how to reduce 0.75hp motor surge current?

[Hayder]

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.

[inetdog]

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.

1. The lowest possible starting draw for the motor will be when the centrifugal pump input or output is closed until the motor has gotten up to speed.
If the shaft barely rotates or does not rotate at all, then there is nothing that you can do to the pump to reduce the starting surge in the motor.
2. One way to reduce the current, which may or may not allow the pump to start, is to feed it from a stepdown or buck transformer during starting to try to reduce the surge current. That will not require you to get rid of a lot of power as heat during the starting process. I would not recommend going below 1/2 of the nominal voltage, especially if the motor will be starting frequently (more than say 5 times per hour) since the starting process will take longer and will heat the motor up more at lower voltage. If you try this, you may need to combine it with a valve as in item 1. The transformer should then be switched out of the circuit by a timer or by monitoring the centrifugal switch in the motor so that the motor gets full voltage once it is up to speed. Running the motor at a reduced voltage while pumping will increase the current drawn by the motor and may cause it to overheat and be damaged.
3. Resistors are not a good idea. You will be wasting power and putting more of a strain on the motor.
4. Sometimes putting a capacitor (similar to a starting capacitor since it must withstand the full line voltage AC) across the motor terminals (in parallel with the applied voltage) during starting so that the inverter only has to supply the motor power and not the reactive current which is a large part of the starting surge. You may be able to leave it connected while running if the power factor of the motor is low enough. I can only suggest trial and error for determining the size, starting with a size in microfarads which is comparable to the size of the starting capacitor. The goal is to make the surge current read with your clamp-on ammeter smaller than what you measure now. It will not change what current goes through the motor but it may reduce the current coming from the inverter.
5. Make sure that the motor is a capacitor start type, since an split-phase motor will be harder to start and will require more current from the inverter. Then check that the starting capacitor size is correct. This may make a difference if the motor is not starting as well as it should on full mains voltage either.

****6. ***** Do not try any of these suggestions if you are not working with a skilled electrician or fully understand what is going on.
# 1 is a safe suggestion.
#2 and following require knowledge, skill and caution. They include some of the actions that a commercial soft start kit might employ.

[Hayder]

well I know the forum Rule, So I am not doing dangerous stuff by myself. Actually I am discussing all this with a local technician/mechanic/professional (who is ignorant of modern techniques but skilled to tackle high voltages). I will discuss your buck transformer Idea with him and will let you know if any positive thing happened.

what If I operate 1hp submersible on this 1000 Inverter?

By the way thankyou for you detailed help.

[SunEagle]

well I know the forum Rule, So I am not doing dangerous stuff by myself. Actually I am discussing all this with a local technician/mechanic/professional (who is ignorant of modern techniques but skilled to tackle high voltages). I will discuss your buck transformer Idea with him and will let you know if any positive thing happened.

what If I operate 1hp submersible on this 1000 Inverter?

By the way thankyou for you detailed help.

It still comes down to how many amps does it take to "cold start" the motor and what is the inverter cut off limitation.

In the industry some motors have a "locked" rotor current draw 7 to 10 times their nameplate normal running current rating. Your 0.75hp motor may require 25 amps for even a very brief time to start rotating but that is enough to exceed your inverter cut off limit.

[Mike90250]

Increase voltage - see if the motor can be re-wired to 220VAC. But you need a new inverter too.
(are you already at 220V ?)

And your batteries could be letting you down, they (and the battery wires) may be passing so much current, the voltage is sagging and the inverter shuts down from under-voltage.

9A @ 220V = 1980w @ 12V = 165A DC Add 20% for inverter losses, and you are right at 200A DC.


My 1/2 hp, 3 wire, cap start, motor pulls 1,000w running, I have nothing that responds fast enough
to measure it's starting current, which is at least 4x running, if not more.

Power Factor may also be an issue, as the inductive motor messes up the phase of the volts and amps, the inverter has to work harder to supply the power.