The direct answer to a fundamental industrial control question: the motor starter is switched on and off by the contactor, which is in turn energized by a separate, lower-voltage control circuit. While the contactor handles the heavy lifting of switching high-current power to the motor windings, the control circuit (comprising pushbuttons, relays, or PLC outputs) handles the logic. Understanding this separation between the power circuit and the control circuit is the bedrock of motor drive selection, troubleshooting, and safe installation.

The Core Mechanism: How the Motor Starter is Switched On and Off by the Contactor

A modern magnetic motor starter is essentially two devices bolted together: a contactor (for switching) and an overload relay (for protection). When you press a 'Start' pushbutton, you are not sending 480V to the button. Instead, you are completing a low-voltage control circuit that energizes the contactor's electromagnetic coil.

Wiring and Terminal Identification

To wire or troubleshoot a starter, you must identify the terminal designations printed on the device. Here is the standard IEC/NEMA terminal map for a 3-phase reversing or standard contactor:

  • Power Circuit (Line to Load): L1, L2, L3 are the incoming line voltage terminals. T1, T2, T3 are the outgoing terminals feeding the motor. The main power contacts bridge these when the coil is energized.
  • Control Circuit (The Coil): A1 and A2 are the coil terminals. Applying the control voltage (e.g., 120V AC or 24V DC) across A1 and A2 creates the magnetic field that pulls the main contacts closed.
  • Auxiliary Contacts (Logic Feedback): 13 and 14 typically denote a Normally Open (NO) auxiliary contact, while 21 and 22 denote a Normally Closed (NC) contact. The NO contact is wired in parallel with the 'Start' button to create the classic 3-wire 'sealing' or holding circuit, keeping the motor running after you release the button.
Bench Tip: Never wire the motor's main power through the auxiliary contacts (13/14). Aux contacts are typically rated for only 10A at 120V AC (control logic). Running motor current through them will weld the contacts shut and destroy the starter.

Motor Type Comparison: Matching the Load to the Drive

Selecting the right starter or drive requires matching the motor's torque curve to the mechanical load. You cannot simply swap motor types without changing the drive architecture. Below is a data-dense comparison to guide your selection.

Motor & Drive Selection Matrix for Industrial Loads
Motor Type Starting Torque Curve Control / Drive Demanded Typical Cost (per HP) Best Load Profile
AC Induction (TEFC, NEMA Design B) 150% of rated torque at 0 RPM; dips then peaks near full speed. Magnetic Contactor (DOL), Soft Starter, or V/Hz VFD. $80 - $150 Centrifugal pumps, fans, compressors, conveyors.
Brushless DC (BLDC) High constant torque from 0 RPM up to base speed. Electronic Speed Controller (ESC) with Hall sensors or sensorless back-EMF commutation. $250 - $400 High-efficiency HVAC blowers, drones, traction drives.
Stepper (Bipolar NEMA 23/34) Maximum holding torque at 0 RPM; torque drops sharply as speed increases. Step/Direction pulse driver (e.g., DM542) with current chopping. NOT interchangeable with servos. $120 - $250 CNC routers, 3D printers, low-speed high-precision indexing.
AC Synchronous (PMSM) Requires complex vector control to start; high dynamic torque response. Flux-Oriented Control (FOC) Servo Drive with high-resolution encoder feedback. $400 - $800+ Robotics, high-speed packaging, precision web tensioning.

Which motor fits this load profile? If your load requires high starting torque to break static friction (like a heavily loaded rock crusher), a NEMA Design D AC Induction motor or a DC Series motor is required. If your load is a variable-torque centrifugal pump, a standard NEMA Design B AC Induction motor paired with a VFD is the most cost-effective and energy-efficient choice. Stepper motors should never be used for high-speed continuous rotation loads due to their severe torque drop-off above 1,000 RPM.

Sizing the Starter and Overload: A Worked Load Example

Sizing a motor starter involves two distinct calculations: sizing the contactor to handle the starting inrush, and sizing the overload relay to protect the motor windings from thermal damage. A common mistake is using horsepower-to-kilowatt conversions without accounting for the specific load's starting duty cycle. We size based on current and NEMA/IEC utilization categories.

Worked Example: 5 HP Centrifugal Pump

Let's size a starter for a 5 HP, 230V, 3-phase AC induction motor driving a centrifugal pump. The motor nameplate states a Full Load Amps (FLA) of 14.8A and a Service Factor (SF) of 1.15.

  1. Contactor Sizing: According to NEC Table 430.250, the standard Full Load Current (FLC) for a 5HP 230V motor is 15.2A. For an AC-3 utilization category (squirrel cage motors, switching off during running), we select a contactor rated for at least 15.2A. A standard Schneider TeSys D LC1D18 (rated 18A at 230V) or a NEMA Size 1 contactor (rated 27A) is the correct choice. Cost for the IEC LC1D18 is approximately $65.
  2. Overload Relay Sizing: This is where journeyman electricians get tripped up. NEC Article 430.32 dictates that the overload must be sized based on the motor nameplate FLA, not the NEC table.
    • Base calculation: 14.8A (Nameplate FLA) × 1.25 (125% rule for 1.15 SF) = 18.5A maximum trip setting.
    • We select an adjustable thermal overload relay (e.g., LRD21, range 12-18A) and set the physical dial precisely to 17.0A to allow for startup transients without nuisance tripping, while still protecting the windings.

For deeper reference on standardizing these sizes across different motor frames, the All About Circuits motor starter guide provides excellent schematics for 2-wire vs. 3-wire control logic integration.

Failure Signatures: Diagnosing Hums, Overheats, and Stalls

When a motor circuit fails, the symptoms at the bench tell you exactly where to put your multimeter probes. Here is a diagnostic framework for the three most common motor starter and drive failures.

1. The Motor Hums but Will Not Start

  • Cause A (Single-Phasing): In a 3-phase system, if one power leg is lost (blown fuse, bad contactor pole), the motor will hum violently and draw massive current on the remaining two legs. Fix: Measure phase-to-phase voltage at the T1-T2-T3 terminals. You should read ~230V across all three combinations (T1-T2, T2-T3, T1-T3). If one reads 0V, the contactor pole is welded open or a fuse is blown.
  • Cause B (Bad Start Capacitor): On single-phase motors, a failed start capacitor removes the phase-shift needed to create a rotating magnetic field. Fix: Discharge the capacitor safely, remove it, and measure capacitance with a meter. A 200μF capacitor reading 12μF is dead. Replace with an identical μF and voltage rating.

2. Chronic Overheating and Overload Tripping

  • Cause A (Voltage Unbalance): A mere 3% voltage unbalance between phases can cause a 20% temperature rise in the motor windings. Fix: Check the incoming utility supply. If the unbalance is on the utility side, you may need to derate the motor or install a phase monitor relay that drops the control circuit (A1/A2) if unbalance exceeds 2%.
  • Cause B (VFD Carrier Frequency): If running on a VFD, a high PWM carrier frequency (e.g., 16 kHz) causes severe eddy current heating in the motor stator. Fix: Lower the VFD carrier frequency to 4 kHz or 2 kHz, or upgrade to an inverter-duty motor with Class H insulation.

3. Stalling Under Load

  • Cause A (Voltage Sag / Brownout): Motor torque is proportional to the square of the voltage. A 10% drop in line voltage results in a 19% drop in available torque. If the load spikes, the motor stalls. Fix: Measure voltage at the motor junction box while under load. If it drops below 10% of nominal, you have excessive voltage drop in the feeder wires and must increase the AWG wire size.
  • Cause B (Mechanical Binding): Bearings or gearboxes are failing. Fix: Decouple the motor from the load. Run the motor uncoupled. If it draws normal no-load current (typically 30-40% of FLA), the motor and starter are fine; the mechanical load is binding.

For a comprehensive deep-dive into how these thermal and mechanical stresses physically destroy windings over time, the legendary Cowern Papers on motor failures remain the gold standard for bench diagnostics and failure analysis.

Safety Warning: Always de-energize, lock out/tag out (LOTO), and verify dead with a properly rated CAT III or CAT IV multimeter before opening any motor starter enclosure or touching A1/A2 control terminals. Control circuits can be back-fed from external PLC sources even when the main breaker is off.