Motor Running above Overload point
Motor Running above Overload point
(OP)
Friends,
Please advise!
We have here a two stage centrifugal pump, 157gpm capacity, at 995 ft head.This pump is in a water injection service supplying three streams, the flow is controlled via valves which are throttled to meet requirement.
The motor driver is 100HP.According to the pump curve this HP is sufficient for the pump, in fact the actual HP required is 85HP.
The problem is the motor is running above the overload (24.3 amps).This is baffling to us , the ammeter was checked,we did a shut off head test ....this was ok,so we ruled out the possibility of pump inefficiency, however on further opening of the valves ,the ammeter reading went as far up as 30 amps, flow went off scale.
What next...what are we not seeing??!!
Please advise!
We have here a two stage centrifugal pump, 157gpm capacity, at 995 ft head.This pump is in a water injection service supplying three streams, the flow is controlled via valves which are throttled to meet requirement.
The motor driver is 100HP.According to the pump curve this HP is sufficient for the pump, in fact the actual HP required is 85HP.
The problem is the motor is running above the overload (24.3 amps).This is baffling to us , the ammeter was checked,we did a shut off head test ....this was ok,so we ruled out the possibility of pump inefficiency, however on further opening of the valves ,the ammeter reading went as far up as 30 amps, flow went off scale.
What next...what are we not seeing??!!





RE: Motor Running above Overload point
RE: Motor Running above Overload point
RE: Motor Running above Overload point
If motor power is correct and motor current is high I would check line voltage. Maybe dyno motor to determine if is operating correctly?
RE: Motor Running above Overload point
On our shut off head test we measured the rpm of the motor,this equalled to 3581 rpm, on opening the control valves we observed the motor speed to be 3561 rpm, bear in mind that the rated speed on the nameplate is 3565rpm.
What we know is the pump is not operating at full capacity.
RE: Motor Running above Overload point
My guess is that you are not seeing the relationship between flow, pressure, and required power. You stated, “We have here a two stage centrifugal pump, 157gpm capacity, at 995 ft head.” What is the flow and pressure when the pump is running in the overload condition you discussed? Have you calibrated your flow meter and pressure indicator? The solution may be as simple as increasing the head with a downstream valve. This might drop the flow down and the associated power.
RE: Motor Running above Overload point
RE: Motor Running above Overload point
Check your voltages.
Low voltage has been mentioned. A motor should function at 10% below rated voltage. That would be 414 volts for a 460 volt motor. The voltage is seldom that low, but you may have a long feeder that is not sized correctly for voltage drop.
A voltage unbalance will cause a disproportionate current unbalance and overloading and overheating.
Check your flow. Many pump-motor combimations have the potential to exceed the rated horsepower of the motor if the flow is excessive.
My first check would be voltage unbalance.
My second check would be low voltage (at the motor, not at the panel if the feeder is long).
My third check would be flow.
respectfully
RE: Motor Running above Overload point
I forgot to mention that during our shut off head test(motor on no load) the ammeter reading was 17Amps, is this indicative of some problem? it seems to me that there is a little difference of Amps between no load and load(24.3A)
This motor is 100 HP and 2300 Volts, frequency 120Hz
For this motor, what should be the no load Amps?
RE: Motor Running above Overload point
There is a difference between shut-in and no load.
At shut-in the load is the sheer in the pumped fluid.
The current is a combination of magnetizing current and load current. These two currents are out of phase with each other and cannot be added directly. This is related to power factor.
The 17 amps indicates to me;
Probably moderate to poor power factor and
moderate to high sheer losses.
I can't say without a lot more information but it could be a reasonable figure.
respectfully
RE: Motor Running above Overload point
RE: Motor Running above Overload point
Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng
www.waterhammer.bigblog.com.au
RE: Motor Running above Overload point
It depends. It looks like the overall efficiency of the motor is below 80%. The current may be reasonable.
If you have a voltage unbalance problem causing excess current under load, it will also be causing increased current at shut-in condition.
In response to stanier's suggestion, #10 cable is almost adequate for 24.3 amps, but is 1.2% below code. Code size is #8 AWG and this should be adequate for extremely long runs at 2300 volts.
Check the current on all three legs and check the voltages on all three legs. Low and/or unequal voltages may well be the problem and should be investigated.
respectfully
RE: Motor Running above Overload point
RE: Motor Running above Overload point
I'm not on sure footing here, however, I would expect some very serious motor issues if one of the phases shut down - probably leading quickly to a motor failure - any thoughts?
I would start the investigation by quantifying the flow rate and differential pressure you are operating at and consolidate that with the pump curve and hp requirments.
good luck.
RE: Motor Running above Overload point
D23