3.4. Calculation of the nominal current
In order to stepper engine gave maximum torque, but it does not overheat, it is important to specify such technical characteristic as
the rated current.
The greater a current in the motor winding, the greater the torque at the axis. It is important to remember that with an increase a
current flowing through the winding, thermal power released by the motor increases. So the engine could operate for a long time
allocated to thermal power (Joule heating) must be less power dissipation. Power dissipation can be calculated on the basis of
documentation on the engine.
Calculation based on the parameters of unipolar full step mode
Power dissipation is equal to , where R
u
- the resistance of the winding in unipolar mode, I
u
- current through the
winding in unipolar mode, n - the number of simultaneous windings.
Consider, for example, ST2818M1006. The table in the documentation shows that in full step mode simultaneously running two
phase (n = 2) in the unipolar mode, i.e. . The motor controllers support only bipolar control mode. To switch from a
unipolar to a bipolar mode, connect each phase windings in series, the resistance will increase, , where R
b
- the resistance
of the series-connected windings in the bipolar control mode.
The motor controllers control algorithm is capable of operating in a microstepping mode and maintains the current so that the first
winding current varies in function , in the other winding current varies in function , where I
a
- current amplitude.
Thermal power released two windings at any time
It follows from the foregoing that the
Calculation based on the parameters of bipolar full step mode
Power dissipation is equal to , where R
b
- the resistance of the winding in bipolar mode, I
u
- current through the winding
in bipolar mode, n - the number of simultaneous windings.
Consider, for example, ST2018S0604. The table in the documentation shows that in full step mode simultaneously running two phase
(n = 2) in the bipolar mode, i.e.
Thermal power dissipated in the motor windings that managed by motor controller, still is
We obtain the equation equating power . We find that
The relationship with an rms current
Alternating current in each motor winding can be characterized by its rms value in the period
. Thermal power of one winding is associated with an rms current through it .
Both windings are identical . The total thermal power of the engine that is run by control by motor controller controller
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