Variable Frequency Drives

Variable Frequency DrivesSaqib SaeedGraduate Trainee Engineer(E&I) - Electrical

Contents

• Introduction• Block Diagram• Building blocks• Modes of operation• VFD Parameters• Some Potential Problems• Harmonics and THD• Recent Improvements in the FFCL system

Variable Frequency Drives

– Standard motors are constant speed and when they are energized they run at a 100% speed no matter the load.

– What if the speed of the driven machine (Fan, Pump) is to be changed?

What is a VFD?

– Variable Frequency Drive (VFD)

– Governing Equation of motor speedSpeed= 120 x f /P

• P=No. of poles

• F=Line Frequency

– How to change line frequency?

Constant =50Hz

Block Diagram

VFD Fundamentals

50 Hz Power

Electrical Energy

ABB

Variable Frequency

To Motor

VFD

RECTIFIER(AC - DC)

INVERTER(DC - AC)

AC DC AC

VFD

Variable Frequency50 Hz

VFD Explored

First, the Converter (usually a diode rectifier) converts three-phase AC power to DC power.

Next, the DC Bus stores and filters the DC power in a large bank of capacitors. Last, the Inverter (usually a set of six IGBTs) switches or inverts the DC power in a

Pulse Width Modulated (PWM) AC waveform to the motor.

Rectifier

• Basic Building blocks of rectification

– Diodes (Uncontrolled)

Thyristors

• Controlled

• Output voltage can be controlled

Gate Pulse

Conduction after Gate

Pulse

Three Phase Rectifiers

Output Voltage

• Output Voltage (dc) = x – Vm=Peak Value of voltage– A= Firing Angle

• Firing at zero gives maximum output dc voltage

IS it a perfect Direct Current?

• Conversion of AC into DC a perfect process?– Ripples

• How to eliminate the ripples?– Filters

DC bus in VFD

Inverter Action

• Switching DC voltage ON and OFF will make it AC

• Filtered output from DC bus is sent to inverter in VFD

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

How switching can convert DC into AC?

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

Pulse width modulation

• Such a waveform is not acceptable– Nowhere near Sine wave

• Contains harmonics– Multiples of fundamentals

• Solution– Pulse width modulation

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

How Often You Switch From PositivePulses To Negative Pulses DeterminesThe Frequency Of The Waveform

Frequency

Vo

lta

ge

Area Under the Curve

Basic Purpose achieved

• Speed of the motor now can be controlled

• Is changing motor frequency alone enough?

Φ=

V/F Control Mode

• Flux = V/F

0

230

400

Volts

Hertz

25 50

400 V

50 Hz= 8

V

Hz

230 V

50 Hz= 4.6

V

Hz

If 230 VAC Power Line:

230 V Motor

400

V Moto

r

V/F Control Mode

• Scalar mode• Drive is unaware of what is happening in the motor

Example:• A 400V scalar drive is told to run a 400V, 50 Hz motor at 50%

speed Following V/F pattern, Voltage applied by the drive will also be half.

• Perfect when at no load.• After loading, motor will run at less than 50% speed• Drive is unaware of it

SolutionVector Control

MotorDrive

Speed setting=50% Speed at no load=50%

Load increase

MotorDrive

Speed setting=50% Speed=40%

No idea what is happening

Vector Control Mode

• Sensor less vector control mode– No feedback through speed sensor– Feedback is derived through motor terminals– Drive need to go through “Auto tuning”

• Vector control with sensor– Feedback through encoder– Better speed regulations up to 0.01%– Faster response to load variations

VFD input Parameters

• Max./Base frequency setting

• Motor rated output

• Motor rated voltage

• Motor rated current

• Carrier frequency

VFD input Parameters

• Frequency Reference setting methods

• Stop Command method

• Start frequency

• Stop frequency (DC Braking starts)

• Torque Boost

• Frequency Skip

VFD input Parameters

• Max./Base frequency setting

VFD input Parameters

• Motor Rated Output

• Motor Rated Voltage

VFD input Parameters

• Motor rated Current

• Carrier Frequency

Frequency Reference Setting Methods

• Potentiometer

• 0-10V input voltage

• 4-20(mA)

Frequency Reference Setting through Potentiometer

Min

Max

Min Max

Frequency

Centrifuge Main Control Panel

Potentiometer

Start/Stop Frequency/Acceleration/Deceleration

Frequency

Start Frequency

Acceleration Time

DC Brake

Stop Frequency

Deceleration Time

Time

Start/Stop Frequency/Acceleration/Deceleration

Frequency

Start Frequency

Acceleration Time

DC Brake

Stop Frequency

Deceleration Time

Time

Stop Command methods

• Coast to stop

• Ramp to stop

Coast to stop

Coast to Stop

Frequency

Run Command

Motor speed

Command is removed

Ramp to stop

Frequency

Run Command Command is removed

DC Brake

Ramp to Stop

Torque Boost

Voltage

Frequency

Rated Frequency

Rated Voltage

Voltage Boost

Frequency Skip

Auto Tuning

• Drive familiarizing itself with motor

VFD here I am IM

Auto Tuning

MotorDrive

Start of Auto tuning

Primary resistanceLeakage reactanceDC Brake VoltageTorque Boost VoltageSlip compensation

Auto Tuning Procedure

Auto tuning procedure PreparationTurn Power ONStart VAT 300

Select the control mode

Motor ratings

Can motor rotate?

yes

NoInput 1: Simple adjustment

mode

Input 2: High adjustment

mode

Input 1: Simple adjustment

mode

Input 2: High adjustment

mode

LED flickers

Start Auto-TuningPress Fwd Revor

RUN LED ON

Auto-tuning End

10s for V/F mode

1min. for vector mode

Auto Tuning Procedure Contd.

DC Injection Braking

• No mechanical Contact• DC is applied at the stator winding• DC causes stator to be become a magnet with

constant field• A voltage is induced inside the rotor causing current

to flow• According to Lens’s law, this current will cause rotor

to stop

Dynamic Braking

• Concept of Braking– Kinetic energy keeping the object moving– Energy cannot be destroyed but can be converted

Kinetic Energy Heat Energy

Mechanical Brakes

Wear and Tear

Dynamic Braking

• Some other form of energy– Electrical

Kinetic Energy Electrical Energy

DiscardUtilize

ResistiveElevators

RegenerativeElectric railcars

Resistive Dynamic Braking in Elevators

Braking resistor for CAN Elevator

VFD

Braking Unit

Resistive Dynamic Braking

• A built in dynamic braking resistor (<15Kw)

• An external DB unit

Potential Problems

• Harmonic Distortion

• Bearing Damage

Total Harmonic Distortion

• Harmonic Content– Deviation of waveform from pure sinusoidal

shape• Present due to non-linear devices (switching)

– Power electronics• THD = x 100

THD Percentage

VFD BUS Non-VFD BUS

Harmonics Spectrum

VFD BUS Non-VFD BUS

Current Waveform Comparison

VFD BUS Non-VFD BUS

519-1992 - IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems

• IEEE Std. 519 (1981) – Revision (1992)

• Deals with harmonics introduced by the static power converters

• Overall THD < 5%

• Any single harmonic < 3%

IEEE standard for THD

AREVA report on THD in MCC (VFD) at NP plant

Voltage (V) R-N Y-N B-N

RMS Voltage 237 237 237

Peak Voltage 362 364 364

THD (%) 4.7 5 5

Harmonics Voltage 11 12 12

AREVA report on THD in MCC (VFD) at NP plant

Harmonic # R-N Y-N B-N

1

3

5 3.2 3.5 3.4

7 2.9 3.1 3.2

11 1.0 1.0 1.0

13 0.9 1.0 1.1

Line Reactors

• Installed ahead of the drive• Protect the drive from sudden disturbances• Reduces the harmonics content introduced by VFD

VFD

High peaks without reactor

Reduction Up to 35% after

reactor

Line reactors installed at NP SS

14 Line reactors are installed, 7 on each side of MCC-VFD

VFD and Line Reactor

Line reactor

VFD

Main Breaker

THD after installation of Line Reactors

Bearing Damage

• Pulse width modulated voltage induces bursts of shaft currents

• Grounded through bearings• Eventually bearing failure

Bearing Damage

Pitting of bearing due to Shaft Currents

Solution for shaft currents

• Shaft grounding through carbon brushes– Wear and corrode– Need maintenance

• Insulated bearings are used– Partial solution– May flow through driven equipment– Insulation may become a capacitor

• Shaft grounding rings– A combination of both

Shaft Grounding Ring

Questions