Transformers & Power Distribution
Transformer principles, tap changers, kVA ratings, efficiency calculations, current and potential transformers, and service entrance equipment.
- Explain the operating principle of transformers and the turns ratio relationship
- Describe the function of a tap changer and its effect on output voltage
- Calculate transformer efficiency using the output/input power formula
- Identify the purpose of current transformers (CTs) and potential transformers (PTs)
- State the minimum main disconnect rating for residential service in Canada
Lección 1
Transformer Principles & Turns Ratio
How Transformers Work
A transformer transfers electrical energy from one circuit to another through electromagnetic induction - without any direct electrical connection between the circuits. The two circuits are called the primary (input) and the secondary (output).
A transformer consists of two or more windings of insulated wire wound around a common magnetic core (usually laminated steel). When AC voltage is applied to the primary winding, it creates an alternating magnetic field in the core. This changing magnetic field induces a voltage in the secondary winding.
Key Transformer Principle
Transformers only work with AC (alternating current). A steady DC voltage produces a constant magnetic field that does not induce a secondary voltage. The changing magnetic field from AC is essential for transformer operation.
The Turns Ratio
The voltage ratio between primary and secondary is directly proportional to the turns ratio:
V_primary / V_secondary = N_primary / N_secondary
Where N is the number of turns in each winding.
Step-Down Transformer
Primary: More turns than secondary
Voltage: Secondary voltage lower
Current: Secondary current higher
Example: 600V to 120V distribution
Step-Up Transformer
Primary: Fewer turns than secondary
Voltage: Secondary voltage higher
Current: Secondary current lower
Example: Generator to transmission line
The primary winding of a step-down transformer has more turns than the secondary. This is a frequently tested concept. More primary turns create a higher primary voltage, and the reduced secondary turns produce a lower secondary voltage.
Power Conservation
In an ideal transformer, the power in equals the power out:
V_primary x I_primary = V_secondary x I_secondary
This means when a transformer steps down the voltage, the current steps up proportionally, and vice versa. A transformer that reduces voltage from 600V to 120V (a 5:1 ratio) increases the current by the same 5:1 ratio.
A step-down transformer has more turns on the primary than the secondary. The voltage ratio equals the turns ratio. When voltage steps down, current steps up proportionally. Transformers require AC to function.