factors affecting maximum current surge from a pump motor

[charliealphabravo]

Hi all,

I am interested in the factors that are considered when the current rating for a motor is determined.

For example, a 110V single phase sump pump motor is stamped 9 amps. What does the 9 amps actually represent.

Can any scenario cause the breaker to see a load from the pump that is higher than that caused when the motor rotor is locked?

Or from another direction, if the motor is operating under a heavy load using a long extension cord, can the load that the breaker sees ever be larger than the load caused by a rotor lockup, irregardless of the length or capacity of the cord?

In short, does the ampacity of the extension cord have any effect on the load measured at the panel under any circumstances??? It seems obvious that it would but I'd like a little help getting some traction on the question.

I have attached a picture of the motor i am looking at.

 

[MiketheEngineer]

My understanding is that at start up you might exceed 9 amps for a short moment. IF you lock it, you definitely will.

If the extension cord is too small, it won't be able to deliver the amps and will overheat and die(burn). But the circuit breaker probably won't pop because it is happy to keep on delivering 15 maps or so. That is why you must never under size a wire. You want the breaker to pop - not the wire.

The old screwed in fuses worked that way. Using a very carefully calibrated piece of wire, if there was an overload, it being the smallest wire would overheat and melt opening the circuit.

 

[dpc]

So, 9 A is the current at rated output (1 hp). If the motor is overloaded, current will be higher (at least for a while).

At locked rotor, current will be many times this.

A long extension cord has a lot of resistance and will cause the voltage at the motor to be less. This could cause a somewhat higher current in the motor.

Maybe if you explained the actual problem or question, you would get better answers.

David Castor

http://www.cvoes.com

 

[charliealphabravo]

yes, thank you all for making a stab at my wanderings.

I am really just asking for my own edification but the operation of the pump is related to my inquiry into the flooding of a basement.

the pump is operating on a general purpose extension cord that is plugged into an outdoor receptacle on a 15 amp circuit on a sub panel served by a residential main panel (frog onna log inna hole...)

The main panel circuit breaker serving the subpanel is throwing after sustained operation of the sump pump (like after 3 or 4 hours). upon resetting the breaker, presumably after allowing the motor to cool, the system will function normally again for a time.

I'm not so much concerned with this phenomenon since it is pretty clear that the sub-panel circuit is overloaded and in fact some other well or pool pump may be causing the intermittent overload on the circuit.

My question is really toward the effect of having the motor on an extension chord. If the capacity and operation of the breaker is ignored...what would a person monitoring voltage or current at the sub panel SEE if the motor was operating under heavy load for an extended period of time? What effect does the ampacity of the extension cord have on this measurement?

thanks again.
cab

 

[jraef]

Lets look at it starting at the other end.

The motor nameplate says something like 9A FLC (Full Load Current) or 9FLA (Full Load Amps), which means if the motor is getting its full rated voltage of 115V and the pump is working at it's rated full capacity, the motor will draw 9A. It can do this virtually forever, or at least until something mechanical wears out and fails.

A pump (not the motor) is designed to provide a certain amount of head (pressure) and flow. The selection of the pump was based on having to provide the required HP (torque at a speed) to lift the water X feet (or meters) and also to move Y gallons (liters) per minute at that height. The size of the pipe and the amount of head determines the maximum flow through the system, and the amount of flow is what determines the HP load on the pump motor. As long and the rated HP of the motor exceeds the required HP of the pump, you are good to go.

If you do something to change the flow however, you change the loading. As an example to that end then, let's say the pump was selected to pump into a 3/4" pipe and lift the water 10' out of the basement. But then let's say someone connected it to a 2" pipe and only had to lift it 5'; more water would flow and the motor would overload. But short of something like that, it's fairly difficult to overload a pump.

But...

If the voltage to the motor is LOWER than rated, then the motor peak torque drops by the square of the voltage reduction. Having too long and/or too small of an extension cord adds resistance to the circuit, and that resistance drops the voltage which reduces the torque capacity of the motor. So if you had a 1/2HP motor and used too small of a too long extension cord, you drop the voltage, which drops the torque, which drops the HP and you end up with a 1/3HP motor. So just like the situation above where we had the pump motor designed for a smaller pipe than was used, you end up in the same place by having the same size pipe but essentially a smaller motor. What happens then is, the motor TRIES to do it's job like a dumb animal but can't, so the motor slows down, which increases the slip in the motor, which causes it to draw more current. That extra current will eventually trip your circuit breaker.

But be aware, the circuit breaker is basically only preventing a fire in the wires. You are incrementally damaging the insulation in that pump motor every time you do this.

"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

 

[7anoter4]

Continuing what jraef explained already, this Little-Giant submersible pump MODEL # 506171 6-CIA-RFS of 1/3 hp [shutoff height 18 ft and rated flow 46 gpm at 5 ft] is droved by a 720 w[0.965 hp]115 V single phase 60 Hz motor. The rated current of the motor is indeed 9 A. See:

http://www.little-giantpump.com/506171_1_3.htm

and:

http://www.franklin-electric.com/media/documents/995201.pdf

But will never draw more than 3.1 A as the pump cannot load the motor more than 1/3 hp.[for 46 gpm and 5 ft height].
By the other hand, at the start the motor will draw about 50 A for milliseconds in any case. If the motor will continue to stay locked and the protection will not clear the fault in time the motor will be burned-out.
I don't think in this condition 10 ft flexible cable will produce such a voltage drop to force the motor to draw more than 9 A in steady state case.

 

[jraef]

Did I miss the 10 ft cord
length somewhere? I agree, that would be inconsequential. Unless maybe it was 24 gauge, in which case it would likey melt and act as a fuse of sorts. Luckily I don't think they sell 24ga extension cords, yet...

"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

 

[charliealphabravo]

i don't think i said but in this case, the sump standpipe is about 13 feet deep plus the extension cord which might be 25 or 50 feet long.

so i think i follow this for the most part. if the cord were to be very long, say 200' or 400', this would cause the motor to see a lower voltage and have less power. but what is the observed effect at the panel if the cord is made very long? does the cord just heat up or does the motor stop or does the breaker throw?

thanks again for the consideration.

(i'm not sure i follow the reasoning about the pump tho. i think that the load on the pump will increase with a taller lift and smaller hose since the load is a function of the head and pipe losses.)

 

[MiketheEngineer]

BTW - I think most local codes frown on using extension cords on sumps. Wire it up correctly and some/all your problems may go away.

 

[charliealphabravo]

my thoughts exactly

 

[7anoter4]

The Power Cord is 18/3 SJOW-A [18 awg 3 cores copper cable] In "Description" [first link] it is noted: 10 ft Power Cord.
The resistance at 20oC is 0.528 ohm/1000 ft and for two conductors at 70oC 10 ft will be 0.012635 ohm. Reactance is negligible [0.00032 ohm].Then the voltage drop will be 0.11 V for 10 A 0.85 PF. At the start the voltage drop will be still less than 1 volt.For 400 ft and 10A the voltage drop will be 4.3 volts and at start for 60A will be 26 V 23% [may be still acceptable].

 

[jraef]

Ah, so the 10' cord is what comes on it, not the extension cord. Makes sense now.

Very thorough response there 7anoter4 (I'm not checking your math though). Mine was more theoretical in response to his wanting to know what effect an extension cord might have.

 

[waross]

I'm thinking
".what would a person monitoring voltage or current at the sub panel SEE if the motor was operating under heavy load for an extended period of time?"
Smoke?

Also, the first 1/2 cycle the current may be much greater than locked rotor current. If the pump cycles off and then restarts, the starting transient may trip a fairly large breaker. I have seen some fairly large transients from small pumps.

Be aware that the pump motor may develop 1/3 HP but if the voltage is low the current must increase to develop the same HP.

Bill
--------------------
"Why not the best?"
Jimmy Carter