Baldor MN770 - Manual (Page 41)

Section 1
General Information
6-4 Troublehsooting MN770
Electrical Noise Considerations Continued
Wires between Controls and Motors
Output leads from a typical 460 VAC 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 as shown in
Figure 6-5. For this waveform, a transient induced in 1 ft. of wire adjacent to motor lead
of a 10 hp, 460 VAC drive. Scope is set at 5 V/div. and 2
m
sec/div.
Figure 6-5 10HP, 460VAC Drive
If the shielded pair cable is used, the coupling is reduced by nearly 90%, Figure 6-6.
Figure 6-6 10HP, 460VAC Drive, Shielded
The motor leads of DC motors contain similar voltage transients. The switching rate is
about 360 times a second. These noise transients can produce about 2V of noise
induced in a wire adjacent to the motor lead. A 30HP, 500VDC Drive, as shown in Figure
6-7. Scope is set at 1 V/div. and 5
m
sec/div.
Figure 6-7 30HP, 500VDC Drive
Again, Replacing a single wire with a shielded pair cable reduces the induced noise to
less than 0.3 V, Figure 6-8.
Figure 6-8 30HP, 500VDC Drive, Shielded