View Full Version : RMS or PEAK

Sun 19 July 2009, 06:42
I have a nema23 gives 4.2A as / /
for the outpout of my current driver, which I use as reference:
PEAK value or RMS value (for exemple: RMS= 4.1 or PEAK= 4.2)
Thank you for helping me

Sun 19 July 2009, 08:32
RMS (Root Mean Square) is the the method that most meters measure AC voltages. I've always had to pay a great deal of money to buy a meter that reads "True RMS". Wikipedia has an excellent section about RMS.

PEAK is the value that is usually associated with DC voltages. It is the RMS voltage X SQRT(2) or the AC voltage multiplied by 1.414.

The voltage going to a stepper motor is controlled by the stepper driver, such as the Gecko G203v that many of us use. In other words, we don't need to worry about RMS or PEAK. We use a formula given to us by Mariss Freimanis, the man behind Gecko drives. The formula is:

32 X SQRT(Inductance) = MAXIMUM DC VOLTAGE for a stepper motor.

If we use that formula, we will have the highest voltage that can be used (under most conditions) without having the motor burn itself up. I like to use a value about 75% to 85% of the maximum voltage so that the motor runs cooler. The main reason is that the local power company often provides AC power that measures 122 VAC. Most power supplies are sold with data sheets that specify 115 VAC. That extra 6% makes me more cautious than most when I select power supplies.

Mon 20 July 2009, 01:20
thank you for your explanation
I do not have a gecko but to drive stepper leadshine
a M880A
the link; http://www.americanmotiontech.com/products/SteppingDrives/MSeries/M880A_Manual.pdf
they are using the current, not voltage.

Mon 20 July 2009, 11:20
Page 1 of the manual shows that the M880A requires a supply voltage between 24VDC and 80VDC, just like a Gecko product.

Page 9 of the manual shows the switch settings to select peak and RMS current values. Using the RMS number (average current) will let the driver supply the motor with its required amount of current and still allow the motor to draw the peak current for very short periods of time.

I've never before seen a manufacturer use RMS and peak current ratings with a DC device. RMS is a specification used with AC power. With a normal AC sine wave, the RMS value is 1 / (SQRT(2)). If you visit http://www.kpsec.freeuk.com/acdc.htm, you'll see an excellent explanation of RMS.

On a Gecko stepper driver, we need to add a "current limiting resistor" to the G20x. The resistor limits the amount of current that a stepper motor can pull through the stepper driver. On the M880A, the switches allow you to select a built-in resistor to do exactly the same thing. The confusion comes from their use of peak and RMS.

Peak current is a rating that is usually found with devices that drive servo motors. Unlike stepper motors, a servo motor has two current levels listed on its data sheet, average and peak. The servo's average current is the current level that it can maintain 100% of the time without overheating. The peak current is the amount that it can draw for a few milliseconds. If you allow a servo to draw its peak current for any substantial length of time, it will overheat and the motor will be destroyed.

Tue 21 July 2009, 08:22
very informative, you were very clear.

Thank you very much!!!

Zapp Automation
Tue 13 October 2009, 02:44
Use the peak figure.
Disregard the RMS.