Torques in Electrical Motors

There are some common torques used to describe and classify electrical motors

Torque is the turning force through a radius and the units is rated in - N.m - in the SI-system and in - lb.ft - in the imperial system.

The torque developed by a synchronous induction motors varies with the speed of the motor when its accelerate from full stop or zero speed, to maximum operating speed.

Locked Rotor or Starting Torque

The Locked Rotor Torque or Starting Torque is the torque the electrical motor develop when its starts at rest or zero speed.

A high Starting Torque is more important for application or machines hard to start - as positive displacement pumps, cranes etc. A lower Starting Torque can be accepted in applications as centrifugal fans or pumps where the start load is low or close to zero.

Pull-up Torque

The Pull-up Torque is the minimum torque developed by the electrical motor when it runs from zero to full-load speed (before it reaches the break-down torque point)

When the motor starts and begins to accelerate the torque in general decrease until it reach a low point at a certain speed - the pull-up torque - before the torque increases until it reach the highest torque at a higher speed - the break-down torque - point.

The pull-up torque may be critical for applications that needs power to go through some temporary barriers achieving the working conditions.

Break-down Torque

The Break-down Torque is the highest torque available before the torque decreases when the machine continues to accelerate to the working conditions.

Full-load Torque or Braking Torque

The Full-load Torque is the torque required to produce the rated power of the electrical motor at full-load speed.

In imperial units the Full-load Torque can be expressed as:

T_fl = P_hp 5,252 / ω_max (1)

where

T_fl = full-load torque (lb.ft)

P_hp = horsepower

ω_max = maximum shaft rotational speed (rev/min, rpm)

Example - The Braking Torque

The torque of a 60 hp motor rotating at 1725 rpm can be expressed as:

T_fl = 60 (hp) 5,252 / 1725 (rpm)

    = 182.7 lb.ft

NEMA Design

NEMA (National Electrical Manufacturers Association) have classified electrical motors in four different NEMA designs where torques and starting-load inertia are important criterions.

Reduced Voltage Soft Starters

Reduced Voltage Soft Starters are used to limit the starting current and reducing the Locked Rotor Torque or Starting Torque and are common in applications which is hard to start or must be handled with care - as positive displacement pumps, cranes, elevators and similar.

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