Motor contactor 2

[Jk1996]

Am I correct in thinking We size contactors for the full load current of the motor not the size of the main fuses supplying the motor circuit?

 

[che12345]

Dear Mr. Jk1996 (Electrical)(OP)18 Oct 21 22:33
" Am I correct in thinking We size contactors for the full load current of the motor not the size of the main fuses supplying the motor circuit?"
Please tell us where (Europe=IEC or US=ANSI,UL, NEMA) are you, or which particular standard you are referring to. Or, could be for marine application etc. The European and the US standards are poles apart, in practice, ratings, selection criterial etc.
Che Kuan Yau (Singapore)

 

[waross]

In North America, they are sized for HP. Motor starters will have a HP rating for each standard voltage.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

Contactors are sized for full load current of motor and the contactor nameplate (Label) indicates recommended maximum back-up fuse rating.
The scheme designer to ensure the recommendation is not violated.
Fuse rating is decided based on motor starting current / duration, permitted number of consecutive starts and of course ambient temperature in the cabinet.

 

[cranky108]

I propose that, if the motor fails, the trend is to replace it with a bigger motor.
I don't agree with that practice, but...
It might be worth looking at sizing the contactor a little larger.

 

[waross]

Many years ago, I worked in one of the first high speed, high efficiency sawmills in the Pacific North West.
Designing such mills was still a work in progress.
Most of our motors were undersized and motor burnout was frequent.
If it was possible, we would have always replaced motors with larger sized motors BUT:
The crew could swap out a motor in 10 or 15 minutes.
Going to a larger sized motor would normally entail a new base plate to align the gear reducer with a physically larger motor.
In many cases, that would lead to gear reducer failure unless the gear reducer was also up-sized.
We were short handed on both the electrical crew and the millwright crew. Out of the whole group, we had only one person with the skill and experience to do a quality job of fabricating and aligning a new motor/gear reducer set.
Due to his above average ability, he was always very busy.
The one time that we did up-size a belt drive motor, the unintended consequence was that the driven machinery failed and shut the mill down for three shifts. The lost revenue down time was estimated at $72,000. This at a time when the head electrician made under $4 per hour.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Dear Mr. cranky108 (Electrical)19 Oct 21 13:16
I propose that, if the motor fails, the trend is to replace it with a bigger motor.
I don't agree with that practice, but...
It might be worth looking at sizing the contactor a little larger.

 

[che12345]

@ Dear Mr. cranky108 (Electrical)19 Oct 21 13:16
"..1) I propose that, if the motor fails, the trend is to replace it with a bigger motor.......I don't agree with that practice, but...2)It might be worth looking at sizing the contactor a little larger."
1. I do NOT agree if a motor fails, replace it with a bigger. Find out the cause of the failure is more important. Not all failures are due to over-loading. In most cases it had been fine for say months/years without problem. However, if it had been consistently over-loading due to increase in load over the years. Replacing it with a bigger is fine, but to what size?
2. I do NOT agree, as in most cases motor failure is NOT due to under-sized contactor. This is evident that in most case we replace the failed motor but noticed nothing wrong with the starter switching function or contactor size.
BTW: NEMA contactors are with plenty of extra capacity and IEC contactors are selected with AC-3 (without plugging or inching) and usually with one million electrical switching or more.
Che Kuan Yau (Singapore)

 

[cranky108]

Putting "Mr" in front of my name implies you are talking to or about my father.

People in some industries don't have an engineer on staff. They replace the failed motor with a larger one.
I've seen it many times.

 

[jraef]

In North America, they are sized for HP. Motor starters will have a HP rating for each standard voltage.

Even in IEC based countries, the contactor for a motor is sized for the motor kW, not just the Full Load Current.

Jk1996,
There is more to switching a motor on and off than just the FLC, using kW or HP ratings account for those extra factors.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[che12345]

cranky108 (Electrical)20 Oct 21 19:17
" ...Putting "Mr" in front of my name implies you are talking to or about my father....People in some industries don't have an engineer on staff. They replace the failed motor with a larger one. ...I've seen it many times.".
1. I used Mr. is a form of [respect]. English is NOT my mother tongue, but I am surprised that you took offence. I am NOT INTERESTED and have nothing to talk to or about your father. STOP.
2. To replace it with a bigger motor is fine, but to what size? Isn't it better to find out the cause of the failure; instead of blindly replacing it with a bigger, if the failure is NOT due to over-loading?
Che Kuan Yau (Singapore)

 

[jraef]

che12345,
I doubt he was "upset", it is a common joke when someone says "Sir" or "Mister" to me, I will look around and say "Did my father just come into the room?" He has been gone for 22 years now and I am now the age he was when he passed, yet I still do that bit whenever I can.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[che12345]

Dear cranky108 (Electrical)20 Oct 21 19:17
@jraef
"...I doubt he was "upset", it is a common joke when someone says...".
jraef, thank you for your kind advice. I am a septuagenarian and English is not my mother tongue. Case closed.
Che Kuan Yau (Singapore)