... or double caged ...) to improve the starting torque for special applications where it is required.

Don't obsess over something that does not matter.

For the SAME HP rating, the lower the RPM, the higher the torque,and conversely,the lower the torque, the higher the RPM, since HP is a product of both torque and RPM.

For a given frequency, the number of poles determines the RPM of the motor.

A variable frequency drive can be programmed, with the proper motor, to give full torque across a broad RPM range,and motors can be designed for a very flat torque curve from nearly zero RPM to Max.

IEC 60034-12 describes Design N, Design NY, Design H & Design HY based on Torques & Starting Methods. Pl. refer to the same.