A transformer is the quiet workhorse of the power system. It has no moving parts, yet it changes voltage up and down, from the power plant to the pole outside your home. Without it, long-distance power transmission would be wasteful and your 120-volt outlets would be impossible. This post explains how it works, how step-up and step-down types differ, and what the grey can on the utility pole does.
How Does a Transformer Work?
A transformer works by electromagnetic induction, and it works only with alternating current (AC). It has two coils of wire, called windings, wrapped around a core of laminated steel sheets. The primary winding receives power, and the secondary winding delivers it. When AC flows in the primary, it creates a changing magnetic field in the core, and that changing field induces a voltage in the secondary, even though the two windings never touch. A transformer cannot work on direct current (DC): a steady DC current makes a steady field, which induces nothing.
Diagram: an ideal transformer. The voltages follow the turns ratio, and the currents follow it in reverse.
The Turns Ratio Rule
The whole behaviour of a transformer comes from one rule. The voltage ratio equals the turns ratio:
Vp / Vs = Np / Ns
Here Vp and Vs are the primary and secondary voltages, and Np and Ns are the numbers of turns in the two windings. More secondary turns means the voltage goes up; fewer turns means it goes down. In an ideal transformer, power in equals power out, so the current changes in the opposite way: when voltage goes up, current goes down. That is the rule Is / Ip = Np / Ns. Real transformers lose a little power as heat in the copper and the core, but large power transformers are still efficient, often above 98 percent.
Step-Up Transformers
A step-up transformer has more turns on the secondary than on the primary, so it raises the voltage and lowers the current. You find them at power plants: a generator may produce electricity at about 13,800 volts, and the step-up transformer raises it to 115,000, 230,000 or even 765,000 volts for transmission. High voltage means low current, and low current means far smaller energy losses in the lines. These are large, oil-filled units.
Step-Down Transformers and the Pole Pig
A step-down transformer has fewer turns on the secondary, so it lowers the voltage and raises the current. You meet it twice on the way home: first in the substation, where transmission voltage is reduced to distribution voltage, typically 4 to 35 kV, and then on your street. The pole-mounted distribution transformer, nicknamed a pole pig, is the final step. In the United States it usually takes about 7,200 volts on the primary and delivers 120/240-volt split-phase power. Common residential sizes are 25 kVA and 50 kVA, and one pole transformer often serves a few homes. Larger buildings use pad-mounted transformers, the green metal boxes on concrete pads, for three-phase services such as 120/208 or 277/480 volts.
Cutaway of a distribution transformer, with wound coils in insulating oil and bushings on top.
What Is Inside a Distribution Transformer?
Inside the tank are the steel core and the two windings, the primary of fine wire with many turns and the secondary of thicker wire with fewer turns. The tank is filled with mineral oil, which insulates the windings and carries heat away to the cooling fins. The porcelain bushings on the lid let the high-voltage wires pass safely through the metal tank.
Key Takeaways
- A transformer transfers energy by induction between two windings on a steel core, and works only on AC.
- Voltage ratio equals turns ratio: Vp / Vs = Np / Ns. More secondary turns means step-up; fewer means step-down.
- Step-up transformers raise voltage for transmission; step-down transformers lower it for use.
- A US pole transformer typically steps about 7,200 volts down to 120/240-volt split-phase power.
Objective Questions
1. How does a transformer work?
a) By moving parts spinning inside it
b) By electromagnetic induction between two windings on a steel core
c) By a battery inside it
d) By a motor driving a generator
Answer: (b) By electromagnetic induction between two windings on a steel core
Explanation: AC in the primary winding creates a changing magnetic field in the core, and that changing field induces a voltage in the secondary winding, even though the two windings never touch.
2. What is the turns ratio rule of a transformer?
a) Vp / Vs = Np / Ns
b) Vp / Vs = Ns / Np
c) Vp x Vs = Np x Ns
d) Vp + Vs = Np + Ns
Answer: (a) Vp / Vs = Np / Ns
Explanation: The voltage ratio equals the turns ratio, where Vp and Vs are the primary and secondary voltages and Np and Ns are the numbers of turns in the two windings.
3. A transformer has 720 primary turns and 24 secondary turns, with 7,200 volts on the primary. What is the secondary voltage?
a) 24 volts
b) 120 volts
c) 240 volts
d) 7,200 volts
Answer: (c) 240 volts
Explanation: Vs = 7,200 x 24 / 720 = 240 volts, a step-down transformer with a 30 to 1 ratio.
4. In the United States, what does a pole transformer usually step down to a home?
a) 480 volts three-phase
b) 7,200 volts
c) About 7,200 volts on the primary to 120/240-volt split-phase power
d) 12 volts DC
Answer: (c) About 7,200 volts on the primary to 120/240-volt split-phase power
Explanation: In the United States a pole transformer usually takes about 7,200 volts on the primary and delivers 120/240-volt split-phase power.
5. What is inside a distribution transformer, and what does the oil do?
a) Only an iron core, cooled by air
b) A steel core and two windings in a tank of mineral oil, which insulates the windings and carries heat away to the cooling fins
c) A battery and a motor
d) A single winding in water
Answer: (b) A steel core and two windings in a tank of mineral oil, which insulates the windings and carries heat away to the cooling fins
Explanation: Inside the tank are the steel core and the two windings, and the tank is filled with mineral oil, which insulates the windings and carries heat away to the cooling fins.
0 comments:
Post a Comment