Post on 09-Nov-2015
description
BASICS OF AC DRIVEBASICS OF AC DRIVE
Why Use Adjustable Speed?Why Use Adjustable Speed?
Machine or Process Requirements
Energy Savings
Automated Factory Concept
Productivity Increases
EfficiencyEfficiency
Power In - Power Out = Power Lost
The less power lost,the higher the efficiency
Power Out
Power In= % Efficiency
TorqueTorque
The turning force
applied to a shaft
tending to cause
rotation.
TorqueTorque
Torque is the product ofForce x Lever Arm Length (Radius)
Clockwise and Counter-Clockwiseefforts are distinguished by
differences in sign, + or -
Why Does
A Motor
Stop?
Stopping Time
STOPPING CHOICES
RegenerationRegeneration
The Motor begins to act like a Generator when:
The CEMF is greater thanthe Applied Voltage (DC)
or
The Rotor is rotating faster thanthe Stator Field (AC)
Power Factor
For Simple Applications:Cosine of the Displacement Angle Between Voltage and Current
However: When one of the waveforms is non-sinusoidal(such as PWM Current)
True Power Factor = Real Power
Apparent Power
Which includes both the Fundamental and Harmonic Waveforms
Synchronous Speed
RPM = 120 x Applied Frequency
Number of Poles per Phase
Base Speed
The Speed Produced by Applying
Motor Nameplate Rated Voltage
at
Rated Frequency
Motor Slip
O rpm 750 rpm 1460BaseSpeed
1500SyncSpeed
40 rpmSlip
Motor Slip
N
Slip Speed = Synchronous Speed Rotor Speed
AC Motor Review
In an AC motor, speed varies by:
RPM = 120FP
Where:F is frequency in hertzP is the # of motor polesSlip is in RPM
- Slip
AC Motor Review
In an AC motor, Torque Varies by:
T = K x ( )2 x I LineEF
Where:K is a constantE is applied voltageF is input frequencyI Line is motor current
AC Motor Review
Horsepower of an AC motor can be determined by:
HP = Torque x Speed
5252Where:
Torque is in lb-ftSpeed is in RPM5252 is a constant
Typical Current & Torque Relationship
for Squirrel Cage Induction Motor
0
% Currentand
% Torque
% Speed
Current
200
400
600
25 50 75 100
Torque
Speed / Torque Curve
0
% Torque
% Speed
Maximum orBreak-Down Torque
100
200
25 50 75 100Synchronous
Torque
150
225Pull-Up Torque
Breakaway orStarting Torque
150% Torque
Rated Torque
PWM Inverter
Constant Input Power Factor Wide Speed Range High Efficiency Ride Through Open Circuit Protection Common Bus Regeneration
Requires Extra Hardware for Regeneration to Line
Complexity Audible Motor Noise
Benefits Limitations
AC InputFixedFrequency
AC Motor
Fixed DC BusInverterDiode AC
OutputVariableFrequencyRectifier
LC
CUSTOM BUILT V/F RATIOS