A kilowatthour meter measures the amount of energy consumed
by a load. AC kilowatthour meters often use an induction-disk type of
mechanism.
The disk revolves at a speed proportional to the rate at
which energy passes through the meter. The metered kilowatthours are indicated
on a set of dials driven by the revolving disk through a gear train.
Solid-state kilowatthour meters use a wide variety of
electronic methods to integrate energy over time. Many solid-state meters also
record other quantities, such as kilovarhours, volts, amperes, and power
factor.
The kilowatthour meter may be used to calculate the power
being used by a load at the moment of testing. To calculate power, count the
seconds for a given number of revolutions of the disk, and then use this
formula:
power (kilowatts) = 3.6 x r x Kh x multiplier/ seconds
Kh - is the meter disk constant in watthours per revolution
and r - is the number of revolutions.
The Kh will be noted on the kilowatthour meter.
The multiplier is 1 unless a meter is installed with
instrument transformers. If current transformers are installed, the multiplier
is equal to the ratio of the current transformer.
For example, 400:5 current transformers have a ratio of
80:1, and so the meter multiplier would be 80. If voltage (potential)
transformers are also installed, the meter multiplier is the product of the
current transformer ratio and the voltage transformer ratio.
A meter connected to 400:5 (80:1 ratio) current transformers
and 14 400:120 (120:1 ratio) voltage transformers would have an overall
multiplier of 80 times 120, or 9600. Some newer electronic meters allow the
user to program the meter with the multiplier.
These meters display actual kilowatthours on the register.
Kilowatthour meters come in several classes. Below is a listing of the common
classes along with the maximum current each can safely monitor.
Class 10 10 A
Class 20 20 A
Class 100 100 A
Class 200 200 A
Class 320 320 A
High-current services would require a Class 10 or Class 20
meter employed with current transformers. For example, a 1000 A service would
use 1000:5 (200:1 ratio) current transformers and a Class 10 (or Class 20)
meter.
Kilowatthour meters typically are rated for either 120 or
240 V potential coils. Higher voltage applications require the use of voltage
transformers.
The following kilowatthour meter application data can be
used only as a general guideline. The number of phases, the number of wires,
the amount of phase-to-phase current, and power-factor balance all have an
effect on the number of stators (or coils) the kilowatthour meter should
have. An unbalanced condition exists if
the phase-to-phase differences in load current or load power factor are great.
Other factors used in selecting kilowatthour meters include
the following:
Type of mountings: socket, bottom-connected, switchboard
1. Voltage: 120, 240, 240/120, etc.
2. Register: clock, cyclometer (like an odometer), digital
3. Type of load current bypass: automatic, manual
There is a high probability of error in selecting or
connecting a kilowatthour meter, especially when using instrument transformers.
If there is any doubt, consult a metering specialist. The high probability for
error also applies to kilovarhour and demand meters.
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