line protection for motors

Hi guys !

I am doing a job for 12 AC motors 1.5 kw therefore I am to choose a line protector for that. can anybody lead me to a way or a chart or experience which can help me in my designation ?

Besides as I am working on a roller table and I want to start the above mebtioned motors one by one for a very short period of 1 or 2 seconds . Does anybody have comments about choosing line protection, motor protction and relay contactors for the job ?

Sorry for my English !

Since you'll have an individual contactor for each motor, you might as well add an overload relay to each contactor to protect the individual motors. I'm not sure you can "share" an overload relay across multiple motors.

Since only one motor will be on at any time, I think you can safely calculate your branch circuit conductor size and overcurrent protection (fuse or CB) requirements based on one motor. If additional motors were accidentally started, the overcurrent device will open.

In the US, we would refer to section 430 of the NEC (National Electrical Code) and other NFPA guidelines. I don't know what the requirements are for your country.

Wait for some more responses as I have a tendency not to thoroughly read questions before answering!

beerchug

-Eric

P.S. Your English is better than most!...

You fail to state enough data for anyone to properly answer.
1. Single or 3 Phase....most important
2. Horsepower
3. Will they ever run at same time....start amps higher than run

There is no "guessing" with motors, the formulas have been around for many years.

This site may help
http://www.reliance.com/mtr/flaclc.htm

You must identify your voltage and number of phases (i.e. a 1.5KW motor will draw 3.4A at 400V 3 phase, and 12.7A at 220V 1 phase).

In the US we size multiple motor branch circuits (for general loading) at 125% of the largest motor current + the sum of all the remaining motor currents (only include motors that are running at the same time).

If all of the motors are identical and you only run one motor at a time then you can size the branch circuit as if it had only one motor. Also you can probably use a single overload relay with multiple motor contactors.

Web links (for US) are:
Fuse sizing: http://www.ferrazshawmut.com/resources/selectafuse.html.
Motor currents in KW and contactor selection: http://www.squared.com/us/products/iec.nsf/DocumentsByCategory/8BCCE5E1665CBFC785256A42005F7375

The only real piece of info missing, in my view, is single phase or three phase. Since 380V is generally three phase, I'll assume three phase. Since this is a roller table and, in view of the amp ranges in your poll, I will also assume all run at the same time. Your text posting seems to say that the motors will be started one-at-a-time.

In that case, textbook amps for those motors is 3.4 amps at full load. If they are premium efficient, could be as low as 3.1 amps. You can figure starting inrush to be 6-8 times nameplate depending on how tight the motor is designed and assuming a good stiff power supply network.

Based on the above, worst case would be 11 motors running and the last one starting. That would be 3.4 x 11 + 3.4 x 8 = 37.4 + 27.2 =
64.6 amps, worst case. (Actually, not worst case since this assumes no motors are overloaded!)

Unfortunately, the above exercise is fine for understanding purposes but probably no good for actual component selection. That is almost surely defined by the electrical codes in existence in your country. Better check them first. If the codes are detailed enough, they may very well deal with your exact situation.

Hope I helped you understand a bit more about how motors work.

I belioeve the NEC has regulations on group motor protection. If you don't have access to this info I suggest you contact a Siemens or Allen Bradley or Cutler Hammer representative you trust. They have products and systems specifically designed for group motor protection, and can give you a lot of guidance.

Thanx and sorry for my late response !

Again I found valuable information regarding my question
Pros even solve the unexplained problems!

Thanks and regards