Adjustable Speed Drives

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Transcript Adjustable Speed Drives

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February 24, 2015
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Advantages of Adjustable Speed Drives
in Process Control
• Motor Driven Pump Systems account for about
20% of Worldwide Energy Use
• Some Plants have usage approaching 50%
• Adjustable Speed Drives can Improve Process
Control and Greatly Reduce Energy Costs
Methods of Variable Flow Control
Fans
Pumps
Damper Control
Throttling Valve
Control
Inlet Vane Control
Variable Frequency
Drive
Variable Frequency
Drive
High Percentage of Plant Connected
Horsepower is for Fans and Pumps
• Opportunity to save energy when you can reduce
CFM or process flow rate using a variable speed
drive rather than using dampers or a control
valve
AC MOTOR
BASICS
Purpose of an “AC Motor”
Electrical
Energy
Mechanical
Energy
The purpose of an AC Motor is to
Convert Electrical Energy into Mechanical Energy
AC MOTOR CONSTRUCTION
Motor Frame
Assembly
Rotor
Assembly
Stator Winding
Assembly
AC Motor Basics
2 Pole Motor
Motor RPM is equal to:
120 * Frequency
# Motor Poles
Note that Frequency
is the only variable to
affect motor speed
Rotating Magnetic Field of a 2 Pole AC Induction Motor
Types of Variable Speed Control
Before AC Variable Speed Drives
• Control Valves, Dampers and Vanes
• Fossil Fuel Engines
• Eddy Current Clutches
• Hydraulic Couplings
• Variable Pitch Sheaves
• Motor Generator Sets
AC Drive
Rectifier
DC Bus
Inverter
AC Power Supply
M
V
V
V
V
T
T
•Rectifier
- Converts AC line voltage to Pulsating DC voltage
T
• Inverter
- Changes fixed DC to adjustable AC
- Alters the Frequency of PWM waveform
• Intermediate Circuit (DC BUS)
- Filters the pulsating DC to fixed DC voltage
Adjustable Speed Drive Availability
• 110V Single Phase Fractional HP
• Medium Voltage (4160V) Thousands of HP
• Most are 460Volt 1-500 HP
Does my load profile vary
sufficiently to justify a AFD?
• If the Speed of the Application is constant DO
NOT USE
• If the load is not running at full speed for a large
portion of the time then Evaluate the Use
• Sweet Spot is a Variable Torque Load
Variable Torque Loads(Fans & Pumps)
Variable Torque Loads
• In these applications:
• Torque varies directly with speed squared
• Power varies directly with speed cubed
• This means that at half speed, the horsepower
required is approximately one eighth of rated
maximum
Variable Speed
 Horsepower Requirements



Flow proportional to speed Q1/Q2= n1/n2
Then: HP2/HP1= (n2/n1)3
Therefore: HP2=HP1(n2/n1)3

At 80% Speed, HP=(.80)3 = .512
Advantages of a Adjustable Speed
Drive
• 1. Energy savings on fan and pump applications
• 2. Better process control and regulation
• 3. Speeding up or slowing down a machine or
process
• 4. Inherent power-factor correction
• 5. Elimination of Other Mechanical Control
Devices
•
Some Considerations When
Applying ASD to AC Motors
Thermal considerations of motor operation with a VFD
should be one of the first areas of attention for successful
application.
• The ability of a motor to cool itself effectively is reduced
as the motor is slowed down. Over-sizing the motor or
providing external forced air ventilation may be required
with extended operation at low speeds and high loads.
• Operation at different speeds can cause mechanical
resonances in driven equipment. These speeds should be
identified and programmed out of the motor’s operating
range.
ASD Considerations
• ASD’s generate harmonic voltages and currents
which can, in some cases, cause undesirable
effects on the electrical distribution system and
affect equipment operation.
• Longer Cable Lengths can cause a voltage
reflection of up to two times the applied voltage
due to standing waves or a ‘‘ringing effect’’, and
becomes more problematic with longer cable
runs (typically greater than 15 m or 50 feet).