2Slow
Hot Rolled
- Joined
- Jun 26, 2006
- Location
- South East Michigan, USA
It has been advised by many (including myself) that one of the parameters used to size any contactor or motor starter in an RPC should be the amount of starting capacitance (based on the 550uF = 50 amps of potential inrush current from the caps ratio) that it is switching.
Part of me thinks the practice of having any contactor or starter that is switching the starting caps having an amp rating for the inrush current it is switching may be overly conservative.
A NEMA sized motor starter must have some inrush capability built into it. Starting a 3 phase motor powered by 3 phase from the utility will usually require about 5-7 times the FLA of inrush current anyway depending upon the motor code.
For example, a NEMA Size 1 starter is rated at 7.5 hp @230V, but only 27 amps. While 27 amps should cover the FLA times the service factor of just about any 7.5 HP 3 phase motor, it would not be sufficient to cover the inrush current due to starting. (multiple times FLA) If the contactor is rated for switching a 7.5 HP motor on three phase, wouldn’t it have to be rated for the inrush current that is associated with a 7.5 HP motor as well?
If you are using a NEMA size1 starter, does that mean you can only switch ~300 uF of capacitance per pole to keep the inrush under the 27 amp rating, or is there some additional inrush capability. If there is additional inrush capability, how much?
-Joe
Part of me thinks the practice of having any contactor or starter that is switching the starting caps having an amp rating for the inrush current it is switching may be overly conservative.
A NEMA sized motor starter must have some inrush capability built into it. Starting a 3 phase motor powered by 3 phase from the utility will usually require about 5-7 times the FLA of inrush current anyway depending upon the motor code.
For example, a NEMA Size 1 starter is rated at 7.5 hp @230V, but only 27 amps. While 27 amps should cover the FLA times the service factor of just about any 7.5 HP 3 phase motor, it would not be sufficient to cover the inrush current due to starting. (multiple times FLA) If the contactor is rated for switching a 7.5 HP motor on three phase, wouldn’t it have to be rated for the inrush current that is associated with a 7.5 HP motor as well?
If you are using a NEMA size1 starter, does that mean you can only switch ~300 uF of capacitance per pole to keep the inrush under the 27 amp rating, or is there some additional inrush capability. If there is additional inrush capability, how much?
-Joe