Ac Motor Speed

AC Motor Speed

AC Induction Motor Speed Control ns =

120 f p

s = 100

ns − na ns

Change AC Frequency ► Variable

speed AC Motor adjustable speed drives are known as ƒ inverters, ƒ variable frequency drives (VFD) , or ƒ adjustable speed drives (ASD).

• So what can we do to control the speed of an AC induction motor? – Change the __________________ (in discrete increments - inefficient & rarely done) – Change the _____________ of the AC signal – Change the _____________

30.1

► Common

ways to vary AC frequency:

ƒ SixSix-step inverter ƒ PulsePulse-WidthWidth-Modulation ƒ Vector Flux

AC Motor Drive Electronics

Pulse-Width Modulation

Rectifier

http://www.sea.siemens.com/step/templates/lesson.mason?ac_drives:3:1:2

http://www.sea.siemens.com/step/templates/lesson.mason?ac_drives:3:1:2

Changing Rotor Slip ► Important

to match the motor to the load

Changing Rotor Slip

Variable Resistor

Main winding

AC

ƒ ensure that a change in motor power gives a desired change in load speed

Rotor

should have a substantial inertial components

Variable Transformer

► Load

High

ƒ inertial torque can "carry" the load through brief periods when motor torque cannot ► Best

used with motors designed for high slip

Main winding

AC Rotor

Medium

Low

Main winding

AC Rotor