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 total or partial rotation of the disk. A pulse device can be retrofitted to
most existing electromechanical watthour meters. These devices cause relay
contacts to close at a rate proportional to load on the meter. Defined
kilowatt demand is derived from relay closure pulses using the expression:
=
n x KWC
kW
T
where:
kW
=
kilowatt Demand
number of pulses occurring over the counting interval
n
=
KWC
=
constant number of kilowatts per pulse (demand multiplier)
T
=
counting interval in hours
KWC, the watthour constant of the pulse devices, is calculated by:
KWC = (R/I) x kh X TF
1000
where:
R/I = number of revolutions of the rotor per pulse. This is a
constant provided by the manufacturer of the pulse device.
TF = transformer factor = CT ratio x PT ratio
k h = watthour constant
Two types of pulse devices are in common use, contact and pulse initiator.
3.1.1 Contact Devices. A contact device is entirely mechanical, consisting
of a multilobed cam, connected into the watthour geartrain. The cam actuates
one or more contact sets. All components of the device are mounted on a
single backing plate that bolts into the meter mechanism under a glass dome.
Three contact configurations are available (Figure 9-12). The different
contact devices available service different electromechanical kW demand
totalizer configurations. Slow-break contacts, for example, are used for
solenoid-operated totalizers and quick-break contacts for motor-operated
units. The type of break mechanism is important when contact closures from a
pulse initiator are interfaced to a microcomputer through a single digital
interface channel. If the sudden drop caused by the cam produces contact
bounce, corrective measures must be taken to ensure accuracy of microcomputer
displays. High-speed logic circuits are sensitive to contact bounce and are
able to resolve a train of pulses from a single bouncing contact switch
closure. This problem can be eliminated using software designed to discard
any data during a specified period of time following a switch contact.
3.1.1.1 Contact Device Drives. Most contact initiators are driven by a
separate worm gear on the meter shaft. Some meters have up to 360 possible
gear ratios using worm gear and sector combinations. Tables for determining
equivalent cam teeth (EqT) can be used, given the number of points on a cam
9-24
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