We've had a few inquiries regarding stepper motor selection for CNC build projects and thought we'd put together a handy guide on demystifying the motor, driver and power supply relationship to better help you make the right choices in selecting the right gear for your build.
How does a stepper motor work?
Stepper motors are DC motors that move in steps. They have multiple coils that are organized in groups called "phases". By energizing each phase in sequence, the motor will rotate, one step at a time. With computer controlled stepping you can achieve very precise positioning and/or speed control. For this reason, stepper motors are the motor of choice for many precision motion control applications. Since steppers move in precise repeatable steps, they excel in applications requiring precise positioning such as 3D printers, CNC and plotters. Operating speeds are often very consistent due to the nature of the stepper motors design and they have maximum torque in low speed operation compared to a conventional DC motor providing a precise platform for motion control applications.
Stepper motors generally come in particular package size categories, these are called NEMA sizing, they refer to categories of motors that share a particular bolt pattern for standard mounting, The larger the NEMA number, the larger the overall motor size.
Let's cover a few of the NEMA sizes below and their general applications for CNC use.
Typically used for 3D printers and budget solid state desktop laser engravers where there is very low demand for torque in both the design of the machine and the lack of require force to push through materials.
Typically used for larger 3D printers with heavier load requirements and many smaller frame CNC routing machines where more torque is required to ensure tooling has enough force to push through material and physically be able to move the machine itself i.e; Gantry.
Typically used for large and heavy CNC router / milling builds generally with steel or cast iron gantries and heavy moving parts. They are a common choice for linear motion control systems that offer very little torque conversion such as Rack and Pinion drives, but we will cover this information below.
Motor Torque & Linear motion system torque
When choosing the right stepper for your application you generally need to consider the rated torque of the motors and any torque conversions provided by your linear motion setup.
Let's take a quick look at different linear motion options and if they offer any torque conversions.
Rack and Pinion drives
Torque conversion: None (unless belt geared)
R&P drives spin a gear over a geared rack for creating motion control. Generally R&P systems are used on long axes where generally other systems will not provide accurate motion control. There is no torque conversion in the design leaving some builders to add geared closed loop belts to improve torque.
Ball Screws & Lead Screws
Torque Conversion: Greatly increased
Depending on the Ball/Lead screws pitch they can have a huge increase in torque at the sacrifice of speed. The higher the torque conversion the slower the overall max speed. Similar to a bench vice, a small amount of pressure to turn the screw can apply lots of torque on the job. Ball and lead screws are often a very precise linear motion control system to use but depending on thickness of the screw may not suit long axes as they begin to whip over long distances causing inaccuracies.
Open belt and pulley
Torque Conversion: None
These open belt and pulley setups are typically found on desktop hobby CNC machines and 3D printers. The design uses an open timing belt meshed to a pulley that is driven by a motor. These are cost effective systems but due to belt stretch they often have a lower degree of accuracy. They also have no torque conversion which means they can move very fast but generally require more powerful stepper motors to drive this setup.
Closed-loop Belt & Pulley
Torque Conversion: None-High
These systems use 2 pulleys synchronized by a closed belt to drive your motion. Depending on the size variation of the 2 pulleys you can generate substantial torque in the system meaning it is possible to use smaller motors to achieve the goal at the sacrifice of max speed. Like most belt systems accuracy is not the best.
Depending on the type of motion control system you choose it will generally dictate the size of the motors you require.
Choosing the right power supply and drivers
So, you've picked your stepper motors and now you want to drive them efficiently, awesome!
The first thing to look at is the rated current and inductance of the motors. This can generally be found in the data sheet of the stepper motor, see example below.
Taking the above motor into consideration we know we need a driver that can deliver at least 3 amps to drive this motor at it's rated current.
You can run a stepper generally at 40% less than its rated current but torque will be greatly affected. So it is recommended to run it as close as possible to the rated current but never over the rating.
Stepper motors are very difficult to destroy and damage, but one sure fire way to destroy them is to exceed their rated current. It is generally recommended to run them at their rated current or slightly below if your driver allows.
Inductance and why it is so important
Taking the above motor into consideration we can see the motor has 4MH (milli henry) of inductance. There is a lot of data explaining how inductance works but as a simple explanation:
A current generated in a conductor by a changing magnetic field is proportional to the rate of change of the magnetic field. This effect is called INDUCTANCE and is given the symbol L. It is measured in units called the henry (H) named after the American Physicist Joseph Henry (1797-1878). One henry is the amount of inductance required to produce an emf of 1 volt in a conductor when the current in the conductor changes at the rate of 1 Ampere per second.
Calculating what voltage is needed to supply your motors
We have designed a simple equation for working out what the ideal minimum input voltage is required to drive your motors to their maximum speed and avoid the motors stalling.
The equation is:
V = 32 x SQRT(Inductance)
If we apply this equation to our motor specs above, this will mean the optimal minimum input voltage is 64V, This value can be higher or lower but to drive the steppers at their maximum speed, a minimum of 64V is needed to achieve this.
This will mean that the stepper drivers required to run this must have a voltage input range greater than the supplied input voltage.
It is important to note that you can successfully run the above motor at 24VDC without issues, we do this very often on our SharpCNC kits and the speeds are acceptable for the machine without any stalling issues.