by Irvine Bell
One point in your last e-mail that is often
the source of misconceptions is
the effect of polarity on traction motors. Many people assume
that traction
motors work like the small electric motors often found in, for
example,
model electric railways i.e the motors have permanent magnets
to provide the
magnetic fields within the motors. These model motors are polarity
sensitive - reverse the polarity of the supply and the motors
reverse their
direction of rotation.
Traditional DC traction motors have electromagnets
to provide the fields.
These motors are not polarity sensitive, because reversing the
supply
polarity reverses the direction of the magnetic fields at the
same time as
the current through the motor reverses, cancelling out the effect
of the
polarity reversal, and the motors continue to rotate in the same
direction.
To reverse these motors it is necessary to reverse the relative
connections
of the armature and field windings - done by a switch(es) under
the control
of the driver.
The latest trolleybus technology [e.g.
ALSTOM's ONIX range] is based on AC
induction type motors. The motors are three phase and their direction
of
rotation is determined by the relative sequencing of the three
phases. The
AC supply to the motors has to be provided from the DC overhead
line supply
by some very complex software driven [microprocessor controlled]
electronic
inverters, based on Insulated Gate Bipolar Transistors, or IGBTs
for short.
I don't know how this type of technology achieves insensitivity
to supply
polarity in trolleybuses [it would not need it in a light rail
system, where
one side of the supply, the track, can be guaranteed grounded].
I suppose it
must involve extra complexity in the electronics and the software:
I don't
suppose it comes automatically in with the technology, as it does
with
traditional DC traction motors.
The Swiss 1,000 Volt system is Lugano.
I believe that its future is very
much under threat because of the difficulty in obtaining new 1,000
Volt
trolleybuses. I know nothing about the electrical equipment of
their
existing vehicles. (This was a question about the varieties in
line
voltage - S.P.)
Traditional British trolleybus equipment
was not sensitive to polarity [or
to grounding / floating] or to the exact voltage of the supply
- intrinsic
features of the relay / resistor / dc motor technology employed.
No changes
were required to operate abroad. Similarly, traditional European
trolleybus
electrical equipment, as far as I am aware, is not polarity or
grounding /
floating sensitive.
I forgot to mention that quite a number
of trolleybuses built for export
from Britain were tested on British systems first. For example,
I have seen
a photograph of a South American trolleybus on test in London.
I would be very surprised if any trolleybuses
anywhere were polarity or
grounding or exact voltage sensitive [although the Lugano 1,000
Volts would
I imagine be too much for most or all nominal 550/600 Volts equipment].
It
would for example, be impossible [or difficult - crossed trolleypoles,
or
dangerous - operators putting poles on the wrong polarity conductors]
to
operate trolleybuses 'wrong line' to get past obstructions if
the equipment
was polarity sensitive. Also for safety reasons, trolleybus equipment
has to
be fully insulated / fully floating. Unlike a railed vehicle,
there can be
no reliable grounding point on a trolley vehicle, because of the
rubber
tyres.
I suppose to be very pedantic, one could
say that no changes were required
to operate, but re-labelling of some electrical equipment I suppose
might be
/ have been required. For example, the two circuit breakers [located
generally over the driver's head] were usually labelled positive
[+] and
negative [-] in British trolleybuses. On a foreign system, these
should I
suppose have had their [+] and [-] labels reversed. I have no
idea whether
this was actually done though.
"Is not it better to use as the route
[system] characteristics the
outgoing
voltage on the substations with one vehicle operating in order
to make an
electrical circuit, as this is the stable value? I fell this should
characterise better the system, because the London one operated
between 550
and 1000 Volts, and there are no reasons why the voltage of 550
Volts should
be believed to be the feature, not 1000 Volts." (This was
my question -
S.P.)
I am not an electrical engineer, but my
understanding is that the reality of
electric traction supply systems is that is it difficult to talk
about the
supply voltage as a single meaningful value. I suspect that the
values
quoted would generally be the no load values at the substation,
but to be
really meaningful, one would need to have figures for the range
of variation
under different circumstances.
Regarding your further questions, my impression
is that all European long
established trolleybus systems operate on about the same, approximately
550
Volts DC, with the exception of the one remaining 1,000 Volt DC
system in
Switzerland. The new systems e.g. Nancy seem to use 750 Volts.
There seem to have been no [electrical]
problems transferring London
vehicles to Spain and there are a number of foreign European trolleybuses
operating in British trolleybus museums now.
I don't know whether the Spanish systems
grounded or floated their
trolleybus overhead. Both arrangements were used in London. Where
trams
shared the same overhead / power supplies in London, the overhead
had to be
grounded. It did not make any difference to the trolleybuses.
Trolleybus
electrical equipment has to be very [electrically] robust and
able to
withstand, for example, sudden breaks [on insulators], lightning
strikes,
polarity reversals, etc. - problems not experienced by equipment
on battery
vehicles.
The ALSTOM ONIX 800 equipment is designed
to operate satisfactorily on
anywhere between about 400 Volts and 750 Volts. The actual AC
motors used on
operate at no more than about 400 Volts anyway, so I would not
expect the
new Athens vehicles to seem underpowered on 550 Volt overhead.
On any traction supply, the actual line
voltage at any one place, from
moment to moment, must vary considerably from the value at the
substation.
It would depend on the distance from the substation [up to perhaps
a couple
of miles] and whether other vehicles in the same electrical vicinity
were
drawing power or even perhaps regenerating. I have been told that
on the
London system, line voltages could go up to as much as 1,000 volts
when a
bus was attempting to put regenerated [braking] power back intro
the
overhead.
The next [July-August] issue of 'Trolleybus
Magazine', should contain an
article, written by myself, describing the ALSTOM electrical equipment
for
the new Van Hool Athens trolleybuses and comparing the equipment
with
traditional [British] relay / resistor control equipment.
All British trolleybus systems [1911 -
1972] operated at a nominal supply
voltage of about 550 Volts DC - the same as British tramway systems
which
were all governed by the same legislation [the legally permitted
maximum, as
measured at the substation, was 650 Volts]. My impression is that
about 550
Volts DC has been, with one or two exceptions, like two 1,000
Volts DC lines
in Switzerland, a world standard for most of this century for
both
trolleybus and tramway systems.
In recent years, the seems to have been
a trend for new trolleybus and
tramway systems to operated at 750 Volts DC. The ALSTOM ONIX 800
range of
trolleybus electrical equipment [as being fitted to the 96 new
Van Hool
trolleybuses for Athens] is designed for up to 750 Volts DC. I
guess that
750 Volts DC is in effect a world standard for new systems. I
am not aware
of any formal e.g. ISO, SAE, EEC, etc., standards governing power
supplies
to trolleybus systems. I assume that it is governed by national
legislation.
In Britain, and I believe, elsewhere, the
convention was/is that the
conductor nearest the kerb [on the left in Britain] was grounded
[if
grounded at all] and the other conductor [the right hand in Britain]
was at
about 550 Volts.
Many British trolleybus systems 'floated'
their trolleybus overhead i.e
there was no ground connection and, theoretically, one could not
get a shock
by touching either conductor and 'ground' at the same time. In
these
situations, the wire nearest the kerb [on the left in Britain]
was held at
about -300 Volts and the other wire at about +300 Volts, giving
a potential
difference between the wires of about 600 Volts. The London system
did this
[once the trams had gone].
4 JL 99
