DIYbanter - View Single Post

In article ,
says...

I think that I've got a pretty good handle on the relative advantages of
static converters, rotary converters and VFDs for operating three phase motors
on
single phase current. With the VFDs coming down in price and their ability to
control motor speed and provide a bunch of other programmable features they
are pretty attractive. The VFD manufacturers warn of two potential problems,
though:

1. They recommend that the VFD be the last thing between the power supply
and the machine - no on/off or reversing switches between the VFD and the
motor.
They warn that use of such a switch may smoke the VFD.

Although it's probably more applicable to metalworking than woodworking
machines, there are times where I'd like to be able to instantly reverse a
motor,
or to bump it on and off.

I've heard that if you use a VFD that is well oversized for the motor in
question, you can get away with the on/off and reversing switches downstream
from
the VFD. Anyone have any experience with this?

You can reverse the motor with the VFD's controls, so
there's no need for a reversing switch/contactor between
the VFD and motor. This has the added benefit of being able
to set the accel/decel rate in the VFD. The accel rate in
most VFDs is set as Hz/sec, so even if you set the accel to
bring the motor up to 60Hz in 1 sec, starting or reversing
at low speeds will be nearly instantaneous. This is how my
mill is set up, making it convenient to drill at high
speed, then turn the speed down to power tap. At 6Hz the
time to reverse is not detectable.

For fastest decel and/or large inertial loads, you need a
braking resistor. Most, but not all, small VFDs come with a
built in resistor. Larger drives will have provision to add
an external resistor.

2. The VFDs - or at least some of them - are reputed not to produce a smooth
output current, but rather one with harmonics or noise that can fry some
older motors. Almost all newer motors are supposed to have insulation that is
rated for inverter use.

How old does a motor have to be before it is cause for worry? How reliable
are older motors when used with VFDs? I'm talking about normal use, and not
using the VFD to run the motor at 30% or 300% of its rated speed.

Though it's true in theory and apparently does happen, I've
never had a problem with this either on my own stuff,
including a 1960s Bridgeport, or on the many drives I've
worked on in industrial settings.

You're in Cumberland or Yarmouth, aren't you? I have both a
lathe and mill running on Mutsubishi VFDs here in
Harpswell. Email me if you'd like to come up and see how
they work in person.

Ned Simmons