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; 05/10/0808:27 PM. Reason: Typo's
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.
Re: Thermal Overload Protection
#177738 05/11/0806:38 AM05/11/0806:38 AM
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.
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.