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#177633 - 05/08/08 09:13 PM Thermal Overload Protection  
dura101  Offline
Joined: May 2008
Posts: 13
I have always been confused by the code when it talks about overload protection.

If I have a 120v motor and it's full load current is 1amp, and it says on the nameplate that the motor is thermally protected, do I have to install an overload for this motor?

-not attended while in operation

I understand the code when it talks about larger motors but the smaller ones still confuse me.

Any help, and discussion on this matter would be appreciated.

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#177681 - 05/09/08 04:57 PM Re: Thermal Overload Protection [Re: sparkyinak]  
dura101  Offline
Joined: May 2008
Posts: 13
Anybody out there?

#177686 - 05/09/08 06:44 PM Re: Thermal Overload Protection [Re: dura101]  
dougwells  Offline

Joined: May 2003
Posts: 1,137
kamloops BC Canada
I would say no that isnt much of a motor

#177719 - 05/10/08 08:24 PM Re: Thermal Overload Protection [Re: dura101]  
Trumpy  Offline

Joined: Jul 2002
Posts: 8,211
SI,New Zealand
Hi there dura,
What is this motor running, in terms of a load on the shaft?

Most single phase motors of this size often have a simple circuit-breaker on the terminal box of the motor, it is meant to trip at a given value of current, not unlike the over-current trip on a multi-plug board, if the motor does trip, you just press the overload and the motor should be able to be re-started.
I've never seen external overloads used on fractional HP motors, but I could be wrong too.

With larger motors, however, they normally have thermistors embedded in the windings, the tails of which come out into the motor terminal box and are connected to an external control circuit and contactor that disconnect the motor, should the windings get too hot.

In this case though, I don't think this application warrrants a thing like that.

Last edited by Trumpy; 05/10/08 08:27 PM. Reason: Typo's

Let's face it, these days if you're not young, you're old - Red Green grin

#177730 - 05/10/08 11:48 PM Re: Thermal Overload Protection [Re: Trumpy]  
twh  Offline
Joined: Mar 2004
Posts: 905
Regina, Sask.
I think the confusion is about a $50 motor protected by $150 of overload devices.

Thermal protection doesn't meet the requirements of 28-302(b) for overload protection unless it is responsive to motor temperature and motor current. I'm not sure that it is.

28-308(c) provides that "a motor constructed so that it cannot be overloaded" does not need overload protection. An example would be a bathroom exhaust fan that has such high resistance that when it stalls, it doesn't draw much current. Low current motors tend to be like that.

I apply that rule so that I don't have to install overload protection on furnace motors. I've seen many thermally protected furnace motors stall and trip the thermal protection. The windings remain intact.

Apply the rule differently and we'll be putting overload protection on bathroom exhaust fans, furnaces, air exchangers, air conditioners, etc.

#177738 - 05/11/08 06:38 AM Re: Thermal Overload Protection [Re: twh]  
dura101  Offline
Joined: May 2008
Posts: 13
Thanks TWH, that is what I was looking for.

#177837 - 05/13/08 02:42 PM Re: Thermal Overload Protection [Re: dura101]  
mr_electrician  Offline
Joined: Jun 2007
Posts: 106
London, Ontario
No you don't. See 28-308(d) This talks about your overload protection allready installed and part of the motor. Probably a thermister.

Never trust an electrician with no eyebrows!!

#177856 - 05/13/08 09:46 PM Re: Thermal Overload Protection [Re: mr_electrician]  
dura101  Offline
Joined: May 2008
Posts: 13
Hi, I can see why you would say that I guess. Lets just say for argument sake that this motor has a little controller and it must be started by pushing the start button, not automatic starting. Overheating protection (thermally protected) is not the same as overload protection.

I know that current thru the windings of the motor create a temperature rise but I always thought that the bi-metalic strip that opens the motor winding was calibrated for a specific temperature. Not the the temperature created specifically by 1.15 or 1.25 times the FLC.

But I do see 28-302(1)(b)- It says that thermally protected motors don't need overload protection just as long as they respond to temperature and current. They must also be able to protect the circuit conductors, control circuit conductors as well as the motor windings.

So if you had a small controller for this motor with a contactor the thermal protection would open the motor windings only. It wouldn't open the control cicuit like a NC contact that is in an overload relay would do.

#177974 - 05/20/08 08:42 AM Re: Thermal Overload Protection [Re: dura101]  
mr_electrician  Offline
Joined: Jun 2007
Posts: 106
London, Ontario
Motors that I have seen that employ thermistors have had the terminals extend to the j box. You could probably use the thermistor wires in your control cct so that when the thermistor senses that the windings are to hot that it would open the control cct.

Never trust an electrician with no eyebrows!!

#178062 - 05/22/08 01:59 PM Re: Thermal Overload Protection [Re: dura101]  
mikesh  Offline
Joined: Jun 2006
Posts: 613
Victoria, BC, Canada
No you don't have to add an overload. Sorry code book is not handy but section 28 does mention thermally protected motors.

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