You can't take an industrial drive and expect it to work reliably in a
train or a tram. But you can take the basic components and build them
into appropriate assemblies for these arduous duties, as one fast growing
Finnish company has discovered. Les Hunt reports
When we consider drives for trains and trams, we might be forgiven for
thinking that this is limited to the bid motors that propel them along
their tracks. However, in today's modern transportation systems the
reliability and efficiency of heating, cooling and ventilation systems
depend much on modern power electronics as the propulsion system itself.
Take Helsinki City Transport (HKL) as an example. Finland's only tram
serving eleven lines. There was a time when the future of this venerable
system was in the balance; buses were the way to go (weren't they?).
But the trams' popularity prevailed and now it would be considered
political suicide for the Helsinki City authorities to even suggest they
be replaced by buses. Popular as they are, however, the HKL rolling stock
is ageing fast. The HKL rolling stock is ageing fast. The oldest four
axle cars are from the 1950s and the articulated trams date from the
1970s. The most recent were built 20 years ago. Hitherto, all auxiliary
equipment, such as ventilator fan motors were dc and the maintenance of
these units was becoming something of a nightmare. Spares were costly and
it was a very labour intensive process keeping them in service. Clearly,
some modernisation was called for. In each HKL tram there were six
ventilation fans with dc motors cooling the passenger compartment, brake
resistor and traction motor. The thinking was that one big inverter
supplying six ac motors was going to be cheaper than several smaller
inverters supplying one motor each, so a 15kW unit was mounted in the
main electrical panel of one of the trams. The existing cabling was
retained because of cost considerations and this connected the various
motors in parallel.
EMC problems very quickly surfaced. Not only was the vehicle's own radio
system badly affected, but-crucially - third party electrical equipment
also suffered interference, including that of a hospital on the tram's
route. It was around this time that HKL's engineering team came across
the work of Vacon Traction, the Tampere, Finland based subsidiary of
Vaasa Control. This company needs of transportation operators where
inverters are concerned, and had developed a range of inverters based on
the component parts its parent company was using to build its large and
successful CX range of industrial inverters. There are special
requirements for inverters that are used in transportation systems. Apart
from the wide fluctuation in ambient temperatures, there is the hazard of
water and dust ingress and wide variation of input voltage - anything
between 24 and 1,000V dc, and between 380 and 500V ac, single or three
phase.
The units need to be robust enough to cope with mechanical shock and
vibration, and they have to provide a range of connection options to meet
the special sealing requirements. Vacon Traction suggested to HKL that
each dc motor be replaced by an ac unit together with its own 1.1-1.5kW
inverter. The inverters are sited close to the motors that they control,
thereby keeping the cabling - and hence the EMC problem - to a minimum.
Vacon TX series inverters were chosen for this particular application.
These IP54 protected units will tolerate +/- 30% fluctuation in the dc
input voltage and feature high transient tolerance plus immunity to the
effects of shock and vibration.
Since the initial trials, some 270 units have been installed and they
have proved their reliability in duty. HKL has also seen a big drop in
its maintenance bill; the only maintenance that is required by the TX
inverter is the replacement of its own cooling fan once every five years.
Examples of the variety of inverters t