Page 457 - Instrumentation Reference Book 3E

440 Electrical measurements

Re x,
Ammeter

Current error
Phase-angle error (degrees) (%)
- 0.4
Phase error - 0.2

0- -0
-0.5 -
- -0.4
15 VA burden
0.8 power factor - -0.6

10 20 40 60 80 100
Full-rated secondary
current (%)
(d)
Figure 20.17 (a) Current transformer; (b) equivalent circuit; (c) phasordiagram of current transformer; (d) current ratio and
phase-angle errors for current transformer.

effects, but the absence of an iron core requires meter. The most important use of the dynam-
the coils to have a large number of ampere turns ometer principle is in the wattmeter (see Section
to provide the necessary deflecting torque. This 20.4.1).
results in a high power loss to the circuit to which
the instrument is connected. The torque-to- 20.2.5 Thermocouple instruments
weight ratio is small and therefore friction effects
are more serious, and the accuracy of these Figure 20.20 shows the elements of a thermo-
instruments can be affected by stray magnetic couple instrument. These are a heating element
fields. Dynamometer instruments tend to be more which usually consists of a fine wire or a thin-walled
expensive than other types of ammeter and volt- tube in an evacuated glass envelope, a thermocouple