Compensating for reactive current

DESIGNING AN INSTALLATION FOR HIGH-FREQUENCY ELECTRIC TOOLS.

Frequency converter with synchronic generator

The best solution, technically speaking, in selecting frequency converters is the combination of asynchronic motors with synchronic generators. The con- verters are single-wave aggregates with an asynchronic motor as drive motor and a brushless innerpole-generator with built-in current generator.

The voltage difference between idling and operation under full load is only approx. 3 % for small transformers at cos f 0.6 – 0.9, for large converters, approx. 4 %. Synchronic converters are not affected by voltage fluctuations in the primary rotary current mains and, in addition, safeguarded against short circuiting. Assimila- tion to the rated voltage can be effected by means of a potentio- meter. Moreover, the converters are maintenance-free for 20,000 machine hours.

Secondary frequency is calcu- lated according to the following formula:

if2 = f1 · p2 /p1

f1 = primary frequency of rotary current mains

f2 = secondary frequency for high-frequency electric tools

p1 = pole pair count of the drive motor

p2 = pole pair count of the generator

Frequency converters with a power output in excess of 4 kVA should generally not be switched into the mains direct but con-

nected by means of star delta switches. When they are switched in directly, a short-term current surge occurs. This surge could overload the lead wires on con- verters over 4 kVA and trigger the serial fuse. When a star delta switch is used, the current surge is reduced since current flow over the star switch is reduced to one third of what it would be with direct switching.

When a star delta switch is used, the coil of the drive motor is switched from the star (switch-on process) to the delta (operating position). It is imperative that a frequency converter that is to be operated on a 400 V mains line with a star delta switch is laid out for 400 V in the delta. If a converter of this kind is laid out for only 230 V in the delta, it can only be switched in to the 400 V mains directly over the star, that is to say, without star delta switching. When laying out a new installation, it is very important to take this into consideration.

Parallel operation of frequency converters

In order to increase the economics of the entire installation and to equalize load peaks, frequency converters can be switched in parallel. This results in optimal assimilation to the equipment being used. When frequency con- verters are linked to synchronic generators, no particular prepara- tions are required to operate equipment in parallel even when power output levels differ.

Every inductive consumer is lumbered with a reactive current that performs no effective work, but only loads the electrical wiring. Frequency converters and high-frequency electric tools are also inductive consumers.

Compensation for reactive current on the secondary side of the converter requires considerable expenditure since each tool must be compensated separately. Depending on the number and the performance level of the individual high-frequency electric tools, a total output factor cos f of 0.5 to 0.85 is to be reckoned with. On the primary side of the frequency converter, the output factor cos f can be significantly improved if the magnetizing current from the drive motor and generator are compensated. Switching in correspondingly rated capacitors makes it possible to counteract the primary side reactance output of the converter almost entirely when idling and to compensate it under load to such an extent as to yield an output factor larger than cos f = 0.9.

Fig. 3

to the tools

a1 = motor guard switch with magnetic and thermal trip a2 = motor guard with thermal trip

b = protective grounding as per VDE 0100