Selecting a contactor, overload relay and breaker for a motor

The short answer

Select three devices in this order. A contactor rated in utilisation category AC-3 for the motor's kW and voltage, never by its AC-1 current. A thermal overload relay whose adjustment range brackets the nameplate full load current with that current near mid-scale, in a trip class that suits the starting time. Then a short-circuit protective device: an MPCB, or an MCCB with a separate overload relay. Arithmetic gets you to candidate devices. Only the manufacturer's coordination table, read at the prospective short-circuit current of that board, tells you the three work together.

Step 1: kW to full load current

FLC = P / (1.732 x V x efficiency x power factor), with P in watts, V the line-to-line voltage, result in amperes.

At 415 V, 50 Hz the answer lands near 1.8 to 2.0 A per kW above about 5 kW, and worse per kW below that, because efficiency and power factor both fall off. Use the calculation to pick a frame while the motor is on order. Use the nameplate to set the overload once it is on site. They differ, and the nameplate wins.

Step 2: the contactor, and why AC-1 is the wrong number

IEC 60947-4-1 defines what a contactor must make and break in each category:

One frame carries several ratings at once. A contactor catalogued as 50 A AC-1 is commonly 32 A AC-3 and 15 kW at 400 to 415 V. The AC-1 number is bigger, so it is the one that gets quoted, and the one that ends up in the wrong bill of materials.

An AC-1 selection fails on a motor because a direct-on-line start makes onto six to eight times FLC for seconds at a time, and every stop breaks an inductive current. Contacts bounce as they close, and at six times current that bounce welds or erodes the tips. The sequence is burnt tips, then single phasing when a pole stops conducting, then a contactor that will not drop out on a stop or an emergency stop. That last one is a safety failure, not a reliability one.

Three adjustments. Jogging, plugging and reversing need the AC-4 rating, typically a third to a half of the same frame's AC-3 rating, and the permitted cycles per hour matter too. Star-delta puts the line and delta contactors in the winding path, so each carries Ie / 1.732, about 58 per cent of FLC, and the star contactor about a third. And AC-3 ratings are declared at a reference ambient, usually 40 degrees C: a closed panel in an Indian summer runs well above that, so the vendor's derating table applies.

Step 3: the overload relay, range and trip class

Pick a relay whose range puts the nameplate FLC near mid-scale rather than at the end of the dial, and set the dial to that nameplate figure. A relay sitting in the delta legs of a star-delta starter is set to 0.58 x FLC, because that is what it sees.

Trip class is the tripping time from cold at 7.2 times the current setting:

Class 10 is the default: centrifugal pumps, compressors, small fans, conveyors that reach speed in a few seconds. Class 20 is for long or high-inertia starts, such as large fans, centrifuges, crushers and extended star-delta. Class 30 is for very high inertia. Class 10A suits a motor with a short thermal withstand.

What decides it is not the load type but two curves. The relay curve must sit above the real start current-versus-time profile of that machine, and below the motor's thermal withstand, which the data sheet gives as a safe stall time hot and cold. Moving from class 10 to class 20 to stop nuisance tripping, without checking the stall time, cooks windings slowly. Confirm phase-failure sensitivity as well: a relay with differential tripping trips faster on loss of a phase, which is the most common way a motor dies.

Worked example: 15 kW, 415 V, 50 Hz, DOL centrifugal pump

With efficiency 0.90 and power factor 0.86, typical for an IE2 or IE3 four-pole machine of this size:

FLC = 15000 / (1.732 x 415 x 0.90 x 0.86) = 15000 / 556 = about 27 A

Say the nameplate then reads 28.5 A. Work from 28.5.

• Contactor. AC-3 rating at or above 28.5 A at 415 V, so a 32 A AC-3 / 15 kW frame. The same frame shows roughly 50 A in AC-1, which is the number to ignore.
• Overload. A 22 to 32 A relay set to 28.5 A, trip class 10, since the pump reaches speed in about two to four seconds.
• Or one MPCB covering 22 to 32 A, set to 28.5 A, its magnetic element fixed near 13 times the setting, around 370 A, which clears inrush without tripping.
• Coordination check. Establish the prospective short-circuit current at the MCC busbar, say 25 kA at 415 V. Open the vendor's table for 415 V, 25 kA, DOL, 15 kW, read the exact breaker, contactor and relay references it names, and note whether that row is type 1 or type 2. If your three devices are not a row in the table, they are not coordinated, however sensible each rating looks on its own.

Type 1 and type 2 coordination

IEC 60947-4-1 defines what a starter may look like after it has been short-circuited at its rated conditional short-circuit current.

Type 1 permits the contactor and overload relay to be damaged. No hazard to people, nothing damaged outside the enclosure, but the starter need not be fit for further service.

Type 2 permits no damage to the overload relay or any other part, except light welding of contactor contacts that can be separated easily without significant deformation. The starter must be suitable for continued use and the relay must still trip correctly.

Both require that no ionised gas is discharged outside the enclosure, that conductors and terminals survive, and that the short-circuit device clears the fault.

Type 2 is what a continuous process needs, and anywhere a spare starter is not on the shelf. Type 1 is legitimate and cheaper where downtime is tolerable, and plenty of OEM machines ship with it. Two constraints get forgotten: the type is declared at a stated prospective short-circuit current, so type 2 at 50 kA says nothing about a board with a higher fault level, and the table covers the tested combination only, so substituting another brand's relay invalidates it.

The mistakes people make

• Sizing the contactor from the catalogue's AC-1 current.
• Setting the overload to the breaker rating, or to the calculated FLC when a nameplate exists.
• Fitting the relay in the delta legs of a star-delta starter and then setting it to the full line FLC. It then protects nothing until line current reaches 1.73 times FLC.
• Choosing an SCPD whose breaking capacity is below the board's fault level because the motor is small. Fault level is a property of the supply, not the load.
• Mixing brands across the starter and assuming coordination transfers.

What to learn next

The starting method decides half of this chain, so compare DOL, star-delta, soft starter and VFD next, then the control circuit that drives the coil, and then control panel wiring standards for how the selected devices get mounted, wired and labelled.