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Overload Setting |
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 Jun 22 2009, 02:21 AM
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#1
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 Group: Members Posts: 1 Joined: 22-June 09 Member No.: 5,593  |
Hello, I am new in this site. Just want to know if its allowed to use one overload for two motors, they are the motors in each side of a Overhead crane. If so, how do I set the overload? Should I add the FLC of each motor? thank you QUOTE (marke @ May 5 2009, 04:40 AM)  Hello Froilan There is a 15% safety margin in the ratings of the motors to allow for tolerances in the calibration of the protection devices. Theoretically, a protection device must not trip at less than rated current and must trip at 115% rated current. Typically, with good protection relays, the average trip point is in the order of 105% rating, but commonly there are 3% - 5% tolerances in CTs etc. If you set the trip point at 115%, you can expect the motors to trip at 120% - 125% on average. This is enough to shorten the life of the motor. If the protection operates late, the motor can operate at a higher temperatures and not necessarily fail immediately. Every ten degrees C hotter you run the motor, you halve it's life, so you may not see the effects of a higher overload setting for some time. Best regards, Mark. |
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Jun 22 2009, 10:33 AM
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#2
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Member Group: Full Member Posts: 78 Joined: 13-June 09 Member No.: 5,502 |
Hello.
On my view - no problem, especially as the same power motors work in parallel and have the same loads. Chose only the correct overload. |
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Jun 22 2009, 10:54 AM
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#3
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Member Group: Full Member Posts: 78 Joined: 13-June 09 Member No.: 5,502 |
So, put it the value what Mark had mentioned (if 10 A each, then 20 A + 15 %). Also, usually, all motors' currents slightly (but in some cases considerably) differ between phases. Such as one I saw recently working with 120/90/140 A. Then, it would be better if there were 2 contactors, and you rearanged phase sequencies for each motor so, that the common currents differ the least (preserving the same rotation direction of the motors, of course).
However, of course, no need for two contactors. |
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Jul 16 2009, 02:10 AM
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#4
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Junior Member Group: Full Member Posts: 7 Joined: 16-July 09 Member No.: 5,871 |
I would not recommend the situation you are suggesting if you are planning on running the motors anywhere near full load current. If you set the overload current relay to 215 % then you could be running 50 % load on one motor and 150 % load on the other motor and not trip. Depending on how the motors are coupled it may be possible to have a bearing failure on one motor, and not catch the problem because your overload is set so high, or if the track the crane is riding on is jamming on one side, but not the other, you could see significantly more load in one motor than the other. Thermal overload relays are relatively inexpensive, and I would recommend using one on the leads to each motor. If you have higher currents than the standard overload relays handle you can run both of the contacts into the same trip circuit.
Hope this opinion is helpful Thanks Dave |
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Jul 18 2009, 06:53 PM
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#5
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Member Group: Full Member Posts: 78 Joined: 13-June 09 Member No.: 5,502 |
Of course you are right Dave. Saved a hundred or so on a proper safety device today may and wiil lead to lost thousands tomorrow. I do not reccomend it also. However, economy on one contactor, one relay....and given the application: if motor is jammed in one shoulder of the cran the motor in another shoulder unable to cope with the load would also be.
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Jul 18 2009, 08:57 PM
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#6
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Posting Freak Group: Moderator Posts: 2,383 Joined: 24-April 02 From: Christchurch, New Zealand Member No.: 1 |
I would only consider this as a last resort, if and only if the motors were identical and were driving the same shaft which they probably will not be. Just a small difference in speed can yield a big difference in load.
Best regards, Mark. -------------------- Mark Empson administrator
Skype Contact = markempson | phone +64 274 363 067 LMPForum | LMP Tech Warehouse | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | LMP Software | Mark Empson Website |
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Jul 25 2009, 04:12 AM
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#7
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Junior Member Group: Full Member Posts: 8 Joined: 25-July 09 Member No.: 5,969 |
Actually can. But we must sure that the 02 motors normally run together to ensure of protection. For the small motor, just adjust the setting range slightly above the trip point. It means that you run these 02 motors, then reduce the setting until the overload relay trips. Then move one step beyond this point.
Regards, |
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