Matching the Driver to the Stepper

Now we come to the most important part: making sure that your motor and driver are compatible.

Mismatched motors and drivers can result in disappointing performance. Or worse: damage to the motor and/or controller.

If you choose unwisely, you might meet this guy:

Know the Driver Specifications

The two most important parameters in the driver specificatons are:

• Voltage - The maximum voltage that the driver can supply to the motor.
  
• Continuous Current - The maximum current that the driver can supply to the motor.
  

"Peak" current ratings are not applicable to stepper motors. Always go by the "continuous" current rating.

Know the Motor Specifications

You will also need to know the electrical specifications for the motor. There are 2 critical parameters:

• Amps per phase - This is the maximum current that the motor windings can handle without overheating.
  
• Resistance per phase - This is the resistance of each phase.
  

A Voltage rating is often stated. It is usually calculated from the two above - but not always. It is better to calculate yourself from the above parameters using Ohm's Law.

Obey the Law!

Stepper motor phases are inductors, so they will resist rapid changes in current flow. But at the end of each step or when not moving, they behave like a purely resistive load and will behave according to Ohm's Law.

Standing still is also when a stepper motor draws the most current. So Ohm's Law allows us to use the motor specifications to calculate the current requirements of the driver.

Voltage = Current x Resistance
or

Current = Voltage / Resistance

These formulas should be strictly applied for all "constant voltage" stepper controllers. This includes both V1 and V2 Motor Shields from Adafruit, and virtually all other L293D based controllers.

But some motors have very low coil resistance. Strictly following those formulas, the drive voltage will be less than 5v and performance will not be good. This type of motor is not a good match for a constant-voltage driver. These steppers require a more specialized controller.

Running Above the Law?

It is not possible to cheat Ohm's Law. If you try, you will have to answer to the Blue Smoke Monster. However, there are some other laws at work here. The expertise at the law firm of Lenz, Faraday and Ohm can help you to increase the performance of your motor.

The stepper coils create a magnetic field when they are energized. According to Faraday's Law, the changing magnetic field induces a current in the coil. And according to Lenz's Law, that current will be in the reverse direction of the current creating the field. This reverse current is known as "Back Electromotive Force" or "Back EMF".

This Back EMF increases the "impedance" or effective resistance of the coil. So Ohm's Law still applies - but to this impedance, not to the simple phase resistance. This impedance limits the current flow through the coil at the beginning of each step.

Chopper Drives

A Chopper or "Constant Current" drive compensates for the back EMF by driving the motor with a higher voltage. It is not unusual to drive stepper motors at several times their rated voltage using a chopper drive.

To keep things safe at these higher voltages, the chopper drive also monitors the current being delivered to the motor and "chops" it before it exceeds a pre-set level.

By starting at a higher voltage, the chopper drive is able to deliver more current to the coils at the start of the step, increasing the available torque. In addition to adding torque at slower speeds, this also allows for higher top-speeds.

Selecting a chopper driver and configuring it for a specific motor requires a good understanding of both the motor and the controller.