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# motor contactor sizing

## motor contactor sizing

(OP)
Hi everybody
I want to get a good way to calculate the right size of contactor that is suitable for 3phase motor ; to be clear I want to know ;should I use the rated current or starting current ?
Thanks for all

### RE: motor contactor sizing

The horsepower rating is the one that matters.

### RE: motor contactor sizing

Are you considering NEMA rated or IEC rated contactors?
NEMA rated contactors are more conservative (hp-to-hp) than their IEC counterparts. There is nothing wrong with IEC contactors, just be aware they rated differently.
GG
David is correct: all motor contactors are rated in motor hp (or motor kW)

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

### RE: motor contactor sizing

The Ampere rating on motor starters is for use as contactors with non-motor loads.

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: motor contactor sizing

Match HP of motor to HP rating of the contactor.

Just be aware that a motor which has a higher than typical FLA might need special consideration. When using NEMA rated contactors you generally can size HP to HP, but with IEC you have to pay more attention to the FLA of the motor. For example, a slow speed motor can have a much higher FLA than normal and require an oversized contactor.

### RE: motor contactor sizing

Hi Lionel,
Did you mean to say "LRA" (locked-rotor) and not "FLA" (full-load) above?
Curious,
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

### RE: motor contactor sizing

Hi GroovyGuy,

High pole-count motors have higher no-load current and full-load current than the more typical 4-pole motors. IEC motors are sized based on power output and the kW ratings printed on the contactors are based on the most common 2-pole and 4-pole motors, so if you have some oddity like an 8-pole or 12-pole machine then selecting the contactor based on the kW rating printed on the contactor will likely lead to an undersized contactor being chosen.

### RE: motor contactor sizing

GroovyGuy - I meant just what ScottyUK posted.

A similar argument probably could be applied to the LRA of certain motors though.

### RE: motor contactor sizing

Scotty & Lionel,
Got it. Thanx
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

### RE: motor contactor sizing

M Rashwsn,
To put what my colleagues have said on non-country specific terms, contactors used for controlling motors must be selected based upon the listed and labeled motor POWER rating (be it HP or kW), not the current rating alone (with a nod to the outliers where the motor FLC is higher than normal). This is because there is more going on than simply the running current, and although not technically unacceptable, if you sized every contactor for the starting current, they would last forever but you may break your budget.

When a contactor is rated for a particular induction motor power rating at your given voltage, it is inherent in the design that the contacts will handle the FLC continuously as well as any LRC that a motor of that size will be expected to draw under normal circumstances.

From that point is where IEC rules and NEMA rule diverge. IEC sizing rules include a specifically detailed duty cycle capability, with limits on the number of cycles per minute and minimum times between cycles before you begin a de-rating process, so the selection involves some minor engineering function to know how they are intended to be used. NEMA designs are intended to take the absolute worst case scenario and survive, after being selected by an electrician with little or no engineering background nor concern for how it might be abused.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

### RE: motor contactor sizing

jraef,

In general terms a NEMA contactor is pretty much equivalent to an IEC contactor sized according to its rating under AC-4 conditions (plugging / inching / jogging). IEC contactors are typically marketed based on the much less onerous AC-3 rating (DOL) and it usually works out that there's a full frame size difference between contactors sized for AC-3 and AC-4 duty. It won't come as a surprise that AC-4 contactors are pretty close in size and cost to NEMA contactors, while AC-3 contactors are considerably smaller and cheaper than a NEMA type.

### RE: motor contactor sizing

Hi Keith,

Thanks.

We have a lot of NEMA stuff where I work as a result of a joint venture between GE and the British company Allenwest in the 1970's which carried the 'Simplex' name. I think the deal was pretty much to produce GE's 7700 series MCC's under license in the UK. The NEMA gear has lasted very well, although at forty-odd years old it is starting to get tired and direct equivalent spares are harder to find without resorting to expensive imports from your side of the water. When we have to replace an old NEMA contactor I tend to use the AC-4 rating as the starting point for identifying an IEC replacement. The additional component cost is more than offset by improved reliability and longevity.

### RE: motor contactor sizing

I'll be keeping the AC3/4 thing in mind as all my customers tend to be cheap and IEC contactors are dirt cheap and more importantly in-stock and online orderable.

I can't think of any jobs I've done that needed plugging or inching type processes but I'll certainly keep that in mind now. Actually, most of the IEC stuff is so inexpensive I usually figure out the required and then go up another size anyway since $10 or$15 doesn't bother my customers. Heck, they're still surprised by the low cost since they all realize they'd be bent-over if they went down to the local electrical supply without a contractor's license and bought the wrong stuff themselves. :)

Keith Cress
kcress - http://www.flaminsystems.com

### RE: motor contactor sizing

The old U-frame motors were pretty rugged. We used to plug them, instant reverse them, jog them and generally abuse them.
I remember one 40 HP with only a start button and a jog reverse button. To stop you hit the jog reverse. If there was a jam-up that had to be cleared quickly the operator would hold down the button until the motor was stopped visually, then start jogging reverse until the jam-up cleared.
Don't try that with a T-frame motor unless you have spare motors in stock.
Some folk made a distinction between jogging and inching.
Jogging was repeated short starts.
In some usage inching was a primitive form of low frequency drive.
I have never seen such a drive but it was covered in some old text books. Inching may be used when a very large machine needed to be precisely positioned for maintenance.
A drum switch with continuous rotation controlled a bank of contactors, closing and opening them is sequence to produce a pseudo three phase source with a stepped wave form. A DC welding machine or similar supply may be used.
I mentioned this method here some years ago and was dissed.
Then one of our Gurus posted a short video of a bank of contactors being used in an inching application as I had described.
The motor rotation follows the rotation of the drum switch giving slow controlled rotation in either direction.
With a slower speed motor it was possible to position the machine to within less than one revolution of the motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: motor contactor sizing

Bill, I once helped build large soft starter to replicate an "incher" system for a large ball mill that had been built in the 40s, used to position the access hatch. It was, after observing it work, essentially using an array of contactors to create a form of cycloconverter! Oh my was that ever ugly, the clacking and banging of contactors to create the pulses was deafening and put on a spectacular light show if you left the cabinet doors open. I'm surprised nobody has put a video of that up on YouTube.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

### RE: motor contactor sizing

That is the context that I remember. Positioning an access hatch.
Did the sift starter give the precision of the original system?
There was a similar video posted or linked on Eng-Tips some time ago.

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: motor contactor sizing

No, the soft starter was not as accurate, but it was close enough most times. What they did was leave the incher in place so that it could be used as a bypass of the soft starter, so they would get close or slightly over shoot it, then use the incher to nudge it forward or back only when necessary. About a year later we rebuilt the incher system for them too, because even though they used it 1/10th the time they had in the past, it was still brutal to the contactors.

You were right Bill, here is the link to where LionelHutz posted his video a couple of years ago. Good memory.

Can't make the video play any more however. Pity...

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

### RE: motor contactor sizing

I can't get the video to play but I am getting the sound.

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: motor contactor sizing

Save it to disk and it plays in VLC player. I needed to add the .avi file extension back on.

Terrible video quality but you get the feel for this strange piece of equipment.

### RE: motor contactor sizing

I'm not getting it and only have the horrid VLC on my phone. :)

Is it worth the hassle to see the vid Scotty? What's it actually showing, just the ball mill or the contactors?

Keith Cress
kcress - http://www.flaminsystems.com

### RE: motor contactor sizing

A few seconds of grainy footage of contactors and the drum switch, and some bright flashes. The sound is what adds the drama to the clip.

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