Stepper Motor Circuit

By Mike Rooke



Contents:



The Stepper Motor Circuit


B2.Spice can be used to display the operating point of a stepper driver circuit. It can also be used to perform parameter sweeps in order to see the correct voltage or current limiting resistor values for L-R drive circuits. The circuit below uses data from the Sonceboz 6600-20-25 Stepper Motor. Four timed switches are used to realise a half phase full step sequence. The darlington transistors sink current from the motor coil windings during each phase excitation.


Stepper Motor Circuit (Download: A/D 2000 | A/D v4)
B2 Spice A/D v4's new dynamic schematic feature shows the
current flow during the first phase excitation.


Stepper Motor Graph (plot overlay has been unchecked). Click HERE to display the overlayed plot. Note: No attempt has been made to model the motors magnetic characteristics in this simuation.

Switches V(n1...n4) show the coil firing sequence. i(vam1..4) displays the current in each phase of the motor.



Calculating Current Limiting Resistors


LR drive circuts typically include a large power resistor to limit the amount of current flowing into each of the stepper coils. The limiting resistor can be found by ((Intended Motor supply voltage - Losses)-Rated Motor Voltage)/Rated Coil current).

To confirm the above calculation and in an attempt to find the ideal value, B2 Spice was used to create a basic model of the stepper motor coils with an additional current limiting resistor.

Finding the resistor value
A single coil was copied from the original circuit and the current limiting resitor was changed until the current reading in VAm1 was close to the manufactures recommendation (460mA) with the supply voltage set to 24V (double). The power option in "show steady state display" was enabled to obtain an instant power rating for the limiting resistor. (Good to double the obtiained value if used in a real driver). The voltage source V1 can be changed from constant DC voltage which was used to obtain the resistor for a static motor to a PULSED DC waveform. The pulse width and period can be adjusted to observe the steppers characteristics at different speeds. (minus the magnetic properties)


24v Motor Supply (Download circuit A/D 2000 | A/D v4)

A sweep was performed for R9_A from 1 to 40 Ohms for a 24V supply voltage. VAm[23] was chosen.


Note: No attempt has been made to model the motors magnetic characteristics in this simuation.

The graph below shows the Original 12V configuration with no current limiting - A plot at 24V which was considered ideal and a plot using the actual resistor in the real driver circuit - 18V Supply and 10R 10W resistor.


Note: No attempt has been made to model the motors magnetic characteristics in this simuation.

While not easily apparent, the driver using the current limiting resistor is able to deliver current faster to energise the coil than the original driver at 12V - This translates to higher torque in the motor and higher speed at the expense of heat generation in the limiting resistor(s).



Continuous Coil Excitation Sequence


The timed switches from the previous model have been replaced with the DC part set to PULSE every 4ms for each coil. Total period for 4 coils is 16ms.



Graph showing all four stepper coils exciting in sequence
Note: No attempt has been made to model the motors magnetic characteristics in this simuation.

Conclusion
While it is difficult to model the actual torque and load present in a stepper, using these example circuits it is possible to alter the parameters and simulate the driving circuit of a unipolar stepper motor.