Motor Starter (DOL) Symbol
Definition: The Motor Starter (DOL) symbol represents a Direct-On-Line motor starter in industrial wiring and schematic diagrams—a control device that connects a three-phase induction motor directly to the full supply voltage for starting, depicted as a block with three line terminals (L1, L2, L3) and incorporating a contactor and thermal overload relay, referenced in IEC 60947-4-1 and ANSI / NEMA ICS 2.
Also known as: DOL starter, direct-on-line starter, across-the-line starter, full-voltage starter, motor starter, magnetic starter.
What the Motor Starter (DOL) symbol means
The Motor Starter (DOL) symbol denotes the complete starting assembly for a three-phase AC induction motor that connects the motor windings directly and simultaneously to all three phases of the supply voltage at the instant of starting. The DOL starter symbol encompasses both the main-circuit contactor (which closes the power contacts when the start command is given) and the thermal overload relay (which monitors motor current and trips the contactor if the motor overloads).
In industrial schematics and single-line diagrams the DOL starter symbol marks the point in the motor feeder circuit where the motor is switched into service. The three line terminals L1, L2, and L3 connect to the incoming three-phase supply; the symbol's output connects to the motor terminals. The DOL method is the simplest and lowest-cost starting method, but it draws a high inrush current (typically 6–8× full-load current) at the instant of starting.
How to identify the Motor Starter (DOL) symbol
The Motor Starter (DOL) symbol is typically drawn as a rectangular block labelled 'DOL' or 'M' with three input lines at the top (L1, L2, L3) representing the three-phase supply terminals. Some schematic styles show the contactor symbol (three parallel switch contacts with a coil) and thermal overload relay symbol (heater element + trip contact) separately within the power circuit, while the DOL label designates the enclosure containing both. The symbol is distinguished from a star-delta starter (which has six motor terminals) and a soft starter or VFD (which shows additional control signals) by its simple three-terminal supply-side connection.
Function in a circuit
A DOL starter works by energising a main contactor coil via a control circuit (start/stop pushbuttons and overload relay auxiliary contact), which closes the three main contacts simultaneously and connects the motor directly to the full three-phase supply voltage. The motor accelerates from standstill to full speed in typically 1–5 seconds depending on motor and load inertia. The thermal overload relay monitors the motor's running current through bimetallic heater elements and opens the control circuit—tripping the contactor—if the current exceeds the overload relay's set trip current for a sustained period.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-4-1 governs AC motor starters and controllers including DOL starters; it specifies utilisation category AC-3 for squirrel-cage motor starting. IEC 60617 uses the contactor symbol with a thermal overload relay to represent the DOL starter in circuit diagrams. Motor starter designators: KM (contactor) and F (overload relay) per IEC 81346. |
|---|---|
| ANSI/IEEE 315 | ANSI / NEMA ICS 2 covers industrial control devices including across-the-line (DOL) motor starters. NEMA starters are classified by size (Size 00 to Size 9) based on horsepower and voltage rating. NEMA starter symbols use a contactor symbol (M) and a thermal overload relay (OL) in the power circuit diagram. |
| Key difference | IEC starters (per IEC 60947-4-1) are rated by current (A) and utilisation category (AC-3); NEMA starters (per NEMA ICS 2) are rated by horsepower and NEMA size number. IEC starters are typically more compact and economical; NEMA starters are more robust for heavy-duty cycling. Both use the same DOL starting principle and similar schematic symbols. |
Terminals / pins
| Pin | Name |
|---|---|
| l1 | L1 |
| l2 | L2 |
| l3 | L3 |
Typical values
Typical DOL starter ranges: 0.18 kW to 132 kW (IEC AC-3), 240 V to 690 V three-phase. Inrush current: 6–8× motor full-load current at starting. Contactor coil voltage: 24 V DC, 110 V AC, or 230 V AC (selectable). Overload relay adjustment range: 0.1 A to 630 A depending on class. Trip class: Class 10, Class 10A, or Class 20 per IEC 60947-4-1.
Where the Motor Starter (DOL) symbol is used
- Pumping stations for water supply, sewage, and irrigation using fixed-speed centrifugal pumps
- Conveyor systems in manufacturing, mining, and logistics where loads have low starting inertia
- Agricultural machinery including grain augers, feed mixers, and irrigation pumps
- HVAC fan motors in air handling units and exhaust systems where starting torque is low
- Compressor units for refrigeration and compressed-air systems with unloaded starting valves
- Machine tool auxiliary drives (coolant pumps, lubrication pumps) requiring simple on/off control
- Industrial process fans and blowers with low-inertia loads and infrequent start/stop cycles
Example
In a pump control panel schematic, a Motor Starter (DOL) symbol is shown with L1, L2, and L3 connected to a 400 V three-phase busbar via a 16 A motor protection circuit breaker (MPCB). The DOL contactor (KM1) is energised via a start/stop pushbutton circuit with a 230 V AC coil. The thermal overload relay (F2) is set to 12 A for a 5.5 kW pump motor, and the overload relay's NC auxiliary contact is wired in series with the contactor coil to trip the starter on overload.
Key facts
- A DOL (Direct-On-Line) starter connects a three-phase induction motor directly to the full supply voltage at starting; it consists of a main contactor (KM) and a thermal overload relay (F) combined in one assembly.
- DOL starters have three supply-side terminals: L1, L2, and L3; the output connects to motor terminals T1/U, T2/V, T3/W. The contactor coil is energised by a separate 24–230 V control circuit.
- IEC 60947-4-1 governs DOL starters under utilisation category AC-3 (squirrel-cage motor starting and stopping at rated voltage); NEMA ICS 2 classifies them by size (00 to 9) based on horsepower.
- DOL starting inrush current is typically 6–8 times the motor's full-load current; this high inrush can cause voltage dips on the supply network and mechanical shock on the load, which is why DOL is limited to smaller motors or stiff supply networks.
- A thermal overload relay set to the motor's full-load current provides overload protection with inverse-time tripping: the higher the overload, the faster the trip. Trip class (Class 10, 10A, 20) specifies the maximum trip time at 7.2× the set current.
- DOL starters are the lowest-cost, simplest motor starting method but are limited to motors where the supply network can tolerate the starting inrush; larger motors (typically >15–22 kW) require star-delta, soft-starter, or VFD starting to limit inrush.
- In IEC schematics the contactor is designated KM and the overload relay F; in NEMA drawings the contactor is designated M and the overload relay OL.
Frequently asked questions
What does the DOL motor starter symbol mean in a schematic?
The Motor Starter (DOL) symbol represents a direct-on-line starting assembly that connects a three-phase induction motor to the full supply voltage in one step. It indicates that the motor is started by directly closing a contactor on the full three-phase supply, without any voltage-reduction or current-limiting mechanism.
What does the DOL starter symbol look like in a wiring diagram?
The DOL starter symbol is typically a rectangular block labelled 'DOL' with three line terminals at the top (L1, L2, L3). In detailed schematics it shows a contactor symbol (three parallel switch contacts with an electromagnetic coil) and a thermal overload relay symbol (heater elements with a trip contact), both within a common enclosure outline.
What is the difference between a DOL starter and a star-delta starter?
A DOL (direct-on-line) starter connects the motor directly to full supply voltage in one step, producing a starting inrush of 6–8× full-load current. A star-delta starter first connects the motor windings in star (reducing voltage across each winding to 1/√3 of line voltage, reducing starting current to about one-third), then switches to delta for running; this reduces inrush current at the cost of reduced starting torque.
What IEC standard covers DOL motor starters?
IEC 60947-4-1 governs AC motor starters and controllers, covering DOL starters under utilisation category AC-3 (squirrel-cage motor starting at rated voltage). The standard specifies making and breaking capacity, mechanical endurance, temperature rise, and overload relay performance including trip classes.
What do the L1, L2, L3 terminals on the DOL starter symbol represent?
L1, L2, and L3 are the three line (supply-side) terminals of the DOL starter, connected to the three phases of the AC supply through an upstream protective device (MPCB or fuse). When the contactor closes, these three phases are simultaneously connected to the motor's U, V, and W (T1, T2, T3) terminals.
Why does a DOL starter cause a high inrush current?
At the moment a DOL starter applies full voltage to a stationary motor, the motor windings act like a short-circuited transformer with very low impedance, drawing a high inrush current of 6–8× the motor's full-load current. As the motor accelerates and develops back-EMF, the current reduces to its normal running level. This inrush can cause voltage dips and mechanical stress on the driven load.
What is the role of the thermal overload relay in a DOL starter?
The thermal overload relay in a DOL starter monitors the motor running current through bimetallic heater elements. If the motor draws more than its rated current for a sustained period (indicating mechanical overload, single-phasing, or blocked rotor), the relay trips and opens an auxiliary NC contact in the contactor coil circuit, de-energising the contactor and disconnecting the motor. This prevents motor winding damage from sustained overcurrent.
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