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Inverter choice


The spindles installed in milling plotters and engraving milling machines are usually induction electro-spindles powered with an inverter. What is the inverter for? The principle of operation of the induction motor shows that the spindle speed in a steady state is proportional to the frequency of the supply current (AC), otherwise as in DC motors, where the rotational speed is proportional to the voltage. As it is known the frequency of the current supply is 50Hz, and therefore the induction motor speed (with one pair of poles) is about 50 rps, which gives about 3000 rpm.


If we want to change the speed, we need to change the frequency of the supply current, or to change the number of pole pairs. Increasing the number of pole pairs allows to reduce the rotational speed of the induction motor. Two pairs of poles give: 50 Hz / 2 = 25 rps = 1500 rpm, three pairs of poles give: 50 Hz / 3 = 16.7 rps = 1000 rpm. However, in this way you can reduce only the rotational speed but in steps rather than smoothly.


The solution to this problem is the use of the inverter. Firstly, the inverter rectifies alternating current to direct current, then it generates alternating current with the voltage and frequency required by the user. The induction motor has a constant amplitude-frequency characteristics, i.e. the ratio of the frequency to the supply voltage amplitude should be constant. This implies that changing the induction motor rotational speed by changing the frequency, we should also change the supply voltage of the motor proportionally. This is so-called U/f control.


There is also another way of induction motor control - vector control. Vector control is a method involving the direct control of the orientation of the stator magnetic field vector on the basis of complex mathematical transformations of the values of currents in each individual motor windings.


Vector control is used primarily to reduce the rotational speed of an induction motor. The advantages of this type of control become apparent primarily in situations where the ratio of the rotational speed to the skid is relatively small. In the situation where the use of the inverter is designed to increase the rotational speed significantly - above 3000 rpm, using a vector inverter does not make sense, and sometimes it may even cause a deterioration of the drive parameters due to the lag in calculating the subsequent orientation of the area by the processor of the inverter.


In addition, the inverter used for the electro-spindle drive should have a much higher switching frequency (at least 20 kHz) than general-purpose inverters, the most common in commerce, where the switching frequency is often less than 6kHz. This creates a current in the shape which is not sinusoidal, but "broken and square", not having much in common with a sine wave. As a result, there are significant losses in the stator of the engine, which  causes overheating of the electro-spindle (especially of air-cooled one) and thus accelerates its wear.


For similar reasons, the maximum nominal frequency generated in the run of the inverter should be at least 2 and preferably 4 times greater than the supply current frequency of the spindle. For example, the 24.000 rpm spindle should be powered with the frequency 400Hz so the inverter should have the maximum range of about 2000Hz.


Many simple CNC machines are equipped with an inverter controlling the spindle completely independent of the control system, coupled only to a signal activating the spindle. This causes a situation in which the CNC control system does not know what is going on with the electro-spindle - whether it is overloaded or overheated and what the current value of the rotational speed is. Additionally, the operator must often set the rotational speed roughly only using the knob on the desktop. The inverter should constantly communicate with the control system to keep providing it with, e.g., the current load of the spindle, which can be used for the dynamic control of the feed rate or for the detection of faults, such as machine stop, before the tool is damaged.