Electrical Noise Considerations - Baldor VS1GV21-1B Installation & Operating Manual

9.4 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 9-1 illustrates noise
suppression for AC and DC relay coils.
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 16,000 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.
Special Drive Situations
For severe noise situations, it may be necessary to reduce transient voltages in the wires to the motor by adding load
reactors. Load reactors are installed between the control and motor. Line and Load Reactors are typically 3% reactance and
are designed for the frequencies encountered in PWM drives. For maximum benefit, the reactors should be mounted in the
drive enclosure with short leads between the control and the reactors.
Control Enclosures
Motor controls mounted in a grounded enclosure should also be connected to earth ground with a separate conductor to
ensure best ground connection. Often grounding the control to the grounded metallic enclosure is not sufficient. Usually
painted surfaces and seals prevent solid metallic contact between the control and the panel enclosure. Likewise, conduit
should never be used as a ground conductor for motor power wires or signal conductors.
Special Motor Considerations
Motor frames must also be grounded. As with control enclosures, motors must be grounded directly to the control and
plant ground with as short a ground wire as possible. Capacitive coupling within the motor windings produces transient
voltages between the motor frame and ground. The severity of these voltages increases with the length of the ground wire.
Installations with the motor and control mounted on a common frame, and with heavy ground wires less than 10 ft. (3m)
long, rarely have a problem caused by these motor-generated transient voltages.
Analog Signal Wires
Analog signals generally originate from speed and torque controls, plus DC tachometers and process controllers. Reliability
is often improved by the following noise reduction techniques:
• Use twisted-pair shielded wires with the shield grounded at the drive end only.
• Route analog signal wires away from power or control wires (all other wiring types).
• Cross power and control wires at right angles (90°) to minimize inductive noise coupling
MN765
Figure 9-1 AC and DC Coil Noise Suppression
RC snubber
0.47
AC Coil
33
Diode
f
DC Coil
Troubleshooting 9-13