Boston VFD Course - Massachusetts Energy Efficiency Partnership

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Boston VFD Course - Massachusetts Energy Efficiency Partnership

Transcript Of Boston VFD Course - Massachusetts Energy Efficiency Partnership

Understanding & Managing Variable Frequency Drives
Presented by: Greg Stark, P.E. September 10, 2014 Sponsored by:
Variable Frequency Drives (VFD’s)
• Popular speed control devices used in industrial, commercial and residential applications. – Huge energy savings potential operating centrifugal fans, pumps and compressors
• Vary frequency of electrical supply to an induction motor to vary the motor speed. – Vary the speed/flow of the operation/application.
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VFD Applications
• Industrial
– Fans, Pumps, Compressors – Conveying Systems
• Commercial
– HVAC Compressors – Pumps and Air Handlers
• Residential
– Variable Speed HVAC equipment – Energy Efficient Washing Machines
How Have We Varied Speed Historically?
• Change Speed
– Belts & pulleys – Chains & sprockets – Gear drives – Multi-speed motors
• Vary Speed
– Variable pitch belts & pulleys – Eddy current clutch – Hydrostatic drives – Wound rotor motor – DC Drives – AC Variable Frequency Drives
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How Have We Varied Flow Historically?

• Size motor/system for maximum flow
• Use throttling devices on fans, pumps & compressors to reduce flow rates
– Valves – Vanes & Dampers

CFM

Typical Airflow Requirements
6000

5000

4000

3000

2000

Min

Avg 1000 Max

0
1 2 3 4 5 6 7 8 9 10 11 12
Month

Considerations?
• Initial Cost
– VFD vs other device
• Maintenance Cost
– VFD vs other device
• Maintenance Issues (Downtime, etc)
• Effectiveness
– How well does it do what I really want/need it to do?
• Others?
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Induction Motor = Constant Speed???
• Synchronous Speed
– Speed the motor’s magnetic field rotates. – Theoretical speed with no torque or friction. – A well built motor may approach synchronous
speed when it has no load. – Factors
• Electrical Frequency (cycles/second) • # of poles in motor Speed = (120 * Frequency)/(# of poles)
• Rated Speed
– Speed the motor runs at when fully loaded and supplied rated nameplate voltage.
VFD Principles of Operation
• Motor speed can be varied by changing the frequency, # of poles, or both.
• Example:
– 4 pole motor @ 60 hertz = 1800 rpm – 4 pole motor @ 50 hertz = 1500 rpm – 4 pole motor @ 40 hertz = 1200 rpm
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Drive Function
• Input:
– 60 hertz AC & rated voltage
• Converter
– Rectifies to DC & changes frequency to desired value
• Inverter
– Converts DC back to AC
• Regulator
– Adjusts voltage level to desired value as a % of speed/frequency value.
– Volts/Hertz Ratio • (480/60 = 8)
• AC Output:
– Desired frequency and voltage for speed requirement.
Torque vs Speed
• What happens to torque when speed is decreased? – Torque increases
• If torque increases, current increases and produces additional heat in the windings.
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However!!!!
• For many types of loads, as speed drops, torque requirements also drop.
• What happens when speed AND torque decrease? – Reduced Horsepower Reduces our energy costs!
Increase Speed?
• What about increasing speed above the motor’s synchronous speed using frequency higher than 60 hertz?
– 4 pole motor @ 60 hertz = 1800 rpm – 4 pole motor @ 70 hertz = 2100 rpm
• Most motors were not balanced to operate above synchronous speed.
• The load may not have been balanced above this speed either.
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Common Applications
• Constant Torque Loads
– conveyor belts, augers, reciprocating pumps & compressors, extruders, gear pumps.
• Variable Torque Loads
– centrifugal fans, pumps, and compressors
• Constant Horsepower Loads
– winding machines
Torque-Speed Curve
• Amount of Torque produced by motors varies with Speed.
• Torque Speed Curves
– Starting Torque – Pull Up Torque – Breakdown Torque
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Torque-Speed Issues
• Some single phase motors have starting and running windings
• The starting windings can not be energized continuously
• If a VFD is used and speed is reduced too much the starting windings burn out.
Constant Torque Loads
• Require the same amount of torque at low speeds as high speeds.
– For a given weight on the belt, the torque to turn the belt is always the same regardless of speed.
• Horsepower increases or decreases as a direct function of speed.
• Examples:
– Conveyor belts, reciprocating pumps & compressors
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Constant Torque Loads
• Horsepower increases or decreases as a direct function of speed.
– A 50% drop in speed produces a 50% reduction in power required to turn the load.
• Energy savings using a VFD to control the speed of a constant torque load is a direct function of speed reduction.
Variable Torque Loads
• Require much lower torque & horsepower at low speeds than at high speeds.
• Power required varies as the cube of the speed.
• Examples:
– Centrifugal fans, pumps & compressors, mixers and agitators.
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Variable Torque Loads
• Horsepower increases or decreases as cubic function of speed.
– A 50% drop in speed produces almost an 88% reduction in power required to turn the load.
• Energy savings using a VFD to control the speed of a variable torque load can be very large due to how centrifugal loads operate.
Constant Horsepower Loads
• Constant horsepower loads include equipment such as grinders, winders, and lathes.
– Since the power required remains the same regardless of torque or speed requirements of the operation, there are no direct energy savings from installing VFD's with constant horsepower loads.
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SpeedTorqueFrequencyPumpsFunction