Page 52
Oscilloscope Safety
There are instances where a simple digital multimeter is not sufficient to trouble shoot a
DC Drive and an oscilloscope must be used. This note will explain how to properly
connect to the drive.
Focus 1, Focus 2 and Focus 3N non-regenerative drives are known as “hot “ chassis
type drives. What this means is that the control circuit is connected directly to the motor
terminals which are in turn tied to the power line through the SCR’s within the drive.
The significance of this is that it is important that no point within the drive circuitry
comes into contact with earth ground. If this should occur it would effectively result in
the power line to be connected directly to earth ground through the control circuitry
(needless to say, destroying the Drive Control Board, blowing fuses etc).
When using a typical oscilloscope powered by the 115vac power line or a battery
powered scope being powered with an AC/DC adapter or any similar line powered
electronic instrument special care must be exercised. By design, the (earth) ground
pin on the power plug is tied directly to the ground clip on the scope probe (and also to
the case of the scope). In order to use the oscilloscope to look at various control signals
and the motor armature voltage, this connection must be somehow eliminated.
This Troubleshooting Information is intended for use by Qualified Electronic Technicians
versed in Motor Drives and Industrial Power devices. The procedures outlined in these
guides expect that the troubleshooter understands the hazards of working with High
Voltage and High power devices and has basic troubleshooting skills and a working
knowledge of troubleshooting equipment
One must not attempt using this information if their
knowledge or skills do not include
working with Motor Drives and High Voltage AC Line power.
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