Electrical Test and Electric Motors

1. Based on construction:

Core type: Windings surround a considerable part of the core.

Shell type: Core surrounds a considerable portion of the windings.

2. Based on cooling:

Oil-filled self-cooled: Small- and medium-sized distribution transformers.

Oil-filled water-cooled: High-voltage transmission line outdoor transformers.

Air Cooled type: Used for low ratings and can be either of natural air circulation (AN) or forced circulation (AF) type.

3. Based on application:

Power transformer : These are large transformers used to change voltage levels and current levels as per requirement. Power transformers are usually used in either a distribution or a transmission line.

Potential transformer (PT): These are precision voltage step-down transformers used along with low-range voltmeters to measure high voltages.

Current transformer (CT): These transformers are used for the measurement of current where the current-carrying conductor is treated as a primary transformer. This transformer isolates the instrument from high-voltage line, as well as steps down the current in a known ratio.

Isolation transformer : These are used to isolate two different circuits without changing the voltage level or current level.

Some points about transformers:
• Used to transfer energy from one AC circuit to another
• Frequency remains the same in both the circuits
• No ideal transformer exists
• Also used in metering applications (current transformer, i.e., CT, potential transformers, i.e., PT)
• Used for isolation of two different circuits (isolation transformers)
• Transformer power is expressed in VA (volt amperes)
• Transformer polarity is indicated by using dots. If primary and secondary windings have dots at the top and bottom positions or vice versa in diagram, then it means that the phases are in inverse relationship.