The Crompton 70 series of AC wattmeters and VArmeters incorporate a DC moving coil taut band indicator with the Crompton designed microcircuit watt transducer PCB to read power on single or three-phase systems. The most frequently selected wattmeter scale marking for common current and voltage transformers are listed on the following pages. In addition, these instruments may be supplied with zero-left or center-zero scales at the same list price.

Scaling
Wattmeter and VArmeter current circuits should have equal carrying capacity because they are frequently connected in series. This means that the sum of the left and right end-scale values of the VArmeters should be equal to or greater than the full scale value of the wattmeter (or have higher end-scale values if the instruments are center or offset-zero). Instruments measuring 10,000 kilowatts and over are marked in megawatts. Center-zero or offset-zero watt and VArmeters are marked “IN” for left deflection and “OUT” for right deflection. Customised scales may be calculated for wattmeters and VArmeters not listed on the charts. Scales for watts must be one of the standard full-scale dial markings shown in the charts.

Calibration
For full load value of watts or VAr, assuming unity power factor:
1-phase 2-wire watts = amps x volts 3-phase 3-wire watts = amps x line-to-line volts x √3
3-phase 4-wire watts = amps x line-to-neutral volts x 3
Minimum scale values are obtained by multiplying resultant watts, using the above formula x 0.7 and selecting next higher standard scale.
For maximum scale value, multiply x 1.3 and select the next lowest standard.
If scale calculates to an exact listed value, use this value rather than the next higher or lower value.
Note: When ordering wattmeters and VArmeters, please specify CT ratio, VT ratio and required scale.

Specifications