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CNC

Last update: 31/07/2015

The choice of a stepper motor it is not always an easy matter. Usually the question is: why should I spend 80-150 US$ for a good nema23 stepper motor when I can find the same motor in a surplus market for 5 (or even less) US$?

 

There are many points you have to consider:

 

Point 1)

a recent motor has been built using different technologies and materials. One of the main problems of a stepper motor is the magnetic hysteresys created by the stator of the motor itself. A recent production motor is always built with materials characterized by a low hysteresys. I can really prove that two motors with the same nominal tension and a same phase/current powered by the same driver and tension have a dramatically different maximum rotation speed (obtaining with our tests a triple maximum speed with a 2006 motor and an 1985 old similar stepper);

  • a new production motor is (90% of the cases) an hybrid motor, this means that we can use it with a unipolar or bipolar controller;
  • a recent motor has an holding torque really higher than an old one. 

A stepper last for a life so the investment could be taken in a serious consideration.

 

Point 2)

If a motor is used to drive a screw, a 200 steps motor can be considered the right choice but if the motor is connected to the axis by a timing belt the power needed by the motor must be, ofcourse, higher, involving questions and other solutions regarding the size of the motor itself (and power supply, and power controller ...). So, if it is not absolutely necessary, use screws to realize the movements.

For small cnc machines (pcb prototype maker, light plywoods) the use of a timing belt can be also considered but, in my opinion, to obtain the necessary accuracy the stepper motors should have a higher number steps/turn. On the market 400/800 steps motor can be easily found at relatively affordable prices for an hobbyst driving them with a microstepping controller to obtain an higher precision.

 

Point 3)

 

stepper motor torcia

The nominal voltage of a motor.

Usually, to obtain good performances by a stepper motor it must be driven at 10-20 times its nominal voltage. Therefore a motor with a very low nominal tension will be more desiderable because a lower voltage will be necessary to obtain the best performance. Many online catalogs show lists of their producs very clearly (like JapanServo's) giving to the buyer a clear situation of theyr production. Whenever possible, the choice should be done for the lower nominal voltage motors (the current can be managed by our controllers). Other companies like Minebea prefers to declare only the nominal current/phase.

In this picture (regarding three different Minebea hibryd motors) it is possibile to notice immediately the differences of speed and torque obtained providing the same voltage (24V).

It is a clear that not always to a motor with an higher phase/current corresponds a faster or more powerful motor. At low frequencies, in this case, the smaller stepper motor presents the best torque performances.

These qualities must be always remembered when we decide to build a cnc machine just to obtain the best performances by the item we have paid for.

 

Most of the stepper motors sold are, nowadays, hybrid motors; this means that they can be used in a bipolar unipolar configuration according to the connections of the driver. While a 6 wires hybrid stepper can be used in a unipolar or bipolar configuration, really interesting is the use of the series or parallel configuration for the 8 wires stepper motor.

connessione unipolare

Unipolar Wiring:

This picture shows how to connect an 8 wires hybrid stepping motor in an unipolar configuration.

Naturally in thi case the motor can be driven by a traditional unipolar driver 

 
   
motore connessione parallela

Parallel bipolar wiring connections:

The parallel connection is normally used when an high torque at high speeds is needed as a result of a lower inductance.

 
   
bipolare

Series bipolar wiring connections:

This kind of connection can be used only with bipolar drivers. It generates an high inductance (and torque) at low speeds. The wires are directly connected in the middle in a series configuration.


The purchase of an hybrid motor, even if a little more expensive, is generally considered a good idea because at any moment the configuration can be changed according to the driver and the speed necessary to drive the cnc machine but it is important even consider the current needed by the same hybrid motor accordingly to the wiring configuration. In this case we have always to pay a look to the pdf of the motor were are always clearly reported the current levels of the different configuration.

Just as example a KL23H251 motor (nema 23) pdf reports:

 

 Voltage

Current

 Resistance

 Torque

 Unipolar

 4.32 

 2.4 A

1.8 

113 

 Bipolar (series)

 6.12

 1.7 A

3.6 

156 

 Bipolar (parallel)

 3.06

 3.4 A

0.9 

156 

 

... and we are talking about the same motor.

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