Lab 8 - Electric Transformer

Transcription

1 Lab 8 - Electric Transformer Safety and Equipment No special safety precautions are necessary for this lab. Computer with PASCO 850 Universal Interface and PASCO Capstone Magnetic Coil and Core Set 100 Ω resistor PASPort 2-Axis Magnetic Field Sensor PASCO Voltage Sensor (DIN to Banana cable) 2 sets of Banana wires One pair of slip-on Alligator clips Introduction A device designed to increase or decrease AC voltages is called an electric transformer. The design of a transformer is based upon a principle of mutual electromagnetic induction. Two coils share the same magnetic field, but they are not electrically connected to each other. When an AC voltage is applied to the one coil (called the primary coil), the resulting alternating current in the primary coil produces a changing magnetic field. The other coil (the secondary coil) thus has a changing magnetic flux. This, in turn, induces an AC voltage in the secondary coil. A core made of a ferrous material such as iron is used in transformers to increase the magnetic flux that influences the secondary coil. This also helps make sure the amounts of flux in the two coils are equal. According to Faraday s Law of Induction, the induced emf (voltage) is proportional to the rate of change of magnetic flux through each loop of the coil (ΔΦ/Δt) and the number of turns (N) in the coil: Specifically, for each coil: E = N Φ B E p = N p Φ B and E s = N s Φ B Since the rate of change in flux through single loop of each coil are approximately the same, E S E P = N S N P = Turns Ratio In a step-up transformer, the number of turns of wire in the secondary coil is more than the number of turns in the primary coil, which makes the voltage induced in the secondary coil greater than the voltage in the primary coil. If the number of turns in the secondary coil is less than the number of turns in the primary coil, the voltage will be reduced. A transformer set up that way is referred to as a step-down transformer. Another important aspect of the transformer is that it cannot create energy. This means the power coming into the primary must equal the power supplied by the secondary. E S P P = P S E P I P = E S I S = I P E P I S

2 Objectives: To verify the relationship of the voltage, current, and number of turns in the secondary and primary coils of a transformer. Part #1: Effect of the Iron Core 1. Open Electric Transformer file (distributed alongside the Lab Instructions). 2. Connect the PASPort Magnetic Field Sensor into PASPort 1 3. Connect 200-turn coil to Output 1 (Red/Black terminals) of the 850 Universal Interface using banana wires. 4. The Magnetic Field Strength tab in Capstone is set up so that Output 1 provides a DC voltage with a maximum current of around 0.55 A. This is ideal for generating measurable magnetic fields without damaging our coils. The voltage is running whenever data is being recorded. 5. Press Record to start a measurement. Tare the sensor while it is away from the coil. 6. Without an iron core in the coil, insert Magnetic Field Sensor into the coil and move it around to find the maximum magnetic field. Record that field and the amount of current that is flowing. This is the measurement of the field strength with an air core. (Note: If the Total Magnetic Field calculation isn t working, you ll have to manually combine the x- and y-components using the Pythagorean Theorem.) 7. Place the coil onto the iron core frame, but don t bolt down the iron frame. Lift up one side of the iron core frame and hold the sensor in the gap that is formed (Figure 1). This allows the magnetic field sensor to approximately measure the magnetic field in the iron core. Record that field and the amount of current that is flowing. This is the measurement of the field strength with an iron core. 8. Repeat the measurements for the 800-turn coil. 9. Describe how the number of turns in the coil and the core inside the coil affect the magnetic field. 200-turn, Air Core 200-turn, Iron Core 800-turn, Air Core 800-turn, Iron Core Current (A) Magnetic Field( T) Table 1. Transformer coil characteristics. (a) (b) Figure 1. Methods of (a) measuring the magnetic field without an iron core and (b) approximately measuring the magnetic field of a coil with an iron core. The core is lifted up a little bit so that the magnetic field sensor can get in between the iron top piece and the iron post.

3 Part #2: Step-Down Transformer 1. Assemble a transformer by placing the 800-turn and 200-turn coils on the iron frame, and bolt down the frame so it doesn t come apart on you. (See Figure 2 on the next page.) 2. For a step-down transformer, use the coil with larger number of turns as the primary. 3. Connect the primary to the voltage source, Output 1 of the 850 Universal Interface (Red/Black banana jacks). 4. Connect the secondary to the 100 Ohm load resistor and to the voltage sensor, in parallel. 5. In Capstone, move to next tab, labeled Step-Down Transformer. This page is set up so that Output 1 is an AC voltage, because transformers only work with AC. 6. Start recording. After a second or two, click Stop. 7. Adjust the axes of the displays so you could see three or four oscillations across the screen. 8. Using the Coordinate Tool, measure the peak voltage of the primary coil, V p. 9. Using the Coordinate Tool, measure the peak voltage of the secondary coil, V s. 10. Using the Coordinate Tool, measure the peak current of the primary coil, I p. 11. Calculate the peak current in the secondary coil, knowing you have a 100 Ohm resistor. 12. Calculate the RMS values of the voltages and currents, and use them to calculate the average power on each side of the transformer. 13. Calculate the ratios in each row (secondary / primary). Convert just the power ratio to a percent, because it is the efficiency of the transformer. Step - Down Transformer Primary Secondary Ratio Turns Peak Voltage (V) Peak Current (A) RMS Voltage (V) RMS Current (A) Average Power (W) Table 2. Measured and Calculated parameters of Step-Down Transformer. %

4 Figure 2. Coils set up as a transformer. The transformer s primary ( in ) is hooked to the Output (Red/Black jacks) of the PASCO Universal Interface, and the secondary ( out ) is hooked to the load. (Image adapted from PASCO instructions.) Part #3: Step-Up Transformer 1. Switch the primary and secondary coil connections, so that the coil with fewer turns is the primary. 2. Switch to the Step-Up Transformer page in Capstone. This page is set up so that Output 1 is a small AC voltage that can be amplified by the transformer. 3. Repeat the measurements of the previous Part, and use a similar Table to present the results. Part #4: Isolation Transformer 1. For this part, use the two 400-turn coils, and repeat the transformer measurements and calculations.

5 Optional Tactile Activity Here s something interesting to try; you don t have to report on this in your Lab Report. Take the bolt out of the transformer. Switch back to the Magnetic Field Strength tab in Capstone. Start the Monitor, and try to lift up on the top piece of the iron core. You should be able to feel the DC magnetic field make the top piece to stick to the lower piece. If you Stop the measurement, you should notice the top piece no longer sticks. Switch to the Step-Up Transformer tab in Capstone. Lift up slightly on the top piece of the iron core. If you re holding it just right, so it barely touches the lower part of the iron core, you should be able to feel the AC magnetic field. While operating the Step-Up Transformer, repeatedly remove and replace the top piece of the iron core. Watch the data on the screen. What happens to the primary voltage? Primary current? Secondary voltage? (If the Monitor is running too long (more than 10 seconds or so), you may have to stop and restart it to see the shift in the data.) Data Analysis 1. From Part #1, describe how the number of turns in the coil and the core inside the coil affect the magnetic field. 2. From Parts #2-4, discuss how well the calculated ratios follow the rules for an ideal transformer.