What is meant by auto transformer and instrument transformer?

 Auto transformer:

Auto transformer is kind of transformer, where primary and secondary shares some common single winding. So basically its one winding transformer

Auto transformer employs only single winding per phase as against two distinctly separate windings in a conventional transformer.

Applications of auto transformer :

-compensating the voltage drops by boosting supply voltage in distribution system.

-auto transformer with a number of tapping are used for starting induction and synchronous motor. 

-auto transformer is used as variac in laboratory or continuous variable over broad  ranges are required. 

Instrument transformer :

It is used in AC system for measurement of electrical quantities i.e. Voltage, current, power, energy, power factor, frequency.

It is also used with protective relays for protection of power system.

Basic function of instrument is to step down the ac system voltage and current.

Types of instrument transformer :

Parameter type :

-current transformer (CT) 

-potential transformer (PT) 

Purpose type:

-measurement instrument transformer 

-protective instrument transformer 

Current transformer (CT) :

Current transformer is used to step down the current of power system to a lower level to make it feasible to be measured by ammeter. 

Primary of CT has few turns. Sometimes bar primary also used. Primary is connected in series with the power circuit. 

The secondary has large number of turns. Secondary is connected to ammeter.

Potential transformer (PT) :

Potential transformer is used to step down the voltage of power system to a lower level to make it feasible to be measured by  voltmeter.

Primary of PT has large number of turns. Primary is connected across the line ( generally between on line and earth). 

The secondary has few turns. Secondary is connected to voltmeter.