Sunday, January 11, 2015

AC Instruments and Meters : Ammeters and voltmeters , Practical ranges for moving iron instruments and The dynamometer wattmeter.

AMMETERS AND VOLTMETERS

Ammeters and voltmeters of the same type operate on the same basic principle. The main difference is that ammeter movements have a few turns of heavy wire and voltmeter movements have many turns of fine wire. Voltmeters also have resistors connected in series with the movement to obtain the desired ranges.

PRACTICAL RANGES FOR MOVING IRON INSTRUMENTS

AC Ammeters

The physical size of ac ammeters, using any type of moving iron movement, deter- mines the current rating of the instrument. The size of the instrument is influenced by the amount of heat to be dissipated and the size of the connection terminals to be supported. In small panel instruments, 100 A is the maximum practical current rating. For large portable instruments, 200 A is the maximum rating. In some large ammeters designed for switch- board use, the current rating may be as high as 600 A.

Larger Current Ratings. Other means must be used to obtain larger current ratings for moving iron ac ammeters. Permanent-magnet moving coil instruments commonly use shunts to obtain higher current ratings. However, shunts are not satisfactory for moving iron instruments. One reason is that the movement is less sensitive and requires a greater voltage drop across the shunt than in dc ammeters. Thus, there is more heat dissipation in the shunt. As a result, the resistance increases in the various parts of the instrument circuit, creating errors in accuracy.

The use of a shunt for moving iron instruments also introduces frequency errors. The inductive reactance of the shunt is low and the inductive reactance of the coil is relatively high. The impedance of the shunt remains nearly unchanged over a range of frequencies. The impedance of the coil, however, changes considerably as the frequency varies. Because the changes in frequency do not affect the coil and the shunt equally, there will be a large error if the instrument is used on a frequency other than the one at which it was calibrated.

Many moving iron instruments are used only for ac measurements. As a result, it is a standard practice to use an instrument current transformer to obtain an increase in the cur- rent range of a 5-A instrument. When the measuring instrument is connected to the secondary of the transformer, the current in the transformer primary will be indicated accurately. The instrument can be calibrated to indicate the primary current. The actual calibration depends on the ratio between the primary current and the secondary current. Detailed information on instrument current transformers is given in a later unit of this text.

AC Voltmeters

When moving iron instruments are used as ac voltmeters, series resistors are used to extend the scale range for voltages up to 750 V. The effect is the same as that obtained when a series resistor or a voltage multiplier is used with permanent-magnet moving coil movements in dc voltmeters.

When ac voltages greater than 750 V are to be measured, larger ohmic resistance values cannot be used. Because of the higher ohmic values, more power would be expended in the resistors. Also, there would be high-voltage insulation problems. Thus, large ac voltages are measured using an instrument potential transformer with the movement. The primary winding and insulation of the transformer are suitable for the higher voltage. The secondary winding is usually rated at 120 V. The ac voltmeter usually has a coil rating of 150 V. In many cases, the instrument scale is calibrated to indicate the primary voltage directly. Detailed information on instrument potential transformers is given in a later unit of this text.

THE DYNAMOMETER WATTMETER

To measure the power in watts with an instrument having a dynamometer movement, the stationary field coils are connected in series with the line. Thus, the field flux depends on the current. The moving coils are connected across the line so that the moving coil flux is proportional to the system voltage. Figure 11–10 shows a typical wattmeter circuit. The resistor is connected in series with the moving coil. It can be shown that the instantaneous torque is proportional to the product of the instantaneous field current and the instantaneous moving coil voltage. The average torque for a whole cycle is proportional to the average of the power pulses. This means that the pointer deflection is proportional to the power as expressed by the following equation:

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Operation of the Dynamometer Wattmeter

The operation of the dynamometer wattmeter is shown by the wave patterns in Figure 11–11. In Figure 11–11A, the current, voltage, and power waves are shown for one cycle when the current and voltage are in phase. Note that the power curve at any instant is positive. When the current and voltage are in phase, the field flux and the armature flux increase and decrease together. These quantities reach their maximum values at the same time. The deflection of the movement pointer represents the average of the product of the instantaneous voltage and current. This value is the true power, in watts, for the circuit.

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Figure 11–11B shows the current, voltage, and power relationships for a circuit where the current lags the voltage by 30°. In this case, the field flux and the armature flux do not reach their maximum values at the same time. The field flux reaches its maximum value 30° behind the maximum value of armature flux. This means that the torque never reaches as high a value as in the case where the current and voltage are in phase. Instead, the torque always has an average value corresponding to the product of the voltage, current, and power factor. Figure 11–11B shows that the average value of the power is less than the value shown in Figure 11–11A for the condition of the current and voltage in phase.

In Figure 11–11C, the current, voltage, and power are shown for a circuit where the current leads the voltage by 30°. The wattmeter indication is the same for this case as for the lagging current condition. Again, the average torque on the movement of the instrument is determined by the product of the instantaneous current and voltage values. Therefore, Figure 11–11C shows that the average value of the power is less than the value shown in Figure 11–11A for the condition of the current and voltage in phase.

Connecting a Dynamometer Wattmeter

When connecting an instrument having a dynamometer movement, the technician must consider the instantaneous direction of current in each of the coils. This direction determines the flux, which, in turn, specifies the direction of the deflecting torque.

The diagram in Figure 11–12 shows the marking (±) next to one of the terminals of

the potential coil circuit (the armature) and also one of the terminals of the current coil (the field). These terminals are connected to the same side of the line to ensure that the deflec- tion has the correct direction.

There are two different methods of connecting the potential coil of a wattmeter. In Figure 11–12, the potential circuit of the wattmeter is not connected directly across the load. Instead, it measures a voltage higher than the load voltage by an amount equal to the voltage drop in the current coil. In other words, the wattmeter indicates too high a value of watts. The extra power is that expended in the current coil. For the connections shown in Figure 11–12, the true power of this circuit is

True power = wattmeter reading - I2 R of current coil

A second method of connecting the wattmeter is shown in Figure 11–13. In this dia- gram, the potential coil is connected directly across the load voltage. The current coil of the wattmeter now reads both the potential coil current and the load current. This reading is due to the fact that the potential coil is really a high-resistance load in parallel with the actual load. In summary, the wattmeter indicates a value that is higher than the actual power taken by the load. The power in excess of that taken by the load is equal to the power expended in the potential circuit of the wattmeter. Using the connections in Figure 11–13, the true power is

imageFor either connection (Figures 11–12 and 11–13) the wattmeter indicates a value slightly larger than the true power. However, using the connections in Figure 11–13, the percentage error will be slightly less because the potential coil circuit is connected directly across the load.

Using a Wattmeter

To use a wattmeter, the rating of its potential coil and current coil must correspond to the current and voltage ratings of the circuit in which the instrument is to be used. For example, a wattmeter may be used in an ac circuit having a low power factor. The wattmeter can indicate a power value within the scale range of the instrument even though the current coil is greatly overloaded. Even if the circuit has a high power factor, the load current may be much larger than the current coil rating. However, because of a low voltage, the watt- meter pointer is on scale. Again, the voltage across the potential coil may be excessive. But the presence of a low current means that the power indication of the wattmeter is on scale. A wattmeter is always rated according to its potential and current coil ratings rather than in watts.

Figure 11–14 shows a voltmeter, an ammeter, and a wattmeter connected into a single-phase circuit. The voltmeter and ammeter readings will show whether the voltage or current rating of the wattmeter is exceeded. Dynamometer type wattmeters are very

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expensive to produce because they must contain a stationary coil for measuring current and a moving coil for measuring voltage. Electronic type wattmeters are rapidly replacing the dynamometer type. Electronic wattmeters employ an electronic circuit to measure the quantities of voltage and current, and supply this information to a common meter movement

or digital readout. Electronic wattmeters are connected into the circuit in the same manner as a dynamometer type wattmeter.