VVVF for Dummies

2014

Mark J T Bowman

VETT

4/2/2014

VVVF for Dummies

VVVF for Dummies

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Mark J T Bowman M-Eng

In the last 20 years 3-Phase drive has become the standard for equipping electric multiple unit trains

and Diesel Electric & Electric locomotives.

There are three stages to the traction system

Figure 1. VVVF Traction drive block diagram

In order to control the motor effectively over a wide speed range the first two systems in the chain need

to be variable, hence the designation given to this type of traction equipment Variable Voltage Variable

Frequency, or VVVF.

The Traction Motor (TM)

Figure 2 Single phase motor shown at 90o & 270

o respectively.

Remembering secondary school physics lessons and Fleming’s left Hand Rule, where the direction of

force is dependent on direction of electricity flowing interacting with the direction of the magnetic flux

which runs from North to South.

The Voltage Chopper The Frequency Inverter

The

Traction

Motor

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As can be seen in figure 2 the polarized rotor of the motor has two positions depending on the direction

of electricity flowing. The change in position of the rotor, shown 180o

apart follows the reversal of the

electricity supplied to the motor. It therefore follows the speed of this change is dependent on the

frequency of change of direction of the electricity supplied in a given time period.

Therefore the speed of rotation is governed by the frequency of supply to the motor, this change of

direction is known as a cycle, and where in the motor diagram illustrated in Fig 2 one cycle would enable

one complete rotation of the motor armature. Cycles per second is defined as an SI unit, the Hertz

named after Heinrich Rudolf Hertz, who was the first to conclusively prove the existence of

electromagnetic waves.

Fig 3 illustrates a circular motion drawn linearly, the X axis shows the degrees of rotation completed, a

complete rotation representing a cycle, comparing this with the position of the motor rotor shown in fig

2 shows the position of the rotor next to the field coils coinciding with the peak waveform at 90o which

is a voltage of 1 and at 270o with a voltage of -1.

Figure 3. A Sine Wave.

Therefore in order to change the speed of the motor the frequency of supply needs to vary. But this is

not the only change that needs to be accommodated in order to control the motor. As can be seen in

the system block diagram in Fig 1 a variable voltage source needs to be presented to the motor.

An explanation of the variable Voltage or VV part.

As those of you familiar with direct current dc resistance control of traction motors are aware, the key

to controlling the motors power on a train is achieved by controlling the voltage seen across the motor,

3 phase motors are the same in this respect.

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Instead of wasting voltage over resistance grids, the VV part of the 3 phase traction system uses a switch

to chop the voltage.

There are three factors involved in controlling the voltage, these are supply voltage, whether the power

is switched on or off and time.

The supply voltage is fixed and cannot be varied, however the behavior of a switch over time is variable,

and it is here the change is made.

The VV side of the traction equipment uses a system called Pulse Width Modulation or PWM, where the

Pulse is the switch action, Width is the resultant time period the power is on, and Modulation denotes to

adjust or adapt a proportion.

The average value of voltage and power is controlled by turning the switch between supply and load on

and off. The longer the switch is on compared to the off periods, the higher the power supplied.

Figure 4. Chopper PWM signal

An example of this is used in home microwaves. Depending upon the power level selected, the

microwave operates differently. Food placed in the microwave and heated at 100% makes the

microwave power on continually for the entire cooking cycle. Evidence of this can be detected by the

hum generated by the microwave tube as cooking commences.

If a lower power setting is chosen then the microwave tube is only energized for a proportion of the

time the cooking cycle lasts, the microwave output cannot be varied it has only two states either full

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power on or off, thus the time period the microwave heats in any one cycle is varied between on and

off.

With 30% power selected on a microwave the tube only hums for 1/3 of the total time the microwave is

on, this gives a gentler heating effect to the food, PWM control acts in a similar fashion. A visual

representation of the power across time is shown in Fig 4 above.

It can be seen that there are two pulses one on denoted by red and one off denoted as blue; and the

difference in time on and time off sequences.

The red or on pulse always starts at the same time, and the blue off pulse ends at the same time,

however it is the diiference between the end time of one and the start time of the other over a given

cycle of time which limits the power passed in that period.

The voltage chopper in a traction system acts at a much faster rate than your microwave so that the

motor does not notice the frequency of the chopping in order to give smooth operation, the way which

the on and off signals are modulated is described.

The total length of both the on and off pulse is the same as can be seen by the visual representation

above say 10 milliseconds, (mS). By moving the time between the on and off pulse in any one cycle the

amount of power delivered is varied.

Fig 4 representation illustrating Duty Cycle it can be seen that the red and blue on/ off pulse repeats

every 10mS but the blue off pulse is cancelled by the repeat of the red on pulse at the second cycle. By

varying the time between the red on pulse and the blue off pulse in any one cycle, the resultant power

passed in that cycle is varied.

The top illustration at 25% at time 0 red on is selected then 2.5mS later blue off is selected with 7.5mS

left until the cycle repeats and red on cancels the blue off state.

The middle illustration at 50% time 0 power rating red on pulse is fired then at 5mS the blue off pulse

switches the power off with a resultant 5mS staying off until the cycle starts again with a red pulse.

Lastly the lower illustration at 75% at time 0 shows the on red pulse followed by the blue off pulse at

7.5mS staying off until the next red on cycle starts at 10mS.

Modulating the time between the digital on off pulses in one cycle controls the power being transmitted

from the supply, this is evidenced by the same combined length of the blue and red pulses over one

cycle, the difference is in the proportion of red and blue or on and off.

Next we consider the variable frequency VF part of the traction inverter.

The simplified explanation of the operation of the motor was given for a single phase machine, where

most motors nowadays for power above 500 Watts are of the three phase type. The block diagram in fig

1 shows three cables joining the traction motor the frequency inverter, each phase is denoted by a

colour Red, Yellow and Blue.

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Like the single phase trace shown in fig 3 the three phases are drawn on a diagram as such.

Figure 5. 3-phase sine wave.

The first stage of the traction inverter the variable voltage stage works off Direct Current dc, but the 3

phase induction motor uses 3 phase and it is the VF part of the traction system which changes the

characteristic of dc to 3 Phase ac voltage.

Looking at the single and 3 phase sine wave diagram in figures 3 and 5 you will see the peaks of each

cycle are at + or – of the 0 centre line.

This means the three phases need to be switched alternatively between the dc + or – feeds from the

voltage chopper.

A representation of how this is done is shown in fig 6 below, the trick is to syncronise the 3 phases so

that they are equally spaced 120 o

apart in order to replicate the supply given from a 3 phase alternator

and ensure a balanced load between all three phases, as shown in Fig 7.

As can be seen the red yellow and blue phases have switches each side connecting them to either

positive or negative, thus producing a pulsed peak in either quadrant above or below the 0 mid-point.

In looking at the sine wave diagram in fig 5 when red phase is at +1 the yellow and blue phases are at -

0.5 with yellow phase heading for 0 or off and blue phase heading for -1.

Figure 6 shows the switch disposition of the traction inverter, the switches only have 2 modes on or off

making the replication of the sine wave form in figure 5 impossible. Therefore with a digital on off state

of switching the phases need to be disposed in a different way to the analogue generated sine wave,

making the -0.5 state of blue and yellow phases when red is +1 impossible.

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Figure 6. Traction Inverter switch disposition

In order for the dc current to remain constant, the current must be equally divided between the output

phases. In the case of a three phase output, this means that each phase must carry the positive dc link

current for one third of the time, and the negative return current for one third of the time. In its

simplest form the output waveforms must be a stepped square wave, as shown below.

Figure 7. Square wave representation of 3 phase from an inverter

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In the diagram above, each output line is positive for 120°, followed by 60° with no current, 120°

negative and another 60° with no conduction. The three line waveforms are symmetric and phase

shifted by 120° with respect to each other.

This method of digital switching mimics as closely as possible the sinusoidal 3 phase output represented

in fig 5.

The switches in the inverter diagram in fig 6 will have the following sequence of switching shown in

table 1 below, for one cycle of operation.

Phase rotation Sw1 + Sw2 + Sw3 + Sw4 - Sw5 - Sw6 -

0o Open Open Closed Open Closed Open

60o

Closed Open Open Open Closed Open

120o

Closed Open Open Open Open Closed

180o

Open Closed Open Open Open Closed

240o

Open Closed Open Closed Open Open

300o Open Open Closed Closed Open Open

360o

Open Open Closed Open Closed Open

Table 1. dc to 3-Phase inverter switching characteristic.

VVVF History.

The earliest use of VVVF traction systems can be traced back to the use of such a locomotive by the

Hungarian Railway operator MAV.

Mav had a long history of innovative electrification and traction systems, being the first in the world to

electrify at 50Hz single phase under the directorship of Kálmán Kandó starting in 1931, Locomotives

were designed using rotary phase converters changing the single phase voltage to 3 phase on board the

locomotive, these locomotives were the class V40 & V60. During World War 2 Rathovszky, extended

these designs and developed the experimental V44 class using a phase and frequency converter, using a

rotating excitation field in the three phase generator allowing variable frequency drive.

The locomotives had four 1000hp traction engines, with Meyfahrt-Secheron transmission. The wheel

arrangement was 2'D2', economical speeds were: 25, 50, 75, 100 and 125km/h. The test runs of the

prototype locomotives were not completed as they were both destroyed in the war.

In 1952 MAV ordered a fleet of Co-Bo locomotives utalising this traction system from Ganz Mavag to

become the 12 strong fleet of V55’s. Original equipment for the Ganz electric locomotives had been

supplied by Metropolitan Vickers, but the cold war in the 1950’s made re supply of equipment from the

UK un-palatable to the communist regime.

Equipment for the production V55 was supplied from within the Soviet Union. Problems with quality

control and the sophisticated electro mechanical phase conversion meant the 12 strong locomotive fleet

was the last to use this system in Hungary, further orders were cancelled. Subsequent locomotives were

supplied as V46 using a simpler Ward Leonard control system, until the supply of discreet power

electronics became available in the late 1960s.

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Figure 8 Ganz Mavag 3150Hp 16Kvac Bo-Co, MAV class V55 No 006 ©Bowman.

The next step in VVVF drive came about with the development of the GTO Thyristor’s capable of high

power applications in the 1980’s The use of these drives has the advantage of being able to use cheap

robust traction motors, but the cost of equipment to drive them outweighed their use up until the 1980s

when BT a private line in Switzerland ordered a fleet of locomotives from SLM using GTO traction drives.

Figure 9. SLM, BBC, ABB 4290Hp 15Kvac Bo-Bo BT Class Re456 No 92 © Bowman

Major problems represented themselves with the bulk of the control equipment needed to produce

VVVF ac drive from a single phase or dc source, this meant the use of the technology on Multiple Unit

rolling stock was not feasible for many years.

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Tests were started in the 1980’s by BR NSE for their next generation of standard rolling stock the

Networker. The equipment was developed at BRUSH Tractions plant in Loughborough & GEC Alsthom

plants at Trafford Park and Preston.

Three sets of prototype equipment were installed on class 210 DEMU using the spare driving trailers and

trailer open standards making a 4-car test train for evaluation purposes. The Class 210 driving trailers

becoming driving motor open standards, furnished with equipment from BRUSH traction and latterly

one of the TOS was furnished with equipment from GEC Traction, with testing being undertaken from

Strawberry Hill depot.

Figure 10. Class 457 Coach 67301 fitted with BRUSH traction equipment.©VETT

Signaling issues

One of the main areas of concern with the equipment was the effect the modern traction systems could

have on signaling track circuits.

There were two issues which could cause a failure of the track circuits involving the traction system,

these being the low impendence connection of the electrical traction supply to the running rails, and

secondly the frequency of on board generated harmonics from the traction equipment being livening up

shorted track circuits.

There were historically two different systems of signaling used on electrified railways, the type used

depended on the system of electrification, dc electrified railways used ac track circuits, ac electrified

railways used dc track circuits. The issue comes where a dc railway utalises VVVF traction equipment, by

its nature converting the dc to ac for the traction motors.

As described in the VV section the VVVF traction system chops the DC voltage with an on off switch,

unlike resistance to drop the voltage to control it. This enables a very low resistance path through the

traction equipment than had previously been the case. If one of the line side traction supply sub stations

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feeding the DC supply has a faulty rectifier or filter, a 50Hz ripple is introduced into the traction supply,

this being transferred through the low impedance VVVF traction equipment into the running rails.

Figure 11. Class 457 Coach 67401, Fitted with GEC traction equipment, showing the

DC link smoothing chokes and the three phase inverter. © Caspian Technology

Fig 12. Class 457 Coach 67401 showing the GEC gate control electronics and test

equipment during test runs from Strawberry Hill depot. © Caspian Technology

This 50Hz ripple could cause the shorted track circuit to be re energized, thus turning the red signal

green. The traction equipment could also generate 50Hz ripple into the return current in the running

rails, used by the track circuits, therefore a safety system needed to be introduced to stop these

occurrences from happening.

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50Hz monitors were installed on the traction package using tuned circuits to detect current at the

frequency which the track circuits operate; detection of 50Hz would open a high speed circuit breaker

disconnecting the traction package and cancelling any 50Hz injection of current into the running rails

and track circuits.

The R&D undertaken was a first for GEC Alsthom, and BRUSH in the VVVF arena; and these companies

were responsible for all aspects of the project from overseeing installation to the class 210 through to

the signaling interference tests.

Upon successful completion of the project Network Southeast ordered 143 sets of equipment from GEC

Alsthom and 194 of a similar design from BRUSH Traction for implementation on the Networker fleet.

Fig 13. BR NSE Class 465 at Slade Green Maintenance depot, the production

example using BRUSH traction equipment, Britain’s first VVVF multiple unit.

© Bowman

VVVF for Dummies

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