Design Constraint

·         All motors must operate at same speed: as they all should start up simultaneously and are accelerated or decelerated at the same rate.

·         System should consider the sequences of treating Altivar as a single point of failure: running multiple motors from one Altivar creates a single point of failure. When the main Altivar fails, all of the motors connected to it fail too.

·         Altivar (VSD) must be upsized according to the connected Load (motors): Upsizing depends on the operating mode.

Operating Mode Sizing

·         All motors start simultaneously: size of the Altivar should be equal or greater than the sum total of connected motors’ full load amperes (FLA) or horsepower.

Example:

Two motors operate at 480 V, one with 50 HP and FLA of 69A and another starts simultaneously with 20 HP and FLA of 39A. A proper sizing of a suitable Altivar is to be greater than or equal (50 HP + 20 HP) 70 HP or to have a total FLA greater than or equal to (39 A+ 69 A) 108 A. 

·         Motors are to be started on different timings: size of Altivar should be calculated with a slight difference where a new factor is to be added to the sizing calculations. Locked rotor amperes (LRA) for each motor that start after the Altivar starting operation is how much current drawn by a motor at startup. If a motor is going to be started after the Altivar starting operation for other motors, Altivar sizing will depend on FLAs of the originally simultaneously started motors and LRAs of the newly started motors.   

 Example:

 If there are three motors operate at 480 V, one with 50 HP and FLA of 69 Amps and another starts simultaneously with 20 HP and FLA of 39 Amps. A third motor is to newly join after the Altivar starting operation in a different timing with 10 HP and LRA of 86.5 Amps. A proper sizing of a suitable Altivar is to be greater than or equal to the sum of the drawn currents from motors (69 Amps (FLA) + 39 Amps (FLA) + 86.5 (LRA) >= 194.5 Amps) which is matched to nearest possible Altivar 250 HP (233 Amps > 194.5). Note that in the case of non-simultaneous starting total sum of HP cannot be valid as 50 HP + 20 HP + 10 HP = 80 HP < 250 HP , so only total sum of motors’ drawn currents is the valid calculation in such cases.

Notice: each individual motor must have its own protection in the form of overcurrent and short circuit devices.

Released for: Schneider Electric Egypt and North East Africa