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Home , Multimeter, Wattmeter

Experiment 1 Basic concept

Objective  To introduce the , the power supply.  To learn to properly use the lab instruments and the correct method of measuring electrical quantities with each instrument.

Basic Information

The Digital Multimeter [DMM] This devise is used to measure values of electrical quantities; such as , current, resistance, etc. The DMM is easy to use, and necessary for all labs.

Voltage measurement

Turn on the multimeter. Using the rotary selector switch, select the voltage function [VDC]. Select the AUTO range mode by making a long press on the range button. Insert the positive (+) (normally red) in the voltage socket and the negative (-) lead (normally black) in the common socket. Place the red probe on the higher voltage point and the black probe on the lower voltage point. The DMM will display the between the probe tips. Voltage measurement between any two points is made in parallel with the components between those two points. If the probes are reversed the reading will be negative of the original value. A has very large internal resistance, which is considered as open circuit (O.C ) during calculations.

Current measurement

Turn on the DMM and select the current function. Place the positive (+) probe in the current socket and the negative (-) one in the common socket. Select AUTO range mode. Connect the tips of the probes in series with the component through which the current is being measured. A positive reading will indicate current direction from the positive (+) to the negative (-) probes. Current measurement through a component is made in series with that component. An has very small internal resistance, which is treated as short circuit (S.C ) during calculations.

The Figure (1-1) shows the connection of the DMM as a voltmeter and an ammeter.

Fig. (1-1): Ammeter and voltmeter connections.

1 Electrical |Dr. Jasem Ghaeb, Eng. Samer Salah

Important Note: Always disconnect the probes of the meter from the circuit before changing the selector switch from current to voltage or vise versa. Failing to do so will damage the meter. Switching off the meter without disconnecting the probes is insufficient for protecting the meter. Connecting the multimeter in an incorrect way, or choosing the wrong selection of switches, may result in personal injury, damage to the multimeter and/or the lab equipment. Observe and obey safety rules and instructions at all times. If in doubt ask your instructor.

Resistance measurement

The part of a multimeter is basically a voltmeter and ammeter. A built-in voltage source is connected across the to be measured and an ammeter measures the current flow. The resistance value is the ratio of voltage to current flow. Resistance should never be measured while it is connected in a circuit. To measure the resistance of a component: switch off the power from the circuit. Disconnect the component from the circuit. Switch the multimeter to measure resistance, and select the AUTO range mode. Touch the probe tips to the end of the ends of the component, and read the value displayed.

Power measurement ()

Electric power is measured by means of a wattmeter. This instrument is of the electrodynamics’ type. It consists of a pair of fixed coils, known as current coils, and a movable coil known as the potential coil. (See Fig. 1-2.) The fixed coils are made up of a few turns of a comparatively large conductor. The potential coil consists of many turns of fine wire. It is mounted on a shaft, carried in jeweled bearings, so that it may turn inside the stationary coils. The movable coil carries a needle which moves over a suitably marked scale. Spiral coil springs hold the needle to a zero position.

Fig. (1-2): A simplified electrodynamics’ wattmeter circuit.

When the current passes through the current coil, it creates an around the coil. The strength of this electromagnetic field is directly proportional to the amount of current passing through it. In case of DC current, the current is also in phase with its generated electromagnetic field. The voltage is dropped across the potential coil and as a result of this complete process, the needle moves across the scale. The needle deflection is such that it is according to the product of the current passing and the voltage dropped, that is, P = VI. The wattmeter simply measures the average power in case if AC power is required.

2 Electrical Machine Laboratory |Dr. Jasem Ghaeb, Eng. Samer Salah

Equipment

 Power supply & loads.  Digital multimeter.  Analog wattmeter.

Procedure :( Measure the following): a) For the fixed voltage 3-phase terminals:

Table (1) Case Value () VL1-2 VL1-3 VL2-3 VL1-N VL2-N VL3-N b) For the variable voltage 3-phase terminals

Table (2) Case Value (volt) VL1-3 * 20% VL1-3 * 80% VL1-N * 20% VL1-N * 80% c) Resistance box measurement:

Table (3) Step Value () 0 1 2 3 4 5 6 7

3 Electrical Machine Laboratory |Dr. Jasem Ghaeb, Eng. Samer Salah

d) For the following circuit, measure the required .

Figure (1-3)

Table (4) Parameter Measured value (Volt) Vs V1 V2 e) For the following circuit, measure the required currents.

Figure (1-4)

Table (5) Parameter Measured value (Volt) is i1 i2

4 Electrical Machine Laboratory |Dr. Jasem Ghaeb, Eng. Samer Salah

f) Alternate the existing magnet inside the coils and fill the table below.

Figure (1-5)

Table (5) Components value (mH) Generated voltage(mV) Coil one Coil two

Questions:

1. Why the measured voltage in part (d) decreased when applied load?

2. What is the equivalent resistance of two (R1=10 Ohm, R2=10Ohm) if:

A. R1 and R2 are connected in series. B. R1 and R2 are connected in parallel.

3. What are the two required parameters to produce EMF in the electrical machine?

5 Electrical Machine Laboratory |Dr. Jasem Ghaeb, Eng. Samer Salah