Sunday, January 11, 2015

AC Instruments and Meters : Clamp-on ammeters and The oscilloscope.

CLAMP-ON AMMETERS

Another meter frequently used is the clamp-on ammeter (Figure 11–38). Most of these meters operate on the current transformer principle. The movable jaw is the core of a cur- rent transformer. The range selection switch connects the meter to different taps on the transformer (Figure 11–39). The conductor around which the movable jaw is connected forms a one-turn primary for the transformer. The secondary is the winding around the iron core. The movable tap changes the turns ratio of the transformer.

If the current is too low to be measured easily, extra turns of wire can be wrapped around the movable jaw. Each turn of wire increases the scale factor of the meter. For example, if two turns of wire are wrapped around the movable jaw, the ammeter reading

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will double. To determine the actual amount of current in the circuit, it would be necessary to divide the reading by 2. If 10 turns of wire are wrapped around the movable jaw, the reading would have to be divided by 10.

The advantage of the clamp-on ammeter is that the circuit does not have to be broken to make measurements. This is a great advantage when it is necessary to check the current draw of a piece of operating equipment.

THE OSCILLOSCOPE

Many of the electronic control systems in today’s industry produce voltage pulses that are meaningless to a volt-ohm-milliammeter (VOM). In many instances, it is necessary to know not only the amount of voltage present at a particular point but also the length or duration of the pulse and its frequency. Some pulses may be less than one volt and last for only

a millisecond. A VOM would be useless for measuring many of these things. It is therefore necessary to use an oscilloscope to learn what is actually happening in the circuit.

The oscilloscope is a powerful tool and will perform many jobs that will not be dis- cussed in this text. The first thing to understand about an oscilloscope is that it is a volt- meter. It does not measure current, resistance, or watts. The oscilloscope not only measures the voltage, it draws a picture of it. What the oscilloscope actually does is measure a volt- age during a particular period of time, creating a two-dimensional image.

Voltage Range Selection

The oscilloscope is divided into two main sections. One section is the voltage section, and the other is the time base. The display of the oscilloscope is divided by vertical and horizontal lines (Figure 11–40). Voltage is measured on the vertical, or Y, axis of the display, and time is measured on the X axis. When using a VOM, a range selection switch is used to determine the full-scale value of the meter. Ranges of 600, 300, 60, and 12 V are common. Having the ability to change ranges permits more accurate measurements to be made. In addition, the oscilloscope has a voltage range selection switch (Figure 11–41). The voltage range-selection switch on an oscilloscope selects volts per division instead of volts full scale. The voltage range switch shown in Figure 11–41 is set for 10 m at the IX position. This means that each of the lines in the vertical direction or on the Y axis of the display has a value of 10 mV. Assume that the oscilloscope has been adjusted to permit 0 V to be.

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shown on the centerline of the display. If the oscilloscope probe is connected to a positive voltage of 30 mV, the trace would rise to the position shown in Figure 11–42A. If the probe is connected to a negative 30 mV, the trace will fall to the position shown in Figure 11–42B. Note that the oscilloscope has the ability to display both positive and negative voltages. If the range switch is changed to 20 V per division, Figure 11–42A would be displaying 60 V positive.

The Time Base

The next section of the oscilloscope to become familiar with is the time base (Figure 11–43). The time base is calibrated in seconds per division and has range values from seconds to microseconds. The time base controls the value of the division of the lines in the horizontal direction. If the time base is set for 5 ms per division, the trace will sweep from one division to the division beside it in 5 ms. With the time base set in this position, it will take 50 ms to sweep from one side of the display screen to the other. If the time base is set for 2 µs per division, the trace will sweep the screen in 20 µs.

Measuring Frequency

Because the oscilloscope has the ability to measure the voltage with respect to time, it is possible to compute the frequency of the waveform. The frequency of an ac waveform can be found by dividing the time it takes to complete one cycle into one (1/f ). For example,

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assume that the time base is set for 0.5 ms per division and the voltage range is set for 20 V per division. If the oscilloscope has been set so that the centerline of the display is 0 V, the ac waveform shown in Figure 11–44 has a peak value of 60 V. The oscilloscope displays the peak or peak-to-peak value of voltage and not the RMS or effective value. To measure the frequency, count the time it takes to complete one full cycle. The waveform shown in Figure 11–44 takes 4 ms to complete one full cycle. The frequency is, therefore, 250 Hz (1/0.004 = 250).

Attenuated Probes

Most oscilloscopes use a probe that acts as an attenuator. An attenuator is a device that divides or makes smaller the input signal (Figure 11–45). An attenuated probe is used to permit higher voltage readings than are normally possible. For example, most attenuated probes are 10 to 1. This means if the voltage range switch is set for 5 V per division, the display would actually indicate 50 V per division. If the voltage range switch is set for 2 V per division, each division on the display actually has a value of 20 V per division.

Probe attenuators are made in different styles by different manufacturers. In some probes the attenuator is located in the probe head itself, whereas in others the attenuator is located at the scope input. Regardless of the type of attenuated probe used, it may have to be compensated or adjusted. In fact, probe compensation should be checked frequently. Different manufacturers use different methods for compensating their probes, so it is generally necessary to follow the procedures given in the operator’s manual for the oscilloscope being used..

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Oscilloscope Controls

The following is a list of common controls found on the oscilloscope. Refer to the oscilloscope shown in Figure 11–46.

1. Power. The power switch is used to turn the oscilloscope on or off.

2. Beam find. This control is used to locate the position of the trace if it is off the display. The beam finder button will indicate the approximate location of the trace. The position controls are then used to move the trace back on the display.

3. Probe adjust. This is a reference voltage point used to compensate the probe. Most probe adjust points produce a square-wave signal of about 0.5 V.

4. Intensity and focus. The intensity control adjusts the brightness of the trace.

A bright spot should never be left on the display because it will burn a spot on the face of the cathode-ray tube (CRT). This burned spot results in permanent damage to the CRT. The focus control sharpens the image of the trace.

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5. Vertical position. This control is used to adjust the trace up or down on the display.

If a dual-trace oscilloscope is being used, there will be two vertical position controls. (A dual-trace oscilloscope contains two separate traces, which can be used separately or together.)

6. Ch 1–both–Ch 2. This control determines which channel of a dual-trace oscillo- scope is to be used, or whether they are to both be used at the same time.

7. Add–Alt–Chop. This control is active only when both traces are being displayed at the same time. The add adds the two waves together. Alt stands for alternate. This alternates the sweep between channel 1 and channel 2. The chop mode alternates several times during one sweep. This makes the display appear more stable. The chop mode is generally used when displaying two traces at the same time.

8. AC–Grd–DC. The ac is used to block any dc voltage when only the ac portion of the voltage is to be seen. For instance, assume an ac voltage of a few millivolts to be riding on a dc voltage of several hundred volts. If the voltage range is set high enough so that 100 V dc can be seen on the display, the ac voltage could not be seen. The ac section of this switch inserts a capacitor in series with the probe. The capacitor blocks the dc voltage and permits the ac voltage to pass. Because the 100 V dc has been blocked, the voltage range can be adjusted for millivolts per division, which will permit the ac signal to be seen.

The Grd section of the switch stands for ground. This section grounds the input so the sweep can be adjusted for 0 V at any position on the display. The ground switch grounds at the scope and does not ground the probe. This permits the ground switch to be used when the probe is connected to a live circuit. The dc section permits the oscilloscope to display all of the voltage, both ac and dc, connected to the probe.

9. Horizontal position. This control adjusts the position of the trace from left to right.

10. Auto–normal. This control determines whether the time base will be triggered auto- matically or whether it is to be operated in a free-running mode. If this control is oper- ated in the normal setting, the trigger signal is taken from the line to which the probe is connected. The scope is generally operated with the trigger set in the automatic position.

11. Level. The level control determines the amplitude the signal must reach before the scope triggers.

12. Slope. The slope permits selection as to whether the trace is triggered by a negative or positive waveform.

13. Int–Line–Ext. The Int stands for internal. The scope is generally operated in this mode. In this setting, the trigger signal is provided by the scope. In the line mode, the trigger signal is provided from a sample of the line. The Ext, or external, mode permits the trigger pulse to be applied from an external source.

These are not all the controls shown on the oscilloscope in Figure 11–46, but they are the major controls. Most oscilloscopes contain these controls.

Interpreting Waveforms

Being able to interpret the waveforms on the display of the oscilloscope takes time and practice. When using the oscilloscope, it must be kept in mind that the display shows the voltage with respect to time.

In Figure 11–47, it is assumed that the voltage range has been set for 0.5 V per division, and the time base is set for 2 ms per division. It is also assumed that 0 V has been set

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on the centerline of the display. The waveform shown is a square wave. The display shows the voltage rises in the positive direction to a value of 1.4 V and remains there for 2 ms. The voltage then drops to 1.4 V negative and remains there for 2 ms before going back to positive. Because the voltage changes between positive and negative, it is an ac voltage. The length of one cycle is 4 ms. The frequency is, therefore, 250 Hz (1/0.004 = 250).

In Figure 11–48, the oscilloscope has been set for 50 mV per division and 20 µs per division. The display shows a voltage that is negative to the probe’s ground lead and has a peak value of 150 mV. The waveform lasts for 20 µs, which produces a frequency of 50 kHz (1/0.000020 = 50,000). The voltage is dc because it never crosses the zero reference and goes in the positive direction.

In Figure 11–49, assume that the scope is set for 50 V per division and 0.1 ms per division. The waveform shown rises to a value of 150 V in the positive direction and then drops to about 25 V. The voltage remains at 25 V for 0.15 ms and drops back to 0 V. The voltage remains at 0 for 0.3 ms before the cycle starts over again. The voltage shown is dc because it remains in the positive direction. To compute the frequency, measure from the beginning of one wave to the beginning of the next wave. This is the period of one complete cycle. In this case the length of one cycle is 0.6 ms. The frequency is, therefore, 1666 Hz (1/0.0006 = 1666).

Learning to interpret the waveforms seen on the display of an oscilloscope will take time and practice, but it is well worth the effort. The oscilloscope is the only means by which many of the waveforms and voltages found in electronic circuits can be understood.

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SUMMARY

• Instruments used to make ac measurements must have three basic components:

1. A spring mechanism to produce an opposing torque

2. A restoring spring mechanism to restore the pointer to a zero reading

3. A damping system to prevent excessive swinging of the pointer

• The selection of an instrument for a particular use is usually based on its ability to

1. indicate RMS or average values, as required.

2. respond within a certain range of volts or amperes.

3. provide linear calibration in the useful range of the scale.

4. indicate values with the desired accuracy.

5. make measurements at a reasonable cost.

DC d’Arsonval movement with a rectifier:

1. can be used for ac measurements.

2. develops a torque proportional to the average value of the current in the moving field; the rectifier makes use of the RMS voltage (1.11 times the average voltage value of a sine wave).

3. is accurate only when pure sine-wave quantities are involved.

4. is used in most multimeters.

a. A multimeter used as a voltmeter has a high-resistance linear scale and uses relatively little power.

b. A multimeter used as an ammeter measures in microamperes or milliamperes.

Magnetic vane attraction movement has a soft iron plunger that projects into a stationary field coil. Also:

1. Current in the field coil produces a magnetic force that pulls the plunger deeper into the coil.

2. The instantaneous value of this force is proportional to the square of the RMS current value in the coil.

3. The torque developed in turning the movement is independent of current direction; thus, the instrument can be used for ac or dc measurements.

4. The scale numbers are crowded at the lower end and expanded for the high end of the scale.

5. This instrument is commonly used in low-cost ammeters.

6. This instrument can be used to make voltage measurements by adding a coil of many turns and a series resistor.

Inclined coil movement is used in portable and switchboard ammeters and voltmeters.

In addition

1. The scale is long and reasonably linear.

2. An iron vane is free to move in a magnetic field.

3. The vane tends to take a position parallel to the flux.

4. Increasing the current in the field coil increases the force aligning the vanes with the coil flux; thus, the shaft turns and moves the attached pointer across the scale.

Repulsion movement employs two soft iron vanes; one vane is attached to the instrument shaft and the other is mounted on the stationary field coil. Also:

1. A repulsion force is developed between the two soft iron vanes because both are affected by the same magnetic field.

2. With no current through the field coil, a control spring holds the movable vane close to the fixed vane.

3. As current increases through the field coil, the like poles of the vanes repel each other and create a torque that turns the shaft.

4. The movement has a reasonably uniform scale and can be used for both current and voltage measurements.

Repulsion–attraction movement produces a greater torque than do most other movements. The following features also apply:

1. The movable vane is attached to the pointer and is repelled from the wide end of the middle fixed vane as the current through the field coils is increased.

2. The repelling force decreases as the movable vane moves to the narrow end of the middle fixed vane; an attraction force increases as the moving vane comes closer to the upper and lower fixed attraction vanes.

3. A scale length representing 250° of angular deflection can be provided.

4. The distribution of values (volts or amperes) along the scale is determined by the shape and separation of the vanes.

5. Meters can be designed to broaden the scale at any point.

Dynamometer movement produces a torque by the interaction of magnetic fields; this torque varies with the amount of current in the stationary coils. Both the fixed and moving coils are connected in series. Dynamometer movement

1. can be adapted easily to make voltage measurements by adding the proper series resistor.

2. is more efficient and accurate than d’Arsonval movement.

3. is seldom used as an ammeter for the following reasons:

a. The lead-in spirals to the moving coil can carry only a limited current.

b. Frequency variations introduce error.

c. The resistance of the two coils in series may produce an undesirably high voltage drop across the shunt.

4. is used for instruments measuring power, power factor, and frequency, and to indicate synchronism in ac circuits.

• Practical ranges for moving iron instruments:

1. When the instrument is used for ammeters, its physical size is influenced by the amount of heat to be dissipated and the size of the connection terminals to be supported.

a. For small panel instruments, 100 A is the maximum practical current rating.

b. For large portable instruments, 200 A is the maximum rating.

c. Some large ammeters for switchboard use are rated as high as 600 A.

d. Shunts are not satisfactory for moving iron instruments.

e. It is standard practice to use an instrument current transformer to increase the current range.

2. When used as ac voltmeters

a. series resistors are used to extend the scale range for voltages up to 750 V.

b. instrument potential transformers are used with the movement for voltages greater than 750 V; in many cases, the instrument scale is calibrated to indi- cate the primary voltage directly.

3. When used as a wattmeter

a. the stationary field coils are connected in series with the line so that the flux depends on the current.

b. the moving coils are connected across the line so that the moving coil flux is proportional to the system voltage.

c. the instantaneous torque is proportional to the instantaneous moving coil voltage.

d. pointer deflection is proportional to the power:

Watts = VI cos 8

e. the power curve at any instant is positive.

Also, when using an instrument with a dynamometer movement, the technician must consider the instantaneous direction in each of the coils. There are two dif- ferent methods of connecting the potential coils of a wattmeter.

a. The potential coil is not connected directly across the load:

True power = wattmeter reading - I2 R of current coil

b. The potential coil is connected across the load voltage:

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For this second method, the percentage of error is slightly less as compared to the first method.

A wattmeter is always rated according to its potential and current coil ratings rather than in watts.

4. When used as a varmeter,

a. it indicates the product of the circuit voltage and the current component 90° out of phase with the voltage; that is, it measures the reactive volt- amperes.

b. the varmeter reads VI cos (8 – 90) or VI sin 8; the wattmeter reads VI cos 8.

5. When used as a power factor meter,

a. a single-phase power factor meter resembles a single-phase wattmeter.

b. the stationary field coils are connected in series with one side of the line.

c. the field coils carry the line current and produce the field flux.

d. it differs from the wattmeter in that it has no control springs.

e. the flux of the moving coils reacts with that of the field coil to produce a torque proportional to the in-phase component of current.

f. the torque magnitude is determined by the amount of quadrature current.

g. the resultant of the torque due to the in-phase component of current and that due to the quadrature component determines the pointer position and shows the lagging or leading power factor value.

• Synchroscope

1. The synchroscope shows the relative phase angle and the frequency difference between two alternating voltages to indicate when two alternators are in phase.

2. This instrument also indicates whether the frequency of the incoming generator is higher or lower than that of the generator already connected to the line.

3. Both the polarized-vane type and the moving-iron type of synchroscope are commonly used.

4. These instruments are designed for operation on single-phase circuits. They may be used with three-phase generators if the phase sequences of the generators are known.

• A commonly used frequency meter is known as the resonant circuit meter.

1. The structure of this meter resembles that of the dynamometer.

2. The two field coils are alike and are connected so that their fluxes oppose each other.

3. Each field coil is connected in series with an inductor–capacitor combination.

Because of the constants of this combination,

a. series resonance occurs below the normal operating frequency in one field coil and above the normal frequency in the other field coil.

b. the current in the armature is the vector sum of the currents in the two field coils. The value of lead or lag depends on which field current is greater.

c. when the frequency is such that the leading and lagging currents are equal, the armature current is in phase with the terminal voltage.

d. the pointer movement is caused by the torque, due to the resultant flux acting on the iron vane; this torque is proportional to the product of the armature flux and the in-phase component of the resultant field flux.

• Recording instruments

1. provide a graphical record of the actual circuit conditions at any time.

2. are grouped into two broad categories:

a. Instruments that record electrical values such as volts, amperes, watts, power factor, and frequency

b. Instruments that record nonelectrical quantities such as temperature

3. use a permanent-magnet, moving-coil-type construction for dc circuits.

4. for ac circuits, may use either the moving-iron or the dynamometer-type construction.

A strip-chart recorder is the most commonly used graphical recording instrument:

1. A strip of paper 4 to 6 in. wide and up to 60 ft long is used for the permanent record.

2. The long chart means that the record can cover a considerable amount of time.

3. The chart can be operated at a relatively high speed to provide a detailed graphical record.

Some recording instruments have a spring-type clock mechanism wound by a small electric motor. This mechanism guarantees that a power failure will not stop the chart

motion until the spring runs down. Some of the larger instruments require a large amount of power for operation. The amount of power required can be reduced by using electronic amplifiers.

• The thermal converter

1. changes ac voltage and current signals into a dc signal in millivolts; this signal is proportional to the product of VI cos 8.

2. can be used to measure ac watts using a dc recording or indicating device.

The thermocouple output (dc millivolt signal) is proportional to ac power in the circuit to which the V and I inputs are attached.

• Watt-hour meter

1. The watt-hour meter determines and adds together (integrates) all of the instantaneous power values to give an indication of the total energy used over a period of time.

2. The watt-hour constant of the meter is the number of watt-hours represented by one revolution of the disc.

3. Each division on the right-hand dial = one kilowatt-hour (kWh).

4. Second dial from the right = 10 kWh.

5. The third dial from the right = 100 kWh.

6. The dial on the left = 1000 kWh.

7. The register ratio is the number of revolutions made by the first gear wheel for one revolution of the right-hand dial pointer.

8. The gear ratio is the number of revolutions made by the meter disc to cause one revolution of the right-hand dial pointer.

9. A full-load adjustment of a meter means that the proper amount of magnetic braking is provided to give the correct speed at the rated voltage and current for a unity power factor.

10. A light-load adjustment is made by moving a shading pole loop to produce a lag in the time phase of part of the potential flux; this adjustment overcomes errors due to friction and torque at light loads.

11. Holes are drilled in the disc 180° apart to overcome the slow rotation (creeping) of the disc at light loads. The disc rotates until one of the hole positions open circuits the eddy currents in the disc. The resulting distortion of the eddy currents produces a locking torque to stop the rotation of the disc.

• Digital multimeter

1. displays reading with individual digits instead of a pointer and scale.

2. has a high input impedance.

3. input impedance remains constant on all voltage ranges.

• The clamp-on ammeter:

1. Most clamp-on ammeters operate on the principle of a current transformer.

2. The range selection switch changes the ratio of the current transformer.

3. The circuit does not have to be broken to measure the current flow.

• The oscilloscope

1. measures voltage and time.

2. can be used to measure frequency.

3. displays a two-dimensional image of the voltage waveform.

4. may use an attenuated probe to permit the measurement of higher voltages.

Achievement Review

1. a. Describe the construction of a magnetic vane, attraction-type instrument movement.

b. Describe the operation of the magnetic vane, attraction-type instrument movement when used as

1. an ac ammeter.

2. an ac voltmeter.

2. Give one advantage and two disadvantages of the magnetic vane, attraction-type instrument movement.

3. Describe the operation of the inclined coil movement, when used as a voltmeter.

4. Explain the operation of a repulsion–attraction instrument movement.

5. A repulsion–attraction instrument movement has a full-scale deflection of 6 mA. The effective resistance of the coil is 3800 D. To use this instrument as a voltmeter with a full-scale deflection of 750 D, a series resistor is added. This resistor has a resistance of 120,000 D. With full-scale deflection, determine the power loss, in watts, in the

a. instrument coil.

b. series resistor.

c. entire instrument.

6. Show the connections for a dc moving coil, permanent-magnet-type movement.

Be sure to show the full-wave rectifier and series resistor used to measure the ac voltage.

7. a. If the losses in the rectifier shown in question 6 are negligible, what reading will the dc voltmeter indicate if the maximum value of the ac voltage measured is 340 V?

b. What factor is used to recalibrate the dc instrument scale to read the effective value of ac volts?

8. a. What are the advantages of using rectifier instruments?

b. What is one limitation of using a rectifier instrument to measure current?

9. Describe the operation of a dynamometer-type movement when used as an ac voltmeter.

10. List several reasons why the dynamometer-type movement is seldom used in ac ammeters.

11. Describe the construction and operation of the dynamometer-type wattmeter.

12. Why is it often more satisfactory to use a two-element wattmeter to measure power in a three-phase, three-wire system than it is to use two single-phase wattmeters?

13. a. The current coil (or the potential coil) of a wattmeter can burn out even though the instrument reading is well below the full-scale deflection. Explain why this can happen.

b. What precautions are taken when determining whether the current coil or voltage circuits are overloaded?

14. a. Show the connections for a two-element wattmeter used to measure power in a three-phase, three-wire system.

b. Describe the operation of a two-element wattmeter used to measure three- phase power.

15. How can a dynamometer instrument be used to measure the reactive power in VARs?

16. Describe the operation of

a. a single-phase power factor meter of the crossed-coil type.

b. a three-phase power factor meter of the crossed-coil type.

17. Explain the operation of a polarized-vane synchroscope.

18. Explain the operation of a resonant-type frequency meter.

19. What is the value of a recording instrument? Explain.

20. a. List the parts of a typical single-phase watt-hour meter.

b. Explain the meaning of each of these terms:

1. Register ratio

2. Gear ratio

3. Watt-hour meter disc constant

21. An analog multimeter has a scale factor of 5000 D/V. If the meter is set on the 300-V range, how much current will flow through the meter if it is connected to 240 V?

22. What is a common input impedance for a digital multimeter?

23. What is the principle of operation of most clamp-on ammeters?

24. A clamp-on ammeter has five turns of wire wrapped around its movable jaw. If the meter is indicating a current of 15 A, how much current is actually flowing in the circuit?

25. An oscilloscope indicates an ac waveform. If one cycle is completed in 4 µs, what is the frequency of the waveform?