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.
Start/stop with the overload and a trip light
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The overload relay has heaters in the motor leads and a contact in the control rung.
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.
Questions people ask
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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