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Section 1
General Information
Troublehsooting 6-5MN770
Electrical Noise Considerations Continued
Even input AC power lines contain noise and can induce noise in adjacent wires. This is
especially severe with SCR controlled DC drives, current–source and six–step inverters.
Figure 6-9 shows a transient induced in 1–ft. wire adjacent to AC input power wire to 20
hp, DC drive. Scope is set at 500 mV/div. and 2msec/div.
Figure 6-9 30HP, 500VDC Drive, Shielded
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 separate conduit for 3 phase input
wires and motor leads. The conduit 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.
If flexible conduit is required, the wires should be shielded twisted-pair. Although this
practice gives better protection than unshielded wires, it lacks the protection offered by
rigid metal conduit.
Special Drive SituationsFor 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. These are often required where a motor housing lacks the necessary shielding
(typically linear motors mounted directly to machine frames) or where the power wires to
motors are contained in flexible cables.
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. Baldor offers a
complete line of line and load reactors that will reduce ripple current and improve motor
life.
Drive Power Lines The same type of reactor as installed on the load side of the control can also suppress
transients on incoming power lines. Connected on the line side of the drive, the reactor
protects the adjustable–speed drive from some transients generated by other equipment
and suppresses some of the transients produced by the drive itself.
Radio Transmitters Not a common cause of noise, radio frequency transmitters, such as commercial
broadcast stations, fixed short–wave stations, and mobile communications equipment
(including walkie talkies) create electrical noise. The probability of this noise affecting an
adjustable–speed drive increases with the use of open control enclosures, open wiring,
and poor grounding.