Buying and Installing a Load Meter

Check out shunts! You will need a load meter (amp meter) connected to a shunt of the right size! The shunt is basically like a little transformer ( just for visualization purposes) that gets an induced flow of charge running through it! The meter measures this! So you can use the same meter for different ranges, It's down to the shunt. Shunts come in different voltages, with different ratios, this gives your needle deflection on the meter. The shunt fits around the phase conductor like a ring (some you may need to connect the phase conductor either side ) and the two wires that come off the shunt, you connect to your meter! Easier still is to read the max load on the motor and ask you local electrical supplier for a load meter and shunt to suit that!

Come on you guys! I'm struggling here.

I'll check into that with Google! Thanks. Didn't know what they were called.

I think you guys are referring to current transformers (often called "CT's"). They are donut shaped small transformers that would encircle the motor leads in this case.

That's the baby I'm on about, a CT or Shunt!

A Current transformer is indeed a transformer, and only works for AC.

A shunt is nothing more than a correctly sized (correct resistance and physical size for heat dissipation) very low value resistor placed in parallel with a current meter, with the combination placed in series with the current-using load. If the meter is an AC meter, it will measure AC current, and if the meter is a DC meter, it will measure DC current.

What does your load meter measure and how ? Is it the mechanical load you apply on the stones or the electrical load of one of your motors?

What do you want to measure in the grinder ? Force between stone and piece or how much the motor is loaded?

Answers are important to give you the correct answer.

Electrical load on the motor. How much it loads the motor which tells us how much pressure we are putting on the stone. Sometimes we have even messed up and put so much force on the stone (against the flywheel) that it stopped the motor. That isn't good to do, but it happens sometimes. Not often, thankfully, it isn't good to yell at the employees.

That's the way it is set up one the hone. I don't see where the shunt is in the wiring though.

Ken

Hi,

But the meters I can buy for $15 don't have a built in shunt. And the motor on the hone is 2HP and on the Flywheel Grinder is 5HP, would that make a difference in the size of the shunt?

Ken

Of course! one horsepower is (approximately)746 Watts. Multiply the hp by 746 to get the power in Watts. Then divide the power in watts by the voltage of the motor in Volts to get the current of the motor in Amperes.

For 5 hp, the power is 5*746=3730 Watts. If the motor is a 3-phase 208V motor, then the average current must be 3730/208=18 Amps. At any given instant, most of this current will flow through one phase winding, and the other two will carry much lower currents. To be conservative, assume all the current goes through the winding whose current you are measuring. Then a 20 Amp scale should work. Many cheap meters are 0-100mA, which is 1/200 of the required current. 199/200 of that current must flow through the shunt, so the resistance/impedance of the shunt should be 1/199th of the resistance/impedance of the meter. I squared R will tell you the power dissipated in the shunt, and indicate the physical size required of the shunt.

Just some advice... Forget the $15 ammeter. Buy a good panel mount AC ammeter, say, 0-25 amps. The shunt will be built into the meter or come with it. It should cost you in the area of $75 to $100. Grainger has many of them in stock. Start making money with your grinder.

Let me see if I have a grasp of it now. If I buy this meter:

And the motor:

will try to pull 14.2A at motor stall, correct? Then this meter should work fine if we only pull the motor down 4-6A. If we tried to pull it down 10A it would explode the stone.

Am I correct in my thinking?

And I put it inline in the 230V line or one of the 110V lines? The 230V (wild leg) line, correct? Or is my thinking wrong there?

I guess it doesn't matter which leg it goes on, just so the meter reads about in the center with our load on it, where it (probably) is most accurate, but at least will be the easiest to see. From me running it and putting a load on it, it doesn't sound like we are putting much of a load on the motor when grinding a flywheel. Nothing like the load we put on the hone when honing.

Maybe the stone on it is "rotten/soft" from not having been ran in over 4 years. When it is gone (looks like it is going fast), we will see how a new CBN wheel works on it, and you sure don't want to load them very much at $200 a wheel.

Where the pressure/load is put on the stone, loading the motor (this photo showing a regular cup type stone):

Thanks for the replies!

Ken

PS We are already making money with it, they have been using these things for 20-30 years without a load meter, but with a load meter, it isn't a matter of "feel", it will be a matter of a gauge "telling" my employees the amount of load on the stone and how quickly they are using it up.

You have the right idea; you just need a little guidance. I'd buy the 0-25 Amp range meter. If you have 230 single phase with a wild leg, the wild leg should measure 170 to 180 volts to neutral or to ground. Do not put the meter in series with this leg. Choose one of the other two legs. From what I can read of the motor name plate, the locked rotor current (stall current) is 82.5 amps. I'd suggest running the grinder at 10 to 12 amps (70% to 85% power) and look at the finish, cutting speed and stone wear. Make your adjustments based on feel and your experience, then tell your operators to not exceed XX amps and to always start the machine unloaded. At full load on the motor, the motor should run at name plate RPM and draw 14.2 amps. As the load is increased beyond this point, RPM will drop and current will increase until you reach zero RPM and 82.5 AMPS. Of course, you would have destroyed the stone at this point, and possibly the customer's flywheel. However, the motor overload should have tripped before this point and prevented all the havoc. Hope this helps.

Just reading over my post and found an error. I meant to say: "If you have 230 VAC three phase with a wild leg..." Sorry for any confusion.

You said: From what I can read of the motor name plate, the locked rotor current (stall current) is 82.5 amps.

I think from looking at the tag, the 82.5 (82 on this tag) is a % of FULL LOAD EFFICIENCY, not Amps? Or am I wrong and can't read correctly?

I think FKIA has it right.

In my previous post, I was assuming 208V 3Ø, (not having seen the motor plate). Increasing the voltage to 230V will drop the rated (5 hp) current (calculated as single phase) to about 16 amps. Three phase is more efficient, therefore the 14.2 Nameplate Amps.

I gather you are not using true 3Ø, but single phase with a capacitor to shift the third wire. This will be less efficient than true 3Ø, so the current will go up somewhat in the two main wires. The 25 Amp meter suggested will have plenty of capacity, while still being readable down to around 2 or 3 Amps.

I suspect that you are correct that you never use anywhere near the full 5 hp, but the inrush current at turn-on could damage the 10 Amp meter.

Okay, will try to answer both of you guys. Just got back from the shop and measured all 3 legs. I guess it depends on the time of day/day of week I measure them. I have measured them before and got: 121/121/234. Tonight I get:111/111/211.

So they must cut the power back some at night, as strange as that seems. I remember when we bought a new air compressor it had a 208V motor on it and they had to get us one with a 230V motor on it. He said you don't see the 230V 3Ø much.

Now, one of you states:...but the inrush current at turn-on could damage the 10 Amp meter.

Ahhhh, things aren't as simple as they seem then. Because on the hone, when you start the motor, NOTHING reads on the meter. ONLY when you load the motor does the meter read anything. The needle does NOT even quiver when the motor is running, just when we run the stones out against the bore and put a load on the motor, then the needle moves.

To me, that IS the way a load meter should work. If there is no load, (motor running free) then no load showing on the load meter.

So, how is the load meter wired on the hone? That is the way I want it wired on the flywheel grinder. Without the stone touching the flywheel it should read ZERO amps.

Interesting to say the least.

It appears the load meter on the hone is NOT just a meter with a shunt in it then. Correct?

To me, a load meter is just like the scale down at the deli. They put the box/plate whatever on the scale and press TARE and then the scale just shows the weight of whatever you are buying.

I don't need to know how many amps the motor takes to run free, I just want a load meter to know how much I am loading it. Thus, a 25A meter wouldn't be needed, and if it was wired correctly, when you started the motor, the meter would show nothing (no load).

Maybe I want to much. :(

BUT, I really do appreciate all the input, I am learning!

Ken

There are two ways to find out how the meter is connected:

1. Obtain a circuit diagram of the unit and study it.

2. Trace the wiring yourself and create a circuit diagram, and study it.

You are clearly NOT running the motor on its intended power supply... Are you using a true RMS meter?

No, just using a cheap Fluke, not an RMS one. I have been there for 30 years, I KNOW we can't run 208 stuff there, it has to be 230 stuff.

What should the meter read with the leads across any two? No matter which two I put the meter leads on, it reads 243V. (Which to me would seem to be 120V x 2?) We have 6 pieces of 230V 3Ø equipment.

I guess I need to back up and ask:

What is the difference between 208V & 230V motors?

Since my power reads lead to ground @ 211V and the others to ground @ 111V, what does that tell us?

FKIA above said a 230V lead should read 170-180V to ground?

Maybe I need to buy the load meter from the grinder manufacturer for $184. I didn't think it would be so complicated to put a load meter on a piece of equipment, but it sure seems to be for some reason.

All the Flukes I've used in the past 20 or so years were true RMS, but I can't say for yours...

We have 6 pieces of 230V 3Ø equipment.

Any shop that has 6 230V 3Ø machines, especially when apparently at least two of them are 5 hp, should have 230V 3Ø power coming into the building. I have the impression that you don't! 230V 3Ø should measure close to 230 V between any pair, and around 133 V any wire to ground. One reason for using 208V 3Ø is that you get 120V 1Ø any wire to ground for all the light duty stuff.

If, as it sounds, you are somehow faking something akin to 3Ø from 1Ø, your motors will have much less than their rated power, and will have much less than their rated efficiency - meaning your power factor will be terrible and your energy bill significantly higher than it could be.

If you do have 3Ø power coming into the building, but are getting the voltage readings you indicated, then you need to hire a qualified industrial electrician FAST, to avoid a fire! One possibility is really bad (or nonexistent) grounding...

Ah! I forgot... Maybe I need to buy the load meter from the grinder manufacturer for $184.

Presumably they have done the research to produce a meter that truly measures motor load for that specific machine. Well worth the extra money to remove doubt, and they will provide exact instructions on how and where to connect it.

The difference between the 208 and 230 three phase supplies is how the power company has configured the secondary windings on the transformers feeding your shop. 120/208 comes from a Wye (star) connected secondary and is the most common. 120/230 comes from a delta secondary with one winding center tapped and tied to ground to provide a neutral. the center tapped delta arrangement is the one with the wild leg. The wild leg calculates to 208 volts when referenced to neutral or ground. Each of the other two legs calculate to 120 volts when referenced to ground. Notice that I said calculate, this is because the 120's are rarely at exactly 120 and the 208 V wild leg is rarely at 208. My experience has been that the wild leg runs between 170 and 185 in my part of the country. But, that's a power company thing and voltages will fluxuate depending on how the power company is loading the secondary and the electricity use of the other customers fed from the same transformer or even from further up the line.

In wanting your ammeter to just register amperes of load put on the motor by the grinding itself, and not to register the motor idle current, you are asking for the addition of some circuitry to negate both the motor inrush (starting) current and the motor idle current. If this is that important to you, I would not want to re-invent this if the manufacturer already has this as an option ready to purchase and install.

Where did the Baldor motor nameplate come from? That's not the one you posted earlier. BTW, motor inrush current/locked rotor current is roughly 6 times full load current. Therefore, 14.2 amps times 6 equals 85.2 - nameplate says 82.5, that's close enough for me.

Of course, you could install the ammeter as I have suggested and while the motor is running in an unloaded condition, just re-adjust the ammeter to read zero.

Picture time: This IS 230V 3phase here at my shop, that is what the power company says anyway:

And notice the wild leg is 212V+ this morning after 08:00am. I have seen it higher. (wild leg to ground, like shown).

Pretty hard to have a bad ground. It was all re-wired just a few years ago when they put the utilities underground.

So, which do we have?

And the meter for $184 is for a different model machine. I would want schematics BEFORE spending that kind of money. :)

Some how, I think we are complicating this all too much. Let's see, we have for 3 phase equipment:

Lathe

Wet grinder

Air compressor

Broach

Seat & guide machine

Hone

Boring bar

Washer

and now the flywheel grinder.

I guess it is 9 pieces, not 6, of 3 phase equipment. And I've wired in 7 of those. Wiring isn't complicated, so why is a dang load meter so complicated.

One thing I DID see a certified electrician do when they changed the box over, they used too small of wire. I got them to go up a size, but they wouldn't go up 2 sizes. And when the equipment runs, the wire is pretty warm. To me, that is just wasting electricity, no matter what the code book says to use for wire.

Pretty dang hard to have too big of wire, too easy to have too small, by going according to the code book.

But they are the experts, right?

Just like in my shop at home, I have 20 120V outlets, wired with 12GA, all going to their own breaker. Overkill? No, I don't have to worry about overloading a circuit like most wire them, 20 outlets on 2 breakers. Geeez.

So, looking at the photos, what kind of power do we have coming into the shop?

And it IS higher after 08:00am... strange, eh?

Ken

Ken,

You have 120/230V three phase supplied from the power company by a center-tapped delta configured secondary. You should have 4 wires coming to the shop (3 hot and 1 neutral). Your voltage readings confirm this. As I stated previously, this is not the most common way power is supplied. Most power companies will supply 120/208 three phase from a Wye (star) connected secondary. My guess would be that your power company is also supplying one or more 120/240 single phase services from the same transformer. You seem to have a good handle on the situation. I also had a 120/230 three phase service some time ago, so I can speak with some knowledge on your situation. As to your preference of using 12 ga. wire for 120V outlets, and each outlet on its own circuit. I do the same. Most of the "electricians" that I know charge per outlet installed and they will usually use 14 ga. wire because it is easier to handle and they will string a few outlets on one wire run to the breaker panel.

Back to the shop - I can also understand your want of having the ammeter only register the grinding load amps. I had a large tractor/trailer repair shop running 24/7 with 57 mechanics for 32 years. I know the difference between knowing how to operate a piece of machinery and how to use it. I will suggest the compromise between the $15 ammeter and the $184 manufacturer's load meter of installing the 0 to 25 amp ammeter in series with one of the 120V motor leads. Start the motor and let it idle. Note the idle current. Using the calibration screw that is accessible through the face of the ammeter glass, turn the screw to adjust the ammeter needle to rest on zero. Subtract the idle current from the full load current of 14.2 amps. This would be the "not to exceed" current during the grinding operation. Of course this "not to exceed" number could be lower if, in your experience, the stone cannot take the full 5 horsepower or if your "feel" dictated something else. You could be able to get away with an ammeter with a different range scale such as 0 to 15. I just am a little unsure if the repeated motor starting/inrush current would bring the internal ammeter shunt to an early grave. Your choice here.

Oh, one other thing. That wire you mentioned that gets pretty warm; change it out.

~john~

We've changed most of them out already, John. That is just nuts, them doing that to save $10 on a job. I guess I'll pick up an inductive Fluke and put it around each wire and see what kind of amps I have and buy an $80 ammeter from Grainer and wire it in. Yes, I HAVE heard that before, even from PGE (Portland General Electric, our supplier) guys, that our setup wasn't the usual run of the mill. But they furnish the same power even when they converted to underground.

They have to do that because we are 230V 3phase? instead of 208V? The 208V I would think would fry our motors, that is the understanding I have gotten out of it from them.

ALTHOUGH:

notice on the bottom of the hone motor tag: USABLE ON 208V. That is the only motor in the shop that says that. Only dual voltage motor we have (208/230, since we don't have any 460V coming into the shop).

My light bill runs right at $300 a month, summer and winter. A LOT cheaper than back east I think.

Click the above 2 photos for larger size that can be read.

Then I can decide on 25A or 10A and zero it out. Good idea.

Thanks, John.

Ken

Thanks for the pix. They help a lot! Looks like a clean, professional installation. Your voltage readings have me baffled, but I've never dealt with a Wye supply like John has, so I'll bow out... (I'll keep watching to see what I can learn - let us know what you end up with.

Dick

Okay Dick, I'll keep everyone posted on what's happening with this small adventure. I'm just a young guy in my 60s, so everything is still pretty much an adventure for me!

John's idea sounds good, probably will go that route.

Regards,

Ken

Ken,

Most three phase, 230 Volt general purpose motors will run just fine on 208 Volts. Just check the current draw under loaded conditions and make sure that the nameplate current (FLA) is not exceeded. Note that your hone motor tag indicates that it is OK to use on 208 volts and lists the full load current at 6.2 amps. The Fluke inductive ammeter is a good tool to have around the shop. So is an infrared thermometer.

I'm just guessing, but when the original service was installed in your shop, either the electrician asked the power company for a 115/230 three phase drop or the power company was already feeding 115/230 three phase to other customers in the neighborhood, or they were feeding single phase customers from a transformer pole that had three phase distribution at the top. They couldn't go the favored Wye route because doing that would provide 120/208 single phase to the single phase customers and that would be unacceptable. So, they opted to change out the single phase transformer with the center-tapped delta arrangement to provide the single phase customers with 115/230 single phase and you with 115/230 three phase from the same transformer bank. Note that all power company voltages are nominal and can be all over the place as you have witnessed first hand with your voltage readings at various times during the day. Most of this variation is beyond the power company's direct control and occurs because of the varying loads placed upon the power company's equipment by its customers up and down the line.

You're right about your power bills being cheaper than mine. I'm running about $1,300 a month on a 600 amp service.

Oh, yes! I'm in my 60's also. If there is a mistake to be made, I made it and thereby paid dearly for my education.

~john~

You may find some help at Load Controls Inc., http://www.loadcontrols.com/ .

I often use their products and their application engineers are very help full in selecting the right product for you job.

Good luck,

Mike

Okay, have found meters for $15. Now need to find a cheaper CT/shunt and I think I'll be in business.

Thanks for the ideas and replies everyone! Glad for the help.

Ken

Update:

Well, stopped at my buddies pawn shop and he never had any Flukes like I was looking for, but did have a Snap-On Bluepoint one (made by Fluke I think) w/ the inductive clip. Snap-On wants 130$ for the meter and 150$ for the clip on and he gave me both for 125$ so I feel pretty good about that.

Then had to complain about the battery said LOW and so he popped a brand new 9V in it. I HATE paying 4-5$ for a dang 9V battery. Glad he threw a new on in on the deal.

Motor starts with a BIG jump!! 30+ amps... WOW. Idles/ no load at 7.9/8.0amps. Put anything more than 2-3 amps load on it and it just GOBBLES the stone up. I pulled it down to 20 amps (about 12 amps load) and it took the meter about 5 seconds to drop back to 10-11 amps... it ate the stone that fast. :(

AND, could NOT hear the motor slow down at all. I could put a 25A meter on it, but don't know how it would like banging the needle off 32A on startup. Hard on the meter?

What an adventure a load meter is. Geeez.

Ken

Ken,

If the motor is drawing 8 amps at idle, that calculates out to using about 2.5 hp just to idle. Seems very high. The motor should not start with a jump unless you mean a jump in current. Mechanically, it should be a smooth spin-up reaching full RPM in about 2 seconds, possibly less. Is the stone directly coupled to the motor shaft or is there a speed reducer/increaser gear train in there somewhere? My thinking is that something is loading this motor. Dry bearings, dry gear train, rodent nest, who knows at this point? Is there a motor brake that could be dragging?

~john~