power factor ?

[jcraft]

In our industrial plant we have about 70 small injection molding machines. We have been hooking energy monitors to these to see what kind energy we are comnsuming and how efficient they are. My question is on the readings we are getting. The machines mainly consist of electric heater bands and a 25 hp pump motor. These machines are about 2 years old and our fitted with premium efficiency motors. When you veiw the monitor on these machines at any given time they pull between 1.5 to 6 Kw which is fluctuating all over the place constantly. When you look at the power factor at the same time, it is constantly changing as well. It can be changing constantly from .3 all the way to .8 while your watching it. My question is, why does the overall power factor vary so much and go so low at any given time. And also, is there anything that can be put on these machines that would increase the power factor during use. On the floor where these machines are located we have capacitor banks hooked up to the mains that bring the overall power factor to about .92 however I am just wondering why it is so low at the machine itself. Thanks

 

[TestBeforeTouch]

How are these heaters controlled? One possibility is that the heater control loop is set too fast. Another possibility is that your instruments are not able to read the distorted waveforms correctly if the heaters are controlled by power electronics such as SCR's. A meter than does not calculate True RMS can be off by over 50%.

 

[jghrist]

The 25 hp pump motor is lightly loaded; otherwise the total load would be much higher. The power factor of a lightly loaded motor is low. The power factor of the heater bands would be high. The overall power factor will vary a lot depending on how much of the total load is heaters and how much is the motor and how much the motor is loaded.

 

[itsmoked]

Yes the low power-factor is caused by the motor during the all the time that the shot is not at its maximum and even when it is at its maximum but the part is maybe on the small side. The horror of it all is that the motor is sized for the largest mold and biggest part with the smallest runners. A condition rarely ever experienced.

I wonder if VFDs would eliminate the LOW power factor and improve efficiency at the same time? You really need a 1hp motor 80% of the time, a 5hp motor 15% of the time, and 25hp 5% of the time.

 

[satish2508]

Dear Colleauges
I have also faced the problem in Steel Plant as load is varying means there is no constant load .When i joined my current organisation at that time power factor was .965 .We have put the capacitor in order to reduce the magnetising current . Withing the span of 3 months our power factor is 0.994 in manual mode without APFC panel . Resulting in reduction of energy consumption by 1.5 kWh per month , Reduced Maximum Demand .

Best Regards

Satish Chauhan
Asst General Manager - Electrical
+911715535312
+9194

 

[jtkirb]

we make a product called "load logic" made for motors in this specific application. A motor unloaded will make the power factor get very off. The only solution is to lower the input voltage to the motor to help save on energy when it is in this state.

You have to look at the pf vs watt relationship to know how bad this is affecting the overall system pf. correction caps can help.

info on load logic is here (not trying to push our product, but more for info):

http://www.load-logic.com/

 

[jraef]

I wonder if VFDs would eliminate the LOW power factor and improve efficiency at the same time? You really need a 1hp motor 80% of the time, a 5hp motor 15% of the time, and 25hp 5% of the time.

Yes, VFDs are used very successfully on injection molding machines for that very reason. There are several companies that specialize in retrofit packages specifically designed for that market because it is such a succesful enegy savings technique, and the payback is very short.

Nola circuit based "energy savers" such as those promoted above were tried in that appication years ago and did not fare well in the long run because of their inherent problems. Some of those issues may have been fixed by now, but the energy savings with VFDs is so dramatically better that it doesn't make sense to use a less effective system even if the component cost is lower.


Here is an article on the technology Plastics Technology emag link

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

The short answer; Everything is normal, you don't have to do anything that isn't already being done.

We used to use a concept called a power triangle.
This is A right angle triangle with the Watts on the base, (Horizontal), The reactive power (volt amps reactive) as the vertical side, and the apparent power (volt amps) as the hypotenuse.
The volt-amps reactive are a characteristic of the 25 HP motor and won't change unless the voltage changes.
The watts are the energy being used. Watts will change as the motor load changes, (If it does), and as the heater load changes. As this load changes, the base line of the power triangle will expand and contract. If you use a scale factor of 1 inch per watt, and plot different readings, the base line will range from 1.5 inches to 6 inches. The altitude will be about 4.5 inches. The Hypotenuse will range from about 5 inches to 7.5 inches.
Power factor is watts over volt-amps.
6 inches over 7.5 inches = .8
1.5 inches over 5 inches = .3
Situation normal.
What can you do about it.
Energy consumption; You need the heat, not much you can do about that. Motor the energy consumed by the motor has two components, work done and heating. If the load on the motor is only 4 HP, then that is all the work you are paying for.
If a motor is lightly loaded, then it is not producing much heat and that is not costing you much.
Power factor; What can you do? You are doing it. The banks of capacitors are cancelling the lagging reactive power of the motors with the leading reactive power of the capacitors.
You could install capacitors on each machine. the cost of capacitors would be very high. The labour cost would be frightening. I know, I went through the calculations the first time I did power factor correction. The system you have is the way to go.
There is a small saving to be made.
With a load of 1.5 KW the power factor is .3
At 480 volts the current will be about 18 amps per phase.
If the power factor is improved to .9, the current will drop to about 6 amps.
Calculate the resistance of one wire from the distribution center to the machine and multiply by 18 amps squared. Multiply by 3 for 3 phases.

Now repeat the calculation for a current of 6 amps squared, times three.
Subtract and divide by 1000. This is how many kilowatt hours you can save for every hour of operation.
Working 40 hr a week 4 weeks a month, I get a saving of about 9.5 Watt Hrs a month saving.
Based on #6 AWG copper, 150 feet. Canadian voltage drop table, transposed.
Like I said, everything is fine.

 

[bgood1]

Two things:
First, the varying power factor, as mentioned by waross, is due to the varying load. Induction motors present a high reactance (.3) as they become unloaded, improving with load toward the nameplate power factor at full load.
Second, the power factor of the motor is constant with constant load and may not be changed. It is the power factor of the "system" or group of components or load that comes into play on "power factor improvement". The motor amps and reactance will be a result of motor properties. To measure pf downstream of the capacitor correction will provide the same results with or without pf caps (slightly tempered with a voltage improvement because of the existence of the caps). To measure upstream of the caps is to measure the system downstream. The motor reactive (90 degrees lagging) amps will simply stop flowing upstream of the caps up to the capacity installed (90 degrees leading or 120 degrees out-of-phase with the lagging). I think of it as "reactive current swap" between the cap bank and the reactive motors. When the leading installed vars are not offset be reactive load, then the system upstream will get the leading reactive current, actually causing an increase in overall flow to the branch installed..