Chosing proper CB for single phase induction motor
Chosing proper CB for single phase induction motor
(OP)
How can I chose proper circuit breaker to be used in a single phase induction motor?
Example case:
Power: 5hp
Voltage: 230v AC
Example case:
Power: 5hp
Voltage: 230v AC





RE: Chosing proper CB for single phase induction motor
RE: Chosing proper CB for single phase induction motor
P=5hp=746*5w=3730w
V=230v
I=P/V=3730/(230*0.75)=21.6A [where pf=0.75]
How the full load current of this motor be 28A? Why CB would be 250% of full load current? Usually, do the CBs not allow 3 to 5 time of rated current?
RE: Chosing proper CB for single phase induction motor
RE: Chosing proper CB for single phase induction motor
Now, my question is Why CB would be 250% of full load current? Usually, do the CBs not allow 3 to 5 time of rated current?
RE: Chosing proper CB for single phase induction motor
RE: Chosing proper CB for single phase induction motor
Only for a short period of time. Circuit breakers have two tripping mechanisms - thermal and magnetic. If you apply continuous current above the rating of the breaker, it will eventually trip on the thermal element. The thermal element has an inverse time characteristic.
250% is the maximum rating allowed. It can be smaller if you like.
RE: Chosing proper CB for single phase induction motor
*(presumed by virtue of you using HP to describe your motor and the absence of the word "Eh?" in your posting).
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RE: Chosing proper CB for single phase induction motor
RE: Chosing proper CB for single phase induction motor
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Chosing proper CB for single phase induction motor
The motor is protected from overload by design. One selects the motor so that it will adequately provide all the power required for all normal loading, with out unduly overheating. The thermal overloads protect the motor windings from thermal damage should the mechanical system fail, say like bearings dragging, and push the motor into overload. The circuit conductors are also protected from overload by the thermal overloads. (Assuming you are under the NEC - but also likely true for the rest of the world) The overloads are selected to trip inside of the motor thermal damage curve. The conductors are selected such that the thermal damage curve is also outside of the overload trip.
The primary purpose of the circuit breaker is not to protect the motor or the conductors in the event of a mechanical failure. That's the job for the overloads.
If the motor faults internally amd disentigrates into a molten pile of slag, there is no motor to protect - only a fire to put out. If the cable suffers a forklift attack, again, no cable to save, only a fire to put out.
The primary purpose of the CB is to protect the structure - limit the electrical heat input keeping the fire going.
The CB is selected such that the motor will reliablily start. From the OP, I suspect the CB will be a UL489. You should be able to find a trip curve and download. Print out an 11x17. Hand plot an expected motor starting curve. The motor could draw 4 to 6 times FLA for the starting time. The CB has to be outside of the motor starting curve.
Hope this helps
So sayeth the worm
RE: Chosing proper CB for single phase induction motor
RE: Chosing proper CB for single phase induction motor
You will need to install thermal overloads.
You may also wish to install a UV trip - although that would be somewhat uncommon on a 5hp single phase
ice
RE: Chosing proper CB for single phase induction motor
A breaker that protects against short circuits will not protect against overloads.
Most overload devices will not operate fast enough to protect against short circuits.
Exception, there may be some devices that have both functions in one device.
Bill
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"Why not the best?"
Jimmy Carter