Re: [CAD_CAM_EDM_DRO] VFD superceeds Motor Starter?

You can put regular thermal overloads in the SUPPLY side for the VFD supply
voltage and wire the auxiliary contacts into the control terminal for
overloads. Also, in order to enable the motor to run, you do put at least two
devices/contacts in the circuit to enable the motor. VFD's are extremely
reliable and SAFE and are used in huge numbers in industry today wherever speed
control is needed. Plus, you can use most VFD's to convert single phase to three
phase power, this is a plus in many cases.

Hope this helps, one warning, NEVER put any type of disconnect device
between the output of the VFD and the motor/load. If there is an open in either of
the power leads between the VFD and the motor, it is likely the VFD will
self destruct or at least be damaged.

Randy Abernathy
Industrial woodworking machinery installation, design, repair and servicing
Over 24 year experience with factory training
Fax: 770-974-5295
Phone: 770-974-5295


In a message dated 1/25/2009 8:41:19 P.M. Eastern Standard Time,
elson@... writes:

scyvt wrote:

> Electromechanical motor starters, as have been in general use for at
> least 70 years, offer motor protection from over voltage, under voltage,
> and over current. From an operator safety standpoint, perhaps the most
> important feature is no-voltage protection; that is, if there is an
> interruption of power, the machine will not automatically start up
> when power is restored.
>
> Modern electronic VFD's offer all the above functions, plus of course
> variable speed and a raft more. But are they considered as reliable as
> the time-tested electromechanical motor starters as far as guarding
> operator safety? In other words, can I use a VFD alone where a motor
> starter was used before?
>

If properly configured, and the control circuits are wired with this
concern in mind, then yes, and it is done in industry all over the place.
In general, a single power component failure might make the motor wires
"live", but would never be able to start the motor turning.
I don't know how they deal with the possibility of a failure in the
control inputs to the microprocessor, but I suspect that the various
safety rating agencies require a design such that at least two
components need to fail for the motor to start without a proper command
from outside.

Jon



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