EE Chapter 7 Measuring Instruments

Transcription

1 EE Chapter 7 Measuring Instruments 7.1 Meter Movements The basic principle of many electric instruments is that of the galvanometer. This is a device which reacts to minute electromagnetic influences caused within itself by the flow of a small amount of current. The most common types of electric measuring instruments employ a moving coil and a permanent magnet. This arrangement is known as the d Arsonval or Weston movement. The coil, consisting of fine wire, is pivoted and mounted so that it may rotate in the magnetic field of the permanent magnet's poles. When a current flows in the coil, a magnetic field is produced. The north pole of this field is repelled by the north pole of the permanent magnet and attracted by its south pole. This will cause the coil to rotate to the right. The magnetic force causing the rotation is proportional to the current flowing in the coil and is balanced against a coil spring. The result is that the distance of rotation will increase as the current flow in the coil increases. The d Arsonval or Weston movement is not suitable for the measurement of alternating current. Such current would produce rapid reversals of polarity in the moving coil that would cause the needle only to vibrate. Under these conditions, no indication could be obtained. A movement similar to the Weston movement, but suitable for a c measurements, employs an electromagnet in place of the permanent magnet. This is called a dynamometer movement. The moving coil can be connected in either series or parallel with the electromagnet circuit. When a movement of this kind is used, the indicating needle will always move in the same direction regardless of the direction of the current through the instrument. This is because the polarity of both the moving coil and the electromagnet changes when the current direction changes; hence the direction of torque (twisting force) remains the same. 1

2 As previously stated, some meters must be constructed with a high degree of sensitivity. The sensitivity is determined by the amount of current required to produce a full scale deflection of the indicating needle. Very sensitive movements may require as little as A to produce a full scale deflection. This value is commonly called 20,000 Ω/V, because it requires 20,000 Ω to limit the current to A when an emf of 1 V is applied. An example of select a voltmeter with proper sensitivity. Two voltmeters with sensitivity of 1,000 Ω/V and 20,000 Ω/V. 7.2 The Ammeter The ammeter is used to measure the current in the circuit. Milliammeters are used to measure current values in thousandths of amperes. The ammeter contains shunt resistance (external or internal shunt) for its operation. An ammeter and shunt are in parallel with each other and in series with the load. The ammeter and its shunt are designed to offer as little resistance as possible in a circuit. If the ammeter is connected in parallel with the power source, it will act as a direct short circuit and cause damage to the ammeter and to the power source. It is essential to choose a meter with a range high enough to ensure that the meter is not overloaded. 7.3 The Voltmeter The voltmeter is used to measure the voltage across any node of the circuit. A voltmeter of the moving coil type actually measures the current flow through the instrument, but since the current flow is proportional to the voltage, the instrument dial may be marked in volts. The meter movement is adapted to the measurement of voltage by the use of series resistances. 2

3 Voltmeters usually have the necessary series resistance build into the meter itself. The voltmeter is connected in parallel with the power source/loads. 7.4 The Ohmmeter An ohmmeter is an instrument to measure the resistance. The type most commonly used by aircraft mechanics and electronics technicians employs a moving coil galvanometer. To make the movement capable of measuring resistance directly, it is necessary merely to provide a source of electric power and a suitable resistance. Before the test is made, the ohmmeter should be adjusted so the scale reading is zero when the probes are connected together and infinity when the probes are separated. This is done by adjusting the meter s resistance to compensate for any variance in battery voltage. To obtain accurate indications, one must always zero an analog ohmmeter prior to use. 7.5 The Multimeter A meter with a single D Arsonval movement that can be used to measure multiple ranges of currents and voltages is called a multimeter. A common type of analog multimeter a combination voltmeter, ohmmeter and milliammeter is abbreviated VOM. The block diagram of a basic multimeter is shown below. 3

4 Moving coil type DC Voltage Measurement Microammeter Selector switch AC Voltage Measurement DC Current Measurement Resistance Measurement Test terminal Test leads The ammeter is a moving coil ammeter. It uses magnetic deflection, where current passing through a coil in a magnetic field from permanent magnet creates the torque and causes the coil to move clockwise. The distance of the coil moves is proportional to the passing DC current. DC voltage measuring circuit 10M 5M 4M 800k 150k 30k 15k 3k 50 μa 2000 Ω meter V Range switch (volts) Test terminals For DC voltage measurement, large values of resistances are connected in series with a microammeter, e.g. 50 μa, 2000 Ω, in order to limit the current passing through it. The voltage obtained can be computed from the passing current and the resistance value. The switch is at 50 V range switch, the total resistance is 800k+150k+30k+15k+3k+2k (meter resistance) = 1,000 kω. If the measuring voltage is 50 V, this results in 50 μa pass through the meter and full scale is achieved. Thus, the sensitivity of this ammeter is 1,000 kω/50 V = 20,000 Ω/V. The higher resistance of the shunt reduces the error. 4

5 AC voltage measuring circuit 3M 800k 165k 32k R1 D2 D1 R2 50 μa 2000 Ω meter Range switch (volts) Test terminals Procedures for AC voltage measurement is similar to DC voltage measurement, however, a rectifier is required in order to convert AC current to DC current before passing through a microammeter. This can be achieved by using two diodes and calibrated by two resistors in order to give RMS values correctly for 1 sinusoidal waveforms. Vrms = Vp = V p 2 DC current measuring circuit 50 μa 2000 Ω mA 1mA 1mA 10mA 50μA 100mA 50μA 100mA Test terminals For DC current measurement, low resistors are connected in parallel with a microammeter and then the current division is applied to calculate the current value. If the switch is set to 100 ma range, the whole current will pass through 2.5 Ω resistor which is connected in parallel with = Ω. If there is 100 ma current passing through test terminals, there will be a voltage drop across this 2.5 Ω resistor, causing the current of 250 mv/ Ω or about 50 μa passes through the meter. A full scale reading is achieved. 5

6 AC current measuring circuit Inductive pickup: The principle is based on electromagnetic induction. Clamp meters measure current by determining the magnetic field around a current carrying conductor. A hall effect probe is a type of inductive pickup that can be used to measure either AC or DC current. Resistance measuring circuit 50 μa 2000 Ω 10k 8k 1.5 V 16k k 53k Rx10 Rx100 Rx1000 Rx1 Test terminals For resistance measurement, a battery in a multimeter and a set of resistors are used for the measurement. When a resistor is connected, the circuit is closed and the current will flow. The resistance value can then be computed. There is a need to do zero adjust prior any measurement when the range is changed. 7.6 The Digital Multimeter Analog multimeters are not hard to find in the used market, but are not very accurate because of errors introduced in zeroing and reading the analog meter face. Modern multimeters are exclusively digital, and identified by the term digital multimeter or DMM. Current, voltage, and resistance measurements are considered standard features for DMMs. Newer equipment can measure many other quantities. Some common quantities are such as inductance, capacitance, temperature, frequency, speed and RMS values. 6

7 Some DMMs even come with software to analyze important parameters. The signal under test is converted to a digital voltage and an amplifier with an electronically controlled gain preconditions the signal. Since the digital display directly indicates a quantity as a number, there is no risk of parallax causing an error when viewing a reading. Better circuitry and electronics have improved meter accuracy. Older analog meters might have basic accuracies of five to ten percent. Modern portable DMMs may have accuracies as good as ±0.025%. The inclusion of solid state electronics has provided a wealth of convenience features in modern digital meters. Fluke digital multimeter features and specifications: Fluke 233 7

8 The NEW Fluke 233 True-rms Remote Display Digital Multimeter gives you ultimate flexibility in unusual measurement scenarios. Place the removable display where you can see it and then put the meter where it is convenient for you no more juggling of leads and the meter while stretching into a tight spot. You can now take measurements in hard-to-reach places, where machines or panels are physically separated from a limit or isolator switch, or in user prohibited areas such as clean rooms or hazardous areas. References: Textbook # 1 EE Aircraft Electricity and Electronics Asst. Prof. Thavatchai Tayjasanant, Ph.D. 8