Issues in VFD Applications

DR.C.SHARMEELA

Assistant Professor in Electrical Engg., A.C.Tech,

Anna University, Chennai -25

E-Mail: sharmeela20@yahoo.com

Cell: 9841363144

Contents

• Introduction

• Applications

• Issues

• Technological solutions

• Case studies

2

The Motor market

• Nearly 5 Lakhs LT AC motors get added to the Indian industry use, each year

• The overall market size for rotating machines is `6300 crores in India- (IEEMA –year 2011)

• India’s import of motors is `690 crores and export is `115

crores worth- (IEEMA year-2011)

• The world motor market is set to grow by 30% in the next 10 years with the advent of new generation IE3 (premium efficiency) motors from Jan,2014

4

Motive energy

Motors account for nearly 73% of industrial energy 2/3rd of the industrial motor applications are centrifugal (utility) machines 1/3rd of the industrial motor applications are for process and material

handling

Major motor applications

5

VFD Market forecast

• Growing demand for electrical efficiency and increasing investment in modernization of infrastructure will drive the global VFD market to USD18,854 Million by 2017, with a CAGR of 8.7% from 2012 to 2017

• Variable Frequency/Speed Drives (VFD/VSD) Market - By Type (AC, DC, Servo), Voltage Range (Low, Medium), Power Range (Micro, Low, Medium, High) and

Application (Pump, Fan, Compressor, Conveyors & Others) - Global Market Trends & Forecast to 2017

6

Existing VFD Technology VFD uses power electronics to vary frequency of input power to the motor thereby

controlling motor speed.

Law of Affinity & Flow control table Major energy saving applications

Centrifugal machines (variable torque loads-2/3rd, i.e.,75% of e.motors) follow the law of affinity

Speed Volume Power

100% 100% 100%

90% 90% 77%

80% 80% 51%

70% 70% 34%

60% 60% 22%

50% 50% 13%

40% 40% 6%

30% 30% 3%

Law of Affinity :

Flow α Speed

Torque α Speed2

Power ( Energy consumption ) α Speed3

Note: Energy saving through speed variation is phenomenal.

Centrifugal loads-Fans, pumps, compressors

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Benefits of VFD

Application examples

11

Scalar or Vector

Scalar Drive

• Constant V/f pattern

• No true torque control

• Not suitable for <1:5 speed

• if your application needs accurate control below 10Hz, scalar may not work for you.

Vector Drive

•Vector drives come in 2 types, Open Loop and Closed Loop

•precise control of speed or torque from exact vector of V & f

•Closed Loop Vector Drive uses a shaft encoder & can develop full torque at zero speed

•Open loop or sensor less vector drive takes the feedback within the VFD. Not good for cranes and hoists

12

Control Technology

• Control Algorithms – Voltage and current control

– PWM

– ----------------------------------------

– Open loop

– Closed loop

- Scalar V/F

- Vector (FOC)

– Direct

– Indirect

– Sensorless

• DTC

– AI techniques

• Control Implementation – DSP, c, p

– PLC, DCS, other

n

Adjustable speed drive

VVVF

Motor

DSP Board

Electronic

gate drive

circuit

Control

Estimation

Algoritm

load

Inverter Rectifier

Torque & Speed

reference

Voltage

and

current

sensing

Comparison of Control Algorithms

Open

loop

Closed loop

Scalar SensVector Sensorless

Low speed perf. Poor good Good (Ind)

Poor (Dir)

Medium

Dynamic perf. V. poor poor excellent V. good

Param. sensitive No No Yes (Ind)

No (Dir)

Yes

Cost V. low Low high low

Sp or pos sensor none simple complex none

Appl to HPDr No No Yes (Ind-Dir) Yes

Carrier frequency

• The output waveform of the VFD is not a pure AC sine wave. It is a simulated waveform that is actually made up of pulses of DC voltage from the VFD's DC bus. These pulses cause vibrations in the motor that are heard as a high-pitched squeal or whine.

• The pulses are generated by the switching of the VFD's output transistors. The switching rate of the output transistors is controlled by the VFD's Carrier Frequency parameter. Increasing the Carrier Frequency will reduce the audible noise from the motor. However, doing so may require derating of the VFD, as increasing the switching rate of the transistors causes the VFD to be less efficient

• Standard range for carrier frequency for many commercial VFDs is 1.25 KHz to 5 KHz

15

VFD Disadvantages

• Less efficient at 100% rated motor speed

• Possible winding insulation breakdown

• Inverter-rated motors recommended

• Harmonics

• Expensive preventive measures from damage

• Shaft current damaging the bearing

• Possible voltage reflected wave from long lead lengths

• Higher first cost

16

Insulation stress

Dual coat wires and Vacuum pressure impregnation for insulation varnishing are recommended along with special care in selection of associated elements

17

Harmonics - Origins

-2.5

-2

-1.5

-1

-0.5

0

0.5

1 1.5

2

2.5

• TOPOLOGY

C

e1

e2

e3

M i I Is

VARIABLE SPEED DRIVES

– Draw high harmonic current

of order 5, 7, 11, 13

– The current is unstable

• CURRENT WAVEFORM • HARMONIC SPECTRUM

H5: - 81%; H7: - 74%; H11: - 42%; ...

0

50

100

H1

H5

H7

H11

H13

H17

H19

H21

H23

Non Linear Load – Three Phase

S = 23KVA; Fc=2.8; THDI=124%

•All the energy efficient devices and drives are nonlinear in nature

•They will draw reactive- and harmonic-components of current

from the source

Power Quality Issues zero sequence

• All harmonics tend to distort the original fundamental 50 Hz sine wave creating operational problems with electrical equipment such as, PLC's, automation equipment, machine tools, and computers. Odd Triplen harmonics, (3rd, 9th, 15th, etc.), do not cancel each other, but add together in neutral conductors of 3-phase, 4-wire systems to cause overheating in panels, neutral conductors, terminations and transformers.

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Negative sequence

• 5th harmonics of sufficient magnitude result in motor inefficiencies and overheating. The negative sequence may produce sufficient counter-torque to cause excessive motor vibration. Generalized harmonic effects include: unexplained operation of protective devices, audible noise interference on telephone circuits, blown fuses on power factor correction capacitors and, erratic operation of generators with solid-state controls.

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Positive Sequence

• 7th harmonics of sufficient magnitude result in losses.

• VFD are deployed in every industry.

• VFD’s for the three – phase load is predominant.

• 5th order and 7th order harmonics are predominant because most of the VFD’s are six-pulse VFD.

• 12, 18, 24 and 30 pulses are available for VFD applications.

How can we reduce the harmonic current?

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• DC link choke within the drive

• Line reactor

• Passive filter

• Active filter

• Multi-pulse converters

• Active front-end

Use of DC choke

• Power factor better than 0.9

• A large DC bus choke is effective at reducing 5th and 7th harmonics, which are the largest.

• A DC choke is internal to the inverter and generally cannot be retrofitted

• The option of using AC line chokes to reduce harmonics even further remains available.

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Passive filter

• A combination of a reactor and capacitor elements

• Tuned

• Connected in a parallel shunt arrangement

• Designed for a specific harmonic frequency (5th)

• Protects multiple drives, including PF correction

25

Active Filter

• Connected in shunt/series

• Good for individual drives <37 kW

• Provides PF correction

• Injects equal and opposite harmonics

• Expensive and Easily adapts to varying loads

26

Multi pulse converters

Multi-pulse drives 12 or 18-pulse converter Fed from equal impedance phase-shifted power sources Harmonics (5th, 7th ) from the first cancels the second A 50% harmonic reduction (up to 85%) Good solution for drives >55 kW

27

Phase shifting Transformer

Harmonic mitigating/Phase shifting/Quasi 12-pulse

transformers Provides substantial reduction (50-80%) in voltage and

current harmonics

28

Active front end Technology

• Unit power factor: AFE can provide a current with a very low harmonic distortion (THD is reduced to 3% when normally is 25-30% in case of a conventional rectifier)

• AC current sinusoidal wave form: very clean energy

• Bidirectional power exchange between AC mains and DC Bus

• bar: AFE is perfectly reversible and so it can recover the energy to the mains saving it

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30

3%

17%

80%

6-Pulse

18-Pulse

AFE

Drives with AFE Technology

The Active Front End configuration continues to dominate sales by a wide margin

• No isolation transformer

• Best harmonic performance

• Best input power factor

Other advancements in technology

• Software for capturing of electronic nameplate of the motor Maps the impedance, inductance & resistance of the motor for

precise reaction with the torque & speed for close loop vector

applications.

• Dynamic braking in the VFD When the rotor is turning faster than the synchronous speed set by

the drive’s output power, the motor is transforming mechanical

energy available at the drive shaft of the motor into electrical

energy that can be transferred back to the drive. This process is

referred to as regeneration.

A more cost effective solution can be provided by allowing the

drive to feed the regenerated electrical power to a resistor which

transforms it into thermal energy. This process is referred to as

dynamic braking.

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Case Study-1: Carried at a tyre manufacturing firm: Bearing defect- Squeegee Calendar: Eq. for converting rubber mix to Calendaring form-Quality of grease

Shaft current did not cause the failure The observation was proved right after

nearly 1 year, when the Gearbox manufacturer inspected and confirmed

damages to 2 sets of gearbox bearings that led to failure of DE bearing

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Case Study-2: Effect of Noise created by VFD VFD noise- Extruder

• 450 kW, 6 pole driven by VFD first at 1.25 KHz- Client complained of high noise- measured at 82 dB

• The drive was run at 5 KHz and the noise reduced to 71 dB

• As per IEC: 60034-9 noise level increases up to 6 dB under PWM supply

• As per IS: 12065, noise limit for 450 kW, is 102dB

• 82dB itself was below this limit. But it was shown to the client that at 5 KHz the noise level is equal to that of a 5.5 kW motor

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Sum up

• VFD selection must complement the motor and the requirement of the driven load

• Causes and effects of PQ issues should be resolved for clean environment while accounting for energy saving

Thank you for your attention

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