"Boltar" wrote in message
om...

ITYF that the noise is more pronounced on older three phase designs.
Current production uses a later three-phase technology which has a less
pronounced "gear change" effect.

The older AC drives used GTO thyristors which operated at a frequency the
human ear can hear and because they had a max operating frequency some
sort of electronic equivalent of gear changing had to occur to let them
drived the motors at the full range of speeds required (don't know the
details
I'm not an electronic engineer). The newer drives use IGB transisters
which
operate at a much higher frequency though if the new stock on the northern
line is anything to go by you can still hear a very high pitched whine.

Yep, that's just about right.

The gear changing is required because it's easier and more desirable [1] to
keep a fixed ratio of device (i.e. GTO) switching frequency to modulation
frequency (the latter is roughly proportional to the motor speed), and you
have a maximum limit on the GTO switching frequency [2].

From start-up you clearly need a high frequency ratio as the motor speed -
hence modulation frequency - is very small. Since GTOs cannot switch at
high speeds (well they can but you need snubbers to slow them down to stop
them blowing up) you cannot maintain a high frequency ratio as the motor
speed increases beyond a certain point, so when the maximum switching speed
has been reached the ratio steps down to the next suitable value. The
modulation frequency remains the same, since the motor speed is the same,
but the switching frequency has been reduced. It is the switching frequency
which you can hear changing through the motor. This process happens many
times as the motor speed increases.

IGBTs can switch more quickly as they don't generally need snubbering, and
hence a higher switching frequency is used. It's just about audible (as
Boltar said, I think it may be the whine you hear on the Northern Line). As
the switching frequency is higher, the ratio of switching to modulation
frequency is greater and can be non-integer. The switching frequency is
therefore fixed, and so you don't hear the gear-changing.

[1] If this ratio is less than approx. 21 and not an odd integer then
subharmonics are a problem. Above approx. 21 it's less of a problem and
non-integer values can be used.

[2] Just to clarify, the devices - whether IGBTs or GTOs - switch to form a
high-frequency square wave. The duty ratio of this square wave is ratio of
the time it spends on to the total period, so if the duty ratio is 1/3, it
spends 1/3 of the period on and 2/3 off. The duty ratio is varied over many
switching cycles to follow a sinewave (in classic examples). However the
motor is inductive, and this has the effect of filtering out the switching
and producing a current proportional to the *average* of the square wave.
This average is proportional to the duty ratio. Hence if the duty ratio
varies as a sinewave, the current will also be approx. sinusoidal. In an
induction motor we need a variable-voltage, variable-frequency sinewave on
each phase. Varying the duty ratio amplitude and frequency (=modulation
frequency) has this effect. The advantage of using pulse-width modulation
(PWM) switching to achieve this is that the switching process is very (90%)
efficient, since the devices only pass high currents at high voltages (hence
burn lots of power) when switching. You could use a linear amplifier (i.e.
a scaled-up audio amplifier) but its efficiency is rarely above 50%, which
is clearly a no-brainer.

Also see http://www.twoof.freeserve.co.uk/TRACTION3.htm for a good summary.

Cheers
Angus

"Angus Bryant" wrote in message ...
frequency) has this effect. The advantage of using pulse-width modulation
(PWM) switching to achieve this is that the switching process is very (90%)
efficient, since the devices only pass high currents at high voltages (hence
burn lots of power) when switching. You could use a linear amplifier (i.e.

How does that efficiency compare with the old DC systems where at full power
the motor was pretty much just connected directly to the power rail(s) as
opposed to the new systems where you still have the electronics driving the
motors?

B2003

In article ,
Boltar wrote:
"Angus Bryant" wrote in message ...
frequency) has this effect. The advantage of using pulse-width modulation
(PWM) switching to achieve this is that the switching process is very (90%)
efficient, since the devices only pass high currents at high voltages (hence
burn lots of power) when switching. You could use a linear amplifier (i.e.

How does that efficiency compare with the old DC systems where at full power
the motor was pretty much just connected directly to the powern rail(s) as
opposed to the new systems where you still have the electronicsn driving the
motors?

B2003

I don't have data to hand, but I imagine the efficiency is very high
for both the old DC systems, and the modern induction motor systems.
Unlike mechanical engineers, electrical engineers are quite good at
making their machines efficient. You'll be looking at 95%+.

The real advantages of the modern AC traction systems over the older
DC systems are the following:

1. DC motors have brushes, which cause mechanical noise and which wear
out and have to be replaced; AC induction motors do not.
2. You get more power per kg of motor with AC induction motors than you
do with DC motors.
3. The control of Ac induction motors is done entirely electronically - there
are no mechanical parts (eg, relays, tap-changers) like the old DC systems
have and, again, which wear out quickly.

Furhter to what other people have said, the 'gear change' sound
is found only in the systems manufactured in the mid-90's. Newer systems -
and here I am thinking of Northern Line tube trains and the Heathrow
Express - have no such 'gear change' sound.

The reason for this is that the AC systems made up to the mid-90s use
a power electronic switch called the gate turn-off thyristor (GTO)
which has a maximum switching speed of only a few kHz. Therefore,
as the train speeds up, the frequency of the PWM square wave keeps
on being taken down a notch so as not to exceed this maximum switching
frequency. More modern systems use insulated gate bipolar transistors
(IGBTs) which can switch up to 20kHz and don't have to have their
freuqency notched down as the train speeds up.

Hope that this makes sense!

David.

In article , D.M. Garner
writes
The real advantages of the modern AC traction systems over the older
DC systems are the following:

1. DC motors have brushes, which cause mechanical noise and which wear
out and have to be replaced; AC induction motors do not.
2. You get more power per kg of motor with AC induction motors than you
do with DC motors.
3. The control of Ac induction motors is done entirely electronically - there
are no mechanical parts (eg, relays, tap-changers) like the old DC systems
have and, again, which wear out quickly.

Plus, surely, no resistances in the circuit (and wasting power) at other
than full settings.

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
Please reply to the Reply-To address, which is:

Angus Bryant wrote:

"Boltar" wrote in message
om...

ITYF that the noise is more pronounced on older three phase designs.
Current production uses a later three-phase technology which has a less
pronounced "gear change" effect.

The older AC drives used GTO thyristors which operated at a frequency
the human ear can hear and because they had a max operating frequency
some sort of electronic equivalent of gear changing had to occur to let
them drived the motors at the full range of speeds required (don't know
the details I'm not an electronic engineer). The newer drives use IGB
transisters which operate at a much higher frequency though if the new
stock on the northern line is anything to go by you can still hear a
very high pitched whine.

I've noticed the sound produced by by the 3 phase drives of some GTO
powered trains can be heard on an AM radio. Perhaps someone here might
like to take a Walkman on one of these buses, to see if you get the same
effect.

Yep, that's just about right.

The gear changing is required because it's easier and more desirable [1] to
keep a fixed ratio of device (i.e. GTO) switching frequency to modulation
frequency (the latter is roughly proportional to the motor speed), and you
have a maximum limit on the GTO switching frequency [2].

From start-up you clearly need a high frequency ratio as the motor speed -
hence modulation frequency - is very small. Since GTOs cannot switch at
high speeds (well they can but you need snubbers to slow them down to stop
them blowing up) you cannot maintain a high frequency ratio as the motor
speed increases beyond a certain point, so when the maximum switching speed
has been reached the ratio steps down to the next suitable value. The
modulation frequency remains the same, since the motor speed is the same,
but the switching frequency has been reduced. It is the switching frequency
which you can hear changing through the motor. This process happens many
times as the motor speed increases.

What are snubbers?

IGBTs can switch more quickly as they don't generally need snubbering, and
hence a higher switching frequency is used. It's just about audible (as
Boltar said, I think it may be the whine you hear on the Northern Line). As
the switching frequency is higher, the ratio of switching to modulation
frequency is greater and can be non-integer. The switching frequency is
therefore fixed, and so you don't hear the gear-changing.

[1] If this ratio is less than approx. 21 and not an odd integer then
subharmonics are a problem. Above approx. 21 it's less of a problem and
non-integer values can be used.

[2] Just to clarify, the devices - whether IGBTs or GTOs - switch to form a
high-frequency square wave. The duty ratio of this square wave is ratio of
the time it spends on to the total period, so if the duty ratio is 1/3, it
spends 1/3 of the period on and 2/3 off. The duty ratio is varied over many
switching cycles to follow a sinewave (in classic examples). However the
motor is inductive, and this has the effect of filtering out the switching
and producing a current proportional to the *average* of the square wave.
This average is proportional to the duty ratio. Hence if the duty ratio
varies as a sinewave, the current will also be approx. sinusoidal. In an
induction motor we need a variable-voltage, variable-frequency sinewave on
each phase. Varying the duty ratio amplitude and frequency (=modulation
frequency) has this effect. The advantage of using pulse-width modulation
(PWM) switching to achieve this is that the switching process is very (90%)
efficient, since the devices only pass high currents at high voltages (hence
burn lots of power) when switching. You could use a linear amplifier (i.e.
a scaled-up audio amplifier) but its efficiency is rarely above 50%, which
is clearly a no-brainer.

Why do you need a sinewave - what's wrong with a VVVF squarewave?

"Aidan Stanger" wrote in message
...

[2] Just to clarify, the devices - whether IGBTs or GTOs - switch to
form a
high-frequency square wave. The duty ratio of this square wave is ratio
of
the time it spends on to the total period, so if the duty ratio is 1/3,
it
spends 1/3 of the period on and 2/3 off. The duty ratio is varied over
many
switching cycles to follow a sinewave (in classic examples). However
the
motor is inductive, and this has the effect of filtering out the
switching
and producing a current proportional to the *average* of the square
wave.
This average is proportional to the duty ratio. Hence if the duty ratio
varies as a sinewave, the current will also be approx. sinusoidal. In
an
induction motor we need a variable-voltage, variable-frequency sinewave
on
each phase. Varying the duty ratio amplitude and frequency (=modulation
frequency) has this effect. The advantage of using pulse-width
modulation
(PWM) switching to achieve this is that the switching process is very
(90%)
efficient, since the devices only pass high currents at high voltages
(hence
burn lots of power) when switching. You could use a linear amplifier
(i.e.
a scaled-up audio amplifier) but its efficiency is rarely above 50%,
which
is clearly a no-brainer.

Why do you need a sinewave - what's wrong with a VVVF squarewave?

Square-wave excitation can be used, but produces a large torque ripple due
to large motor current harmonics. Sinewave excitation produces practically
constant torque as the current harmonics are much smaller. However as the
number of phases increases in an induction motor, any torque ripple produced
by square-wave excitation decreases and it becomes more attractive (mainly
because the inverter switching losses are almost non-existent). Someone in
my lab did a PhD on it a year or two ago, looking at total drive (inverter
and motor) losses.

Angus