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Old 03-13-2013, 07:22 PM   #1
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I was taught that all motors over one horsepower should be thermally protected externally(by installer) because manufacturers usually provide internal thermal protection on fractional HP motors.
How true is this? Do I always have to provide overloads on every motor that doesn't say "TP" ...it seems a bit expensive for a $100 motor and $200- $300 worth of heaters and contactor
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Old 03-13-2013, 08:58 PM   #2
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Originally Posted by Threefaze View Post
I was taught that all motors over one horsepower should be thermally protected externally(by installer) because manufacturers usually provide internal thermal protection on fractional HP motors. How true is this?
Usually is not always. There is no rule that says they have to, I have seen plenty of FHP motors that did NOT have internal thermal protection. In fact MOST 3 phase motors do NOT come with it.

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Do I always have to provide overloads on every motor that doesn't say "TP"
Yes. You cannot ASS-u-me that it MIGHT be there, you can only deduct that if the nameplate does not EXPLICITLY say that the motor is thermally protected, then it is not. embedding the thermal protectors is something that costs them money, so believe me if they paid to include it, they will brag about it.

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...it seems a bit expensive for a $100 motor and $200- $300 worth of heaters and contactor
$100 motor, I assume you mean single phase? If it's manually controlled, all you need is a manual motor starter and for 115V motors, it has only 1 heater element. $77
http://www.grainger.com/Grainger/SQU...385?Pid=search

And forget the cost of the overload relay or even the motor, what does it cost for the machine to not be making money with whatever it is supposed to do? For example the machine makes you $100 / hour, you smoke an unprotected motor and have to wait just one day for the replacement to arrive, you just lost $800.
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Last edited by JRaef; 03-13-2013 at 09:01 PM.
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Old 03-13-2013, 09:09 PM   #3
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Thank you
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Old 03-13-2013, 09:58 PM   #4
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Also, a fuse or set of fuses is code compliant for O/L protection, though O don't recommend it. Too easy to install the largest size that will fit in the fuseholders.
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Old 03-13-2013, 10:06 PM   #5
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And forget the cost of the overload relay or even the motor, what does it cost for the machine to not be making money with whatever it is supposed to do? For example the machine makes you $100 / hour, you smoke an unprotected motor and have to wait just one day for the replacement to arrive, you just lost $800.
So long as it is only a quick swap.. If the machine works 24/7 and you need to wait til the next morning for the replacement, you are now at $2,400 in production losses.. There go any savings for not installing overloads...
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