EMC problems due to lifts
How can you ensure electromagnetic compatibility (EMC) in lifts in the susceptible environment of the Westdeutsche Rundfunk (WDR) broadcaster?
In the meantime, such problems not only occur there, but also in complex office buildings. This is because many susceptible means of communication are in use there too.
Various measures and combinations of filter techniques were realised to guarantee EMC in the susceptible environment of the WDR as part of an elaborate pilot project at the WDR building involving Osma-Aufzüge, Klinkhammer Steuerungen and Fuji Electric. Sophisticated measurements documented the effectiveness of the measures.
The increasingly frequent use of non-linear electrical consumers – such as switching power supplies, electronic ballast or regulated controls for devices of all kinds – results in electrical interference. This can then impair operational processes in the building.
The devices involved operate with synchronised semiconductor elements and generate non-sinusoidal electricity with a high harmonic distortion factor and steep current gradients in the grid. This results in high, non-sinusoidal currents, also on the neutral conductor or PEN conductor. The use of frequency converters in lift technology results in a high load (disturbance current) in the PE and PA systems, initiated by parasitic capacitance of the lines and motors. This load represents a potential source of interference for the technical equipment of a broadcaster’s building.
Susceptible microphones …
Broadcasting buildings contain many electrical systems with different requirements on their electromagnetic operating environment. Microphone and loudspeaker installations as well as radio receiver systems are the most susceptible. The EMC requirements for the first environment (EMC standard EN61800-3 Category C1) are set as minimum condition for interference-free operation of these and comparable technical systems.
However, lifts with a current of more than 25A per phase, which meet the EMC standard for lifts and escalators according to DIN 12015, already belong to EMC Category C3. However, these limits are unacceptable in such a susceptible environment.
Solutions such as internal or external filters on the grid side of the converter and shielded motor cables, which are connected with low impedance, have become established to meet the customary EMC requirements. However, the requirements of broadcasting organisations far exceed these standards.
Discharge interference current
Systematic separation of PE and PA with the aim of discharging the interference current arising via the PE and "keeping the PA clean" is an important measure for reducing sources of interference. The pulsed filter discharge current produced by the frequency converter is discharged to the earth via the PE in the lead.
The PA must be as free as possible of interference current. To achieve this in the separation of the PE and PA, the sources of interference at the frequency converter output (pulse width modulated voltage/current, frequency converter switching frequency) must be eliminated by a suitable filter.
The use of sinus wave filters permits practically sinusoidal current at the motor, allowing the capacitive discharge current at the motor to be largely eliminated. However, these high performance filters render use of a high quality controller, which is taken for granted in modern lifts, extremely difficult. Previous attempts by converter manufacturers to cope with this problem have either been unsuccessful or associated with very high costs and effort.
Interference is no longer acceptable
As can be seen in the diagram, PA and PE are consistently separated down to the earth rod. All sources of interference should be discharged via the PE included in the power supply lines. For this purpose, it is in turn important that the power supply line to the lift is designed throughout with five conductors.
When it comes to a lift connected to the mains of a building, the interference sources are probably not critical. However, if several lifts exist or there are correspondingly susceptible means of communication in the building, PA and PE unavoidably become mixed and the interference is no longer acceptable for the operator.
This then makes elaborate filtering necessary. Only a complex network analysis that also takes high frequency sources of interference into account will reveal the interference.
By Klaus Arnolds and Bastian Dettlinger
Klaus Arnolds was head technician for conveyance technology at the WDR. Following his retirement, he is working for Hundt-Consult.
Bastian Dettlinger is branch manager of Hundt-Consult in Cologne.