Control Systems Application Guide
All electrical and electronic devices can be susceptible to interference by voltage transients and/or electrical noise signals. Variable speed AC and DC drives can not only be affected by these signals but in many cases may be the primary sources of such signals.
There are many types of “interference” signals and ways that these signals can invade and adversely affect sensitive electronics. Some examples follow:
1.) Radiated signals:
Radiated signals are usually high frequency RF (radio frequency) signals that like radio waves can radiate through the ether until they find an antenna to receive them. Just about any length of wire or ungrounded metal mass can act as an antenna. Problems occur when the amplitude or power level of the received “noise” signal is great enough to overcome a normal low power signal being carried by a wire or electrical conductor. The noise can distort by adding to or subtracting from the normal signal level.
2.) Coupled signals:
Coupled signals can be connected from the signal source to the receiving circuit via capacitive and/or inductive components. Capacitors and inductors are commonly used electronic components; what we’re talking about here are instances of these components being accidentally created or mimicked by improper wiring practices and lack or mis-use of suppression and filtering components.
3.) Conducted signals:
Conducted signals simply follow wire conductors directly from the source to the receiver. All of these signals can be continuous and repeatable or random and transient – depending on the source.
Some common sources of noise in industrial applications are:
– Welders and DC Drives; Power converters using SCRs to convert AC to DC are switching on and off current flow through inductive motor and transformer loads. Switching “ON” loads cans create fast dips or notches in the AC line voltage while switching “OFF” allows the inductive currents to create large voltage spikes or transients reaching hundreds if not thousands of volts.
– Many AC inverter drives include SCR type power sections in their front end circuitry while their output sections use power transistors switched at high frequencies that with associated harmonics can radiate like radio transmitters. Transients in the thousands of volts are created within the motor and on motor lead wires to the degree that inverter duty motors must use specially insulated wiring and construction techniques to survive.
– Arcing across the contacts of switches, relays, contactors, etc. are miniature lightning bolts – also due to switching current flow through inductive loads.
4.) What to do?
First read and follow the instructions and manufacturer’s recommendations concerning the installation and use of their product. In general, use the following guidelines.
– For low level signal wiring such as that from potentiometers (pots), tachometers, encoders, etc. , use shielded cable and run in separate conduit from switched AC logic and power wiring. Always follow the manufacturer’s guidelines for the shield wire connection but, in general, connect the shield at the signal receiving circuit end only. Clip off and insulate the other end so that it cannot eventually vibrate around and come in contact with grounded metal. This is true even if the shield connection at the receiving circuit end is to ground. This can prevent a “ground loop” type of signal distortion.
– When low level wiring cannot be run in separate conduit, provide as much physical separation from power wiring as possible. Where the two types of wiring must cross, cross at right angles.
– When possible, transmit low level signals as “process current” signals, i.e. 4 to 20mA levels. This higher power signal level is less susceptible to interference.
– Use line reactors, isolation transformers and Drive Isolation transformers. Refer to Section B on ISOLATION and Section O on Reactors and DITs. DIT types are optimized for use with drives to provide fault current limitation and isolate the distortions caused by the drive from affecting other equipment on the power system. Line reactors also prevent line distortions and provide fault current limitation but do not provide isolation.
– On AC Inverter installations, use output reactors or output filters designed for this purpose. Reactors can be used between the drives and motors to “filter” high voltage transients and add protection to the motor internal wiring and connecting wires.
– On AC Inverter installations, use VFD (Variable Frequency Drive) cable in connecting the motors. This cable is specially designed to withstand and contain the transient energy.
– Use dedicated control voltage (115VAC) transformers instead of taking control voltage from a hot leg and neutral of a 230 VAC, center tapped drive isolating transformer. Again, this keeps the distortions caused by the drive from affecting other equipment on the same voltage supply.
Topics in this motor control application guide include: reducing electrical noise and voltage transients. Read other useful motor control applications and tips by clicking on “Back to Index” below.