---
title: "Instrument transformer vs power transformer"
subject: "Electrical Machines"
chapter: 2
key: "semester_4/electrical_machines:instrument-transformer-vs-power"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/instrument-transformer-vs-power
site: BCT Engineering
---

# Instrument transformer vs power transformer

Electrical Machines · Chapter 2: Transformer

- Asked in: 2067 Mangsir (old course), 8 marks
- Video: [What is Instrument Transformer | Current Transformer and Voltage Transformer](https://www.youtube.com/watch?v=Ik2r-TImy5Y) — Electrical lectures
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/instrument-transformer-vs-power
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-2

## Question

What is meant by an instrument transformer? How they differ in principle of operation from that of power transformer? Explain with suitable diagram and mathematical expressions.

## Short answer

Instrument transformers (CT and PT) step down high currents/voltages for measurement and protection with low VA burdens. Unlike power transformers, CTs operate near short circuit, PTs near open circuit; design emphasizes accuracy (low ratio and phase-angle errors) over efficiency, and core flux density is low.

## Answer

An **instrument transformer** is a special transformer used to step down high voltages or currents to safe, measurable values for meters and relays. It isolates the measuring circuit from the high-voltage line. There are two types: the **current transformer (CT)** and the **potential transformer (PT)**.

#### Types and working

**Current transformer.** The primary has few turns (often a single bar) and is connected in *series* with the line. The secondary has many turns and feeds an ammeter or relay coil (low impedance burden). Its secondary is nearly short-circuited. Secondary current is 5 A or 1 A.

$$
I_1N_1 = I_2N_2 \Rightarrow \frac{I_1}{I_2} = \frac{N_2}{N_1} = \text{CT ratio}
$$

**Potential transformer.** The primary is connected *across* the line, and the secondary feeds a voltmeter or pressure coil (high impedance burden). It works almost at no load, with secondary voltage 110 V.

$$
\frac{V_1}{V_2} = \frac{N_1}{N_2}
$$

```text
  Line ----+----- Load       Line ------+------
           |                            |
          [CT]--(A)                    [PT]--(V)
```

#### Difference from the power transformer

| Point | Instrument transformer | Power transformer |
|---|---|---|
| Purpose | Measurement and protection | Transfer of power |
| Rating | A few VA (burden) | kVA to MVA |
| Primary current (CT) | Decided by the line current (series) | Decided by the secondary load |
| Flux | CT: flux depends on line current; PT: constant | Constant at all loads |
| Secondary condition | CT: nearly short-circuit; PT: nearly open | Varies with the load |
| Main design aim | High accuracy (small ratio and phase-angle error) | High efficiency, good regulation |
| Core flux density | Low, to keep exciting current small | Close to saturation |
| Secondary open | Dangerous for CT | Safe |
| Errors | Ratio and phase-angle errors matter | Not a design concern |

#### Errors

For a CT, the exciting current $I_0$ is drawn from the primary but does not appear in the secondary, so the actual ratio $I_1/I_2$ differs from the turns ratio $N_2/N_1$ (**ratio error**), and $I_2$ is not exactly 180° from $I_1$ (**phase-angle error**). Both errors are kept small by a low-loss, high-permeability core operated at low flux density and by a low burden.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A current transformer is connected:
   A. In series with the line, with a nearly short-circuited secondary
   B. Across the line, with open secondary
   C. In parallel with the load
   D. Across the secondary of a PT
   Answer: A. Few primary turns in series; the secondary feeds an ammeter. (Key point: CT connection)

2. Opening the secondary of a CT is dangerous because:
   A. The primary current falls to zero
   B. Flux is set by line current, so with no balancing mmf it rises, producing a very high voltage
   C. The burden doubles
   D. The ratio becomes N1/N2
   Answer: B. Unlike a power transformer, primary current is fixed by the line, not by secondary load. (Key point: CT open secondary)

3. A potential transformer works:
   A. With short-circuited secondary
   B. With a 5 A secondary
   C. Almost at no load with a 110 V secondary and high-impedance burden
   D. With low-impedance burden
   Answer: C. The voltmeter draws very little current. (Key point: PT)

4. Instrument transformers are designed mainly for:
   A. High efficiency near saturation
   B. Maximum power transfer
   C. Unity regulation
   D. Accuracy, with low flux density and small ratio and phase-angle errors
   Answer: D. Power transformers aim at efficiency and regulation. (Key point: Design aim)

5. Ratio error in a CT arises because:
   A. Exciting current is drawn from the primary but does not appear in the secondary
   B. Secondary resistance is zero
   C. Primary has many turns
   D. Flux is zero
   Answer: A. So I1/I2 differs from N2/N1 and phase is not exactly 180 degrees. (Key point: Error source)

## Related in Electrical Machines

- [Practical vs ideal transformer, phasor diagram](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Apractical-vs-ideal-transformer-phasor) — Electrical Machines, chapter 2
- [Loaded transformer and phasor diagram for inductive load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Atransformer-loaded-operation-phasor-inductive) — Electrical Machines I, chapter 2
- [Transformer no-load and load operation, constant core flux](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aideal-transformer-noload-load-constant-flux) — Electrical Machines I, chapter 2, asked 5×
- [Transformer no-load operation and ideal transformer properties](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Atransformer-no-load-ideal-properties) — Electrical Machines I, chapter 2
- [Real transformer no-load operation and amp-turn balance](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Areal-transformer-noload-amp-turn-balance) — Electrical Machines I, chapter 2
- [Ideal transformer principle and emf equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aideal-transformer-principle-emf) — Electrical Machines I, chapter 2, asked 6×

## Related in other subjects

- [Static vs dynamic mathematical model](https://www.bctengineer.com/question.md?key=semester_8%2Fsimulation_and_modeling%3Astatic-vs-dynamic-math-model) — Simulation and Modeling, chapter 2, asked 3×
- [Physical vs mathematical models](https://www.bctengineer.com/question.md?key=semester_8%2Fsimulation_and_modeling%3Aphysical-vs-mathematical-model) — Simulation and Modeling, chapter 2
- [Transformer operation; flux constant at any load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Atransformer-load-noload-flux-constant) — Electrical Machine, chapter 2, asked 3×
- [Frequency counter operation with block diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Afrequency-counter) — Digital Logic, chapter 11, asked 14×