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.
- Size contactor to motor FLA and duty category
- Set overload relay to nameplate full-load current
- Wire the latching control circuit with NC stop dominance
- 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 complete guide for the full map, or continue with star-delta starting and breaker types compared. For the arithmetic, open the Electrical Power Calculator.
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 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
- Read next: pid controller explained: proportional-integral-derivative in practice
- Also in this track: single-phase vs three-phase power: the real differences
- Continue with: power factor explained: why real power is not apparent power
- Calculate as you go: power calculator · transformer turns ratio tool · cable sizing helper
- 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.
Extended Application Notes
This section expands the practical application of dol starter circuit: direct-on-line motor starting beyond the worked example, into the situations builders actually meet. Component substitution: when the exact specified part is unavailable, the substitution logic follows the governing parameter of this design, not the nominal value, and the verification step after any substitution is to re-measure the one quantity this guide identified as critical. Batch variation: components vary, and the design margins recommended in the sections above absorb that variation; if a second build behaves differently, the difference itself is diagnostic and points to the tolerance that dominated. Environmental limits: temperature, supply variation and ageing each push a real circuit away from its bench behaviour, and the recommended practice is to test the extremes deliberately rather than discover them in the field. These notes exist because the bench taught them, repeatedly, and each one was once a real troubleshooting session that ended in understanding.
Failure Analysis in Depth
The mistakes section above lists the traps; this section explains why each trap exists and how to recognize it early. 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. Each of these failures has a signature that appears in measurement before it appears in smoke: a reading that drifts, a waveform that differs from the prediction, a temperature that climbs faster than the calculation. The discipline this guide teaches is to measure at the first sign, not at the last, and the sections above give the specific instrument and setting for each check. Failure analysis is not pessimism; it is the fastest curriculum in electronics, because a fault understood once is a fault prevented forever.
Pre-Build Checklist
Before powering any build of this design, run the list: every component value verified against the specification above, the critical measurement points identified and accessible, the instrument modes and ranges chosen in advance, the expected values written down beside the bench, and the power source current-limited for first application. The checklist takes two minutes and replaces the most expensive class of beginner error, which is not ignorance but confidence outrunning verification. Builders who adopt the checklist across the guides in this track report first-apply success rates that feel like cheating, but it is not cheating, it is engineering.
What Comes Next
Having worked through this guide, the natural next steps are the adjacent guides in the track index above, each of which assumes exactly the vocabulary this page built. The calculators linked in the tools section verify every number in seconds, and the complete guide at the head of this track maps the entire curriculum. Read once, build once, measure always: that is the method this site teaches and the method every section above followed before publication.
Theory in Practice, Extended
The theory section of dol starter circuit: direct-on-line motor starting deserves one more pass with the bench in mind, because knowing a relationship and applying it under constraint are different skills. In application, the relationship is never isolated: it interacts with tolerances, with temperature, with the behaviour of adjacent stages, and with the measurement itself. The extended practice is to take the governing formula from the sections above and stress it, deliberately. Push the input to the edge of its specified range and watch the output follow the prediction, then push past it and watch the prediction break, because the edge of the specification is exactly where the formula stops being the whole story. That boundary, found on the bench rather than in the datasheet, is the real knowledge this guide offers beyond the mathematics.
Component Sourcing and Substitution Notes
Real builds meet real supply chains, and this section addresses the practical reality. The specified components in this guide were chosen for the reasons stated in the design sections, but equivalent parts from reputable manufacturers almost always serve, provided the governing parameters match, not merely the nominal ones. The substitution checklist: match the parameter this guide identified as critical, verify the package and pinout against the physical part before layout, check the datasheet revision for silent changes, and re-run the verification measurement after installation. Avoid unbranded surplus and marketplace components for anything this guide treats as safety-relevant; the failure mode of a counterfeit is not degradation, it is unpredictability, and unpredictability defeats every other design decision in the chain.
Instrumentation for This Design
Every measurement recommended in this guide maps to a specific instrument configuration, and this section consolidates them. Voltage checks: DC range selected before probing, leads verified against a known source, meter burden considered when the node is high impedance. Current checks: circuit broken at the defined point, meter inserted with the correct range and fuse status confirmed first. Waveform checks: probe compensated against the reference before any amplitude claim, ground lead kept short, bandwidth sufficient for the edge rather than the repetition rate. The instrumentation discipline matters more than the instrument class, and a modest instrument used correctly outperforms an expensive one used casually, a claim this site demonstrates throughout its measurement guides.
Documentation Template for This Build
Close the loop the way professional builds do: record the design values from this guide, the as-built values including every substitution, the measured results beside the predicted ones, and the deviation notes that explain every gap. The template is short, a single page, and it converts a successful build into a reference that survives component changes, firmware updates and the passage of months. Every guide on this site was built and documented exactly this way before publication, and the discipline is offered here as part of the curriculum rather than an afterthought. A build that is documented is twice built, once in copper and once in confidence.
Last updated 23 August 2026