The winding of a brushless servomotor can lose its insulation because of the speed of the drive output voltage and/or because of the over voltage created by the LC parasitic elements of the motor power cable. This paper analyses the phenomenon and suggests the necessary installation solution. The output voltage Vcp of a servo drive is a series of pulses with magnitude either Vb or -Vb, where Vb is the drive d.c. bus voltage, and with pulse width modulation (Figure 1). Its pectrum exhibits frequencies up to 1/ (.tR), that is up to some mega Hertz: the motor power cable acts as a high frequency transmission line.

Figure 1 Output voltage of a servo drive.

 

In conclusion, we point out that this problem is automatically solved if the drive/power able/servo motor supplier is the same and the cable length of the specific application is not greater than the maximum value tested by the same supplier.

If the DV/DT or the peak voltage is greater than the guaranteed value, it is necessary to connect an inductor LF between the drive output and the power motor cable. This inductance, which shouldn’t saturate with the servo motor overload current, forms a low pass filter with the cable parasitic capacitance, thus increasing the voltage pulse rise time tR, and reducing the DV/DT and Vm values (Figure 4). A conventional three-phase or individual phase ron-cored inductor is suitable; allowance should be made for additional core loss because of the high frequency fPWM.

The use of an inductor with 0.3-0.6% of voltage drop is sufficient to limit the DV/DT to a safe value:

                                

 where is the maximum value of the drive d.c.bus voltage (see the Note at the end of the paper);

 Is the motor power cable length;

 Is the apacitance between two power conductor of the motor cable, measured at 1kHz by means of a RLC bridge (100-160pF/m for cable section of 1.5-10mm2; it also depends on the insulation material). The waveform of Figure 5 gives an example of the described phenomenon: in this case the peak voltage is 1050V and the rate of rise is 6.3kV/ s.

Figure 6 shows the positive effect of a three-phase inductor LF=80 H: the peak is reduced to 785V and the  DV/DT  to 1.5kV/ s.

Figure 5 Servomotor voltage.

Figure 6 Servomotor voltage with inductor

Note

The maximum value reached by the d.c. bus voltage of a drive with resistive braking is from 1.2 to 1.4 times the nominal value (it depends on the specific manufacturer). The d.c. bus voltage of a drive with regenerative braking increases, during the servo motor deceleration and a blackout of the a.c. supply, only if the machine manufacturer has designed the motor to stop by means of a braking resistance connected to the d.c. bus.

In this case the maximum value of the d.c. bus voltage is from 1.1 to 1.2 times the nominal value.

 

For more information vist. www.automotioninc.com

References

[1] J.Bonal , G.Sèguier .Entraînements électriques à vitesse variable-Vol.3, Ed.Tec&Doc, 2000.

[2] L.Bonometti . Convertitori di potenza e servo motori brushless , UTET Ed.Delfino , 2001.

[3] B.Drury.The Control Techniques drives and controls handbook ,IEE , 2001.

[4] A. von Jouanne, P.N, Enjeti.Design considerations for an inverter output filter to mitigate the effects of long motor leads in ASD applications, IEEE Trans. on Industry

Application, Sep/Oct 1997, pp.1138-1145.