Baldor MN770 manual Electrical Noise Display

Electrical Noise Considerations All electronic devices including a Series H Control are vulnerable to significant electronic interference signals (commonly called ªElectrical Noiseº). At the lowest level, noise can cause intermittent operating errors or faults. From a circuit standpoint, 5 or 10 millivolts of noise may cause detrimental operation. For example, analog speed and torque inputs are often scaled at 5 to 10 VDC maximum with a typical resolution of one part in 1,000. Thus noise of only 5 mv represents a substantial error.

At the extreme level, significant noise can cause damage to the drive. Therefore, it is advisable to prevent noise generation and to follow wiring practices that prevent noise generated by other devices from reaching sensitive circuits. In a control, such circuits include inputs for speed, torque, control logic, and speed and position feedback, plus outputs to some indicators and computers.

Causes and Cures Unwanted electrical noise can be produced by many sources. Depending upon the source, various methods can be used to reduce the effects of this noise and to reduce the coupling to sensitive circuits. All methods are less costly when designed into a system initially than if added after installation.

Figure 6-1 shows an oscilloscope trace of noise induced (as the coil circuit is opened) in a 1±ft. wire located next to a lead for a Size 2 contactor coil. Scope input impedance is 10KW for all scope traces. Maximum peak voltage is over 40V. Unless well filtered this is often enough noise to ruin the output of a productive machine.

Figure 6-1 Electrical Noise Display

Relay and Contactor Coils Among the most common sources of noise are the ever±present coils of contactors and relays. When these highly inductive coil circuits are opened, transient conditions often generate spikes of several hundred volts in the control circuit. These spikes can induce several volts of noise in an adjacent wire that runs parallel to a control±circuit wire.

To suppress noise in these AC coils, add an R±C snubber across each relay and contactor coil. A snubber consisting of a 33KW resistor in series with a 0.47￿f capacitor usually works well. The snubber reduces the rate of rise and peak voltage in the coil when current flow is interrupted. This eliminates arcing and reduces the noise voltage induced in adjacent wires. In our example, the noise was reduced from over 40 V zero±to±peak (V0P) to about 16 V0P as shown in Figure 6-2.

Figure 6-2 R-C Snubber Circuit

6-2 Troublehsooting MN770