PDA

View Full Version : Peak or RMS

m.marino
23-05-2012, 10:16 PM
Does anyone know if the Amperage on the nema23(4) 3Nm stepper motors is listed in Peak or RMS?

The international standard is to list it in RMS but we are dealing with China here so that is not a for sure in any way.:joker: I am just wondering as it greatly affects how you set up the drivers. Which set up properly is not a problem and should have smooth running steppers. Step up the wrong way and you are either running hot and going to cause their useful life to shorten a good bit or you will be under powering them massively and there by have step loss issues and such. Just asking as It would be useful to know.

Michael

Jonathan
23-05-2012, 11:13 PM
Assuming the motors are wired in bipolar parallel then set them to the nearest to 4.2A peak. Keep an eye on the motor temperatures to start with - up to 80°C on the case is OK.

m.marino
24-05-2012, 09:07 AM
That is assuming that the current listed is Peak and not RMS/phase amps. Which is what I really want to know is which is it. Not the assumption of use it as Peak and therefore add to the safety margin of the motors operating life span. IF it is phase/RMS then using 4.2 as peak reduces the current that motor can use to .707 of what RMS would be.

Let me write it this way:

Which is the 4.2A on said motor? Peak or Phase? Why?

Peak is 1.414 * RMS (or put list the inverse RMS = Peak * .707)

IF it is Peak then fine set accordingly and everything should run fine.

IF RMS then either set as RMS (IF Driver lists RMS Amps) or do the math to arrive at what peak current you can use for said RMS.

IF set as Peak and IS RMS/Phase then you have a problem RMS/phase being .707 of Peak means that you take 4.2 * .707 = 2.9694A is your operating Amperage (RMS/Phase) IF that was peak; this section we are assuming RMS so you have reduced you usable power by 30%.

Now for folks coming into this discussion and not having a clue why it is important:

RMS/Phase is the current you stepper motors run at under normal full current load (no micro-stepping no stand by mode).

Peak is the max that the motor can handle without damage to the motor from use under these conditions (normally for sustained periods).

While you can use RMS as peak it decreases the effectiveness of the motor as well as it's torque.

This is why I am chasing this information down to find out which it is so I can set up the best option on my machine for power usage.

Michael

m.marino
24-05-2012, 09:21 AM
After a cal to Zapp (Thanks for your time Gary), according to him set them as Peak on M752 type series drivers. Other drivers or full stepping may require different settings.

Again good things to know for the best signal to the motors.

Michael

Jonathan
24-05-2012, 04:29 PM
Just to confuse matters further :topsy_turvy:

Which is the 4.2A on said motor? Peak or Phase? Why?

Peak is 1.414 * RMS (or put list the inverse RMS = Peak * .707)

...
RMS/Phase is the current you stepper motors run at under normal full current load (no micro-stepping no stand by mode).

It's rather odd that in the stepper driver datasheets they state the RMS current and the current multiplied by 1.414 for peak current since the current waveform varies so much from sinusoidal. It depends on the driver and the speed the motor is running at - on some drivers the current is just chopped, so without microstepping it's closest to a square wave. With microstepping the waveform approximates a sinusoid (but quite poorly at the low microstepping settings we use) and for better drivers at medium-high speed they do transform to a sinusoidal waveform, but still an approximation. The only waveform for which the peak value is 1.414*RMS is a sinusoid, so this formula generally doesn't apply...yet they still confuse the ratings by including it.

Anyway..

The current rating on the stepper motor is the maximum constant current you can apply to the coil/phase due to the thermal limit (i.e. in steady state). Since in the 3Nm motors this rating is 2.1A, they're 4.2A in parallel so you know the current cannot be greater than 4.2A so it's safe to use 4.2A peak (4.09A) is nearest on PM752.

4.2 * .707 = 2.9694A is your operating Amperage (RMS/Phase) IF that was peak; this section we are assuming RMS so you have reduced you usable power by 30%.

Power is proportional to the square of the voltage, (P=I^2*R etc) so reducing the current by 1/2^0.5 reduces the power by the square of that, so (1/2^0.5)^2=0.5. Therefore you have reduced the available power by 50%. However the motor torque is (roughly) proportional to phase current, so you would be correct in saying the torque is reduced by 30% in this hypothetical situation.

blackburn mark
03-09-2012, 10:48 AM
can you run your eye over this for me jonathan :)
my motor spec
6722
new drivers
6723

i assumed id be setting peak for 2.84 (closest?)
am i going to smoke anything?

Jonathan
03-09-2012, 10:56 AM
Setting to 2.84A as you can't get 2.8A is only 3% more heat, which is negligible so you'll be fine. It's always a good idea to keep an eye on the motor temperature to start with just to be sure.

blackburn mark
03-09-2012, 11:02 AM
cheers man x

It's always a good idea to keep an eye on the motor temperature to start with just to be sure.

im a nervous bugger when it comes to electric.... ill be like a gun-slinger on the E-stop :)

blackburn mark
03-09-2012, 11:25 AM
another conundrum
my bob has EL817 opto isolators all over it and my drivers state "Opto-isolated signal I/O"
the drivers came with this bob thrown in?
i thought it was unwise to have optos on both?
any thought anybody?

Jonathan
03-09-2012, 11:28 AM
i thought it was unwise to have optos on both?
any thought anybody?

Yes, double isolation is an additional pointless delay, but if the breakout board is well designed the difference should be negligible. Try it and if it doesn't work just bypass the breakout board isolation on just the outputs.

blackburn mark
03-09-2012, 11:41 AM
thanks Jonathan, im guessing all might be well with them sending as part of the kit
i might have a go at putting these in tomorrow

i'm hoping that the "sinusoidal output" is going to make the low speed noise a bit more sociable its the main reason i bought them.... got to be better than the TB board im using now....

blackburn mark
04-09-2012, 04:16 PM
after grinding my teeth for a couple of hours i found pin 3 had been pushed back into the cable and not making contact :)
iv burnt out my 12v atx power supply for the atom board :)
took the skin off my finger and poked my eye... we are up and running again :)

these digi drivers are shite compared to the 5axis TB6560 china specials!!!!! :)

6726
bit of a squeeze.... i'm quite pleased, not done much testing yet but they easily run quicker on the same voltage, the resonance isn't as bad (they play more of a tune now) and i didn't hear one lump of a lost step on a 20min run
there's no noise at all when the motor isn't turning (the tb6560 had the motors hiss like a boiler)
motors don't seem to get any hotter.... so far all seems well :)

the only disadvantage is they maintain power to the motors when i hit the "reset" where as the TB cut all power so i will have to do a bit of faffing when plugging the 4th axis motor in

all in all, quite pleased with them so far, fingers crossed :)

JAZZCNC
04-09-2012, 04:58 PM
the only disadvantage is they maintain power to the motors when i hit the "reset" where as the TB cut all power so i will have to do a bit of faffing when plugging the 4th axis motor in

Are you using the enable pins.? If so then don't and think you'll find it drops the power.?

blackburn mark
04-09-2012, 05:57 PM
Are you using the enable pins.? If so then don't and think you'll find it drops the power.?