Section 1
General Information
Troubleshooting 5-9
MN715
Electrical Noise Considerations
All electronic devices are vulnerable to significant electronic interference signals
(commonly called “Electrical Noise”). 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 10VDC 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.
Relay and Contactor Coils
Among the most common sources of noise are the 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. Figure 5-1 illustrates
noise suppression for AC and DC relay coils.
Figure 5-1 AC and DC Coil Noise Suppression
AC Coil
DC Coil
RC snubber
0.47
m
f
Diode
-
+
33
W
Wires between Controls and Motors
Output leads from a typical 460VAC drive controller contain rapid voltage rises created by
power semiconductors switching 650V in less than a microsecond, 1,000 to 10,000 times
a second. These noise signals can couple into sensitive drive circuits. If shielded pair
cable is used, the coupling is reduced by nearly 90%, compared to unshielded cable.
Even input AC power lines contain noise and can induce noise in adjacent wires. In some
cases, line reactors may be required.
To prevent induced transient noise in signal wires, all motor leads and AC power lines
should be contained in rigid metal conduit, or flexible conduit. Do not place line
conductors and load conductors in same conduit. Use one conduit for 3 phase input
wires and another conduit for the motor leads. The conduits should be grounded to form
a shield to contain the electrical noise within the conduit path. Signal wires - even ones in
shielded cable should never be placed in the conduit with motor power wires.
