Why no internal overloads on 3 phase Motors?

[Ingenieur]

Practicality?

if remote mounted you would need to run control wires back to the starter which is the point of interruption
or have another contactor in the motor

I guess you could rig something up for smaller motors
keeping it in the controller seems to make sense for ease of maintenance, troubleshooting and adjustment

the people who developed this science are pretty amazing

 

[kwired]

Practicality?

if remote mounted you would need to run control wires back to the starter which is the point of interruption
or have another contactor in the motor

I guess you could rig something up for smaller motors
keeping it in the controller seems to make sense for ease of maintenance, troubleshooting and adjustment

the people who developed this science are pretty amazing

It is/can be practical in smaller motors and has been mentioned there are some out there. No control wires needed, the motor line current passes directly through the overload just like it does when using similar device in a single phase motor, and if there is overload condition it does interrupt current to the motor.

Biggest limitation Jraef mentioned is they monitor the three leads of the wye point of a wye wound motor and will not work on a delta wound motor, or would at least need different design if they were to work on a delta wound motor.

All one needs to do is interrupt two of the three leads and you have opened enough of the circuit that the motor will not draw any current.

 

[Ingenieur]

It is/can be practical in smaller motors and has been mentioned there are some out there. No control wires needed, the motor line current passes directly through the overload just like it does when using similar device in a single phase motor, and if there is overload condition it does interrupt current to the motor.

Biggest limitation Jraef mentioned is they monitor the three leads of the wye point of a wye wound motor and will not work on a delta wound motor, or would at least need different design if they were to work on a delta wound motor.

All one needs to do is interrupt two of the three leads and you have opened enough of the circuit that the motor will not draw any current.

Guess that's why I said perhaps on smaller motors lol

I can't recall seeing a 3 ph motor without ext ol's
perhaps a hermetically sealed compressor?

why would delta or wye matter?
3 power leads either way with the i differing by sqrt 3 for a given hp
the neutral only carries a small imbalance
???

 

[kwired]

Guess that's why I said perhaps on smaller motors lol

I can't recall seeing a 3 ph motor without ext ol's
perhaps a hermetically sealed compressor?

why would delta or wye matter?
3 power leads either way with the i differing by sqrt 3 for a given hp
the neutral only carries a small imbalance
???

No source neutral involved, the internal wye point of the motor is made in the overload. One could do a delta motor but would need six leads to connect to the overoad, by making the wye point of the motor windings within the overload you get all three winding currents passing through the overload with only three leads necessary.

Only place I ever recall running into such an overload was on some farm duty poultry house fan motors and has been 20+ years ago, so I'm not saying they are common, but is possible and they do or at least once did exist.