Page 170 - Industrial Power Engineering and Applications Handbook

6/150 Industrial Power Engineering and Applications Handbook

Features A.C. drive Variable Speed fluid coupling D.C. drive
21 Harmonics They generate high harmonics nk ? There arises no such Higher than even a.c. drives,
1 (Section 23.6(b)) and must be phenomenon because of commutator arcing
suppressed by using filter circuits which it produces at high
frequencies
22 Unfriendly environmental Not suitable. The static controls must No problems as it is a sealed Not suitable at all, as the motor
conditions such as be located away from such areas in unit. But for controls which itself cannot be relocated in
dustladen areas, fire well-protected rooms may be located separately safer areas
hazardous, corrosive and
contaminated locations
23 Heating (i) Moderate in transistor controls Very high at lower speeds and Moderate
but requires extensive cooling
(ii) Excessive in thyristor
controls. The higher the rating
the higher the temperature
rise. They require extensive
cooling arrangements which
may be external or forced

24 Maintenance and May be high. For efficient Very low maintenance. Where a Downtime due to brushes is
downtime maintenance high-skilled operators mechanical ruggedness is about 1 hour only in 6 months’
and their proper training are essential, required, a fluid coupling to a year’s time. However,
besides stocking enough spares as provides a more reliable meticulous monitoring of
recommended by the manufacturer. solution brushes and commutator
Yet the expert services of the condition is important and so
manufacturer may sometimes be also the environmental
necessary. At times, the spares may conditions. Controls will also
not be readily available or it may need similar attention as for
not be possible to repair them ax. drives. In a breakdown, the
immediately. In such cases either it motor cannot be run in any way
is a total shutdown or the controls (unlike ax. motors) and would
have to be bypassed and the machine lead to a total shutdown
run on DOL
__ _. -~
25 Cost consideration A costly arrangement It is generally cheaper than a Quite economical compared to
static drive in all ranges an a.c. drive. The following
may give a rough idea:
Up to 10 kW - price difference
not significant
Above 10-40 kW- a.c controls
may be costlier by 3MO%
Above 40-130 kW - a.c.
controls may be costlier by
40-50%
In the range of 500 kW and
above - a.c. controls may be
costlier bv 2.5 to 3 times and
even more in still higher ranges