Noise problems with DC motor system


Thread Starter

Dan McGrath

I am having problems with a Bodine permanent magnet DC drive 1/4 hp 130 vdc SCR drive putting noise on its 120VAC supply power supply. This noise is effecting some sensitive equipment being fed by the same 120VAC source. I have tried some low cost EMI/RFI filter we had on hand on both
the sensitive equipment and on the motor control board but this did not help. An isolation transformer to just feed this motor control did eliminate the problem but is an expensive solution in time and cost. Can anyone recommend an appropriate filter or approach?

Dan McGrath

Johan Bengtsson P&L Automatik AB

An EMI/RFI filter blocks really high frequencies, the noise you see here is probably of a somewhat lower frequency than the filter blocks. A filter built with the same principles in mind but for a lower frequency could do the job.

Try out something like this:

------- C
L | L

Where sqrt(L*C) is something around one tenth to one fourth of the AC period. The higher the better generally but don't get too close to the AC periodic time.

As a rule for deciding the individual values of L and C, keep the impedance about equal for the AC frequency you have.

Let the EMI/RFI filter stay in place, this will block the real high frequencies not effectively blocked with "bigger" components as those

/Johan Bengtsson

P&L, the Academy of Automation
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]

Randy Hayward

Your problem is common with DC drives. DC drvs use an SCR ( or similar) switching directly on the front end of the drv. Doing so will sometimes introduce a from of distortion in the power supply side, commonly called " Voltage Notching". The only sure way to elliminate interference with other sensitive equipment is to use a seperate power supply ( like a control transformer ) for the drv or the other equipment. You can also use isol xfmr's or reactors and possibly achieve the same results. Reactors will not always work and depends on your systems impedence and capacity.

Hope this helps,

Randy Hayward, C.E.T.
Automation Drives Specialist