Ok, I know that this may seem like a totally insane question, but is there any other way to correct a lagging power factor other than caps or a synchronous motor? Reason I ask is because our maintenance manager (mechanical background) swears that an engineer at the local power board told him that we could stagger our machine startups to correct our poor power factor. Now I may not be a genious, but this in no way makes sense to me.

Any info would be appriciated.

Thanks,
L.M.E.

 

I think the correct term is mitigate rather than correct the lagging power factor. Power factor is current related so the higher starting currents will drop the power factor even lower. Staggering the startup keeps the current draw lower thereby mitigating the lowered power factor. Not a total solution but a cheap one.

Fred Townsend

 

Maybe if you staggered each stratup to be at a point where the power draw from each previous motor is the lowest, so the load will balance. Then of course you'h have to synch all the motors... more trouble than it's worth.

 

First, there are two ways to get a power factor below 1:
1. phase shift, as produced by induction motors
2. distortion of the current's waveform. This is produced by all sorts of electronic equipment that generate their DC supply by simple rectifier/capacitor circuits.
This second cannot be compensated by capacitors alone, but it can by (expensive) LC filters.

Newer inverters and DC supplies use a technique called active Power Factor Compensation.
Here the rectifier is quickly switched on and off, taking thus little blocks of current. The average current has a sinusoidal shape and is in phase with mains voltage.
So if you would use such inverters to start and run your motors, your would be vary near to 1.

In principle it would be possible to use a combination of such an active PFC rectifier and an inverter for PFC only. The inverter had to feed back an AC current of such form and phase to compensate overall power factor of an installation.

I don't know whether such devices are yet in the market.

 

Responding to Fred's comments:

1) Power Factor Improvement is the more correct term.

2) PF capacitors will have virtually no effect on PF improvement during start. Typically they are sized to offset a little less than the no-load reactive power of a motor. More than that could expose the motor to overvoltages when they're operated at no-load.

3) Any PF penalty imposed by a utility would relate to long-term exposure to low PF. Lower PF during starting, even if staggered, would be of such a short duration as to be negligible. It is not a viable solution.

4) Check utility for exact measurement / duration of imposed penalties.

5) Improved PF, as well as substantial energy savings, can be realized by replacing grossly oversized motors wth ones sized more closely to their driven loads.

Regards,
Phil Corso, PE
Boca Raton, FL
[[email protected]] ([email protected]) {[email protected]}

 

Hi All,

I suspect that since staggered start doesn't fix PF problems it's likely the utility engineer was actually talking about peak demand limiting which is also of considerable concern to electricity suppliers and also to users if they're on a peak demand tarrif.

Staggering motor start reduces the surge load on your supply and is gernally therefore a "good thing" but not for PF reasons.

Regards

Peter Whalley
Magenta Communications Pty Ltd
Melbourne, VIC, Australia
e-mail: peter*no-spam*@magentacomm.com.au
delete *no-spam* before sending

 

Even peak demand is based on time duration., i.e., kWhr / time. Furthermore, demand is related to real power, and peak current, which results in peak kVA, or apparent power.

Regards,
Phil Corso,
PE Boca Raton, FL [[email protected]] ([email protected]) {[email protected]}

 

You can not control power factor by staggered switching but you can reduce electricity bills to small extent.
If your electric bill is based on a tri vector meter then these below are possible.

The electric bill is based on the total KVA multiplied by price per KVA plus energy charges at total KWH multiplied by KWh unit price. In addition there is a penalty charge for low power factor. Most places it is charged at 1% of the total bill for every point below 0.80pf. The PF is calculated as KW/KVA in some places and KWH/KVAh in other places.If you start the machines in sequence then you may not be raise the KVA in the TVMeter than necessary. It may give you a small reduction in KVA and it is very difficult to do so if the machines are lot and if you do not have control over production.

The savings is the lesser KVA multiplied by unit rate per kva.

The best option is to have capacitors connected to each machine if you have many machines or one at every DB. Please note you will blow up capacitors connected in the main switch board if you accidentally power them on without inductive loads on the Switch board (say after a holiday) due to ringing effect. You can not have more than 200 rkva capacitors switched live into mains without at least 30% inductive load.

regards,
Sekar

 

Sekar,

I've never heard of ay penalty based on an instantaneous value. They are usually based on an integrated interval... eg, 15 minutes. Thus, while staggered starting sequence will ameliorate voltage-drop due to a "weak" power supply, it will not have an impact on pf penalty.

Now, the addition of capacitors switched with individual motors is a viable solution to consider. But, this too requires further analysis.

Regards,
Phil Corso, PE
Boca Raton, FL
[[email protected]] ([email protected]) {[email protected]}

 

Hi L.M.E. Your man is right. Any motor that starts up does so at real bad power factor. If you start many motors at the same time it has a cascading effect. Thus yes it makes perfect sence do stagger startup of motors, especially if you have powerfactor correction installed, passive or active. The controlled starting will minimize your need for powerfactor correction.

Johan Hees