Three-Phase Transformer Circuits

Since three-phase is used so often for power distribution systems, it makes sense that we would need three-phase transformers to be able to step voltages up or down. This is only partially true, as regular single-phase transformers can be ganged together to transform power between two three-phase systems in a variety of configurations, eliminating the requirement for a special three-phase transformer. However, special three-phase transformers are built for those tasks, and are able to perform with less material requirement, less size, and less weight from their modular counterparts.

Experiment-Basic Concept and Test Equipment-chapter-8

 Experiment-Circuit with a Switch

PARTS AND MATERIALS

• 6-volt battery
• Low-voltage incandescent lamp (Radio Shack catalog # 272-1130 or equivalent)
• Long lengths of wire, 22-gauge or larger
• Household light switch (these are readily available at any hardware store)

Switch

Household light switches are a bargain for students of basic electricity. They are readily available, very inexpensive, and almost impossible to damage with battery power. Do not get "dimmer" switches, just the simple on-off "toggle" variety used for ordinary household wall-mounted light controls.

CROSS-REFERENCES

Lessons In  "Basic Concepts of Electricity"

LEARNING OBJECTIVES

Ammeter design

Ammeter design

Ammeter

A meter designed to measure electrical current is popularly called an "ammeter" because the unit of measurement is "amps."

Taking the same meter movement as the voltmeter example, we can see that it would make a very limited instrument by itself, full-scale deflection occurring at only 1 mA:In ammeter designs, external resistors added to extend the usable range of the movement are connected in parallel with the movement rather than in series as is the case for voltmeters. This is because we want to divide the measured current, not the measured voltage, going to the movement, and because current divider circuits are always formed by parallel resistances.

Experiment-Basic Concept and Test Equipment-chapter-7

 Experiment-Power Dissipation

PARTS AND MATERIALS

• Calculator (or pencil and paper for doing arithmetic)
• 6 volt battery
• Two 1/4 watt resistors: 10 Ω and 330 Ω.
• Small thermometer

The resistor values need not be exact, but within five percent of the figures specified (+/- 0.5 Ω for the 10 Ω resistor; +/- 16.5 Ω for the 330 Ω resistor). Color codes for 5% tolerance 10 Ω and 330 Ω resistors are as follows: Brown, Black, Black, Gold (10, +/- 5%), and Orange, Orange, Brown, Gold (330, +/- 5%).

Do not use any battery size other than 6 volts for this experiment.

The thermometer should be as small as possible, to facilitate rapid detection of heat produced by the resistor. I recommend a medical thermometer, the type used to take body temperature.

CROSS-REFERENCES

Voltmeter design

Voltmeter design

Voltmeter

As was stated earlier, most meter movements are sensitive devices. Some D'Arsonval movements have full-scale deflection current ratings as little as 50 μA, with an (internal) wire resistance of less than 1000 Ω. This makes for a voltmeter with a full-scale rating of only 50 millivolts (50 μA X 1000 Ω)! In order to build voltmeters with practical (higher voltage) scales from such sensitive movements, we need to find some way to reduce the measured quantity of voltage down to a level the movement can handle.

Let's start our example problems with a D'Arsonval meter movement having a full-scale deflection rating of 1 mA and a coil resistance of 500 Ω:

Experiment-Basic Concept and Test Equipment-chapter-6

 Experiment-Nonlinear resistance

PARTS AND MATERIALS

• Calculator (or pencil and paper for doing arithmetic)
• 6-volt battery
• Low-voltage incandescent lamp (Radio Shack catalog # 272-1130 or equivalent)

CROSS-REFERENCES

 "Ohm's Law"

LEARNING OBJECTIVES

• Voltmeter use
• Ammeter use
• Ohmmeter use
• Use of Ohm's Law
• Realization that some resistances are unstable!
• Scientific method

SCHEMATIC DIAGRAM

Experiment-Basic Concept and Test Equipment-chapter-5

 Experiment-Ohm's Law

PARTS AND MATERIALS

• Calculator (or pencil and paper for doing arithmetic)
• 6-volt battery
• Assortment of resistors between 1 KΩ and 100 kΩ in value

I'm purposely restricting the resistance values between 1 kΩ and 100 kΩ for the sake of obtaining accurate voltage and current readings with your meter. With very low resistance values, the internal resistance of the ammeter has a significant impact on measurement accuracy. Very high resistance values can cause problems for voltage measurement, the internal resistance of the voltmeter substantially changing circuit resistance when it is connected in parallel with a high-value resistor.

At the recommended resistance values, there will still be a small amount of measurement error due to the "impact" of the meter, but not enough to cause serious disagreement with calculated values.