DOL Starter Control Circuit
This is a free printable dol starter control circuit: download the diagram as SVG or open it and print to paper or PDF.
A DOL (direct-on-line) starter control circuit uses a start pushbutton, stop pushbutton, hold-in (seal-in) auxiliary contact, and overload relay NC contact to safely start and protect a three-phase motor.
The direct-on-line starter is the simplest and most common method of starting a three-phase squirrel-cage induction motor. It connects the motor directly to the full supply voltage on start-up, producing the motor's full locked-rotor starting torque but also drawing its full locked-rotor starting current — typically five to seven times the full-load current, for a brief period during acceleration.
The power circuit is straightforward: three-phase supply through a fuse or MCCB to the main contactor (KM1) power contacts, then through the overload relay (OLR) thermal elements to the motor terminals. The control circuit, however, contains the essential logic that makes the starter safe and functional.
The control circuit operates at a lower voltage — typically 230 V AC derived from one phase and neutral, or from a control transformer stepping down to 24 V or 110 V for safety. The stop pushbutton (SB1) is wired normally closed (NC) in series with the circuit. The start pushbutton (SB2) is wired normally open (NO) in parallel with the hold-in (seal-in) auxiliary contact (KM1a).
When SB2 is pressed, current flows through the NC stop button, through the NC overload relay contact (OLR-NC), and through SB2 to the contactor coil (KM1). The contactor energises, closing its main power contacts to start the motor, and simultaneously closing its auxiliary contact KM1a. When SB2 is released, current continues to flow through KM1a, which is now closed in parallel with SB2 — this is the hold-in or seal-in function. The contactor remains energised after the operator releases the start button.
To stop, the operator presses SB1, opening the NC stop contact and de-energising the coil. The motor coasts to rest (or brakes if a mechanical brake is fitted). KM1a opens with the contactor, so the circuit returns to its initial state awaiting the next start command.
The OLR NC contact is wired in series between the stop button and the contactor coil. If the overload relay trips due to sustained overcurrent (motor overloaded or stalled), the OLR NC contact opens, de-energising the coil and disconnecting the motor — identical to pressing the stop button. The circuit remains de-energised until the OLR is manually or automatically reset.
How to wire dol starter control circuit
- Draw the power circuit Show the three-phase supply (L1, L2, L3) connecting through a fuse set or MCCB to the three main contacts of contactor KM1, then through the three thermal elements of the overload relay (OLR) to the motor terminals (U, V, W). Add the protective earth conductor throughout.
- Identify and draw the control supply Take the control supply from one phase (e.g. L1) and neutral, or from a control transformer. Show the control fuse (CF) in the live conductor of the control supply. The control circuit operates between this supply live and neutral (or two phases if no neutral is available).
- Wire the stop pushbutton (SB1) in series From the control supply live terminal, connect to one terminal of the NC stop pushbutton SB1. The other terminal of SB1 connects to the next element in series. Label clearly: 'NC — break to stop'.
- Wire the start pushbutton (SB2) and hold-in contact in parallel From the output of SB1, connect to one terminal of the NO start pushbutton SB2. In parallel with SB2, wire the auxiliary NO contact of contactor KM1 (KM1a). The combined output of SB2 parallel with KM1a connects forward to the OLR NC contact.
- Wire the overload relay NC contact in series Connect the OLR NC contact in series between the SB2/KM1a parallel combination and the contactor coil (KM1). Label 'OLR-NC — trips on overload'. The output of the OLR NC contact connects to one terminal of the KM1 coil.
- Connect the contactor coil to the control neutral The remaining terminal of the KM1 coil connects to the control circuit neutral (or to the other control phase). This completes the control circuit loop. Optionally, a run indicator lamp (HL1) in parallel with KM1 coil and a fault lamp (HL2) in series with the OLR trip contact can be added for local indication.
- Add optional remote stop or interlock contacts Additional NC contacts from remote emergency stops, door interlocks, or high-temperature protective switches can be wired in series with SB1 in the control circuit. Each additional NC contact de-energises the starter when opened, providing a fail-safe interlock chain.
Specifications
| Motor starting current (DOL, typical) | 5× to 7× motor rated full-load current (Istart/In) |
|---|---|
| Control circuit voltage (common options) | 24 V AC, 110 V AC, 230 V AC (from control transformer or phase-neutral) |
| Contactor utilisation category (squirrel-cage motor) | AC-3 (IEC 60947-4-1) |
| Overload relay current setting | Set to motor rated full-load current (In) per nameplate |
| Overload relay trip class (most industrial applications) | Class 10 or Class 20 (IEC 60947-4-1) |
| Start pushbutton colour | Green (IEC 60073 and IEC 60947-5-1) |
| Stop pushbutton colour | Red (IEC 60073 and IEC 60947-5-1) |
| Emergency stop button colour and shape | Red mushroom head on yellow background (IEC 60947-5-5 / ISO 13850) |
Safety warnings
- A DOL starter must not be used as the means of isolation from the supply. An upstream lockable isolator or circuit breaker must be installed, and locked off with a safety padlock before any work on the starter, motor, or driven machine.
- The overload relay is not a short-circuit protection device. The upstream fuse or MCCB provides short-circuit protection. Installing an OLR set above the motor rated current does not protect against short circuits and will not adequately protect the motor winding against sustained overloading.
- After an overload relay trip, do not reset and restart the motor immediately. Allow the motor to cool for the period specified by the motor manufacturer before restarting, to prevent cumulative thermal damage to the winding insulation. Investigate and correct the cause of overloading before restart.
- The high starting current of a DOL-started motor can cause significant voltage dips on the supply network. Verify with the local electrical utility that DOL starting is permitted for the motor size. Exceeding the utility's permitted DOL motor size can cause voltage disturbances affecting other consumers.
- All motor starter installations must comply with applicable electrical codes including IEC 60947-4-1 (contactors and motor starters), IEC 60364, BS 7671, AS/NZS 3000, or NEC Article 430 as applicable, and must be installed by a licensed electrician where required by law.
Tools needed
- Multimeter (AC voltage and continuity)
- Clamp meter (for measuring starting and running current)
- Insulated screwdrivers (flat and Pozidriv)
- Terminal driver set
- Cable stripper and ferrule crimping tool
- Phase rotation meter (to verify correct motor rotation before commissioning)
- Tachometer (for speed verification)
Common mistakes
- Wiring the hold-in contact (KM1a) in series rather than in parallel with the start button — this means the motor runs only while the start button is held, and releasing it stops the motor.
- Setting the overload relay above the motor's rated full-load current 'to avoid nuisance trips' — this defeats the thermal protection and allows the motor to overheat to destruction during an overload condition.
- Omitting a fuse on the control circuit, leaving the control transformer or control supply wiring unprotected against a short circuit in the pushbutton wiring or contactor coil.
- Wiring emergency stop buttons as NO contacts in series with the start button (only preventing a new start) rather than as NC contacts in series with the stop button (providing an immediate stop of a running motor).
- Using a contactor rated for AC-1 (resistive load) for a motor application — motor starters require a contactor rated for AC-3 (squirrel-cage motor) or AC-4 (plugging/inching) utilisation category per IEC 60947-4-1.
Troubleshooting
- Motor starts but immediately stops when start button is released
- Cause: Hold-in auxiliary contact (KM1a) is not wired, is wired incorrectly, or the auxiliary contact block is faulty Fix: Verify the NO auxiliary contact on KM1 is wired in parallel with the start pushbutton. Test the contact by measuring resistance across its terminals while manually pressing the contactor armature — it should show near-zero ohms when closed. If the contact block is faulty, replace it.
- Motor trips on overload relay at less than full motor load
- Cause: Overload relay current setting is too low; phase loss causing overcurrent on remaining phases; motor has high ambient temperature; motor insulation degraded causing excessive current draw Fix: Measure actual motor running current with a clamp meter on all three phases. Verify the OLR is set to the motor's rated full-load current nameplate value. Check for phase loss — a single-phased motor draws very high current on the two remaining phases. Investigate motor condition if current is above rated at normal load.
- Contactor chatters (rapidly opens and closes) when energised
- Cause: Control supply voltage too low (below rated coil voltage), loose control circuit connection causing intermittent supply, or contactor coil is beginning to fail Fix: Measure control supply voltage at the coil terminals while the motor is running. If voltage is significantly below the coil's rated voltage (typically below 85 % of rated), the control supply transformer is undersized or there is excessive impedance in the supply. Also check for loose terminal connections in the control circuit.
Frequently asked questions
What is the hold-in (seal-in) contact in a DOL starter?
The hold-in or seal-in contact is an auxiliary NO contact on the main contactor (KM1a) wired in parallel with the start pushbutton. When the contactor energises on pressing Start, this contact closes, providing an alternative current path that keeps the coil energised after the start button is released. Without it, the motor would only run while Start is held down.
Why is the stop button wired normally closed (NC) and the start button normally open (NO)?
The NC stop button is in series with the control circuit — pressing it breaks the circuit and stops the motor. This 'fail-safe' wiring means that a broken or disconnected stop button wire also de-energises the contactor, which is the safe state. An NO start button requires a deliberate action to energise the circuit; an open circuit (broken wire) simply means the motor cannot start — also the safe condition.
What is the overload relay (OLR) and why must its NC contact be in the control circuit?
The overload relay uses bimetal thermal elements or an electronic current sensor to detect sustained overcurrent that would overheat the motor. When the thermal element trips, its NC contact opens, which de-energises the main contactor coil and disconnects the motor. This protects the motor from thermal damage due to overloading, phase loss, or stalling. The OLR is not a short-circuit protection device — that is the role of the upstream fuse or MCCB.
What is the starting current of a DOL-started motor?
A squirrel-cage induction motor starting direct-on-line draws its locked-rotor current, typically five to seven times its rated full-load current (often expressed as starting current multiplier or Istart/In). For example, an 11 kW / 380 V motor with rated current of 22 A may draw 132 A to 154 A during the starting period. This high current persists for the duration of acceleration, which can be one to five seconds for most industrial loads.
When should a DOL starter not be used?
DOL starting is unsuitable when the high starting current causes unacceptable voltage drops on the supply network, when the mechanical shock of full-voltage starting damages the driven machine or coupling, or when the driven load requires a gradual speed ramp. In these cases, star-delta starters, autotransformer starters, soft starters, or variable frequency drives (VFDs) are used instead. Most electrical utilities specify maximum motor sizes permitted for DOL starting on their network.
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