ProcirelDOL Starter Circuit: Direct-On-Line Motor Starting
The direct-on-line starter is the simplest motor starter. A contactor slams the motor across full supply. Up to roughly 7. 5 kW (rules vary), the grid tolerates the inrush; beyond that, softer methods take over.
> At a glance: 7 minute guide · part 5 of 10 in the electrical engineering complete guide track · includes a worked example and a quick-reference table.
## Power and control circuits
Here is the working theory in one pass. The power path runs through a contactor and an overload relay. The control circuit stop (NC), start (NO), contactor coil and holding contact latches the contactor on. The overload’s NC contact breaks the coil circuit on sustained overcurrent.
| E | l | e | m | e | n | t |
| — | — | — | — | — | — | — |
| R | o | l | e | | | |
| Contactor | Switches motor power, rated for inrush | | | | | |
| Overload relay (OLR) | Trips coil circuit on sustained overload | | | | | |
| Start (NO) + holding contact | Latching seal-in circuit | | | | | |
| Stop (NC) | Breaks the latch | | | | | |
| MCB/fuses | Short-circuit protection | | | | | |
| Under-voltage release | Drop-out on supply dips | | | | | |
## Why inrush matters
What this means at the bench. An induction motor at standstill looks like a transformer with a shorted secondary. 5–8 × rated current until it accelerates. Lights flicker, contactors must be rated for the duty. Frequent starts overheat the windings limiting starts per hour matters.
## Protection and limits
The overload relay protects against sustained overload, not short circuits that is the breaker or fuses’ job. Under-voltage release drops the contactor on supply dips, requiring a deliberate restart. DOL suits fans, pumps and conveyors below the local starting-current threshold.
## How to apply this in your build
Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly.
1. Size contactor to motor FLA and duty category
2. Set overload relay to nameplate full-load current
3. Wire the latching control circuit with NC stop dominance
4. Test trip by simulating overload before commissioning
### Worked example
A 5. 5 kW motor with FLA 11 A: DOL inrush hits ~80 A for a second. On a weak rural supply that dip resets electronics across the farm the same motor on a star-delta starter draws a third of that. Run the numbers yourself with the Electrical Power Calculator and the result should agree to within rounding.
> Practical note from the bench. Every motor circuit we document carries a locked-rotor current estimate next to the FLA sizing decisions follow from those two numbers.
## Field mistakes we see again and again
– Setting the overload to “trip later” after nuisance trips find the cause instead
– Using the overload relay as short-circuit protection
– Ignoring starts-per-hour limits on fan duty
## Key takeaways
– Power and control circuits the foundation of this guide; revisit it if any measurement here surprises you.
– Why inrush matters the foundation of this guide; revisit it if any measurement here surprises you.
– Protection and limits the foundation of this guide; revisit it if any measurement here surprises you.
## Who this guide is for
Beginners get a single focused topic instead of a whole textbook chapter. It assumes the track’s earlier pages in the electrical engineering complete guide path. Intermediate readers use it as a reference the table, the worked example and the mistake list answer the questions that come up mid-build. If you teach, the structure (theory, application, example, failure modes) maps cleanly onto a lab session.
## What you need before starting
Nothing exotic: the parts or tools named in the guide, a multimeter. The Electrical Power Calculator open in a tab. Size contactor to motor FLA and duty category before you begin the guide assumes it and keep the quick-reference table above within sight while you work through the steps.
### Quick reference card
| Aspect | Where to find it in this guide |
| — | — |
| Core theory | Power and control circuits |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Field mistakes we see again and again |
## How this fits the electrical engineering complete guide track
This guide is one stop in the structured learning path. Start from the [electrical engineering complete guide](/tutorial/electrical-engineering-complete-guide) pillar page for the full map, or continue with [star-delta starting](/tutorial/star-delta-connection-guide) and [breaker types compared](/tutorial/mcb-mccb-rccb-breakers). For the arithmetic, open the [Electrical Power Calculator](/tools/electrical-power).
## Frequently asked questions
When must I stop using DOL?
When inrush causes unacceptable voltage dips or the utility restricts starting current typically above ~7.5 kW.
Why does my contactor chatter?
Weak control supply or a failing coil economy circuit measure coil voltage under load.
Is there a calculator for this?
Yes the [Electrical Power Calculator](/tools/electrical-power) tool runs the formulas from this guide instantly, client-side, with no signup.
## Related guides and tools
– The complete electrical engineering guide: [Electrical Engineering complete guide](/tutorial/electrical-engineering-complete-guide)
– Read next: [pid controller explained: proportional-integral-derivative in practice](/tutorial/pid-controller-explained)
– Also in this track: [single-phase vs three-phase power: the real differences](/tutorial/single-phase-vs-three-phase)
– Continue with: [power factor explained: why real power is not apparent power](/tutorial/power-factor-explained)
– Calculate as you go: [power calculator](/tools/electrical-power) · [transformer turns ratio tool](/tools/transformer-turns-ratio) · [cable sizing helper](/tools/wire-gauge-awg)
– From here, the natural continuation is the next guide in the track index. It assumes exactly the vocabulary this page built and adds the next layer of practice.
## Verification routine
The fastest way to internalise this topic is to change one variable deliberately and predict the result before measuring. Wrong predictions are the curriculum, they show exactly which mental model needs revisiting, and the bench grades honestly.
Component substitution is a legitimate experiment as long as it is deliberate. Swap one part, predict the effect, measure, and record. That single habit converts a parts bin into a teaching lab and makes every future guide in this track faster to absorb.
## Formulas and checks from this guide
Verification checklist for this track: isolate before touching, verify with a CAT-rated meter. Re-check protective device ratings after any load change. Mains discipline is a habit, not a step, and every guide here assumes it.
Bookmark this page against your next build in the track. The checklist above is the same one used across 27 guides in this series.
## Notes from the bench
Inrush from motors or supplies. Curve C breakers tolerate it, and sizing follows the datasheet.
Isolated low-voltage work, yes. Mains and panel work needs a qualified electrician, every time.