Skills / Motor Controls & Starters / Introduction to Motor Controls & Starters / Motor Controls and Starters: What They Do and How to Read a Print
Motor Controls & Starters

Motor Controls and Starters: What They Do and How to Read a Print

45 min read Training Guide

Contactors, overloads, control voltage, and the three-wire start-stop circuit every industrial electrician knows.

Table of contents

What the work looks like

Motor controls are how electricians start, stop, and protect the motors that run everything in industry. The simplest motor control is an across-the-line starter: press start, a contactor coil energizes, the contactor closes its power contacts, and the motor runs. Press stop, the coil de-energizes, the contacts open, the motor stops. Add an overload relay to protect against drawing too much current and you have the foundation of industrial motor control.

You will wire, troubleshoot, and replace contactors (Allen-Bradley 100 series, Siemens 3RT, Eaton XT, ABB AF), overload relays (eutectic alloy, bimetallic, or electronic), control transformers (480V to 120V step-down), pilot devices (start/stop buttons, selector switches, pilot lights), and terminal strips. Typical voltages: 480V three-phase on the power side, 120VAC on the control side (sometimes 24VDC for PLC-controlled systems).

Job titles: Controls Technician, Motor Control Technician, Industrial Electrician. Rates $28 to $50 per hour depending on region and apprentice vs journeyman.

Safety and tools

The three-wire start-stop (seal-in) circuit is the most common motor control you will see. Learn it cold:

  1. Push the start button (NO contact, momentary): current flows through the stop button (NC contact), through the start button, through the overload contact (NC), and energizes the coil.
  2. The coil pulls in an auxiliary contact (the seal-in) across the start button.
  3. Release start: current keeps flowing through the seal-in. Motor keeps running.
  4. Push stop: breaks the circuit. Coil drops out. Seal-in opens. Motor stops.

Safety: MCC buckets have dangerous voltages even when racked out (backfeed from control circuits, capacitors, or adjacent buckets is possible). Follow lockout/tagout. Use voltage-rated gloves when verifying dead on the line side of the disconnect. A Fluke 87V is the standard meter; use the CAT III 1000V or CAT IV 600V setting.

Tools: Fluke 87V meter, Klein insulated screwdriver set, long-nose pliers, side cutters, wire strippers, ferrule crimper for IEC control wiring, and a magnetic tool tray so you do not drop a tool into a live bucket.

Your first exercise

Draw a three-wire start-stop circuit on paper:

  • L1 and L2 on the 120VAC side of a control transformer.
  • Stop button (NC) in series.
  • Start button (NO) in parallel with an auxiliary contact M-aux.
  • Coil M.
  • Overload contact (NC) in series with the coil.

Trace the current path when you push start and when you push stop. If you cannot follow the path, draw it again. This is the circuit you will see on 90 percent of motor control prints.

Where to go next

Build on Motor Controls with PLC Programming (Introduction to PLC Programming), Variable Frequency Drives, Reading Electrical Schematics (Introduction to Schematic Reading), Industrial Electrical Systems, and NFPA 70E Arc Flash Safety. If you move toward automation, add HMI Programming and Industrial Networking (EtherNet/IP, Modbus TCP).