5.2 Motor Control Circuits & Overload Protection

Power Circuit vs. Control Circuit

Every motor starter contains two electrically separate circuits that a Registered Master Electrician must be able to tell apart on a wiring diagram:

• Power circuit — Carries the full motor current, from line through the contactor's main contacts and the overload relay's current-sensing elements to the motor. Conductors are sized for the motor's full-load current (FLA); this is the circuit covered in 5.1's DOL, star-delta, and autotransformer wiring.
• Control circuit — Carries only the small current (often well under 1 A) needed to energize the contactor coil. It may run at line voltage or, more commonly on larger equipment, at a reduced voltage — commonly 24 V or 120 V — supplied by a control transformer stepped down from the power circuit. Conductors here are sized for the coil and pilot-device load, not for motor current.

Mixing these two up on a diagram is a common exam trap: the pushbuttons and pilot lights of the control circuit never carry the motor's running current, no matter how large the motor is.

Basic Control-Circuit Elements

A standard control circuit for a DOL starter contains:

• START pushbutton — normally open (NO); pressing it momentarily completes the coil circuit.
• STOP pushbutton — normally closed (NC); pressing it opens the coil circuit and drops out the contactor. STOP is wired in series ahead of START so that a stop command always overrides a start command.
• Seal-in (holding) contact — an auxiliary NO contact on the contactor itself, wired in parallel with the START pushbutton. Once the contactor picks up, this contact closes and holds the coil circuit energized after the operator releases START — this is why a momentary pushbutton can start a motor that then keeps running.
• Overload contact — a normally closed auxiliary contact from the overload relay, wired in series in the coil circuit, described in more detail below.
• Pilot lights — typically a RUN light wired across the seal-in contact (or an auxiliary contact) to show the motor is running, and sometimes a separate TRIPPED or fault light wired from the overload relay's alarm contact.

Thermal Overload Relays: How They Protect the Motor

A thermal overload relay protects the motor winding insulation from damage caused by sustained overcurrent — a jammed conveyor, a partially seized bearing, single-phasing, or simply an undersized motor for its load. It does this by continuously sensing motor current, most commonly through small heater elements connected in series with each phase in the power circuit. As current flows through a heater, it heats a bimetallic strip (or, in electronic overload relays, a current sensor feeds a microprocessor that models the same heating behavior). The hotter the strip gets, the more it bends, until — at a current and time combination set by the relay's design — it trips a normally closed contact in the control circuit, dropping out the contactor coil.

This is fundamentally a time-current response, not an instantaneous one: a small overcurrent takes a long time to trip the relay, while a large overcurrent trips it quickly. Overload relays are rated by trip class — Class 10, 20, or 30 — indicating the maximum number of seconds the relay allows before tripping at 6× the relay's current setting. Class 10 trips fastest (within about 10 seconds), suiting motors with little thermal margin; Class 20 and Class 30 allow progressively longer starting times for high-inertia loads such as large fans or crushers.

Sizing an overload relay starts from the motor nameplate full-load current (FLA), never from horsepower alone. Where the nameplate service factor is 1.15 or higher (or the motor is marked for no more than 40°C temperature rise), overload relays are commonly set up to about 125% of FLA; motors with a 1.0 service factor or less thermal margin are typically set closer to 115% of FLA. Setting the relay too low causes nuisance tripping during normal operation; setting it too high defeats the protection and risks cooking the winding insulation long before the relay ever trips.

Overload Protection vs. Short-Circuit Protection

It is critical not to confuse these two protection functions:

An overload relay is deliberately slow so that it can ride through normal starting inrush without tripping; it is not built to interrupt a short-circuit-level fault safely, and it carries no interrupting rating for that purpose. That job belongs to the fuse or breaker upstream, sized and rated for interrupting fault current — the subject of the next section.

Interlocking

Interlocking prevents a control action that would create an unsafe or damaging condition — most commonly, preventing a forward contactor and a reverse contactor from closing at the same time, which would short two line phases together through both sets of contacts. Two forms are used, often together:

• Electrical interlock — a normally closed auxiliary contact from the forward contactor is wired in series with the reverse contactor's coil circuit, and vice versa. If forward is engaged, its auxiliary contact opens the reverse coil circuit, so reverse simply cannot pick up until forward drops out.
• Mechanical interlock — a physical lever or latch links the two contactors so that if one is mechanically closed, it blocks the armature of the other from closing, regardless of what the control wiring is doing.

Reversing starters, and any application with two contactors that must never close together, use both electrical and mechanical interlocking as a belt-and-suspenders safeguard — one protects against a wiring fault, the other against a welded or stuck contact that electrical logic alone could not catch.