Special Drive Situations; Drive Power Lines; Radio Transmitters - Baldor 29M Installation And Operating Manual

Section 1
General Information
Electrical Noise Considerations

Special Drive Situations

Drive Power Lines

Radio Transmitters

MN1230
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 5-9 shows a transient induced in 1 ft. wire adjacent to the AC input power wire of a
20 hp, DC drive. Scope is set at 500 mV/div. and 2 sec/div.
Figure 5-9 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. 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.
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. 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. Manufactured by such companies as TCI (Milwaukee, WI) and MTE
(Menomonee Falls, WI) these reactors also reduce ripple current in the motor windings
and often improve motor life. For maximum benefit, the reactors should be mounted in
the drive enclosure with short leads between the control and the reactors.
The same type of reactor 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.
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.
Troubleshooting 5-15