# 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.

1

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

2

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?

3

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

4

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.

5

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.

6

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

7

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

8

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.

9

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.

10

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.

1

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

2

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?

3

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

4

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.

5

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.

6

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

7

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

8

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.

9

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.

10