Page 213 - Root Cause Failure Analysis
P. 213
Inverters 201
Digital Oscillators
Most inverter systems incorporate a digital oscillator. The feedback provided by this
device is essential for applications where speed stability is required.
Fault-Indication System
A multimotor inverter system should have sufficient diagnostic capability to detect
and display the reason for an unplanned system shutdown. It is desirable for the
inverter to display the first fault that occurred.
Other Instrumentation
It is necessary to have voltage, frequency, and current readouts or meters on the front
panel of the inverter. Most new inverters have digital displays, which allow the output
to be displayed in series or simultaneously.
OPERATING METHODS
Proper operation of an inverter system varies with the type of inverter and the specific
application. However, all systems have some common operating parameters, such as
startup and braking.
Startup
Motors controlled by inverters must be started across the line, which refers to con-
necting them across the inverter output while the inverter is operating at running fre-
quency. The inverter must be capable of handling a motor’s starting current in-rush
without affecting the other motors controlled by the inverter.
Braking
If the application requires the motors to be slowed down faster than they would nor-
mally coast to a stop, then dynamic braking is required. This can be achieved with a
multimotor inverter system in several ways. When a motor is removed from the
inverter output, direct current (DC) from a separate power supply can be connected
through one phase of the motor being stopped. Controls must be designed to shut off
the DC power when the motor comes to a complete stop. If not, the motor’s windings
will be damaged.
Another method of braking uses a 60-Hz utility power supply connected to the motor.
This 60-Hz power is connected in opposite-phase rotation from the normal direction
of the motor. This reverse power provides the braking required to stop the motor.
Again, provisions must be made to shut off this power supply as soon as the motor
comes to a complete stop.
