how to reduce surge current of motor?

Badar · December 2013

Hi friends,

I want to run a 0.75hp centrifugal water pump on 1000 watt inverter (They claim to bear 2000watt surge for few seconds, but I dont believe).
I checked this motor with my clamp meter
Surge Current : 9 Amps on 220V
Normal Current: 2.5 Amps
Water Level is about 50 feet

When I try to run this motor on 1000 Inverter, motor shaft tries to rotate and Inverter trips.

So I am in need to reduce starting current so that I can operate it on my 1000 inverter.

Option:
Soft Starter: Unavailable in local market.

Submersible: Not an option "so far"
Thermistors: Not available in market on such big size

Resisters: to reduce starting current.
Now Problem is that how what size of resister should I place so that my Inverter could handle this? so I know I=V/R, but I dont know how to use this and even dont know the resistance. Any idea ????

Note: reduction in water output is not a matter of concern for me.

BB. · December 2013

Re: how to reduce surge current of motor?

I would be surprised if your motor would run on any 1,000 watt AC inverter...

One of the problem is that induction motors, in general, have less than ideal efficiency and poor power factor. If we use some "typical numbers", we can see what VA (volt*amp) rating would be needed:

0.75 HP * 746 watts * 1/0.85 efficiency * 1/0.67 Power Factor = 982 VA

Very close to the maximum VA/Watt rating of the inverter...

Another thing to look at is the DC wiring to the input of the AC inverter.... Measure the input voltage to the AC inverter and see if your battery bank and wire is capable of supplying sufficient current at rated voltage. If you have a 2,000 Watt surge, the maximum input current for a 12 VDC input inverter would be:

2,000 watts * 1/0.85 inverter efficiency * 1/10.5 volts cutoff = 224 Amps

That requires a very healthy sized battery bank and short/heavy copper cables for the DC power run (to keep voltage drop low).

Some AC inverters also do better with inductive loads too--An AC motor at starting is very inductive can be very hard for many inverters too.

-Bill

waynefromnscanada · December 2013

Re: how to reduce surge current of motor?

Or is it .75 hp motor in the traditional sense, as in capable of doing 3/4 hp actual work AFTER all electrical losses. Most water pumps and industrial motors are still rated this way, which would indicate your motor could be sucking back at least 1200 VA, with a start surge of between 2 and 3 times that.

Cariboocoot · December 2013

Re: how to reduce surge current of motor?

I would buy an inverter capable of running the pump rather than try to make the pump run off the inverter.

Power factor on a motor can be really bad, like 0.60, causing that 550 Watts running (just about 3/4 of 746 Watts which is one electrical HP) to be more like 917 Volt Amps to the inverter. That's near peak capacity of a 1kW inverter and leaves no room for running anything else. The same goes for the start-up demand: that could be exceeding 3kW.

Putting in resistance to reduce the current draw doesn't help the motor spin up and do the work it needs to do. A starting and running capacity pair can take some of the PF burden off, but only if the pump motor can be fitted with the right ones.

A little note on surge capacity of inverters: most of them say they will surge to 2X their rated capacity, but they won't say for how long. In too many cases (especially low-priced inverters) it's like 0.5 seconds, making the "surge capacity" meaningless in the real world. Good companies will tell you not only how much extra power demand they can withstand, but for how long.

I think you'd be happier with a 24 Volt 2-3 kW inverter to run that pump. Or you spend the money replacing the pump. Can't squeeze blood out of a turnip.

Badar · December 2013

Re: how to reduce surge current of motor?

well well well....
Nothing going well except your kind help.
Thank you for your replies and detailed answers.
I really ignored poor PF.
one more thing I ignored was poor winding of motor.
Well this motor is off-topic now.
...................................
I share an experience with you. Today I checked my "Donkey Pump" and it was of
starting Current: 7.5 Amps
Running Current: 1.7 Amps
Perhaps it was a german motor.
I tried to start it with 1000W inverter. Inverter shut down. I rotated its wheel with hand and when it was rotating, I switched ON the inverter, this time it was fine. Inverter was capable to drive this motor.
It means Inverter is not capable to bear any surge current.

...................................
Now my question is

If I use 1hp submersible pump in place of capacitor start centrifugal pump?
Is there any surge current issue with these submersible pumps?
Will my inverter drive this submersible pump?

Actually in my area submersible pumps are not used. I have to buy it from capital city Islamabad which is about 1000Kms away. so no return will be possible.....

Cariboocoot · December 2013

Re: how to reduce surge current of motor?

Badar wrote: »

...................................
Now my question is

If I use 1hp submersible pump in place of capacitor start centrifugal pump?
Is there any surge current issue with these submersible pumps?
Will my inverter drive this submersible pump?

Actually in my area submersible pumps are not used. I have to buy it from capital city Islamabad which is about 1000Kms away. so no return will be possible.....

Submersible pumps have the same motor issues as non-submersible. It all depends on the pump motor how easy it will be to start and run.

Ken Marsh · December 2013

Re: how to reduce surge current of motor?

Badar wrote: »

Hi friends,

I want to run a 0.75hp centrifugal water pump on 1000 watt inverter (They claim to bear 2000watt surge for few seconds, but I dont believe).
I checked this motor with my clamp meter
Surge Current : 9 Amps on 220V
Normal Current: 2.5 Amps

2.5 amps would be the approximate no load running current for a 0.75 Hp single phase motor, depending on what type of motor.
Your pump must be way unloaded.

If you could come up with a 3 phase motor, you could run it with a VFD (Varriable Frequency Drive).
It would start with 0 amp inrush. Current ramps up as the motor speeds up.
Expect 10 to 20% less running current since 3 phase motors are more efficient.

mike95490 · December 2013

Re: how to reduce surge current of motor?

As Ken says, there are also VFD controllers, that can take single phase, convert to three phase, and that will soft start your motor.

But, do your pumping needs really require a .75hp motor ? Is that just what the shop had 300 of sitting in stock ? What is the head and volume of water you NEED to pump? A single house does not use more than 5 GPM, with all the low-flow faucets and toilets codes are enforcing. There are many more options for VFD, with .5 hp motors

When PF and efficiencies are taken into consideration, my 1/2hp pump, is actually pulling 1,000w from my 6KW inverter, when it makes it's 3 hr daily run.

http://www.grainger.com/category/adjustable-frequency-drives/motor-supplies/motors/ecatalog/N-bom

waynefromnscanada · December 2013

Re: how to reduce surge current of motor?

mike95490 wrote: »

When PF and efficiencies are taken into consideration, my 1/2hp pump, is actually pulling 1,000w from my 6KW inverter.

My pump, shown my avatar, is powered by a 1/4 hp induction motor name plate rated at 5 amps, and it had terrible PF. Being friends with an electric motor shop I was able to do some trial and error testing with various "motor run" capacitors wired across the supply to the motor. I was able to bring the PF up to roughly .97 which I was very happy with. The Capacitor is wired between the pressure switch and the motor, so the only time it's on line is when the motor is running. The motor by the way draws about 325 watts. Less when the pressure is low, rising as the pressure increases, as it's a positive displacement twin piston pump., 300 GPH.
Oh, and many years ago I added capacitor start to this motor. Huge difference in starting speed and surge.

Walterk · February 2014

Re: how to reduce surge current of motor?

Sometime ago it was mentioned you can decrease a pump starting surge current with a capacitor. Does anybody know if this can be done with a submersible 2 wire well water pump? I have one that is killing my generator only at start due to a high surge current... once it gets going it's no problem

Walterk · February 2014

Re: how to reduce surge current of motor?

I'm talking about 230V AC 2 wire submersible water well pump

waynefromnscanada · February 2014

Re: how to reduce surge current of motor?

Unfortunately 2 wire submersible pumps are hard starters if operated on limited power supply. Great for grid power, not so for off grid.
Three wire pumps however come with a separate control starter box that has within it, a properly sized start capacitor, and they are much easier to get going.
Unfortunately the two wire pumps have their start circuitry sealed within the motor and cannot be modified. The 3 wire units, since they already have a start cap, don't need to be modified, they're already there.
Another, and better (but probably considerably more expensive) alternative, is a pump with an inverter motor. These pumps start slow, without surge, and gradually ramp up to full speed. Ideal for off grid.

Dave Angelini · February 2014

Re: how to reduce surge current of motor?

Walterk wrote: »

I'm talking about 230V AC 2 wire submersible water well pump

Like the Grundfos SQ series? These pumps have no surge greater than the nameplate. Excellent for offgrid applications or if you do not want surging from a pump. A 1/2 HP 10 gallon per minute pump is listed as 2.9 amps and that is what it will draw.

mike95490 · February 2014

Re: how to reduce surge current of motor?

Walterk wrote: »

Sometime ago it was mentioned you can decrease a pump starting surge current with a capacitor. Does anybody know if this can be done with a submersible 2 wire well water pump? I have one that is killing my generator only at start due to a high surge current... once it gets going it's no problem

Only shot, is to add another 100' of extension cord to the generator. That adds a bit of resistance, limiting the peak current a bit, but makes the "start" winding in the motor work a little bit harder. And it may not help, you might need 150'.

waynefromnscanada · February 2014

Re: how to reduce surge current of motor?

mike95490 wrote: »

Only shot, is to add another 100' of extension cord to the generator. That adds a bit of resistance, limiting the peak current a bit, but makes the "start" winding in the motor work a little bit harder. And it may not help, you might need 150'.

Interesting you should say that. Friend of mine was having a problem at his machine shop where one of the big single phase motors would occasionally practically explode the main breaker when they tried to start it. The instant initial inrush of current / crack of the whip thing.
Some who should know suggested shorted windings and a lot of other things. I finally convinced him to at least TRY a 125 foot length of cable in series with the motor. Problem solved. None of the former "experts" could even remotely figure out how that could possibly work, and in fact thought it should have made the problem worse.

Cariboocoot · February 2014

Re: how to reduce surge current of motor?

"Experts" who think adding resistance to a circuit will exacerbate a problem of high current have failed electricity 101.

As Mike points out there is the down side of the starting windings working harder/longer which can affect motor life. Usually you want the thing spun up to speed ASAP, which is why there's all that current draw to begin with. Starting caps can only go so far to reduce the problem. VFD gets around this issue nicely, but can't always be incorporated.

In all honesty quite a lot of the motors out there are very poorly designed because the makers didn't care. When you have 'unlimited' grid power available it doesn't matter how efficient your motor is, right? :roll:

Texas Wellman · February 2014

Re: how to reduce surge current of motor?

If you look in the Franklin AIM manual you will find that 2-wire motors require approx. 50% more starting power than the starting current that 3-W pumps pull.

http://www.franklin-electric.com/aim-manual/page-5.aspx

Walterk wrote: »

I'm talking about 230V AC 2 wire submersible water well pump

BB. · February 2014

Re: how to reduce surge current of motor?

And to add to Texas Wellman's comment... Remember we try to design your off grid power system to work for many years in the future.

A new battery bank and clean connections may start that 2 wire pump today... 5 years down the road with an aging battery bank with a discharged battery bank at night--It may fail to start that 2 wire pump motor.

Conservation/reducing loads is a good way to reduce those frustrating issues years down the road (and needing to start the genset so folks can take a shower in the evening).

-Bill

RCinFLA · February 2014

Re: how to reduce surge current of motor?

I have played with soft-starter will limited benefits. They are similar to a time dependent dimmer using triacs that reduce the motor voltage for a startup period.

The bottom line is you don't get something for nothing. Yes, the soft-starter reduces peak startup current but it makes up for it by lasting a longer period of time.

I have a 3/4 HP sump pump that draws about 18 amp startup surge at 120vac, 8.5 amps full load while pumping, dropping to 5.5 amps when tank runs dry and motor has no load. Power factor during loaded run is a remarkable 0.95, dropping to 0.45 at no load.

With a loaded power factor of 0.95 and 8.5 amps draw, that translates to 970 watts real power for a 3/4 HP motor at full load (746 x .75 = 560 watts of real shaft delivered equivalent). The difference between 970 watts input power and 560 watts output shaft mech. power is primary heating of motor.

Since it is a light duty cycle sump pump, the motor is likely wound with smaller gauge wire then a continuous run motor, accounting for much of the heating losses.

waynefromnscanada · February 2014

Re: how to reduce surge current of motor?

RCinFLA wrote: »

I have played with soft-starter will limited benefits. They are similar to a time dependent dimmer using triacs that reduce the motor voltage for a startup period.

Thanks for that explanation, I never did know the theory behind their operation. Now I do.

Walterk · February 2014

Re: how to reduce surge current of motor?

RCinFLA wrote: »

I have played with soft-starter will limited benefits. They are similar to a time dependent dimmer using triacs that reduce the motor voltage for a startup period.

The bottom line is you don't get something for nothing. Yes, the soft-starter reduces peak startup current but it makes up for it by lasting a longer period of time.

I have a 3/4 HP sump pump that draws about 18 amp startup surge at 120vac, 8.5 amps full load while pumping, dropping to 5.5 amps when tank runs dry and motor has no load. Power factor during loaded run is a remarkable 0.95, dropping to 0.45 at no load.

With a loaded power factor of 0.95 and 8.5 amps draw, that translates to 970 watts real power for a 3/4 HP motor at full load (746 x .75 = 560 watts of real shaft delivered equivalent). The difference between 970 watts input power and 560 watts output shaft mech. power is primary heating of motor.

Since it is a light duty cycle sump pump, the motor is likely wound with smaller gauge wire then a continuous run motor, accounting for much of the heating losses.

Thank you everyone for all this good discussion...Funny that you mention a soft starter.... I looked for one to solve this problem with my generator and Franklin people told me there isn't one for a 2 wire pump just as Wayne mentioned in his original post. There would be one for a 3 wire submersible pump... the generator faults out for an undervoltage fault and trying to run it that way blew a fuse on my furnace...The franklin people also told me the problem might be in the fact that a standby generac 11kw generator is putting out an inverter type "square" sinusoidal instead of the old style sinusoidal....I have no idea what that has to do with the pump needing much more than 7KW at startup and it looks like I'm stuck short of changing the pump or generator which either one is an expensive option at this point....does anybody else have any other ideas?

Texas Wellman · February 2014

Re: how to reduce surge current of motor?

Wait...you're saying that an 11 kw generator can't start a Franklin 2-wire pump? The largest 2w is 1.5 HP. Something ain't right.

vtmaps · February 2014

Re: how to reduce surge current of motor?

Walterk wrote: »

the generator faults out for an undervoltage fault and trying to run it that way blew a fuse on my furnace.
<snip>
does anybody else have any other ideas?

Are you saying that the problem with the generator only occurs when the furnace is running AND the pump turns on? If so, I have some other ideas :cool:

--vtMaps

Cariboocoot · February 2014

Re: how to reduce surge current of motor?

11kW is a lot of generator. It shouldn't notice anything your household can throw at it.
Maybe you should be taking a close look at it and the wiring. Like Texas Wellman said, something is wrong.

Walterk · March 2014

Re: how to reduce surge current of motor?

vtmaps wrote: »

Are you saying that the problem with the generator only occurs when the furnace is running AND the pump turns on? If so, I have some other ideas :cool:

--vtMaps

Yes so far I only tested with a furnace on a quite a few flourescent lights on and a couple of refridgerators...I assume the other load was less than 3kw...what are you thinking if forced air furnace was on?

Walterk · March 2014

Re: how to reduce surge current of motor?

Walterk wrote: »

Yes so far I only tested with a furnace on a quite a few flourescent lights on and a couple of refridgerators...I assume the other load was less than 3kw...what are you thinking if forced air furnace was on?

Yes and it is only a 3/4 hp franklin 2 wire pump

vtmaps · March 2014

Re: how to reduce surge current of motor?

Walterk wrote: »

Yes so far I only tested with a furnace on a quite a few flourescent lights on and a couple of refridgerators...I assume the other load was less than 3kw...what are you thinking if forced air furnace was on?

I'm thinking that the combined load of the pump and the furnace is too much for the generator. Are your pump and furnace 120 volts or 240 volts? if they are 120 volts, how are they wired from the generator (are they on opposite legs of the split phase power)?

--vtMaps

BB. · March 2014

Re: how to reduce surge current of motor?

You could also have a falling AVR--I believe that failing capacitor can cause very strange AC wave forms for the generator.

In theory, a standard AC Alternator (generator) should output a pure sine wave.

In practice, the different loads and designs of the alternator head/AVR will have affects.

-Bill

mike95490 · March 2014

Re: how to reduce surge current of motor?

I've discovered 2 appliances that don't like one generator.

The garage door opener electronics trigger auto reverse when running the 6/1 Listeroid
The Oven ignitor circuit won't run off of the Subaru/Robin/Hatz generator.

Your mileage may vary !

SolarPowered · March 2014

Re: how to reduce surge current of motor?

Badar wrote: »

Hi friends,

I want to run a 0.75hp centrifugal water pump on 1000 watt inverter (They claim to bear 2000watt surge for few seconds, but I dont believe).
I checked this motor with my clamp meter
Surge Current : 9 Amps on 220V
Normal Current: 2.5 Amps
Water Level is about 50 feet

When I try to run this motor on 1000 Inverter, motor shaft tries to rotate and Inverter trips.

So I am in need to reduce starting current so that I can operate it on my 1000 inverter.

Option:
Soft Starter: Unavailable in local market.
Submersible: Not an option "so far"
Thermistors: Not available in market on such big size

Resisters: to reduce starting current.
Now Problem is that how what size of resister should I place so that my Inverter could handle this? so I know I=V/R, but I dont know how to use this and even dont know the resistance. Any idea ????

Note: reduction in water output is not a matter of concern for me.

Soft start wont help you that much. I have softstart on a sunforce 1000watt inverter. It will initially get most motors spinning under 8.5 amps. Motors that demand higher into 10~13 amps soft start ramps them up then the inverter crashes and throws trouble code. I just watch the volts drop from 120V to 117V, then I know its game over.

You might need a bigger inverter.

waynefromnscanada · March 2014

Re: how to reduce surge current of motor?

Walterk wrote: »

Yes and it is only a 3/4 hp franklin 2 wire pump

These hp ratings are causing confusion, because traditionally, the hp rating on these and almost all other motors referred to the shaft output power. Now however, it seems either the manufacturers are using input power to the motor in an attempt to inflate the hp ratings for sales purposes, or at least some folks are using it to rate the motors they own. Perhaps it's time to start listing the amps and voltage listing of these motors so we're all singing from the same hymnbook?.

how to reduce surge current of motor?

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