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Choosing a Power Supply for Your Stepper Drive...

When choosing a power supply for a stepping motor driver there are performance and sizing issues that must be addressed. An undersized power supply can lead to poor performance and even possible damage to the stepping motor driver, which can be both time consuming and expensive. However, bipolar chopping stepping motor drives are quite efficient and may not require as large a supply as you might suspect. Motors have windings that are electrically just inductors, and with inductors comes resistance and inductance. Winding resistance and inductance result in a L/R time constant that resists the change in current. It requires five time constants to reach nominal current. To effectively manipulate the di/dt or the rate of charge, the voltage applied is increased. When traveling at high speeds there is less time between steps to reach current. The point where the rate of commutation does not allow the driver to reach full current is referred to as Voltage Mode. Ideally you want to be in Current Mode, which is when the drive is achieving the desired current between steps. Simply stated, a higher voltage will decrease the time it takes to charge the coil, and therefore will allow for higher torque at higher speeds. Also a characteristic of all motors is Back EMF, and though nothing can be done about back EMF, we can give a path of low impedance by supplying enough output capacitance. Back EMF is a source of current that can push the output of a power supply beyond the maximum operating voltage of the driver and as a result could damage the stepper driver over time. Bipolar chopping steppers are very current efficient as far as the power supply is concerned. Once the motor has charged one or both windings of the motor, all the power supply has to do is replace losses in the system. The charged winding acts as an energy storage in that the current will re-circulate within the bridge, and in and out of each phase reservoir. While one phase is in the decaying stage of the chopper, the other phase is in the charging stage, this results in a less than expected current draw on the supply. Stepping motor drivers are designed with the intention that a user's power supply output will ramp up to greater or equal to the minimum operating voltage. The initial current surge is quite substantial and could damage the driver if the supply is undersized. If a power supply is undersized, upon a current surge the supply could fall below...

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