Page 222 - Electric Drives and Electromechanical Systems

218 Electric Drives and Electromechanical Systems

FIG. 8.6 Pull-out torque-speed characteristics of a typical stepper motor: the dips in the curve are resonance
points. In order to achieve torque throughout the speed range, the maximum load torque T L should be less than
the resonance dips, in addition the peak torque at standstill must be greater than the load torque or the motor
will stall.

the manufacturers in the form of pull-out characteristics, which show the maximum
torque that can be developed at any speed (see Fig. 8.6). If the applied load torque
exceeds the torque that can be generated by the motor, the motor will pull out of
synchronism with the magnetic field, and it will stall. From Fig. 8.6, the following points
can be noted:
The motor is capable of operating with a load of T L , up to a speed of N max steps
1
s . Above this speed, the motor will not start.
There are significant dips in the pull-out-torque curve at a number of speeds.
These dips are caused by resonance between the motor and the excitation
frequency.
The impact on motor winding inductance needs to be considered as function of
motor speed. At low motor speeds the phase currents are effectively rectangular.
However, at high speeds, the time constant for the phase current’s rise and
decay will become a significant proportion of the winding’s excitation time
(see Fig. 8.7). Hence, the effective phase current, and hence the torque produced,
will be reduced. In addition, high motor speeds result in an induced stator voltage
which also distorts the current waveform. This is particularly marked with hybrid
stepper motor because of the presence of the permanent magnet in the rotor.

As shown in Fig. 8.7, the phase currents of a stepper motor are almost rectangular
at low speeds, allowing the pull-out torque of a motor to be determined from the
static-torque-rotor-position characteristics for a particular excitation scheme. The
pull-out torque can, within certain limits, be dependent on the driven inertia. With a high
load inertia, the pulsating variations of the motor torque will only lead to small variations
of the motor speed. Under these conditions, the pull-out torque can be considered to be