66 Electrical Power Systems Technology
several basic principles. The user should assure that the chart roll has
enough paper to last throughout the duration of the time that it is to be
used. The ink supply should be checked. If ink is needed, the well should
be filled to the proper level. Also, the pen should be checked for proper
pressure on the roll chart and for accurate adjustment along the incre-
mental scale of the roll chart. The user should also assure that the meter is
properly connected to the external circuit (which is needed for making the
desired measurement).
MEASURING ELECTRICAL POWER
Electrical power is measured with a wattmeter. A meter movement
called a dynamometer movement, shown in Figure 3-5, is used in most
wattmeters. Note that this movement has two electromagnetic coils. One
coil, called the current coil, is connected in series with the load to be mea-
sured. The other coil, called the potential coil, is connected in parallel with
the load. Thus, the strength of each electromagnetic field affects the move-
ment of the meter pointer. The operating principle of this movement is
similar to that of the moving-coil type of movement, except that there is a
fixed electromagnetic field rather than a permanent-magnetic field.
When we measure DC power, the total power is the product of volt-
age times current (P = V × I). However, when measuring AC power, we
must consider the power factor of the load, since P = V × I × pf). The true
power of an AC circuit may be read directly with a wattmeter. When a load
is either inductive or capacitive, the true power will be less than the appar-
ent power (V × I).
MEASURING ELECTRICAL ENERGY
The amount of electrical energy used over a certain period of time
may be measured by using a watt-hour meter. A watt-hour meter, illustrat-
ed in Figure 3-6 relies upon the operation of a small motor mounted inside
its enclosure. The speed of the motor is proportional to the power applied
to it. The rotor is an aluminum disk that is connected to a numerical reg-
ister, which usually indicates the number of kilowatt-hours of electrical
energy used. Figure 3-7 shows the dial-type face plate that is frequently
used on watt-hour meters. Other types of watt-hour meters have a direct