Section 1: Introduction; Plant Design - Motors - Siemens 7SR17 Rho Manual

Chapter 7) 7SR17 Rho Applications Guide

Section 1: Introduction

1.1 P D - M
LANT ESIGN OTORS
Three phase AC motors use the synchronous or induction principle and have wide ranging power outputs from a
few kW to several MW.
Three phase induction motors are employed for all general purposes, typically in fixed speed applications to drive
machinery, pumps, fans, compressors, conveyors, hoists etc. Induction motors are also used with variable
frequency inverters as controlled speed machines.
In a squirrel cage induction motor the 3-phase supply voltage produces a current in the stator winding which sets
up a rotating magnetic field. This field flux cuts the short-circuited rotor conductors and induces a current in them.
The interaction of the current and flux produces a torque which causes rotation.
LV motors are typically switched by contactors and HV motors by circuit breakers. Circuit breakers will make and
break fault current whereas contactors will make but not break fault current. This means that tripping by a
contactor must only be undertaken when the current is less than the contactor capability e.g. when the condition
detected is overload, unbalance or stalling.
Induction motors behave as transformers with shorted secondary winding until the rotor begins to move.
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