Some advice on selecting contactors for use in direct line starters

Contactors for DOL starters
The selecting contactors used in direct line starters (DOL) are normally selected by their AC3 classification, i.e. starting a cage-type induction motor and cutting off the power supply to the motor once. after the motor has reached its maximum speed.

Some advice on selecting the contactor to use in the direct starter
The other most common categories of use are AC4, starting and stopping a cage-type induction motor before it reaches its maximum speed, sometimes referred to as “slow forward” or “running at full speed”. Idle ‘, AC2, switching the stator power supply to a wound rotor motor where the starting circuit automatically inserts a resistance into the rotor circuit on each start.

Cataloged evaluations //
Cataloged notes are published on the basis of known service conditions, typically //
Note n ° 1 – Ambient temperature outside the start-up enclosure between -5 and 35 ° C on average (with a maximum not exceeding 40 ° C)
Note # 2 – Operating rate to be specified by the manufacturer, typically 120 starts per hour.
Note # 3 – Duty Cycle or Duty Category – see Table 1 for typical switching current values ​​during type and in-service testing, p. ex. is the starter a reversing type that should reverse an already running engine, or does the reversing take place only after the engine has stopped? In the latter case, only AC3 rated contactors can be used.
Note # 4 – Reader acceleration time, the ability of the selecting contactor to carry the starting current, must be considered. AC3 contactors have the capacity to withstand eight times the rated current for at least 10 seconds (this applies for rated voltages up to 630 A. Above this, the value is six times the rated current).
Note # 5 – Any special contact life requirement.
Note # 6 – The type of short-circuit protection device to be installed in series with the starter power supply and the classification of the type of protection to be obtained.
Note # 7 – Any special coordination requirements, eg. Are there any residual current devices that, upon detection of a fault condition, may attempt to open the contactor with a current greater than its breaking capacity.
Note # 8 – Any special requirements for attaching copper, PVC or rubber insulated cables to the contactor terminals, eg. ex. Using certain types of high-temperature insulation such as XLPE allows the cable to run much hotter than expected.
Normal practice is to design for use with a 70 ° C cable therefore the savings possible on using XLPE may not be exploited as the core temperature can reach 250 ° C. Therefore, the wiring would act. as a heat source and not as a heat sink.


Category of use Current as a multiple of the operational current (I e )
The normal running Verification operation
to do Pause to do Pause
AC1 – Non-inductive or slightly inductive loads such as furnaces and heat loads 1 1 1.5 1.5
AC2 – Starting slip-ring motors. Connection with rotor resistor switched on 2.5 2.5 4 4
AC3 – Starting of squirrel cage motors, switching of running motors 6 1 * 10 8
AC4 – Starting of squirrel cage motors, progressive connection 6 6 12 10
All tests were performed at supply voltage for normal operation except as indicated below and at 105% voltage for check operations. For details, power factors, etc., see specification IEC 947 / BS EN60947.

  • At 17% of the supply voltage.
    All of the above may require special attention and may require the selection of a contactor with an AC3 rating greater than that initially suggested by the cataloged values. Final or in-service contactors used in automatic transformer starters should be selected using AC3 ratings. Star and intermediate selecting contactors should be selected according to catalog recommendations.
    For stator-rotor starters, stator contactors should be selected using AC2 ratings. The ratings of rotor contactors are normally specified by the contactor manufacturer as enhanced ratings, as they are only connected to the circuit during start-up.
    Another important consideration when selecting contactors is to ensure that the proposed wire conductor can be accommodated through the contactor terminals. Most manufacturers provide this information in their catalog with the different types of terminations available.

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