Until now, the use of the some control diagram was only possible for motors fed from a low-voltage network due to the lack of technical and economic solutions acceptable for the medium-voltage zone.

Currently, these solutions exist, and therefore the medium-voltage motors too can be fed directly with variable frequency voltage. Existing installations for medium-voltage motors supplied with variable frequency use an intermediate reduced voltage circuit and an increased frequency converter outlet voltage (Figure 1.11).

Transformer T2 operation at variable frequency requires the magnetic induction to be kept at a value that should not exceed the saturation limit.

As in the case of the asynchronous machine, to keep the induction equal to the rated induction, for frequencies under the rated frequency it is necessary to meet the condition between the terminal voltage and supply voltage frequency.

In the specific case shown in Figure 1.11, the condition is met, being required by the asynchronous motor operation. Obviously, the condition  has to be met by the transformers of another type that can operate at frequencies other than the rated frequency.

One case often met is that where pieces of equipment are designed for 60 Hz operating systems. Their utilization in 50 Hz frequency systems requires the supply voltage to be reduced some 20% against their rated voltage.

The pieces of equipment with magnetic circuits designed to operate both to 50 Hz and 60 Hz have larger losses with the 50 Hz supply. For low-power equipment the losses are irrelevant.

For equipment of larger power it is required to consider operation at reduced frequency. Identically, the utilization of parallel-connected coils, at a frequency other than the rated frequency, requires analysis of the conditions when saturation limits are exceeded and losses increase.

Induction keeping to the rated value requires control of the voltage to terminals.

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