Page 106 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
104 Chapter Three

rated current may cause a drop in voltage, depending on the type of
excitation, of up to 20 percent. During the accelerating period the cur-
rent reduces but the power factor increases, and the peak starting
power taken may well be of the order of 200 percent of rated power for
a very short period. A large motor will have a run-up time longer than
the voltage recovery time and once the voltage regulator has restored
the initial dip it will take care of the comparatively slow changes in cur-
rent and power factor that occur during the accelerating period.
More is written about the effect of motor starting in an earlier sec-
tion titled “Sizing the Engine and Generator.”
Contactors associated with motor control gear can experience diffi-
culties. If motor starting causes the voltage to drop momentarily to
below say 80 percent, there is a danger that the contactor will fail to
close its magnetic circuit, with disastrous consequences for the contacts
and the operating coil.
If a generator that is already supplying some running motors expe-
riences a step load, the resulting voltage dip will be increased by the
running motors. This is because ac induction motors run at almost con-
stant speed and therefore behave as constant power machines; if there
is a drop in the supply voltage there must be a corresponding rise in
current which increases the step loading. The effect is contrary to that
produced by resistive and most other loads, which take less current
during voltage dips. The effect will be noticeable only if the motor load-
ing is significant, say in excess of 50 percent of the generator rating.

Power Factor Correction Capacitors
If an installation includes bulk power factor correction equipment, con-
sideration should be given to disconnecting it when the standby supply
is in use. The essential load will be less than the total load and there may
be overcorrection unless the capacitor bank can be reduced. A generator
supplying a capacitive load is likely to produce overvoltages, certainly
the voltage regulator will have less control. When the standby supply is
in use there are no tariff implications of a low power factor, and bulk cor-
rection seems to have no benefit except to reduce the generator current.

Regenerative Loads
The regenerative load most likely to be encountered in buildings is a lift.
Apassenger elevator will usually feed power back into the supply system
when raising an empty car or lowering a full car. When the normal sup-
ply is in use, surplus power can be absorbed by other building loads, or
be fed back into the supply system, but when the standby supply is in use
different conditions apply. The elevator manufacturer can advise the

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.