contactor size for switching motors

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b78wang

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Im having problem looking for a contactor for switching motors.
I have to use Siemens contactors to switch a 1hp 460VAC 1.8AMPS 3 phase motor. The smallest contactor i can find from Siemens' website is rated 5 HP 460/575 V.
My question is if it's ok to use a higher rated one. What are the drawbacks. Also if anyone can point me to a more suitable contactor for the job.

I'm open for any suggestion. please be gentle to my post:D! Thank you!
 
texie said:
I agree but the OP question leads me to wonder what he is using for overload protection.
Click to expand...

As in: does he really need a motor starter with replaceable or adjustable overloads rather than just a contactor?
And if he is actually talking about a starter, does it accept low enough setting overloads?

Tapatalk!
 
GoldDigger said:
As in: does he really need a motor starter with replaceable or adjustable overloads rather than just a contactor?
And if he is actually talking about a starter, does it accept low enough setting overloads?

Tapatalk!
Click to expand...
What he likely means is that in Siemens, as in most other IEC manufacturers, the smallest contactor sold in the "standard"* product lines is rated for 9A and has a rating of 5HP maximum at 460V. So assuming he knows how to read a catalog properly, that size contactor will have the ability to be connected to an OL relay that can be adjusted down to 1.8A or less. Now if he uses a contactor rated for 50HP, no standard OL relay that will attach directly to it will be adjustable down to that low of a setting but in a pinch, one could conceivably use a stand-alone mounted OL relay that is that small, it just would not make sense unless the contactor was free or something.

The operative point however is that contactor motor size ratings are always MAXIMUM, there is technically no reason, other than those stated, to be concerned for having it be too large for the motor.

* Several manufacturers have what are called "mini-contactors" that are rated slightly lower, the smallest is often 5A and 3HP at 460V, then they have smaller OL relays that attach to them. Siemens as it happens does not have that line of contactors. They have mini-contactors (3TG) but they are rated at 400V maximum and have no direct connected OL relay available, so they are difficult to use as starters.
 
cloud is clear!

cloud is clear!

Jraef said:
What he likely means is that in Siemens, as in most other IEC manufacturers, the smallest contactor sold in the "standard"* product lines is rated for 9A and has a rating of 5HP maximum at 460V. So assuming he knows how to read a catalog properly, that size contactor will have the ability to be connected to an OL relay that can be adjusted down to 1.8A or less. Now if he uses a contactor rated for 50HP, no standard OL relay that will attach directly to it will be adjustable down to that low of a setting but in a pinch, one could conceivably use a stand-alone mounted OL relay that is that small, it just would not make sense unless the contactor was free or something.

The operative point however is that contactor motor size ratings are always MAXIMUM, there is technically no reason, other than those stated, to be concerned for having it be too large for the motor.

* Several manufacturers have what are called "mini-contactors" that are rated slightly lower, the smallest is often 5A and 3HP at 460V, then they have smaller OL relays that attach to them. Siemens as it happens does not have that line of contactors. They have mini-contactors (3TG) but they are rated at 400V maximum and have no direct connected OL relay available, so they are difficult to use as starters.
Click to expand...
I very appreciate this textbook detailed answer! appreciate everyone else's effort too. great experience posting on this forum:thumbsup:
 
As Jraef mentioned 9 amps for IEC is the smallest size you will likely ever find. If they made a lower rated one it would likely be fit into same frame with same coil - only difference would be the contacts themselves as is often the case with anything rated from 9 amp to usually 25 amp.
Cost differences would not vary by much and could even be more for the smaller unit, not because of materials used but because of sales volume.

Then you run into similar thing with another range like 32 to 50 amps are all built with same frame and coils just have different contacts installed.
 
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