3 phase line .....

I don't even know how to list a title for this.I'm not very smart about this,so please be kind.I Run my machine shop off grid (cnc's),I would like to monitor the amp draw on the generator AND stop a non essential motor from starting until line amps are at acceptable level.I think I'm at the right place,if not just let me know,thanks for your consideration.Rob

Use a normally closed current switch on one line of the generator and hook the NC contacts in series with the starter for the device(s) you want to shut down when the current from the generator exceeds a certain level. In the event the current through each leg isn't "balanced", use three current switches and hook them all in series. Whenever one exceeds the preset limit, the devices will shut down

...or acquire an analog output CT (Current Transformer) which's characteristics match your functional min/max line current values, install it over the bus to be monitored and pick up the CT's analog output within an analog input module of your choice, module installed within the PLC system which controls the motor starter.
If the current control system does not have analog capabilities, the CT could be connected to a simple analog input smart relay (such as RedLion LPAX/MPAX) which could be programmed as to close a contact when the CT's output is within the intended limits; the respective contact will be part of the motor starter's control circuit.

What you are contemplating is called "load shedding". Contact your electric utility. They probably have some source recommendations for equipment and energy service companies to help you, and they may even have a rebate program that will pay for some of the equipment!

http://www.cee1.org/

What you are contemplating is called "load shedding". Contact your electric utility.

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Hmmm...because he is running off-the-grid with a generator, the local utility may not be very interested in offereing any help. It probably depends on the personality of the utility Electrical Engineer, how overworked-and-underpaid he is feeling today, and whether he gets a lot of brownie points for good public relations. The majority of utility engineers that I have had the occasion to work with over the years would not touch an off-grid generator with a 10' pole, fearing legal liability issues that might come up later (for which the local government would not be financially responsible).

If I had a machine shop and needed to shed load at times, I probably would set up a PLC, as Dmargineau said, with an anlog input that monitors the total current, but have several outputs for my largest shop motors and other equipment, that could be shut down or prevented from starting temporarily (HVAC maybe would also be a possible load-shed).

...AND stop a non essential motor from starting until line amps are at an acceptable level.

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Do you just want to prevent motors from starting, or also to shut down any that are already running? The either/or block is easier, but the "prevent from starting" is usually the most desired route, because it avoids upsets and surprises.

I wrote a "load-startup-prevention" Omron ZEN PLC program for a large church complex several years ago which seemed to work well. It did startup-prevention for 11 large HVAC units, only allowing one to start in any 5-minute period. The time was adjustable using the internal PLC timer. Below is a link to the "I need a furnace startup ladder program" thread from member "randalo". The easiest-to-read best version of the Church Heating Sequencer PLC program is in Post #48. This old program could easily be modified to run on a PLC with at least 1 analog current input. A comparison instruction looking at "load current greater than xxx", where xxx is the generator load limit, could be substituted for the thermostat inputs in the ZEN program. Also the program could be modified to prevent startups in a preferred order, so that the most critical loads were blocked last.

http://www.plctalk.net/qanda/showthread.php?t=7012&highlight=church+heating

Here is the fail-safe wiring diagram for the Church load-control ZEN program. Each of the 11 units had an input relay and an output relay. To keep the costs down, I used the 120 volt version of the ZEN (no DC power supply needed) and used 120 volt relays with Normally Closed contacts. The ZEN PLC could be shut down or removed and the HVAC units would not know the difference, except that the power bill would increase due to the higher 15-minute peak demand load. Ideally the time period should have been 15 minutes, but the church elders felt like 5 minutes was the most that some hot old lady would be willing to wait for cooling.

Put a over current switch in the mains line from the generator. normally used before every motor.
set it in automatic reset mode.
use this switch in series with the normal stop button of the non preferent group.
When amps are too high the switch will stop the NP group and impossible to start as long as current is too high.

if you want to start automatic when current is lowenough you can use the OC switch as starter stopper, but this could cause hunting.
or use second OCswitch with much lower setting. to start NP group.

Shooter, your hard-wired version would be simpler and cheaper. Maybe if DEFM is more familar with wiring than with PLCs, he would prefer this hard-wired solution.

With a hard-wired solution, it might not be as easy to change the group of equipment to be start-blocked. With a PLC program, adding or removing motors from the critical list would be a quick software change, not a wiring change. As I said, "if I had a machine shop" that had the problem, I would use a PLC. With a cheap operator touch panel to go along with the PLC, the motors to be blocked could even be selected from a menu list.

Thinking some more, given an off-grid limited power supply, and an owner not proficient in PLC programming, a better solution might be a wiring scheme that used selector switches (activated by a generator current switch) to select which motors are start-blocked during a gernerator overload. That might be the best of all methods. To make some wiring schematics would require more information about the system, the generator capacity, the voltage levels, the shop equipment list with the current requirement for each piece, and an order of load shedding for the group of motors to be blocked.

The problem with many methods is that the manufacturer's control circuit for each machine would have to be modified, and that could void some warranties. To do the job without changing existing equipment would require an external contactor for each CNC machine.

I've done something similar to what Lancie1 is talking about. I used a small Siemens display and a S7-200 PLC that was fed off UPS power. It monitored a power fail indication and a run indication from each motor. It also had an output for each motor run indication tied into the motor start circuit. The display allowed you to set the time after a power failure to bring motors back into operation that were running prior to a power failure. You also had the ability to assign each motor to a group and change the the time between group A B or C would come back in service.

Wow all you folks are wonderful.I think if I explain a little more detail you guys will hone in on the solution that fits my needs and abilities. I've put on line a 10 hp air comp (shop air hose) that loads and unloads,so I don't have a large "hit or Dump" when psi is reached or needed.It is started first and this keeps the 54kw in the sweet spot & loads the 125 kw to a happy spot.I run the rest of the shop.5 cnc's a auto saw and mics manual equipment- no problem. The 125 runs (if) I intend to run a 25hp cnc lathe,same routine as above,just add in big lathe-life is good. now the rub,I would LOVE to put two 5 ton ac units on line too.If the big lathe hits when the air comp loads AND one or both the ac's hit I fear the 125 won't keep up.I heard of a device I can buy-read plug & play-that will stop the ac's from starting until the cost is clear (for lack of a better term).Your imput is greatly appreciated,Rob

Are you completely Off-Grid or just possibly only for 3-phase loads? Are the AC units existing or a new project? If they are a new project and you have 1-phase On-Grid power available, On-Grid connecting to the 1-phase power may be an option. Maybe you could use three 3 ton units instead of two 5 ton units.

DEFM said:

I think if I explain a little more detail you guys will hone in on the solution that fits my needs and abilities.

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Now I am going to make lists of your details to help me see this better:

Loads:
a) 10 hp air comp
b) 5 cnc's a auto saw and misc. manual equipment-
c) 25hp cnc lathe
d) two 5 ton ac units

Generators:
a) 54kw
b) 125 kw

Logic:
Starting: The 10 hp air comp is started and loads and unloads on demand keeping the electrical demand more steady and this keeps the 54kw in the sweet spot & loads the 125 kw to a happy spot. I think includes existing equipment, not just the compressor?

Running: The 125 runs when I add the first load (I intend to run a 25hp cnc lathe) Just add in that load, and life is good as long as you wait a couple or ten seconds before you add in the next one.

Likely, the loads and time delays need to vary and certain loads need higher priorities for starting.

Yes, I would say this might qualify for a PLC rather than a simple timer circuit or dreadful programmable relay.

One of these:

http://www.automationdirect.com/adc...C)/Current_Transducers/AC_Current_Transducers

and:

http://www.automationdirect.com/adc..._(Stackable_Micro_Brick)/PLC_Units/C0-02DD2-D
+
http://www.automationdirect.com/adc...Stackable_Micro_Brick)/Power_Supplies/C0-01AC
+ (for more than 4 digital inputs or outputs):
one or more of these:
http://www.automationdirect.com/adc..._Series_PLCs_(Stackable_Micro_Brick)/DC_I-z-O

Toss in some interposing relays to protect the delicate ones in the PLC. And buy their plain serial cable if you aren't comfortable or don't want to make one, and the software is free. If you need a USB to serial adapter go with a Keyspan (part number eludes me right now...).

Then your imagination and available man hours are the only limits to what you can make it do.
The free software is pretty decent and the hardware is tiny but powerful as far as instruction set, memory and speed.

Okie,you pretty much pegged it.The air comp loads the genny,then because it never realy shuts off,the amp swing is manageable.It's never happened that all cnc's and air comp "hit" at the same nanosecond.I do have a 220v line from power co. because balancing load with single phase was just impossible.I'm going to call these folks you linked to me because I don't have a clue,transducers,interposing relays,serial cables,Oh my.I mean I can do basic wiring 3p or 1p,but when something goes wrong with the cnc,I keep my paws off all those boards and such-it's less expensive that way .I'll let you guys know what I find out and I can't thank you enough,Rob.

I do have a 220v line from power co. because balancing load with single phase was just impossible.

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Please explain that statement. A 220 volt line from the power company would normally BE a single phase supply. Are you saying that you use the 220 volt utility line to balance your load, or that you can't use it?

The thing about using generators is that they work best if you have an AC battery that you can switch in with the generators to help them out with any heavy load for a few minutes. I know, all of you are screaming that there is no such thing as an AC battery. Yes, in the normal sense that is true. There are 2 or 3 exceptions. The best AC battery yet invented for normal folks is a residential power line that is not metered for power factor or demand charges. With such an infinite power source (up to the current limit), it can be used as ride-through power for the generators, if a synchronizer can be found to synchronize the generators with the power line. A single phase-to-3 phase converter can be used along with the synchronizer, to parallel all 3 sources, and allow the power line to help out when needed. Instead of a converter, a 1-phase line can be used along with some capacitors to run a 3-phase motor.

That scheme is probably too complicated for this situation, now that I think about it. So are several others, so maybe shedding the load with a PLC is the best option. I think it will work best if the loads are ones that can be anticipated a few minutes ahead. Air conditioner loads fit that category - they can be held off several minutes until the other loads decrease. There may be other manual shop equipment where the job can be delayed for 5 or 10 minutes.

I do think this load controller will need to be very flexible and have the ability to easily and quickly swap the order of loading with a few key clicks as opposed to making wiring changes.

Ok I'll explain,when I first started all power in shop came from genny.I got two 400 amp panels.I learned that by putting this breaker here and that breaker over there (referring to the panels),I could make the digital amp meter on genny very even.Throw in 15 florescent light fixtures and a couple small drill presses for countersinking,a belt sander,ect...,and my balance went to-not so good-the genny and the cnc's were not happy.I procured some burial wire and with the back hoe ran a 220v 1p line to the shop.I run all the 110v stuff and a buzz box,also 5hp air comp for when genny not running. BTW some of that is or,as in air comp OR buzz box.The rest of your post is completely over my pay scale.I heard about a batt bank to help with demand and dump,kinda like giant capacitors,-I think.I went with genny because I was out on the street(lease was up,landlord had new tenants willing to pay a lot more),my barn/building was about done -no power,so the 54 was bought and away we went.I'm going to call automation direct and talk to tech support and see what they say.I'll let you guys know what happens,Rob