STARTER SIZE

[vanskat]

What, appart from the usual considerations should be taken into account when sizing a starter.
In other words, besides worst case what are the consequences of over and undersizing starters

 

[gordonl]

The only caution I can share is when using ground fault or overcurrent protection make sure that you have enough delay in the protection to allow a fuse to clear before the starter operates.

Starter interupting capacity is generally low compared to system fault levels, so the fuse has to clear faults of large levels. But time delay fuses can have a substantial delay for currents above the interupting rating of the starter, so a time delay is required on the protection so the fuse clears before the starter is requested to operate.

The larger the starter the higher the interupting rating. (From memmory I think it is ten times the steady state current rating)

 

[dpc]

I think it depends on the starter type. There are major differences between the NEMA standards and IEC standards for starters. IEC starters must be more closely matched to the motor size and application. NEMA starters are generally much larger for the same horsepower rating and may be more "forgiving" in terms of overloading.

I agree with Gordonl that the contactor has a much lower interrupting rating than the fuse or circuit breaker ahead of it.

 

[Marke]

You must provide branch protection (either fuses or a circuit breaker) sized for the cable that you use. Over sizing the branch protection could cause a fire unless you oversize the cable as well.
The starter itself will last longer if you oversize it. Generally contactors are given a rating based on a specified number of operations and duty. The aging of the contacts is essentially a thermal characteristic. Increasing the rating of the contactor will reduce that heating anp allow for more operations. Reducing the size of the contactor will reduce the number of operations and will eventually cause the contacts to weld closed.
The starter incorporates some form of motor protection. This must be sized to protect the motor being used. Oversizing the protectio is no problem provided it can be set to the rating of the motor. If it can not be set down to the rating of the motor, there is the posibility that the motor will experience an overload that will cause motor failure or degradation of life expectancy. Mark Empson

http://www.lmphotonics.com

 

[TheDOG]

The most important consideration when sizing starters is to size based on current (amps) NOT motor power (Hp or Kw). Motors of the same power but different manufacture may have FLC's of up to 20% difference!
As Marke stated, contactor life is given at a nominated current, duty and operations (usually 1 million). Reputable contactor suppliers will be able to provide de-rating curves for their contactors ie. as switching current increases, expected number of operations decrease.

 

[JBD]

The major areas of concern are (not in any particular order):

Number of starts per hour, and number of starts per minute.
Length of start (acceleration time)
Jogging (rapid start and stop, like a crane)
Plugging (rapid forward then reverse)

Power rating, can be in amps, horsepower, or kilowatts and voltage.

Type of overload protection (i.e. thermal or solid state, fixed or adjustable)

Parts, do you want to fix/repair or toss out and buy new.

Physical size and enclosure requirements.

Note: in the NEMA vs. IEC debate, the biggest performance difference is the inherent oversizing of NEMA devices. For a 480V 10HP (nominal 14A) motor NEMA uses a 27A device while an IEC device would be rated 18A.