NJM3517
7. To change actual motor rotation direction, exchange motor connections at P
A1
and P
A2
(or P
B1
and P
B2
).
8.
Half-stepping.
in the half-step mode, the power input to the motor alternates between one or two phase
windings. In half-step mode, motor resonances are reduced. In a two-phase motor, the electrical phase shift
between the windings is 90 degrees. The torque developed is the vector sum of the two windings energized.
Therefore, when only one winding is energized, which is the case in half-step mode for every second step, the
torque of the motor is reduced by approximately 30%. This causes a torque ripple.
9.
Ramping.
Every drive system has inertia which must be considered in the drive scheme. The rotor and load
inertia plays a big role at higher speeds. Unlike the DC motor, the stepper motor is a synchronous motor and
does not change speed due to load variations. Examination of typical stepper motors’ torque versus speed
curves indicates a sharp torque drop-off for the start-stop without error curve. The reason for this is that the
torque requirements increase by the cube of the speed change. As it can be seen, for good motor perfor-
mance, controlled acceleration and deceleration should be considered.
s
COMMON FAULT CONDITIONS
• V
MM
supply not connected, or V
MM
supply not connected through diodes.
• The inhibit input not pulled low or floating. Inhibit is active high.
• A bipolar motor without a center tap is used. Exchange motor for unipolar version. Connect according to figure 3.
• External transistors connected without proper base-current supply resistor.
• Insufficient filtering capacitors used.
• Current restrictions exceeded.
• L
A
and L
B
used for continuous output at high currents. Use the RC network to set a proper duty cycle according
to specifications, see figures 19 through 24.
• A common ground wire is used for all three power supplies. If possible, use separate ground leads for each
supply to minimize power interference.
s
DRIVE CIRCUITS
If high performance is to be achieved from a stepper motor, the phase must be energized rapidly when turned on
and also de-energize rapidly when turned off. In other words, the phase current must increase/decrease rapidly at
phase shift.
s
PHASE TURN-OFF CONSIDERATIONS
When the winding current is turned off the induced high voltage spike will damage the drive circuits if not properly
suppressed. Different turn-off circuits
are used; e. g. :
Diode turn-off circuit (figure 12)
— Slow current decay
— Energy lost mainly in winding resistance
— Potential cooling problems.
Resistance T O C (figure 13)
— Somewhat faster current decay
— Energy lost mainly in R-Ext
— Potential cooling problems
Zener diode T O C (figure 14)
Relatively high V
Z
gives:
— Relatively fast current decay
— Energy lost mainly in V
Z
— Potential cooling problems
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