Stepper Motor Drivers specify the amount of Current that
they output to a stepper motor in either RMS or Peak Current.
Which of the two is better?
RMS, which stands for Root Mean Square, is a fundamental
measurement of the magnitude of an Alternating Current (AC)
signal. The RMS value equals the amount of Direct Current
(DC) required to produce an equivalent amount of heat in a
same sized load. The shape of the alternating Current AC waveform
is not important. RMS values simplify the calculation of average
power and energy. However, Peak values, which only give the
maximum value of the AC signal, require more information and
can only be compared to RMS values if the shape of the waveform
is known.
For example, when the AC waveform is an ideal sine wave,
the relationship between RMS and Peak Current is as follows:
RMS Current = Peak Current x 0.707 or Peak
Current = RMS Current x 1.414.
The relationship between RMS and Peak stepper Current depends
on the driver’s configuration. The three common modes
of operation for Stepper Motor Drivers include Full-Stepping,
Half-Stepping, and Micro-stepping. If you were to view the
Current waveform of these three modes on an oscilloscope,
they would all appear different representing the relationship
between the RMS and Peak Current:
- Ideal Full-stepping waveform (Figure
A.)
- Ideal Half-stepping waveform
(Figure B.)
- Ideal Micro-stepping waveform
(Figure C.)
All stepper motors are rated in RMS Current, and when Full
Stepping, 200 steps/rev, there is no difference in stepper
drivers that output RMS Current and those that output Peak
Current. The stepper driver simply outputs the Current value
that the motor requires. For the following example, assume
1.0 Amp RMS:
Figure A.
However, when Half Stepping, 400 steps/rev, the RMS Current
and Peak Current are not equal, as shown in Figure B. Unless
the peak driver raises its output Current by 15% (as shown
in figure B.) the motor will not receive its rated 1.0 Amp
RMS Current, and is therefore not generating all of the torque
that its capable of providing.
Figure B.
The Microstepping waveform illustrates an even larger difference
between RMS and Peak Current. In this case the Peak Current
setting must be raised by approximately 41% (as shown in Figure
C.) to equal the required 1.0 Amp RMS motor current.
Figure C.
RMS Current controlled drives will send the motor the selected
RMS value of the Current independent of the Current waveform.
This may not be the case with Peak Current controlled drives,
which output different RMS Currents dependent on the waveform.
In conclusion, because stepper motors only specify torque
curves and maximum ratings in RMS Current, using a stepper
driver that is also specified in RMS Current instead of one
that is specified in Peak Current simplifies the matching
of a stepper motor to a stepper driver in any application.
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