Which inverter for the CNC?

Spindles installed on milling plotters and engraving machines are most often induction electrospindles powered by an inverter. What is the inverter for? The principle of operation of an induction motor shows that the spindle speed in steady state is proportional to the frequency of the supply (alternating) current, unlike in DC motors, where the rotational speed is proportional to the voltage. As you know, the frequency of the current in the network is 50Hz, therefore the rotational speed of the induction motor (with one pair of poles) is approx. 50 rev / sec, which gives approx. 3000 rev / min.

If we want to change this speed, we have to change the frequency of the supply current, or change the number of pole pairs. Increasing the number of pole pairs makes it possible to reduce the speed of rotation of the induction motor. Two pole pairs are 50Hz / 2 = 25rpm = 1500rpm, three pole pairs are 50Hz / 3 = 16.7rpm = 1000rpm. However, in this way it is only possible to reduce the rotational speed, not smoothly but in steps.

The solution to this problem is the use of an inverter. The inverter first rectifies alternating current to direct current, and then generates an alternating current with the voltage and frequency required by the user. An induction motor has a constant amplitude-frequency characteristic, i.e. the ratio of frequency to amplitude of the supply voltage should be constant. It follows that by changing the rotational speed of the induction motor by changing the frequency, we should also proportionally change the voltage supplying the motor. This is called u / f control.

There is another way to control an induction motor – vector control. Vector control is a method of direct control of the stator magnetic field vector orientation on the basis of complex mathematical transformations of the current values ​​in individual motor windings.

Vector control is used primarily to lower the speed of an induction motor. The advantages of this type of control are manifested primarily in situations where the ratio of rotational speed to slip is relatively small. In a situation where the use of an inverter is intended to significantly increase the rotational speed above 3000 rpm, the use of a vector inverter does not make sense, and sometimes it even causes deterioration of the drive parameters due to the inverter’s failure to keep up with the calculation of subsequent field orientations.

In addition, the inverter used to drive the electro-spindle should have a much higher switching frequency (at least 20 kHz) than general-purpose inverters most commonly found in the trade, whose switching frequency does not often exceed 6kHz. This produces a current that is not a sinusoidal shape, but a “broken square” shape having little to do with a sinusoid. As a result, significant losses appear in the stator of the motor, which causes overheating of the electro-spindle (especially air-cooled) and, as a result, acceleration of its wear.

For similar reasons, the maximum nominal frequency of the waveform generated in the inverter should be at least 2, and preferably 4 times greater than the frequency of the spindle current. For example, a 24000 rpm spindle should be supplied with a 400Hz frequency, so the maximum range of the inverter should be about 2000Hz.

Many simple CNC machines have a spindle control inverter that is completely independent of the control system, coupled only with the spindle activation signal. This causes a situation where the CNC control system does not know what is going on with the spindle. Is it overloaded, overheated, what is the current speed value? In addition, the operator often “by eye” has to set the revolutions only by using the knob on the desktop. The inverter should communicate with the control system on an ongoing basis, for example, the current spindle load status, which can be used for dynamic control of the feed rate, or for detecting emergency states, e.g. machine stoppage before the tool is damaged.