Motor FAQ

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Revision as of 18:23, 21 March 2010 by Sircastor (talk | contribs) (Stepper drivers)
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This page tries to answer most of the frequently asked questions related to the choice and operation of drive motors used by the Reprap.

Stepper motors vs Servo Motors vs DC Gear Motors

I'll copy and paste a quote/excepts later.

Electric motors all start with a simple concept. You apply an electric current to a coil, and it generates a magnetic field. That field causes the shaft attached to rotate. This is all an electric motor is. Through some engineering, variations on this have been created to allow some further control.

DC motors DC (Brushed) motors= easiest to use, just connect straight to battery, only two wires. Hardest to control, you need an external feedback mechanism (optical or magnetic encoder, linear encoder, etc) and H bridge motor driver to get variable speed and direction.

DC gear motors A DC gear motor is a DC motor which has an extension built on to it to "gear down" the rotation. Essentially, a series of gears to make the rotation of the motor move more slowly for particular uses. DC gear motors are normal DC motors which have been geared down to decrease their speed and increase their torque. The Solarbotics GM series are a popular type because of their low cost, but are constructed with plastic gears. Because these gears can only handle so much force, they include a torque limiting slip clutch to protect the plastic gears. If this clutch is disabled, the gears may break.

Servo Motors (Hobby) Servo Motors are DC motors coupled with internal gearing and some control electronics that allow you to move the motor to a precise position. Servos function using information produced by a computer or a microcontroller called Pulse-width-modulation. By sending a certain repeating signal to the Servo, it will move to that certain point. In terms of a reprap, a servo would need to either be modified or purchased as a "full rotation servo". In a reprap, servos require closed loop feedback system and usually involve an optical or magnetic rotary encoder. They generally will offer higher torque than comparable steppers. Servo motors use three wires, Ground, Power, and PWM.

Stepper Motors A stepper motor is another kind of special motor. This motor is powered and sent a signal to move forward x number of "steps". The current RepRap printer uses a common NEMA 17 stepper motor, which usually has 108 steps in it. These are operated in an "open loop". Stepper motors have 4 to 8 wires, and require a more complicated with two or four phases.

Stepper Motors

There is a good article on wikipedia explaining the technology behind stepper motors. The physical size of stepper motors are usually described via a US based standard called Nema, which describes the bolt-up pattern and shaft diameter, the reprap site has an article explaining the standard.In addition to the Nema size rating, stepper motors also also rated by the depth of the motor in mm, the longer the motor typically the more powerful. Stepper motors also have a step size rating, of 1.8 degrees per full step, 4 steps within each cycle. The step size, divided into 360 degrees gives the number of steps per revolution. Some stepper motor controllers generate 'microsteps' by generating a sine cosine waveform for the stepper coils. The microsteps become less accurate then the full size steps, but allow finer control and smother operation. Also check the motor torque and the current draw to compare stepper motor strengths.

The pages related to building a Mendel has a list of suppliers of stepping motors.

The power of a motor is usually proportional to the physical size of the motor, The Darwin version of Reprap primarily used NEMA 24 motors, whereas the Mendel version is designed to use either NEMA 14 or NEMA 17 motors. The more commonly used size is NEMA 17 as it is easier to find NEMA 17 motors with sufficient torque compared to NEMA 14.


The Mendel officially requires 0.137Nm torque (1400 g-cm or 1.215 lb-in) for the X, Y and Z axis. Recent designs for extruders almost exclusively require stepper motors as well, but no requirements for torque has been given in those designs.

Stepper motor's do not offer as much torque or holding force as comparable DC Servo motors or DC Gear motors. Their advantage over these motors is one of positional control. Whereas: DC motors require a closed loop feedback mechanism, as well as support circuitry to drive them, a stepper motor has positional control by it's nature of rotation via fractional increments.

Power and current

All stepper motors will have a certain specifications for voltage and current, typically 2.8V and 1.68A, as long as the stepper driver/controller does current control you can use any supply voltage greater than the motor's rated voltage. In fact, a large difference is advantageous to the top speed of the motor. If the motor dirver/controller does not do current control, you must use a supply voltage fairly close to the motor voltage (no more than 2x the voltage specified by the manufacturer) or the motor will overheat and burn out its winding insulation or demagnetize its rotor.

The 2.3 version of the Reprap axis controllers do have current control.

Stepper drivers vs Stepper Controllers

To run a stepper motor, two things are normally required: a controller to create step and direction signals(at +-5V normally) and driver circuit which can generate the necessary current/amperage to drive the motor. In some cases: a very small stepper may be driven directly from the controller, or the controller and driver circuits may be combined on to one board.

PWM and Stepper Drivers

From Wikipedia:[[1]]: Pulse-width modulation (PWM) is a very efficient way of providing intermediate amounts of electrical power between fully on and fully off. A simple power switch with a typical power source provides full power only, when switched on. PWM is a comparatively recent technique, made practical by modern electronic power switches.

Stepper Drivers normally work by chopping up a supply voltage using an embedded PWM chip. These chips do require minor support circuitry which is the primary thing you pay for when you buy a stepper driver. The PWM chips themselves usually have a unit price below $10USD depending mostly on their rated current.

Some example chips include:
Chip Verified Max Amperage Comments
[L293D Yes .6amp(s) Multiples can be stacked on top of each other to divide up amperage.
[A3967] No .75amp(s) Slightly underpowered, at only 750mA/Phase
[A4983] Yes 2amp(s) Can get very warm, active cooling is needed
[Allegro 3977] No 2.5amp(s)
[TB6560] No 2.5-3amp(s)

Stepper drivers

Sourcing stepper motor drivers can be a bit difficult, the 2.3 stepper drivers for the Reprap is very hard to purchase pre-assembled, sourcing the individual parts and assembling the controllers can be done with just a little bit of skill, for those without skills or materials to assemble the boards, generic stepper drivers purchased from the web. In Europe it will usually be more cost-effective to purchase pre-assembled boards compared to purchasing the individual parts and perform a DIY assembly.

Alternative sources for stepper drivers
Manufacturer Verified Location Max Amperage Microstepping Comments
[RepRap Stepper Motor Driver 2.3] Yes US 2amp(s) Half Listed for comparison.
[EasyDriver (A3967)] Yes US .75amp(s) 1/8 Slightly underpowered compared to other drivers, at only 750mA/Phase. Has plenty sufficient power for Mendel. Recommended.
[Polulo (A4983)] Yes US 2amp(s) 1/16 Can get very warm, active fan cooling or passive small heatsink is needed above ~.5A. Recommended.
[4 Axis Stepper Motor Driver Controller (A3977)] Yes US 2.5amp(s) 1/8 4 stepper drivers on a single board.
[DIY CNC] No GB 2.5amp(s) 1/8 Can drive 1 stepper, discount when buying several.
[Arduino Motor Shield] No US .6amp(s) ? Requires Arduino as controller. Can drive 2 servos, 4 DC, or 2 (bipolar or unipolar)steppers, Website notes that you can increase the Amperage max by piggy-backing (soldering a chip onto a chip) another L298D chip on top of the first (and notes that this could be done twice...)
[TB6560AHQ based] No GB 2.5-3amp(s) ? can drive 4 or 5 steppers depending on model
[Stepper Driver 2.3 Clone by kymberlyaandrus Yes US 2amp(s)] Half Same schematic but physically smaller than the original version. The trimpot doesn't have a start/end point so adjusting the current can be more difficult than other boards. The terminal blocks are nice because they don't require making special connectors.
[Gecko Drive Yes US 3.5amp(s)] ? Can drive 4 steppers
[Nanotec SMC11 Yes GER 1.4amp(s)] 1/16 with cooling until 2.5amps

Micro stepping

Microstepping between the pole-positions is made with lower torque than with full-stepping, but has much lower tendency for mechanical oscillation around the step-positions and you can drive with much higher frequencies.

If your motors are near to mechanical limitations and you have high friction or dynamics, you won't receive much more accuracy. When your motors are 'overpowered' and/or you don't have much friction, then you can transfer the higher positioning accuracy to moving accuracy too.