There is hardly any other product with the same life expectancy as a lift. Even after many years, a lift system is capable of continuing to give good service if carefully maintained. On average, wearing parts do require replacing around once every 25 years.
However, experience has shown that the lift itself frequently still continues to do its job without cause for complaint. One of the reasons for this is that elevator gearboxes used to be designed to include generous reserves, which provided they were regularly serviced, allowed them to achieve an extremely long service life.
As well as maximum safety and a high degree of availability, today’s lift user demands extreme stopping accuracy and low energy consumption, alongside low noise emissions.
Many of these requirements can be addressed and major improvements achieved simply by retrofitting a frequency inverter. Even if the original drive system is retained, this can offer a number of substantial advantages, including
• Shorter travel times and consequently greater lift availability, due in particular to optimized timing of the landing approach.
• Constant speeds in the upward and downward direction with a loaded or unloaded car.
• Good positioning accuracy within a tolerance of less than ± 1 mm irrespective of load.
• Energy savings of more than 50% compared with pole-changing motors.
• Gentle-action running characteristics relieving stress on the mechanical components.
Standard solutions are now available permitting the integration of new frequency converters into existing control systems which are reasonable in cost and quick and easy to implement.
A frequently more complicated procedure is mounting an incremental encoder on the existing lift. As the pole-changing motor can continue to be used, there is no preparation in place either for integration of the encoder in the motor shaft or for mounting the necessary torque support in the housing. This is why it often makes sense when modernizing an existing system to operate the motor without an encoder. This is generally termed encoderless or open-loop operation.
Encoderless operation calls for a high-grade frequency converter which is capable of using a precise integrated mathematical motor model to calculate the function and response of the motor in advance and use this information for precise motor control. For the installer on site, simple converter parameterization is of decisive importance.
In the case of the ZETADYN 3 illustrated here, only motor data given on the motor rating plate such as rated voltage, rated current, rated speed needs to be entered. This data is sufficient for operation of the lift.
Only once all the mechanical components have been installed and optimized does precision adjustment take place. This is done using the efficient ZETAMON software which affords not only a rapid overview of all the important communication signals between the control and regulation system, but also the most important motor data such as voltages, currents and speeds.
Older motors do not come equipped with the necessary degree of insulation for converter operation. In addition, it is no longer possible to guarantee that the insulating materials in place are providing an adequate protective effect after extended periods of operation. The degree to which this effect has deteriorated depends on the level of stress to which the motor has been exposed in terms of both hours of service and thermal stress over its lifetime.
When working with a inverter, there two additional factors which bring about added motor stress: The rate of voltage rise dU/dt and the occurring voltage peaks. Due to cable capacities and inductivities, these parameters become increasingly unfavourable as the length of the cable between the motor and frequency inverter increases.
If only for this reason, motor cables should be kept as short as possible, particularly in the case of renovation projects. Experience has shown that if these minimum standards are observed, these older motors generally go on working reliably for many years.
Also in the case of renovation projects, not only the mechanical components but also the quality of the electrical installation determines the ride quality of the lift. As well as keeping the motor cable lengths to a minimum, a number of basic rules of thumb have been listed below which are, incidentally, also applicable for new installations.
• Use a high-quality frequency inverter suitable for encoderless operation of lifts
• Arrange all the components on a mounting panel affording good metallic contact
• Ensure that the motor cable and encoder cable are correctly shielded
• Use ferrite rings in the inverter output cables
• Physically separate the input and output cables to the inverter
• Ensure correct timing when actuating the motor contactors
• Use RC elements at the contactors
• Use varistors in the brakes
• Ensure that the cable to the brake chopper or resistor is shielded
To sum up:
It is possible and sensible to use modern frequency converters to modernize even old geared machines. The success of completed modernization projects clearly indicates that this technology can be used without hesitation.
