METER MOVEMENTSTest Instruments & Measuring DevicesA common variation of the D’Arsonval movement is the Weston movement, which usesessentially the same principle built to a more rugged construction by employing jeweled supportsfor the core and employing a heavier winding in the electromagnet. Remember that theD’Arsonval movement is a DC device and can only measure DC current or AC current rectifiedto DC.ElectrodynamometerMovementThe electrodynamometer movement (Figure 2) has the same basic operating principle as theD’Arsonval meter movement, except that the permanent magnet is replaced by fixed coils. Themoving coil and pointer, which are attached to the coil, are suspended between and connectedin series with the two field coils. The two field coils and moving coil are connected in seriessuch that the same current flows through each coil.Figure 2 Electrodynamometer MovementCurrent flow through the three coils in either direction causes a magnetic field to be producedbetween the field coils. The same current flow through the moving coil causes it to act as amagnet exerting a force against the spring. If the current is reversed, the field polarity and thepolarity of the moving coil reverse, and the force continues in the same direction. Due to thischaracteristic of the electrodynamometer movement, it can be used in both AC and DC systemsto measure current. Some voltmeters and ammeters use the electrodynamometer. However, itsmost important use is in the wattmeter, which will be discussed later in this module.ES-14 Page 2 Rev. 0
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