Well pump drawing excessive current only when on portable generator...
- Thread starter sbrehler
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shortcircuit2
Senior Member
- Location
- South of Bawstin
The minimum portable generator I have to use on my installations to power submerged well pumps is 6500 watt. I have had a problems with the Honda 6500 Inverter type not being able to start a well pump...so 6500watt with surge capacity is needed.
At 400ft deep and power needing to run though the house electrical system to get there, voltage drop will add to the problem.
The generator is to small...
At 400ft deep and power needing to run though the house electrical system to get there, voltage drop will add to the problem.
The generator is to small...
donaldelectrician
Senior Member
OK OP , what size wire from the house to the wellhead , pump ?
Don
Don
Yes, I was getting 240V Line to Line. Each leg was pulling approx. 27A. I didn't think to check voltage as the pump was trying to run. I assumed a good 130V then went to check amperage.
I did just come across a new finding. When I ran the system with my generator (a Coleman Powermate 5500 - which has never given me a lick of trouble) I noticed that it wouldn't run without partial choke when connected to the sub-panel. I thought my carb had gunked up, but I'm now running it perfectly without choke exercising it here at my house. Now, I don't remember if I had the pump on and tried to get it to go, but I think I'll go back hook it into the system and see where I get the choke/no-choke trouble starts. My guess will be when the pump is on...
Now, if I do get a low voltage reading (which I feel that I may) along with the high current, what might the trouble be. Why would the generator's voltage drop so much that it pull so much current?
Thank you all for your inputs...
BTW, his generator is a 6500W.
I'm starting to think voltage drop. I really tried to watch the generator when it tried to run the pump. But, as I remember, it didn't seem to bog down all that much. Would it sound right if it were able to provide that much current (if only for a minute or so) while it tried to kick in the pump...it's just that the voltage dropped off?:?
He still didn't measure the voltage though. Remember that in hp ,(power) of a motor is based upon a given current at a specific voltage. If the voltage drops what do you think the current willneed to be tin porder to provide the same HP, the current must increase.
I found it not all that unusual that something be replaced assuming that it was defective and get the same result with the replacement. If try he genset was purchased new and he exchanged it for another was the first one defective?
By simply taking a voltage measurement would tell you if the genset is able to maintain voltage while supplying the required starting current. Should the rpm drop remember that the frequency also drops.
All too often it is replace this, that's not it, well then I'll replace that, oh that"s not it either, then I'll try this until you finally get to the root of the problem after which you have replace a lot of good parts.
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Okay, brain fart...
I know why the voltage may drop off (voltage drop). But, and please excuse me, but I'm not a math whiz...if it is a 1/2 hp pump (372W), and I am drawing 27 amps, would not the voltage drop to just over 13V? 327W/27A= 13.77V? That seems extreme for voltage drop to me. I'll check the wire size to the pump tomorrow and report back...
Again, thanks!
sparkyrick
Senior Member
- Location
- Appleton, Wi
Check the voltage when you start the pump. The generator may not be capable of providing enough current at the correct voltage to start the pump with that much head pressure.
You would be correct if this was a simple resistive load where I=E/R, when resistance doesn't chance and you reduce the voltage current will decrease.
BUT, this is a motor which you want to produce power. Without considering loss 1hp=746 watts. And to make things simple watts law is power in watts(P)=volts(E) x amps(I) or I=P/E.
If you want a 1hp motor to produce 1hp it will need 746watts of electric power in order to do so. Should the voltage be 120v the amp being drawn to produce that power will be 6.2 amps. Now, reduce the voltage and the amps must go up I=P/E. If you want that motor to produce 745 watts(1hp) of power and you reduce the voltage the amperes must increase.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
But it works fine when connected to the utility supply - generator is not capable of delivering the surge capacity needed to get it started.
Remember the utility generator is likely in the megawatt range, this little motor doesn't make it flinch (not even a tiny bit) when trying to start.
But a 5500 watt generator supplying (maybe) a 1000 watt load is going to draw the voltage down, this will also slow the prime mover down (the engine) which will also bring the frequency down. And it gets even worst at starting time because the nameplate rating is what it draws when supplied at rated voltage and frequency, and at full rated load, startup is at zero speed - we need to accelerate the motor before we can think about normal rated load characteristics too much. Next thing you know you are supplying a 230 volt rated motor with maybe only 170 volts and maybe the frequency dropped to 45 or 50 hertz. That motor is going to draw a lot of current compared to if the voltage and frequency were much more stable, plus it will take much longer at those levels to accelerate it - if it even produces enough torque to accelerate at a reasonable rate. Starting an unloaded motor will be more successful then a loaded one in these conditions.
Many of these motors are capacitor start, but have a potential relay to drop out the start capacitor - is possible correct voltage to operate the potential relay and drop out the start capacitor is never reached under the low voltage and low frequency conditions which only worsens things as it pulls higher current when the start capacitor is in the circuit.
But actual current depends on efficiency and power factor also, and will always be higher then what simple watts calculations produce, as there is no 100 percent efficient motors.
As mentioned you need to measure volts while trying to start should the voltage be dropping to say 170 like I mentioned before 170 * 27 = 4590VA. Much more then what a 1/2 hp motor should draw (after reaching normal operating speed), but you have not reached normal operating speed yet, are possibly running at a low frequency as well, and overall circuit impedance is what is limiting the current, if you could just maintain frequency - maybe it would allow for faster acceleration, but this is a limitation of a small source.
donaldelectrician
Senior Member
K Wired ... The fact that it works on Utility Source and not on both Generators is troublesome . Do you think the generators are not big enuff ? I still think he has a wire size / length VD problem to add to whatever other problems he has with this setup .
Don
The simple task of measuring the voltage from the gernerastor would have save a lot of posts.
I agree with the utility supply and that a "symptom" of why the generator will not run the motor is because of a supposed voltage drop.
Why is there a resistance to simply using a common voltmeter to check the voltage on the gernerator. It's like pulling teeth.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Wire size/length does add some resistance, but we have lower voltage to begin with when it draws the generator down compared to how much it draws utility down in same situation, so we see even lower voltage when supplying from the generator then from utility. Remember the utility prime mover is a huge turbine back at a power plant, this half HP motor doesn't have any noticeable impact on it's inertia, it it does have a huge impact on the inertia of the prime mover of the small genset though.
What I am saying is same motor/ same conductor size/length - maybe only causes a 5 - 10 volt drop max and frequency is maintained when starting on the utility - and because this drop is not all that much it allows for faster acceleration of the load and quicker voltage recovery once the load accelerates. Now connect to the smaller source that can't maintain that kind of voltage or frequency and you get even more voltage drop when starting - maybe drops clear down to the 170 volt level I mentioned - this is low enough it will be about impossible to ever accelerate that motor to proper speed.
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NEMA motors are designed to operate satisfactory at +-10% of he NP voltage. In not knowing how the pump manufacturer designed their motor +-10% may be a good place to start.
It is kind of hard to address this issue not knowing what the actual gernerator voltage was wehern attempting to start the pump motor. By knowing that fact it would answer a lot of we questions. Thus far it is only being assumed that it sugs which I think we all agree.
It is kind of hard to address this issue not knowing what the actual gernerator voltage was wehern attempting to start the pump motor. By knowing that fact it would answer a lot of we questions. Thus far it is only being assumed that it sugs which I think we all agree.
Okay. Here is what I came across today...
I started his generator (5600W - 8500W Surge) and measured 255V (60Hz) at the terminals of the pump's breaker. When the pump kicked in the first time I read a drop to 180V momentarily, then it increased to 193V -just below 10% of 255V. with the generator still slightly sagging. I measured the amperage at about 30A (just like before). Forgot to check frequency again. Does this confirm what most of you have been saying, primarily, that the generator(s) are unable to keep pace?
I mentioned all of this to my client and told him that given the sub-panel and the massive number of circuits that he has in there (and were disconnected during testing) that he should consider a permanent standby system instead of "load managing" it with his portable. Given that, and just for my own curiosity, what would a minimum system need to be to be able to run a pump like that. Again, just for my own info...I'll let whoever does the gen-set install figure out all the load calcs.
Thank you all again...
I started his generator (5600W - 8500W Surge) and measured 255V (60Hz) at the terminals of the pump's breaker. When the pump kicked in the first time I read a drop to 180V momentarily, then it increased to 193V -just below 10% of 255V. with the generator still slightly sagging. I measured the amperage at about 30A (just like before). Forgot to check frequency again. Does this confirm what most of you have been saying, primarily, that the generator(s) are unable to keep pace?
I mentioned all of this to my client and told him that given the sub-panel and the massive number of circuits that he has in there (and were disconnected during testing) that he should consider a permanent standby system instead of "load managing" it with his portable. Given that, and just for my own curiosity, what would a minimum system need to be to be able to run a pump like that. Again, just for my own info...I'll let whoever does the gen-set install figure out all the load calcs.
Thank you all again...
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
The frequency drop is also exacerbating this, because it almost ENDURES that the motor will not get up to the speed at which the starting caps kick in. So yes, it confirms that your generators are not up to the task. Like many things, the ratings are likely more "marketing" than engineering values.
It certainly smells that way. If the genset slowed down the frequency would drop also which would compound the issue the way that I see it.
In sizing gensets kw costs money so they are generallysized quite tight which may result in an issue such as yours, something that would not be problematic with a utility source that can support the load.
rt66electric
Senior Member
- Location
- Oklahoma
Inrush and load...
Inrush and load...
The typical inrush current for a motor is 6X the running amps.
For a 10 amp motor it takes 60 amps to start. ( does your gen have 60amps)
I'll try to compare a "electric motor" to a "gasoline engine".
Think of a fully loaded 3/4 ton pick-up truck .... Put it in third gear , then , rev the engine to 3000 rpm, and dump the clutch.
is the same as----- throwing the switch "on" a electric motor-starter.
zero-to-full speed in 1 sec.
how many HP is needed to get the truck to 60 MPH in one second vs. maintaining a 60 MPH speed??
A pure resistive load (incandescent lights/heater elements) does not have inrush.
Inrush and load...
The typical inrush current for a motor is 6X the running amps.
For a 10 amp motor it takes 60 amps to start. ( does your gen have 60amps)
I'll try to compare a "electric motor" to a "gasoline engine".
Think of a fully loaded 3/4 ton pick-up truck .... Put it in third gear , then , rev the engine to 3000 rpm, and dump the clutch.
is the same as----- throwing the switch "on" a electric motor-starter.
zero-to-full speed in 1 sec.
how many HP is needed to get the truck to 60 MPH in one second vs. maintaining a 60 MPH speed??
A pure resistive load (incandescent lights/heater elements) does not have inrush.
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
...however tungsten lamps do... cold resistance is much lower than hot (ex: a 100 w lamp might be 10 ohm cold and 140 ohm hot, give or take).
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
One voltage measurement is missing, the generator O/P terminals on pump start up. It would confirm volt drop through the sub panel, etc.
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