Mindy Lin Cheng
Engineering Applications
Specialist
Jeannie Robinson
Manager
Lin Engineering
Santa Clara, Calif.
It's hard to stay up to date when
you are floating in a sea of technical jargon. However, it's imperative that engineers know
the terminology associated with
the areas in which they work.
And that's especially true if their
assignments take them outside
their engineering discipline. For
example, a mechanical engineer
specifying a stepmotor should
understand the associated mechanical and electrical terms.
Stepmotors need the right current if they are to work correctly.
Without it, the motors can overheat, miss steps, and even freeze in
their tracks. Yet the one electrical
specification that most confuses
all engineers, from the recent graduate through seasoned veteran, is
the rating for stepmotor current.
No doubt this happens because
stepmotor-current ratings come in
many forms such as amps/phase,
amps RMS, average current, and
even amps peak current.
An understanding of how
power is applied to a stepmotor to make it step gives insight
into the different current ratings.
Most stepmotors have two electrical windings or phases labeled
A and B. The phases are placed at
a magnetic angle of 90° apart. Current flows through the windings
generating a magnetic field that
forces the permanent magnet in
the rotor to align with the field.
Power for the windings comes
from the stepmotor driver. The
most common drive technique
uses a bipolar, current-controlled
method. Bipolar means the driver
periodically reverses the polarity
of voltage applied to the winding.
Current controlled, as its name
implies, varies the amount of current the motor sees. When power
is first applied, the permanent
magnet in the rotor of the stepmotor aligns with the magnetic
fields generated by the windings.
To step the motor, current in one phase is turned off. The
change in direction of the magnetic field forces the rotor to align
with the powered phase. With
that, the motor has taken its first
step. Power to the first winding is
turned back on, but with opposite
polarity. This reverses the magnetic field of that phase. The rotor
takes its second step to align with
the new polarities. The second
phase is turned off for the third
step, and then its polarity is reversed for the fourth step. Overall,
the motor takes eight steps before
the polarity sequence repeats.
An obvious problem with
this method is that one phase is
turned off every other step. The
lack of phase current reduces
motor torque. To compensate
for this drop, the driver boosts
current in the powered phase.
The higher current flow in the
powered phase keeps torque the
same for all steps.
At first glance, it appears that
current through the powered
phase should double. Such is not
the case, however. When both
phases are powered, the rotor
locks midway between them. That
places the rotor at a 45° magnetic
angle to each phase. The strength
of the magnetic field felt by the
rotor is the sin 45° for one phase
and cos 45° for the other, or only 70.7% of each phase.
When one phase turns off, the rotor aligns with the powered
phase. To keep torque the same,
the single-powered phase must
develop a magnetic field strength
of 2 X 70.7%, or 141.4%. The current through the single-powered
phase must be 1.414X higher
than its value when both phases
are powered.
Because current changes with
each step of the motor, it's not
possible to specify a single value
of current when the motor is running. Current ratings in stepmotors stem from the amount of
power and, thus, heat that the
motor winding can handle. Power,
of course, is calculated using the
square of the current multiplied
by resistance, or P = I2R. Resistance in this case is the resistance
of the motor windings. For each
motor a value of I is chosen such
that I2R does not exceed the standard power rating of the motor.
Because current is constantly
changing in an operating motor, a statistical method is used
to calculate the current's effect
on motor power. As power is a
function of current squared, the
method used is called the Root
Mean Square, or RMS, method.
Using this technique, the value of all currents are squared, the average of their squared values found,
and then the square root is taken
of that average value. The calculation shows that the RMS value of
a stepmotor equals the amount of
current when both phases have
power. Thus, for stepmotors, average current, RMS current, and
amps RMS are identical ratings.
Labels on motors and motor
data sheets typically list an amps/
phase rating. amps/phase specifies
how much average current each
winding or phase can handle without burning out the motor. It should
be obvious that this value is the
same as the amps RMS rating.
Peak current or amps peak is
the highest current that can flow
through the motor. As previously
shown, the peak current is 1.41 X
amps RMS. Drivers and controller
products cannot supply currents
higher than their design permits.
Therefore, they specify their current rating in terms of its peak
value. Recent changes now have
some companies adding both
amps peak and amps RMS to their
driver data sheets to make it easier for engineers to match what
the motor can handle.
The key relationship to remember is that amps peak = 1.41
X amps/phase (or amps RMS).
Regardless whether you remember the reason behind the 1.41 constant, understanding
the relationship between amps
peak and amps/phase is crucial
because, for most manufacturers, stepmotors only list amps/
phase while drives only spec
amps peak. Understanding that
difference lets you talk the same
language to drive and stepmotor makers alike.
Calculating the RMS value
Power dissipation in stepmotors is a function of the square of the
phase current times the winding resistance, I2R. However, phase
currents in a turning stepmotor are constantly changing. To determine power dissipation in the motor involves calculating the average power over a specific period of time.
The current pattern repeats every eight steps, so averaging the
power level of all eight steps should provide the average power. To
find the average power, take the average of the square of the current — a process known as the root-mean-square or RMS value. To
help with the calculation, assume the current for each phase is 1 A
when both phases are powered, and 1.41 A when only one phase
has power.
Calculating the RMS value of current for the eight steps gives:
Notice that the RMS value is the same as when both phases are
powered. Thus the amps/phase, amps RMS, and the average current are all the same value, and that the amps peak value is 1.41X
higher.
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