What sort of Electromagnetic interference is produced by the fast switching of SCR's in a DC drive? What sort of effect does this have on the switching itself? What sort of protection is recommended for low voltage signal inputs of the controller?

-- Justin Ward (jw@hotmail.com), November 09, 2000

Answers

The fast collapse or rise of voltage across an SCR switch when it turns ON and OFF and a rapid change in current causes EMI, the same as with any other switch device. SCRs in a typical dc drive will switch on and off in 1-5 microseconds with a voltage rise/fall rate of 250-800 volts/microsecond (dv/dt). Normally switching occurs 6 times per line cycle, or 360 Hz on a 60 Hz line. A 12-pulse drive will switch 2X this rate. The rate of change in current (di/dt) is limited by reactive devices built into the equipment and/or the effective inductance of the utility connection. In many cases the inductance is supplied by the leakage reactance of a power isolation transformer. In part, it is the inductance of the utility connection that causes line notches.

EMI in the form of switching noise and line notching are created directly at the SCR bridge. Any inductive device in the utility path will divide this voltage disturbance down in direct proportion to the ratio of the inductances. A common isolation transformer supplied from a 5X KVA sized utility connection on the primary side will attenuate line notching to be negligible. This is covered in IEEE- 519 documents and help guide literature. However, switching dv/dt is capacitively coupled from power circuit wiring to drive chassis and other circuits. This is radiation in the most direct form that creates the flow of noise currents in other objects. Radiation like this will also emanate from dc output lines to the motor armature and field. This sounds bad, but is relatively quite simple to suppress. Keep output wires in bonded metalic conduit. Ground bond the drive chassis to the input transformer core. Ground bond the drive chassis to the output wiring conduits and dc motor frame. Enclose the drive in a ground bonded metal cabinet. Keep the cabinet door closed. The noise suppression means is to simply give unwanted noise currents a preferred path to flow, rather than through other sensitive circuits or radiate out into space.

Other signal wiring within the drive equipment cabinet will always be subjected to some electrical noise. If you follow the ground bonding plan outlined above, and use differential analog signaling with twisted pair shielded wiring and there should be no problems. Ditto for any serial communication wiring. 24 Vdc logic circuits should also not be affected by local EMI. But relay switching noise can be worse than any SCR caused EMI. All analog, logic, and digital circuitry of control equipment should pass the EN50082-2 electrical noise immunity standards. By contrast a typical ac inverter drive uses IGBTs. These switch with dv/dt rates in the 5,000 to 10,000 volts/microsecond range. The switch repetition rate is often 2,500 to 15,000 Hz. The combined effect is the creation of 30 B 1,600 times more EMI than a dc SCR drive! But the noise suppression techniques are relatively the same. Provide a preferred path for the noise currents to flow. This is exactly what all those EMI filters and added reactors do.

DonV

-- Don Vollrath (dvollrath@magnetek.com), November 09, 2000.