---
title: "Need of starter and induction motor starting methods"
subject: "Electrical Machines I"
chapter: 5
key: "semester_4/electrical_machines_i:induction-motor-starter-methods"
source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/induction-motor-starter-methods
site: BCT Engineering
---

# Need of starter and induction motor starting methods

Electrical Machines I · Chapter 5: Three-Phase Induction Machines

- Asked in: 2082 Kartik (new course), 6 marks
- Video: [IM 17 Starting Methods of 3 Phase Induction Motor 1](https://www.youtube.com/watch?v=0wgZEqjbgWQ) — Lectures in Electrical Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/induction-motor-starter-methods
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-5

## Question

Explain why do we need a starter to start an induction motor? Also explain different methods of induction motor starting.

## Short answer

At start, high inrush current (5–7× FL) can cause voltage dips and winding stress. Starters limit current: DOL for small motors; auto-transformer and star-delta reduce voltage; rotor resistance for slip-ring motors gives high torque; soft starters/VFDs provide smooth control.

## Answer

A **starter** is a device that limits the starting current of an induction motor and gives overload and no-volt protection.

#### Why a starter is needed

At standstill, slip $s = 1$, so the rotor behaves like a short-circuited transformer secondary. The rotor emf is maximum and the only opposition is the small leakage impedance:

$$
I_{2,st} = \frac{E_2}{\sqrt{R_2^2 + X_2^2}}
$$

The stator draws **5–7 times full-load current** at rated voltage. This causes:
- a large voltage dip in the supply line, disturbing other loads;
- heating and mechanical stress on the windings;
- tripping of protective devices.

Small motors (below about 5 kW) can be started direct-on-line; larger ones need a starter that reduces the voltage or adds rotor resistance during starting.

#### Starting methods

**1. Direct-on-line (DOL) starter** — full voltage applied through a contactor with overload relay. Simple and cheap; starting current 5–7 × FL. Used for small motors.

**2. Stator resistance (reactor) starting** — resistors in series with each stator phase reduce the voltage to $xV$. Current falls to $x$ times, but torque falls to $x^2$ times. Resistors are shorted after run-up. Wasteful; used rarely.

**3. Auto-transformer starting** — a 3-phase auto-transformer applies a tapped voltage $xV$. Motor current falls to $x$ times, line current to $x^2$ times, starting torque to $x^2$ times. Better than resistance starting; used for large motors.

**4. Star–delta starting** — for motors designed to run in delta. At start the winding is connected in star, so each phase gets $V/\sqrt{3}$; after speed builds up it is switched to delta.

$$
I_{line,\,Y} = \tfrac{1}{3} I_{line,\,\Delta}, \qquad T_{st,\,Y} = \tfrac{1}{3} T_{st,\,\Delta}
$$

Cheap and common for medium squirrel-cage motors.

```text
 Supply ---[TPDT switch]--- Stator
          Start: Y          Run: Delta
```

**5. Rotor resistance starting (slip-ring motors only)** — external resistance in the rotor via slip rings reduces starting current and, by increasing $s_m$ towards 1, **increases** the starting torque. Resistance is cut out in steps as the motor speeds up.

**6. Soft starter / VFD** — thyristor (or inverter) control raises voltage (or frequency) smoothly. Low current, smooth acceleration; used in modern installations.

| Method | Motor type | Start current | Start torque |
|---|---|---|---|
| DOL | Cage (small) | 5–7 × FL | High |
| Auto-transformer | Cage | $x^2$ × DOL | $x^2$ × DOL |
| Star–delta | Cage (delta-run) | 1/3 × DOL | 1/3 × DOL |
| Rotor resistance | Slip ring | Low | High |

## Check yourself

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

1. Why is a starter needed for large induction motors?
   A. At s = 1 the rotor acts like a shorted transformer secondary, drawing 5-7 times full-load current
   B. The motor has no torque
   C. Back emf is too high
   D. Frequency is too low
   Answer: A. High starting current causes voltage dips and heating. (Key point: Why starter)

2. With tap x, how do motor current and starting torque change?
   A. Both fall to x times
   B. Motor current falls to x times and torque to x^2 times
   C. Both stay the same
   D. Torque rises
   Answer: B. Torque is proportional to V^2; line current falls to x^2 times. (Key point: Auto-transformer starting)

3. What does star-delta starting do to line current and torque compared with DOL delta?
   A. Both fall to 1/3
   B. Both fall to 1/sqrt(3)
   C. Current falls to 1/3 but torque is unchanged
   D. Both rise
   Answer: A. Phase voltage is V/sqrt(3), giving 1/3 for both. (Key point: Star-delta)

4. What does external rotor resistance do at start in a slip-ring motor?
   A. Lowers current and increases starting torque by moving sm towards 1
   B. Lowers current and lowers torque
   C. Raises current
   D. Has no effect
   Answer: A. Higher R2 improves starting pf and torque. (Key point: Rotor resistance starting)

## Related in Electrical Machines I

- [Need of DC motor starter and three-point starter](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-starter-necessity-three-point) — Electrical Machines I, chapter 4, asked 4×
- [Need of starter and four-point starter](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-motor-four-point-starter) — Electrical Machines I, chapter 4
- [Starting, no-load and full-load current of slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aslip-ring-0p8-4ohm-400v-ratio4) — Electrical Machines I, chapter 5
- [Starting current and rotor resistance for 150 kW slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-150kw-3000v-starting-current-30a) — Electrical Machines I, chapter 5
- [Torque of 4-pole induction motor at 1400 rpm](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-4pole-2ohm-8ohm-5nm-1400rpm) — Electrical Machines I, chapter 5
- [DC series motor for starting heavy loads](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aseries-motor-heavy-load-starting) — Electrical Machines I, chapter 4

## Related in other subjects

- [DFRT, starting methods and capacitor start-run motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Adfrt-starting-methods-csr) — Electric Machines II, chapter 3
- [Starting line current of capacitor start motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Acapacitor-start-line-current) — Electric Machines II, chapter 3
- [Soft start variable AC voltage starter](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Asoft-start-ac-voltage-starter) — Utilization of Electrical Energy, chapter 3
- [High starting current; 3-point starter](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-motor-starter-high-starting-current) — Electrical Machines, chapter 4, asked 4×