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Facteur de service

Service factor (SF) is a multiplier on a motor’s nameplate that says how far past its rated load it can be pushed, under rated conditions, without immediate damage. An SF of 1.0 means no margin; 1.15 means it can deliver 15 % more than its rated horsepower for a while.

A margin, not a rating

Service factor is there to absorb the unexpected — a load a little heavier than planned, a voltage a little low, a hot day. It is not extra horsepower to design around. A motor running in its service factor runs hotter than it was designed to, and heat is what ages insulation: run it there all the time and it will fail years sooner than it would at rated load. The margin also assumes rated voltage, frequency and ambient; a motor on a drive, at altitude or in a hot room may have less or none.

Many nameplates also print SFA — service-factor amps — the current the motor draws when it is loaded to its service factor. It is the ceiling a clamp reading should stay under.

Sur le chantier

Clamp the running motor and compare with the plate. At or under FLA, it is loaded as designed. Between FLA and SFA, it is into its margin — find out why before calling it fine. Over SFA, it is overloaded. How the overload protection is set against these figures is a code-book and manufacturer question, not a service-factor one.

One worked, so the idea sticks

Reading a plate

  1. The plate says 10 HP, SF 1.15. Most it should ever deliver: 10 × 1.15 = 11.5 HP.
  2. FLA 28 A, SFA 32 A. You clamp 30 A on each phase: the motor is into its service factor — working, but hotter than design.
  3. If a plate gives no SFA, FLA × SF (28 × 1.15 ≈ 32 A) is a rough stand-in. Use the printed SFA whenever there is one.

Practice numbers. Read the real ones off the motor in front of you.

Questions fréquentes

Can I size a motor so it runs in its service factor all the time?

You can, but you are buying a shorter life. Service factor is meant for occasional overload. Size the motor so its normal load sits at or under rated horsepower and leave the margin for the bad days.

Does service factor apply on a VFD?

Often not, or only partly. A motor on a drive runs hotter from the drive’s waveform and loses cooling at low speed, so many makers state an SF of 1.0 on inverter power. Check the motor’s data for inverter duty.

Is service factor the same as efficiency or power factor?

No. Efficiency is how much of the input becomes shaft power; power factor is how much of the current does real work. Service factor is only about how much overload the motor can take.

Voir aussi

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