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Development

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 ControlFans 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”

ElectricalEnergy

MechanicalEnergy

The purpose of an AC Motor is to Convert Electrical Energy into Mechanical Energy

AC MOTOR CONSTRUCTION

Stator Winding Assembly

Stator Winding Assembly

RotorAssembly

RotorAssembly

Motor Frame Assembly

Motor Frame Assembly

AC Motor Basics

Rotating Magnetic Field of a 2 Pole AC Induction Motor

Motor RPM is equal to:

120 * Frequency

# Motor Poles

2 Pole Motor2 Pole Motor

Note that Frequency is the only variable to

affect motor speed

Note that Frequency is the only variable to

affect motor speed

Types of Variable Speed ControlBefore AC Variable Speed Drives• Control Valves, Dampers and Vanes

• Fossil Fuel Engines

• Eddy Current Clutches

• Hydraulic Couplings

• Variable Pitch Sheaves

• Motor Generator Sets

AC DriveDC Bus

• Rectifier

- Converts AC line voltage to Pulsating DC voltage

• 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

V

TT

V V V

T

AC Power Supply

Rectifier Inverter

M

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 WhenApplying ASD to AC Motors

Thermal considerations of motor operation with a VFDshould be one of the first areas of attention for successfulapplication.• 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).