Cable length

All these drives are based on pulse width modulation (PWM) and hence would produce overvoltages at the inverter output and require overvoltage protection for cable lengths of 100 m (typical) and above, depending upon the steepness of the wave (Section 6.14.1). 

If the surge is very steep, say, with a front time of 0.2 jjs. or less, and the cable length is short, say, lO m (presumed), it may inflict all its severity to only the entrance coil of the machine and sometimes even only a few entrance turns of this coil. However, with the technological improve-ment of switching devices, the arc prestrikes are now less predominant or non-existent and hence, the switching surge may not be as steep as described here. 

In case of static drives also generating simihir switching surges, their manufacturers provide the safe cable lengths as a standard practice. Section 6.14.1. 

The tests may be conducted on a few ratings with different sizes and lengths of cables to simulate the near actual condition of the installation. One should be more conservative than liberal on cable lengths to obtain safe... 

For cables such as in elevators or cranes, the service life might be based on 95% of the fatigue life of the material. In such circumstances, a composite cable would clearly last far longer than a metal cable. Moreover, the composite cable length, if measured, would be found to be longer than originally as time passes (an elevator would have to be periodically adjusted to stop precisely at each floor). Such measurements when recorded would be an evidential record of the cable per-... 

Furthermore, for lower level temperatures well below mininum PVC decomposition temperature, it was assumed that no more than 20 % of the cable length, viz. 80 m, was... 

Fig.19. Sieve with integrated cables (length 21 mm) and connection pads (light micrograph)...

Due to these restrictions the intrinsic safety of a fieldbus will be established in a simple way. However, practice has shown that a rather small amount (4. .. 6) only of field devices depending on cable length can be operated via a single bus following this proceeding. The reason is the limitation of capacitance C and inductance L. In fact, a power supply with high output current provides the current for many field devices (10 mA each). However, the values for Cc and L0 still permissible are accordingly low. In contrast, a... 

So, it can supply one or more system modules according to their current demand. The DC/DC convertor mentioned above is fitted to the individual modules as required. In this example, cable lengths do not exceed some metres, so that, in practice, the small inductance permissible does not impose a restriction. 

Complicated power distributions may be caused by reflections on bus systems with long cable lengths (see Fig. 6.239) ... 

Figure 6.240 Power distribution on a cable (power versus position). Cable length 400m Resistance matching condition (100 ohms) Inductive termination ZA = 200 pH.

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