ammeter - 110v vs. 220v?

[TRX]

I'd like to install an ammeter on my lathe to see how much the motor is loaded when making heavy cuts. According to the spec sheet for the lathe, the motor is rated at 220v and pulls a maximum of 8.5 amps.

10A meters seem to be common enough, but is there a difference between a meter for 110v and one for 220v? Wouldn't a 220v meter need to read the average of both legs?

An example of the meters I'm looking at is here: https://www.ebay.com/itm/AC-10A-Ana...315869?hash=item1ead86dddd:g:rbsAAOSwAvJW8mwd

 

[sberry]

I got to admit that the price is right. I might meter it for giggles but can run a common machine like that by feel and put your hand on it from time to time. It's not a bad thing to have meters but the rest of the world doesnt. You gonna have hands full with tools.

 

[sberry]

As I said, not a bad thing but it's the kinda thing that if it needed it would have come factory.

 

[dogdog]

https://en.wikipedia.org/wiki/Shunt_(electrical)

 

[alfredeneuman]

First of all there are no such things as 110 and 220. (They are 120 and 240).

A 240 circuit will have equal current on both legs, so no averaging needed.

With a wire-through meter like the one pictured, you just need to check whether or not it's rated for the higher voltage.

 

[pattenp]

Amps is amps, 120V vs. 240V doesn't matter other than the max applied amps and voltage that the meter is rated for.

 

[theoldwizard1]

Technically (okay, those not interested can move on) amps are measured by the voltage drop across the shunt resistor (usually less than 1V). In this case, the shunt resistor is built in.

Exceeding the current rating for any sustained period of time will lead to the "magic smoke" leaking out !

 

[TRX]

So, under the same load, one leg of a 220v motor will pull the same amount of amps as a 110v motor?

 

[alfredeneuman]

So, under the same load, one leg of a 220v motor will pull the same amount of amps as a 110v motor?

No! A 240 volt motor will draw half as much current (amps) as would a 120 volt motor.

 

[TRX]

So, if my 220v motor is pulling its full rated 8.5 amps, the meter, being on only one leg, will show 4.25 amps?

 

[dogdog]

So, if my 220v motor is pulling its full rated 8.5 amps, the meter, being on only one leg, will show 4.25 amps?

doesn't work like that

 

[Mr. T]

So, if my 220v motor is pulling its full rated 8.5 amps, the meter, being on only one leg, will show 4.25 amps?

No, it would show 8.5 amps.

I mean no offense at all, but if you’re struggling with the basic theory of how amps and volts work I’d look into an option that uses a current transformer to measure amp draw so that you are able to leave the wiring of your lathe alone.

Also this way there is no “engineering” involved when you spec out what type of device you need.

To add to that, I’m not sure I’d trust the eBay $5 special to handle inrush current on motor start.

 

[sberry]

If there is 10A at 120 each leg will be 5 on a 240. Actual power the same, 1200 watts.

 

[alfredeneuman]

The current on the 2 wires that you test will be equal, regardless of the voltage.
Amps in = Amps out. 8.5 amps will in will equal 8.5 amps out.
If you ran your 8.5a 240 volt motor on 120, it would measure 17 amps on both wires

Edited:
Ground Fault Interrupters are a good example.
They compare the current going in with the current coming out.
They trip when more is going in than coming out.
That means the current is going elsewhere, such as a human body.

 

[dogdog]

If there is 10A at 120 each leg will be 5 on a 240. Actual power the same, 1200 watts.

I don't think single phase 240 works like that...

 

[alfredeneuman]

I don't think single phase 240 works like that...

Yes, it does

Table 430.248 Full-Load Currents in Amperes, Single-Phase
Alternating-Current Motors

The following values of full-load currents are for motors running at
usual speeds and motors with normal torque characteristics. The voltages
listed are rated motor voltages. The currents listed shall be permitted
for system voltage ranges of 110 to 120 and 220 to 240 volts.

---------------Horsepower---------------

.................1...........2.........3.........5
115----------16a.......24.......34........56
Volts

230----------8.0.......12.......17........28
Volts

 

[wyliesdiesels]

Wow lots of confusion on this thread.

If a 240v motor is pulling 8.5a, the current measured on each 120v leg will be 8.5a...

 

[dogdog]

Wow lots of confusion on this thread.

If a 240v motor is pulling 8.5a, the current measured on each 120v leg will be 8.5a...

exactly....^^^

To complete the circuit... current / electron flows from one leg to another... if 10AMP flows between the L1 and L2 it will measure 10AMP on L1 or L2 at the circuit for the motor...(in op's case it draws 8.5@240V supposedly)... there is no magic ground or neutral (other paths) these current flows through...

The example given is a 1200Watt as in Power... power consumption on 120V would be 10AMP, and on a 240V circuit for the same power consumption would be 5AMP... basic math P=I*I*R or P=I*V... there are no branch circuit or any other KVL / KCL into this calculations. Even for this Alternating current circuit... and It doesn't matter which direction it flows through, there is only one path.

If I was the OP, I would get the 20AMP model... even though that the 10AMP might give better resolution... no I wouldn't worry about the in-rush current... unless you are stopping and starting ... but the link on the wikipedia shows how these shut meters work...

 

[sberry]

I thought I said that, maybe didnt.

 

[Sevenhills1952]

If I were the OP, and only concerned with watching the load, imho there is an easier way. Why not a light bulb? If you were to take two 120 volt bulbs in series across 240v, or just one leg to neutral with one regular incandescent light, wired of course at the motor circuit, it's bound to dim noticeably under load by the voltage drop.
Light could be used to assist with work too.

 

[walta]

Before you buy something expensive get a clamp on amp meter and see if the added load changes your current readings.

I was testing an air compressor and looking at current draws at 0 PSI and 125 PSI and found very little difference between the two readings.

https://www.harborfreight.com/digital-clamp-meter-96308.html


Walta

 

[The Cobbler]

you should be able to tell from the sound,resonance , the chips coming off & the overall performance of the cut whether or not you're taking to deep a cut

 

[u3b3rg33k]

So, under the same load, one leg of a 220v motor will pull the same amount of amps as a 110v motor?

yes.

No! A 240 volt motor will draw half as much current (amps) as would a 120 volt motor.

why you gotta go and lie to the guy? that's just not cool. voltage really isn't relevant here, so you could make your point to series/parallel windings without getting preachy.

TRX, yes, the current drawn on a hot leg should always equal the return path, so unless you have some kind of ground fault, it should be balanced.

 

[dogdog]

I thought I said that, maybe didnt.

yea it is, I read it again and it is....... dunno what I ate

 

[alfredeneuman]

why you gotta go and lie to the guy?

Read post #17

 

[matt_i]

Ammeters used to confuse me too.

But, an ammeter is an integral part of the circuit with *all* current passing straight thru it, same current as the copper (?) conductors going to and from it.

The ammeter has a fixed micro-resistance to it, which correlates with a micro-voltage drop across it depending on the current applied.

And that micro-voltage drop is "measured" by the gauge, and the needle & scale calibrated to read actual amp draw.


In regard to motors, keep in mind that P = V * I, a motor being a horsepower-constant device, so if V1 * I1 = V2 * I2, then doubling the voltage halves the current. Ohm has told us this is so

 

[wyliesdiesels]

So, if my 220v motor is pulling its full rated 8.5 amps, the meter, being on only one leg, will show 4.25 amps?

doesn't work like that

dogdog is correct here.

If there is 10A at 120, each leg will be 5 on a 240. Actual power the same, 1200 watts.

I don't think single phase 240 works like that...

The way he worded it made it sound very confusing.

However, he is correct.

All he is saying is that current will be halved when ran on 240v vs running on 120v....

So, under the same load, one leg of a 220v motor will pull the same amount of amps as a 110v motor?

yes.

Aww no.

When voltage is doubled current is halved.

So for example, a 3HP single phase motor @ 230v is 17a and @ 115v its 34a.

So this means on 240v, each hot let would measure @ 17a.
However, on 115v, the single hot leg would measure @ 34a.

Have you ever looked at an FLC chart?

Might want to start...

why you gotta go and lie to the guy? that's just not cool. voltage really isn't relevant here, so you could make your point to series/parallel windings without getting preachy.

Say what? He didnt lie to the guy. Voltage IS relevant here.

You clearly need to do some studying...

TRX, yes, the current drawn on a hot leg should always equal the return path, so unless you have some kind of ground fault, it should be balanced.

right but thats not what were talking about here.

Were talking about amperage in relation to voltage on an inductive load.

You really could look at table 430.248 to get better educated in the matter...

 

[Sevenhills1952]

As the one fellow said here...ohms law.
I had it day 1 in engineering and again in automotive class.
E is voltage
I is current
R is resistance

And power (Watts) is voltage times current.

So a simple example is a 60 watt light bulb.
Using 120 volts bulb draws 1/2 amp or 500mA (1000mA=1 amp).
Lower the voltage like a light dimmer, the current and power drops.
So any one unknown thing you can easily figure out what's missing. If a car light bulb on 12 volts draws 2 amps that's 24 watts. Also ohms law, 12/2=6...so that's 6 ohms resistance. A 4 ohm resistor across a 12 volt battery = 3 amps, and 3 amps X 12 volts= 36 watts.
As the other fellow said if you have two 1hp motors, one wires 120v, other wired 240v, the 240v one draws half the current...but since P=EI...the wattage will be the same (half current X twice voltage).

 

[u3b3rg33k]

dogdog is correct here.





The way he worded it made it sound very confusing.

However, he is correct.

All he is saying is that current will be halved when ran on 240v vs running on 120v....





Aww no.

When voltage is doubled current is halved.

So for example, a 3HP single phase motor @ 230v is 17a and @ 115v its 34a.

So this means on 240v, each hot let would measure @ 17a.
However, on 115v, the single hot leg would measure @ 34a.

Have you ever looked at an FLC chart?

Might want to start...

Say what? He didnt lie to the guy. Voltage IS relevant here.

You clearly need to do some studying...



right but thats not what were talking about here.

Were talking about amperage in relation to voltage on an inductive load.

You really could look at table 430.248 to get better educated in the matter...

I did mis-read his question though. if you are looking at the per WINDING amp draw on a dual voltage motor, it is the same regardless of the operating voltage, unless you have the operating voltage wrong.

my point was current in wether it's L - N or L1 - L2 is the same as current out. bickering over nominal voltage is pointless.

 

[rjcnaples]

A “ feed through” meter will be rated in voltage and max amps it can handle. Just make sure it’s 240 volt rated at how many amps you need or larger. A 240 volt circuit uses (2) 120 volt phases connected in series with the load. Install the meter in series with the feed to the load.


Sent from my iPhone using Tapatalk

 

[dogdog]

If I were the OP, and only concerned with watching the load, imho there is an easier way. Why not a light bulb? If you were to take two 120 volt bulbs in series across 240v, or just one leg to neutral with one regular incandescent light, wired of course at the motor circuit, it's bound to dim noticeably under load by the voltage drop.
Light could be used to assist with work too.

Not sure what an amp meter would indicate in a lathe do.. so it draws some current... if I were the OP... I think for that real estate... a rpm display kit would be more useful.... than amp gauge... besides that I have always confused on the current limiting and a voltage dropper of a device in a circuit . OP's machine draws max 8.5AMP not sure if it is 240v or 120V that is pretty big wattage light bulb to match for current limiting.

something like this.. but you get the idea...

https://www.ebay.com/itm/MachTach-M...301954?hash=item4b4fad4242:g:RYQAAOSw-7RVFEqY

 

[wyliesdiesels]

I did mis-read his question though. if you are looking at the per WINDING amp draw on a dual voltage motor, it is the same regardless of the operating voltage, unless you have the operating voltage wrong.

my point was current in whether it's L - N or L1 - L2 is the same as current out. bickering over nominal voltage is pointless.[/COLOR]

right but Op was talking about metering current on each leg which DOES change based on operating voltage.

Need to know the correct current so the ammeter can be properly sized.

 

[Sevenhills1952]

I like that tachometer idea...didn't know they made them, not a bad price either.

Not sure what an amp meter would indicate in a lathe do.. so it draws some current... if I were the OP... I think for that real estate... a rpm display kit would be more useful.... than amp gauge... besides that I have always confused on the current limiting and a voltage dropper of a device in a circuit . OP's machine draws max 8.5AMP not sure if it is 240v or 120V that is pretty big wattage light bulb to match for current limiting.

something like this.. but you get the idea...

https://www.ebay.com/itm/MachTach-M...301954?hash=item4b4fad4242:g:RYQAAOSw-7RVFEqY

 

[American Locomotive]

I got to admit that the price is right. I might meter it for giggles but can run a common machine like that by feel and put your hand on it from time to time. It's not a bad thing to have meters but the rest of the world doesnt. You gonna have hands full with tools.

Most modern lathes I've seen have ammeters. If they're CNC, they always have spindle load meters. An ammeter would be a useful tool if you're turning a long piece while taking a heavy cut.

Also judging by how many lathes I've seen that don't have their original motor, an ammeter should probably have shipped with the lathe.

 

[dogdog]

Most modern lathes I've seen have ammeters. If they're CNC, they always have spindle load meters. An ammeter would be a useful tool if you're turning a long piece while taking a heavy cut.

Also judging by how many lathes I've seen that don't have their original motor, an ammeter should probably have shipped with the lathe.

So what is the use for an amp meter reading on a lathe, so you take a heavy cut, and the motor bogs down a little, draws some current... what does that mean? a real question.... I really don't know.

 

[Bert_]

So what is the use for an amp meter reading on a lathe, so you take a heavy cut, and the motor bogs down a little, draws some current... what does that mean? a real question.... I really don't know.

Usually put a mark on it indicating max motor current. If you go over the mark for an extended time you risk damage to the motor. IE your cut was too heavy. You need to take lighter cuts or burn up the motor.

 

[wyliesdiesels]

Usually put a mark on it indicating max motor current. If you go over the mark for an extended time you risk damage to the motor. IE your cut was too heavy. You need to take lighter cuts or burn up the motor.

How would the motor burn up?

Do lathe motors not have overload protection?

 

[American Locomotive]

How would the motor burn up?

Do lathe motors not have overload protection?

It can be easy to go well over nameplate current on a smaller lathe. The cheap klixon-type thermal overloads smaller motors use often allow the motor get extremely hot before tripping. They're more like fire-prevention devices than motor protectors. Some smaller motors even utilize a thermal fuse buried in the windings, so if you get the thing too hot, the motor is junked.

 

[TRX]

Well, this question turned out like putting a dollar in the coke machine and getting a rabid ferret instead of a Dr. Pepper.

I'll just buy the freakin' meter and see what it reads when I stall the motor again. I figure the answer is worth five bucks.

 

[sberry]

Why stall the motor?