09-05-2014, 04:55 PM
Electric Motors
Introduction
Electromechanical device that converts electrical energy to mechanical energy
Mechanical energy used to e.g.
Rotate pump impeller, fan, blower
Drive compressors
Lift materials
Motors in industry: 70% of electrical load
DC Motors – Components
Field pole
North pole and south pole
Receive electricity to form
magnetic field
Armature
Cylinder between the poles
Electromagnet when current goes through
Linked to drive shaft to drive the load
Commutator
Overturns current direction in armature
AC Motors – Synchronous motor
Constant speed fixed by system frequency
DC for excitation and low starting torque: suited for low load applications
Can improve power factor: suited for high electricity use systems
Synchronous speed (Ns):
Motor Load
Three methods for individual motors
Input power measurement
Ratio input power and rate power at 100% loading
Line current measurement
Compare measured amperage with rated amperage
Slip method
Compare slip at operation with slip at full load
Reduce Under-loading
Replace with smaller motor
If motor operates at <50%
Not if motor operates at 60-70%
Operate in star mode
If motors consistently operate at <40%
Inexpensive and effective
Motor electrically downsized by wire reconfiguration
Motor speed and voltage reduction but unchanged performance