A full voltage starter is an electromechanical controller that applies the full available line voltage directly to a motor's terminals the moment the start command is initiated. In a real circuit, it changes the state from open to closed across all phases simultaneously, subjecting the electrical system to the motor's maximum locked-rotor inrush current while delivering peak starting torque to the mechanical load. Beginners and even some seasoned techs commonly confuse it with reduced-voltage methods like soft starters or variable frequency drives (VFDs), incorrectly assuming all motor starters ramp up voltage gradually to protect the grid.
The Mechanics of Direct-On-Line Starting
Often referred to as a Direct-On-Line (DOL) starter, this setup is the simplest and most rugged way to start an AC induction motor. It consists of three primary components: a disconnect or circuit breaker for short-circuit protection, a magnetic contactor to switch the power, and an overload relay to protect the motor windings from sustained thermal damage.
When you press the start button, the contactor coil energizes, pulling the main power contacts closed. The motor sees 100% of the line voltage instantly. Because an induction motor acts like a transformer with a shorted secondary when the rotor is stationary, it draws a massive amount of current to establish the magnetic field and break the mechanical inertia of the load. This inrush, known as Locked Rotor Amps (LRA), typically sits at 600% to 800% of the motor's Full Load Amps (FLA).
Sizing the Components: A Numeric Walkthrough
Sizing a full voltage starter requires looking past the motor's running current and designing for the brutal inrush conditions. Let us walk through a concrete example using a standard industrial motor.
The Load: A 10 HP, 460V, 3-phase AC induction motor driving a direct-coupled centrifugal pump.
Nameplate Data: FLA = 14.0A, LRA Code Letter G (approx. 6.0x multiplier), Service Factor 1.15.
- Calculate Inrush Current: 14.0A (FLA) × 6.0 = 84A LRA. The contactor must safely make and break this current without welding its contacts shut.
- Select the Contactor (NEMA Sizing): NEMA standards rate contactors by horsepower and voltage. A NEMA Size 1 contactor is rated for 27A at 460V (up to 10 HP). While a Size 1 is technically compliant, the 84A inrush will cause significant contact pitting over time. For high-inertia or frequently cycled loads, stepping up to a NEMA Size 2 (rated 45A at 460V) provides a massive increase in electrical contact life.
- Size the Overload Relay: The NFPA 70 (National Electrical Code) Article 430.32 dictates that overload protection for a motor with a 1.15 service factor must be set at no more than 125% of the FLA.
14.0A × 1.25 = 17.5A. You would select a solid-state overload relay (like an Allen-Bradley 193-EIO) or a bimetallic heater element dialed exactly to 17.5A.
| NEMA Size | Max HP (460V) | Max FLA (460V) | Typical Application |
|---|---|---|---|
| Size 00 | 1.5 HP | 4.3A | Small fans, fractional HP pumps |
| Size 0 | 5 HP | 10.8A | Conveyor drives, small compressors |
| Size 1 | 10 HP | 27.0A | Standard machine tools, HVAC blowers |
| Size 2 | 25 HP | 45.0A | Large pumps, heavy-duty air compressors |
| Size 3 | 50 HP | 90.0A | Industrial chillers, rock crushers |
Where You Meet This in Practice
You will encounter full voltage starters in almost every industrial, commercial, and agricultural facility. They are the default choice for any application where the mechanical load can tolerate a sudden torque spike and the electrical grid can absorb the inrush current without excessive voltage drop.
Common installations include:
- Centrifugal Pumps and Fans: These loads have low starting torque requirements, meaning the motor accelerates quickly through the high-inrush zone, minimizing thermal stress on the starter.
- Conveyor Belts (Unloaded): If started without material on the belt, the inertia is manageable.
- Small Air Compressors: Typically under 10 HP, where utility companies do not yet enforce strict inrush limits.
However, utility companies often draw a hard line. Many rural electric cooperatives and municipal utilities forbid full voltage starting for motors larger than 15 HP or 50 HP (depending on the local transformer infrastructure) because the sudden current draw causes visible light flicker and voltage sags for neighboring customers. Always check your local utility's motor starting guidelines before specifying a DOL starter for large horsepower.
Real-World Scenario: The 15 HP Compressor Voltage Dip
Theory is clean, but jobsite physics are messy. Here is a scenario that highlights the hidden costs of full voltage starting.
The Setup: A custom CNC machine shop installed a new 15 HP (460V) reciprocating air compressor to keep up with production. The shop was fed by a dedicated 100 kVA padmount transformer. The compressor was equipped with a standard NEMA Size 2 full voltage starter.
The Numbers: The 15 HP motor had an FLA of 21A and an LRA of roughly 126A. Because reciprocating compressors are high-inertia loads, the motor took a full 3.5 seconds to accelerate to full speed, meaning it drew over 100A for several seconds during every start cycle.
The Outcome: Every time the compressor's pressure switch called for air, the shop's 5-axis CNC machines threw 'Control Voltage Brownout' alarms and paused mid-cut, ruining expensive aerospace titanium parts. The facility manager blamed the CNC manufacturer.
What Went Wrong: The full voltage starter caused a 12% voltage dip on the secondary side of the 100 kVA transformer during the 3.5-second acceleration window. The CNC machines' internal 24V DC power supplies dropped out at a 10% line dip. The fix was not to upgrade the transformer, but to swap the full voltage starter for a solid-state soft starter. By programming the soft starter to limit the starting current to 300% of FLA (63A), the voltage dip was reduced to under 4%, and the CNC machines never faulted again.
Full Voltage vs. Reduced Voltage Starters
Knowing when to abandon a full voltage starter for a reduced-voltage alternative is a critical design decision. Refer to the Electrical Engineering Portal's comprehensive guide on motor starting methods for deeper utility-side calculations.
| Criteria | Full Voltage (DOL) | Soft Starter | Variable Frequency Drive (VFD) |
|---|---|---|---|
| Starting Inrush Current | 600% - 800% FLA | 200% - 400% FLA (Adjustable) | 100% - 150% FLA |
| Starting Torque | 150% - 200% (Peak) | Reduced (Proportional to voltage squared) | 150% (Full torque at zero speed) |
| Mechanical Stress | High (Shock loading) | Low (Smooth ramp) | Lowest (Controlled ramp) |
| Relative Cost (10 HP) | ~$350 - $500 | ~$800 - $1,200 | ~$1,500 - $2,500 |
| Harmonic Distortion | None | None (Bypassed after start) | High (Requires line reactors) |
Troubleshooting and FAQ
Why does my full voltage starter's overload relay trip immediately on startup?
If the overload trips within the first second, you likely have a short circuit or a grounded winding, not an overload condition. The overload relay is designed to mimic the motor's thermal mass; it takes time to heat up. Instantaneous tripping means the magnetic trip element of the upstream breaker should have caught it, or you have a severe mechanical jam locking the rotor. Check the motor windings for continuity to ground with a megohmmeter.
Can I use a full voltage starter on a single-phase 240V motor?
Yes, but the architecture changes. Single-phase DOL starters use a 2-pole contactor and require a specialized single-phase overload relay that monitors both hot legs. If you only monitor one leg and a ground fault occurs on the unmonitored leg, the motor will burn out. Always ensure both Line 1 and Line 2 pass through the current sensors of the overload block.
What causes the contactor contacts to weld shut?
Contact welding happens when the starter is commanded to close into a dead short, or when the motor is 'jogged' (rapidly started and stopped) repeatedly. The massive inrush current arcs across the closing contacts, melting the silver-alloy tips together. If you need to jog a load for alignment, you must upsize the contactor by at least one NEMA size or switch to a heavy-duty IEC AC-4 rated contactor designed specifically for inching duties.
