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Summary: “Bypassing an ASD:
What's Involved" With
trilingual summary
When a motor is
supplied from an inverter for adjustable-speed operation, any
malfunction in the inverter requires the motor to be shut down. In
many applications, the motor can be kept in fixed-speed operation
through a bypass contactor that routes line power around the disabled
inverter. Manufacturers of small adjustable-speed drives frequently
include such a bypass circuit in their equipment. For larger sizes
(typically above 150 kilowatts), separate bypass controls are
available.
The simplest
bypass scheme uses two contactors, often combined in the form of a
simple reversing motor starter. One connects the motor to the
inverter for normal drive operation; the other (normally open) routes
line power around the inverter to the motor. Opening the first
(either manually or automatically) closes the second.
That second
contactor may be called upon to start the motor from rest, and must
also provide overload protection for the running motor. Its rating,
however, is not the only concern. The motor must also be able to
start the driven machine from fixed-frequency line power. Some
motors, designed for coming up to speed on rising frequency from the
inverter, may be incapable of doing that.
The
two-contactor circuit does not permit the electronic power supply
itself to be entirely isolated from the line for servicing. That
requires a third contactor between the electronics and the supply
line, which can allow the motor to remain in operation while the
drive apparatus is isolated.
In any bypass
scheme, interlocks and time delays are needed to ensure personnel
safety and to prevent reclosing the circuit between motor and
inverter when the motor is at full speed. The probable out-of-phase
relationship between inverter output and motor residual voltage can
be damaging to electrical and mechanical components in both motor and
inverter.
Although few
industry standards have dealt with drive bypass circuitry, the
National Electrical Code in the United States now has requirements
for conductor sizing and motor overload protection during bypass
operation, and the National Electrical Manufacturers Association has
published an "Application Guide" offering helpful information on
bypass usage.
From “Bypassing
an ASD: What's Involved," published in Electrical Apparatus
December 2005
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