real poer question
real poer question
(OP)
This is puzzling me so hope you can help. We have injection molding machines in our plant. We have a power analyzer hooked up to one to monitor the power consumption and stuff. Looking at the load profile for about 2 hours on this machine, it draws anywhere from 2.5 kw to 20kw depending on how the heater bands and pump motor are cycling. We have occasional peaks up to 25 kw for 1-2 sec. intervals. the data was taken at 1 second samples. The strange part is that during this 2 hour period about 4 - 6 times for one second the real power is actually negative. We looked at the actual data table and for one second it goes negative then it is back up to the standard kw. This is what we read from the data table for this one second duration. Apparent= 16.6kva, real power = -1.44 kw, Ractive = 16.54 Kvar and PF = -.09 ( Pf is usually .4 to .87). This is puzzling why the Kw would be negative and the PF goes into the negative for such a short duration. Any light shed on this would be wonderful. Thanks






RE: real poer question
RE: real poer question
RE: real poer question
RE: real poer question
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RE: real poer question
RE: real poer question
Do you have severe voltage dips in the plant?
I would guess the pump to be 20 Hp to 25 HP. Is that close?
One explanation may be that some external mechanical force is overspeeding the motor for a moment. Is this physically possible with your pump? Do you have a valving sequence that can do strange things to the flow through the pump?
If you had a mechanical load, and showed me those readings, I would say the load was overhauling the motor, but with a pump it is puzzling.
RE: real poer question
RE: real poer question
RE: real poer question
RE: real poer question
I don't have much real world experience with such a matter. But I know theoritically there is no reason why a motor can't act as a generator from time to time depending on what the load is doing.
RE: real poer question
1 An over-hauling load drives it faster than syncronous speed.
2 There is a fault on the system. Hence my query as to severe voltage dips in the plant.
What-ever the cause, I'm going to sit back and listen for a while. This may be a good learning opertunity for some of us.
yours
RE: real poer question
RE: real poer question
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: real poer question
RE: real poer question
I have taken oscillograms and recordings in industrial power systems that show induction motors supplying power to other loads. Usually it is when an upstream breaker or transfer switch opens. It never lasts long. (5- 10 cycles except for large motors > 2000 kW.)
I have seen where starting a large motor seems to suck power from the operating motors on an isolated system powered by diesel generators. Frequency dropped during the motor starting so the on-line motors would be running faster than synchronous speed momentarily and act like induction generators. It would be very unusual to see that occur on a utility supply, unless something in the process caused the pumps to accelerate.
I think you are right -Meter error due to low currents is the most likely explanation.
RE: real poer question
I agree with you.
The text books used to say 1 or two cycles.
In support of your observation of 5 to 10 cycles duration;
I think that the time is set by the motor coasting down. Remember that when it is acting as an induction generator, the energy that is regenerated is aupplied by the rotational inertia of the motor. The inertia doesn't last long. Large motors > 2000 kW. have a lot of inertia.
Many years ago, I took a class out to the shop to demonstrate regeneration, or induction generation if you prefer.
We coupled a DC motor to an AC motor. Our various machines were in the 3 Hp. to 5 Hp. range. We connected a load bank in parallel with the motor. We connected an analyser in the circuit. We started the motor. We then energised the DC motor.
As we increased the torque and the speed we had a good example of the motor current reducing and then actually transfering power to the resistance bank. Then the power to the motor and resistance bank was interrupted, whether intentionally or accidentally I don't remember. The voltages and currents all changed, but stabilized at new values and continued stable. The motor was still delivering power to the load bank.
Years later, different institution different class, different mix of equipment in the shop. I tried to repeat the demonstration. The system would regenerate while the motor was being over driven, but when the main power was interrupted the voltages and currents decayed to zero almost instantly.
I have seen a couple or other examples over the years.
I am trying to visualize how valve operation could push a pump over speed. Any scenario I can imagine is not very probable or believeable.
I'm with you on meter error on this one. Possibly a sampling error.
yours