Valhalla 2300, Operation Manual

Page: 82 / 115

APPLICATION NOTE AN101
PROTECTING THE 2300 DIGITAL POWER ANALYZER
FROM INDUCTIVE LOADS
101-1. General
The 2300 Series of Digital Power Analyzers is
ideally suited for measuring the power drawn
or supplied by large electric motors,
generators, transformers, or other types of
inductive devices. The testing of highly
inductive loads such as these requires that
precautions be taken to protect the 2300 from
damage due to inductive kickbacks. Very
high voltage spikes may be produced when
the motor or transformer is disconnected from
the source. Spikes may also be produced
when a resistive load is disconnected from an
inductive source, e.g. a transformer or
generator. These spikes can easily exceed the
common mode and series mode specifications
of the 2300.
101-2. Protection Methods
There are two methods of protecting the 2300
from inductive kickbacks. These methods are
described in the following paragraphs.
101-2-1. Switch Protection
In this method a switch is placed between the
2300 and the inductive load. When
disconnecting, the load switch SW1 should be
opened first. Switch SW2 can now be
opened, disconnecting the 2300 from the
source. Figures 1, 2 and 3 detail this type of
protection for single and three-phase loads.
101-2-2. Spark Gap Protection
In many cases the switch protection method is
not practical. Spark gap protection is the
preferred method of protecting the 2300 from
inductive kickbacks because the protection is
not dependent on the operator. The spark
gap(s) dissipate the energy from the inductive
kickback. Figures 4, 5 and 6 detail this type
of protection for single and three-phase
connected loads. When the source is also
inductive, e.g. a generator, additional spark
gaps should be used. In all cases the spark
gaps should be connected close to the load
(source) and as far from the 2300 as practical.
All connections should be made using heavy
gauge wire. The spark gap voltage rating
should be selected at two to four times the
working voltage, 1500 volts maximum. The
energy rating in joules depends on the load.
For motors, a good rule of thumb is 10 joules
per horsepower. For transformers, the energy
rating can be calculated from the equation:
Where:
Energy =
the stored energy in the inductor in Joules
L = the inductance in Henries
I = the current used to charge the inductor
(supply current) in amps
If the inductance of the transformer is
estimated, use a spark gap of higher energy
rating than calculated for a margin of safety.
Spark gaps are available from C.P. Clare,
among others.
)
LI
(
2
1
= ENERGY
2