7/30/2019 Torque of ThreePhase Induction Motor Explained

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Torque Of Three-Phase Induction Motor Explained (photo by empoweringpumps.com)

http://electrical- engineering- portal.com/torque- of- three- phase- induction- motor- explained April 12, 2013

Torque Of Three-Phase Induction Motor Explained

Edvard

Introduction to torque

The rotating force that a motor develops is called torque.

Due to t hephysical laws of inertia, where a body at rest tends to remain at rest, the amountof torque necessary to start a load(starting torque) is always much greater than the amountof torque required to maintain rotation o f the load af ter it has achieved normal speed.

The more quickly a load must accelerate from rest to normal rotational speed, the greatermust be the torque capability of the motor driver.

For very large inertia loads or loads that must be accelerated quickly, a motor having a highstarting torque should be applied.

The National Electrical Manufacturers Association (NEMA) provides design letters toindicate the torque, slip, and starting characteristics of three-phase induction motors.

They are as follows:

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7/30/2019 Torque of ThreePhase Induction Motor Explained

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Design A

Design A is a general-purpose design used forindustrial mot ors. This design exhibits normaltorques and full-load slip of approximately 3 percent and can be used for many types ofindustrial loads.

Design B

Design B is another general-purpose design used f or industrial motors. This design exhibitsnormal torques while also having low starting currentand a full-load slip of approximately 3

percent. This design also can be used for many types of industrial loads.

Design C

Design C motors are characterized by high starting torque, low starting current, and low slip.Because of its high starting torque, this design is useful for loads that are hard to start, such asreciprocat ing air compressors without unloader kits.

Design D

Design D motors exhibit very high starting torque, very high slip of 5 to 13 percent, and lowstarting current. These motors are excellent in applications such as oil field pumping jacks andpunch presses with large f lywheels.

Variable-torque and Constant-torque motors

Variable-torque motors exhibit a speed-torque character-ist ic that varies as the square of thespeed.

For example, a two-speed 1800/900-rpm motorthat develops 10 hp at 1800 rpm producesonly 2.5 hp at 900 rpm. Variable-torque motors are of ten a good match for loads that have atorque requirement that varies as the square of the speed, such as blowers, fans, and

centrifugal pumps.

Constant-torque motors can develop t he same torque at each speed; thus power output f romthese motors varies directly with speed. For example, a two-speed motor rated at 10 hp at1800 rpm would produce 5 hp at 900 rpm.

These motors are useful in applicat ions with constant-torque requirements, such as mixers,conveyors, andpositive-displacement compressors.

Induction Motor How it works? (VIDEO)

Cant see this video? Clickhere to watch it on Youtube.

Resource: Electrical Calculation Handbook John M. Paschal

http://www.youtube.com/watch?v=HWrNzUCjbkkhttp://electrical-engineering-portal.com/industrial-applications-brushless-servomotor