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Motor control · lesson 4 of 12

The overload relay

A motor overload relay watches the motor current and, if it is too high for too long, opens a normally closed contact in the control circuit. That contact drops the starter coil; the starter opens the motor. Trip one below and follow what happens.

Marche/arrêt avec surcharge et voyant de déclenchement

X1X21ArrêtMarcheMOLM1, 2Démarreur de moteur2MEn marche3OLSurcharge

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À faire

Le relais de surcharge a des éléments thermiques dans les conducteurs du moteur et un contact dans l’échelon de commande.

Two halves in one device

Physically the overload relay bolts on under the contactor. Its sensing part — heaters on an old thermal unit, current transformers on an electronic one — sits in the motor leads, carrying the full motor current. Its switching part is a tiny contact block, and that contact is what you see in the control rung marked OL. On the ladder you only ever see the contact; the heaters are drawn on the power diagram.

Many overload relays also have a normally open "alarm" contact, which closes when it trips. The lab uses it for an amber Overload light — so whoever walks up to a dead machine can see why.

Why it rides through a start

A motor draws several times its running current for the first second or two of a start. A fuse sized to the running current would blow every time. The overload relay is built to be slow — it trips on heat building up over time, not on a spike — so it lets a normal start through and catches a motor that is stalled, dragging, or running on two phases. What rating to set it at comes from the motor nameplate and your code book, not from this page.

Before you reset

An overload trip is the motor telling you something. Resetting and restarting without finding out why — a seized bearing, a jammed conveyor, a blown fuse on one phase — usually just trips it again, and each trip cooks the windings a little more.

Sur le chantier

An overload trip is information. The temptation is to push the reset and walk away; the habit worth building is to find out what the motor was complaining about first.

Common mistakes

  • Resetting repeatedly without looking. Every trip is heat that has already been through the windings. Three resets in a row is a decision to replace a motor later.
  • Setting the dial by eye. The setting comes from the motor's nameplate figures and your code book. Nothing on a web page — this one included — can give you that number.
  • Confusing the overload with short-circuit protection. The overload is slow on purpose so a normal start gets through. The fuse or breaker upstream is what handles a dead short.
  • Leaving the OL contact out of the control rung — the relay then senses an overload and has nothing to open.
  • Assuming reset restarts the motor. On a three-wire circuit it does not: the seal-in dropped when the coil did, so somebody has to press Start. That is a feature.
  • Swapping in whatever heater or relay is in the van. The sensing element is matched to the motor, not to the contactor.

On a real panel

  • A repeated trip on one machine is usually mechanical — a dragging bearing, a jam, a belt over-tensioned — or a lost phase, not a faulty relay.
  • A motor running on two phases pulls badly unbalanced current. Measure all three legs before you blame the relay.
  • Ambient matters on a thermal relay: a panel in the sun trips earlier than the same relay in a cool room. Electronic relays care less.
  • Hand and automatic reset are different devices and different consequences. Automatic reset on a machine somebody stands next to is a decision, not a default.

Prove it before you leave

  1. Read the nameplate. Write the figures down. Set the relay from the book against those figures, not from memory.
  2. Trip the relay by hand with the control energised. The coil must drop out immediately.
  3. Reset it. The motor must not start on its own — somebody must press Start.
  4. Run the motor and measure all three legs. Note them on the panel schedule so the next person has a baseline.

Questions fréquentes

Does the overload relay open the motor leads?

No. It senses the motor current in the motor leads, but what it opens is a small normally closed contact in the control circuit. That drops the starter coil, and the starter’s main contacts open the motor.

Why does the motor not restart when I reset the overload?

In three-wire control, resetting only closes the OL contact again. The seal-in dropped out when it tripped, so someone has to press Start. On two-wire control (a float switch, say) a reset CAN restart the motor straight away — something to be aware of.

Is an overload relay the same as a fuse or breaker?

No. Fuses and breakers protect the conductors against short circuits and heavy faults, and act fast. An overload relay protects the MOTOR against running too hot for too long — a jammed load, a lost phase — and deliberately rides through the normal starting surge. Sizing either is a code-book job.

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