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06-07-2011, 02:04 PM #1
Limiting in-rush amperage on motor start-up
I was wondering if using start caps on 3p motors would be a way to limit in-rush current on start up? Would a VFD do it better? The motors in question are 3-5hp. Any ideas?
06-07-2011, 02:19 PM #2
I don't believe caps would even work on a three phase motor. A frequency drive can be programmed for a timed start (ramp up) which would control a 'across the line' massive inrush.
Many European machines I have seen come with Wye-Delta starting contactors. This ramps the motor up to speed and limits the current inrush.
In my plant, PG&E would monitor peak inrush and you would be charged accordingly. Any means to limit this peak was attempted, even starting large loads in some type of sequence as opposed to all at once.
06-07-2011, 04:34 PM #3
What exactly is the problem?
A soft-start can be achieved with VFDs, but time-delay fuses are the old-school way of dealing with motors and start-up currents.
My educated guess is you are stalling a rotary converter, or a genset?
Helpful minds need to know...
How is this 'failure' manifesting? Perhaps the contactor heaters are grossly under-sized?
06-07-2011, 04:42 PM #4
A VFD can be programmed to provide a long start up time which limits the inrush current. A 'time delay fuse' merely allows a longer overload time before it blows.
06-07-2011, 05:05 PM #5
VFD, yes - with the abovementioned long ramp-up time.
06-07-2011, 05:43 PM #6
Well, according to a number of RPC manufacturers, you CAN use capacitors to reduce inrush current.
Some (GWM for instance) claims that they can reduce the inrush current to only 33% of the across the line starting value.
And this technology can be applied to individual motors as well as an RPC.
So, it seems that by the use of electronic trickery, capacitor banks can be utilized to mitigate inrush current.
06-07-2011, 06:09 PM #7
I am not sure how capacitors can be applied to limit inrush, but remember a discharged capacitor acts as a dead short on charging. Other methods of reduced inrush starting are expensive. Wye-delta requires special motors and expensive control. Autotransformer starters are also expensive.
Why the need for reduced inrush on 3-5 hp motor?
When you tote up the cost of the various methods, vfd on an individual motor is the cheapest and most simple. Multiple motors, start them one at a time.
06-07-2011, 07:24 PM #8
My RPC works fine, but it does use start capacitors (10hp w/Steveco PR). I was thinking in terms of using "alternative" energy for my shop. As a whole it doesn't work so easily and convenience is still an issue. Lighting, yes. Heat, yes. AC, well a good fan, yes. Machinery, not so much. But by breaking down a big problem into it's component issues, maybe I can find a solution that will work for me or at least limit my draw from the grid. The latter is the most likely outcome but that's fine. Putting a VFD on every machine doesn't sound like a very succinct solution, but adding a few start caps and a potential relay does if it'll reduce the in-rush. I was thinking start caps would supply the in-rush needed but recharge themselves at a more reasonable pace. A resistor might be necessary to regulate the cap recharge. I know I can add the caps, but will it work? I still want the instantaneous on.
06-07-2011, 09:26 PM #9
If your goal is to reduce your electric bill, I don't think slow-start is really going to help.
And I'd be interested to see a diagram that shows how a capacitor would store energy
to soft start a three phase motor being run off a three phase supply - which is what
seems to be the OP's situation.
06-07-2011, 10:55 PM #10
I'm not looking for a soft start. I want instantaneous start without a big line draw. I'm trying to figure out if in-rush could be limited/supplied by caps. In-rush would have to come off batteries otherwise and that's a pretty big yank. Too big really. From my experience that's what start caps do- store energy and release instantaneously. Starting my 10hp RPC with start caps doesn't even flicker the lights. A resistor to meter the caps recharge might not be necessary or might be an oversimplification, I don't know. The next step, generating and storing enough energy to run a 5hp motor all day, or several, is where your "can't do it" attitude is going to be correct. It's not really about saving money either, at least in the short run. But it is somewhat about figuring out the limits of "distributed power generation" (Google it, they own it).
But, I really just wanted to know if my thinking was correct about start caps supplying the in-rush.
06-08-2011, 07:35 AM #11
NOT an expert, but from what I've read, your RPC startup inrush can be adjusted with the capacitors used for starting. But it does this by slowing the start time. And this is not going to change the inrush on the motors you are running off the RPC.
You want instant on, you will have the inrush. Not sure how long a cap will hold it's charge, but leaving them in a charged state is dangerous when you start working on them and they discharge through you. From experienc, they pack quit a wallop!
Using the caps to help provide the inrush (if it works) would also be problematic as far as switching. They cannot stay connected to the motor coils on either the motor you're running or on the RPC, as they will discharge through these coils when it's shut down.
06-08-2011, 08:42 AM #12
There seems to be confusion about the function of capacitors. On DC, you can slowly charge a capacitor and discharge it quickly, such as in an electronic flash. On AC the charge/discharge cycle happens with every AC cycle. They will not store energy to be used later. You can use capacitors to limit inrush by connecting small ones in series with the motor, in which case their function is somewhat like resistors. You can also produce some unwanted effects. For example, a capacitor in series with an inductor can produce a resonant circuit that develops much higher voltages than the line applied to it. The starter on my 3 hp Roto-Clone dust collector has a 480 V coil. To run it on 240V I connected a 2 mfd capacitor in series. That develops enough voltage to start the pull in. It changes when the contactor closes, but by then it doesn't need the higher voltage to hold it. The behavior of a starting motor will be different, but that is an example of things that can happen. My suggestion is that you get some coaching in basics before trying anything like that.
The high inrush is what develops the torque to accelerate the motor quickly. Limited inrush means soft start and slower acceleration. TANSTAAFL.
Most soft start systems use wye-delta motors or transformers to reduce the starting voltage until it gets some speed up.
06-08-2011, 11:23 AM #13
It's done all the time in very VERY large motors, it's called "Capacitor Assisted Starting" and used to be much more common than it is now. But it is VERY tricky, the capacitors MUST come out of the staring circuit when the motor reaches 80% speed or you will get a severe resonance and voltage surge, as previously suggested. When you have that very VERY large motor, start time is almost always long and you have the ability to monitor speed to determine the timing. But on a 5HP motor? No way.
Besides, what's the purpose? Despite COUNTLESS anecdotes about avoiding "peak demand charges", the reality is that you are NOT charged for motor inrush currents in 99.999999% of locations. Lots of people selling gizmos want you to go on believing that myth, but it just is not true. Demand Charges are based on a sliding window of time, usually 15, 20 or 30 minutes. Anything that happens for less than that time frame is all but insignificant in the energy calcs that result in the peak demand determination because only the overall average energy consumed in that window is considered, not the peak power moment within it. Capacitors, Soft Starters, Wye-Delta, Autotransformer, Part Winding starting, all of them have NOTHING to do with saving on peak demand charges. They are sometimes REQUIRED by the utility because they must avoid voltage drops, and they may penalize you for not following their requirements, but not in the form of a Peak Demand charge. Generally though, utilities will typically have a minimum HP at which reduced voltage starting is required, anywhere from 15 to 100HP, and even then you can usually ask fr a variance if your motor is not going to start very often. If you have a residential service, they don't have any requirements, but your service capacity may be the limitation anyway.
If your goal is to avoid a voltage drop, a VFD is a MUCH better choice. A VFD can accelerate almost any load without ever exceeding the FLC of the motor. A soft starter cannot quite get there, although they can help, but at 5HP, a soft starter is likely to cost almost the same as a VFD now, so there is no point given that the VFD has added benefits.
06-08-2011, 09:32 PM #14
There are lots of ways to reduce start current, but a VFD is the most versatile, at least. Just so long as you don't need a start up (capacitor charging) circuit for the VFD..... otherwise star-delta resistive start etc work. Autotransformer reduced voltage start works particularly well, but is expensive, often more so than the VFD and ancillary stuff.
The VFD can be cheap, until you start attaching other things, like cap charging, etc, even boxes and possible special disconnects.
I may have to borrow that method at some point.
06-09-2011, 01:08 PM #15
06-09-2011, 10:45 PM #16
06-10-2011, 07:29 AM #17
06-10-2011, 09:06 PM #18
Capacitor in series
The capacitor-coil concept CAN be used as a voltage divider/multiplier... but as others said, starting a 5hp motor is really not going to show any benefit... particularly if you're looking for instantanious start. The facts are pretty simple... a body at rest, tends to stay at rest, and in order to accellerate it, the amount of energy input required, from the inception, to realization, is a set thing... but the timeframe over which that energy is applied, determines how quickly the body will experience change in velocity. The shorter the accel time, the higher the motor current. This is why slow ramp-up on a VFD keeps line current surges down... all about integrals and derivatives... and of course, the first rule of legal action: Can't get blood out of a turnip, therefore, trying to will result in a net yield in the negatives.
06-11-2011, 11:43 AM #19
06-11-2011, 02:39 PM #20