how to select overload for 3 phase

we NEVER place protective device(such us Thermal Overload Protection/TOR) inside motor winding.

We Always and MUST put the protection device after MCCB (for low voltage) whether the motor connected in delta or why. so in this case, current that flow through of TOR is Current Line ( I line) NOT I phase.

If you are using a motor, and we must assume you are, then the only valid selection process is to read the motor nameplate current information. No formulae are necessary. If the motor nameplate says 7.2A, then you use an overload relay set for 7.2A. It is no more difficult than that.

jraef you are right over load setting depends on Motor name plate information in this part of world the The Motor starters are supplied with built in over loads generally thermal over loads which are not that effective because by the time temperature for thermal tripping relay reaches the set temperature and trips the motor cable end termination on the motor would have caused the damage,hence i always set the overload trip slightly less than the name plate value to safeguard my self

CRM

Your overload must be set higher that the FLA in order to eliminate nuisance trips. In the US we reference the NEC section 430.32(2and thermal overloads need to be sized @ 170% for 9A or less, @ 156% for 9.1A- 20A, and 1450% @ greater than 20A

JRaef,

In the US we reference the NEC section 430.32(2) in order to eliminate nuisance trips that would result from sizing the overload at the FLA. 170% less than 9A, 156% 9.1A- 20A, and 140% greater than 20A

Dear Sir,

We should not go by the name plate motor rated current in selecting over load relay of motor.We must select the overload relay current rating , based on the actual load of the motor supposed to deliver.

V.Ambarani

OL is always placed in the line.

You would use a thermal overload rated @ 12.24 A (reference NEC 430.32(2) 170% less than 9A. Reference NEC Table 430.52 for the shortcircuit and ground fault percentage ratings dependant upon the type of device utilized.

NEC 430.32(2) is for a Thermal Protector, which is integral with the motor. Selection of an Overload Device, which is integral with the motor starter, use NEC 430(1). That section states that the device shall trip or be rated at no more than the following percentages of nameplate:

125% for motors with a SF of 1.15 or greater

125% for motors with a marked temp rise 40 degrees C or less

115% for all others

Sorry, that should have been 430.32(1), not 430(1).

To finish that quote by Lincoln:"Bring me a list of all the ones that I can fool all of the time"

Good point, however, 430.31 is dependant upon the service factor of the motor.

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125% for motors with a SF of 1.15 or greater

125% for motors with a marked temp rise 40 degrees C or less

115% for all others

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is this setting not depend on manufacture curve ?

is this setting already consider motor starting (that can reach 600%I FLA) ?

tx...

The setting is based on the NEC requirements and the max allowed under these conditions. Percentages are applied to manufacturers FLA, as indicated on ea motor.

Inrush should only be seen for fractions of a second and thus should have no bearing on the OLs.

Are you selecting for a single motor?

If so, determine the brand of motor starter you have, and consult the OL chart for that particular make, and use the motor load amps to determine.

If calculating main feeder calculation for a group of motors, use 125% of the largest motor, and 100% of all other motors in the group. Remember, fuses protect wiring, thernal overlaods protect the motor.

The motor you select for the job should always be capable of operating a less than its maximum kW ie if your mechanical load requires 3-4 kW then 4 kW is the minimum 5.5 may be a better choice. Use the name plate current rating as the MAXIMUM setting of your overload. If the overload trips at this then get a bigger motor. Operating a motor at more than its rated current will dramatically shorten its life. It is a good idea to check the operating current of the motor when it is operating under load and if lower than the name plate adjust the overload down to match. This will better protect the installation in case mechanical or other problems. Browse the schneider and AB sites for starters and overloads and ABB or Crompton for Motor info

TRhe overloads need to be sized greater than the FLA on the nameplate due to the fact that this rating is often exceeded due to inrush current at start-up.

To get the best O/L protection measure the actual maximum current by clamp meter and set the O/L at 125% of the current reading.

The name plate indicates the max current and your motor will not necessrily run at max load.