Big electric motors and little VFD's
 

I got a good deal on a couple of little VFD's. They take 120 volt
single phase and put out 240 volt 3 phase and are not very big, but
they are only rated for 1/4 hp. And me without any 1/4 hp three phase
motors.

But at the scrap yard, I ran across a 1/2 hp three phase motor that
had a key taped in place on the shaft. May not have ever been used.
There were no wire nuts on the power leads and it did not look like
they had ever had a wire nut on them.

So can you run an unloaded motor off a VFD that is only rated at half
the motors hp? Well the answer is that you can not at least with the
VFD that I was trying. After a second or so, the vfd would stop and
show an error code. But wait, the motor can be connected for either
240 or 480 volts. Meaning it has two sets of windings. Connect is
parallel for 240 volts and in series for 480 volts.

Or you can connect just one set of windings, and voila a 1/4 hp
motor. Works with the VFD just fine.


Dan

Big electric motors and little VFD's
 

So can you run an unloaded motor off a VFD that is only rated at half
the motors hp? Well the answer is that you can not at least with the
VFD that I was trying. After a second or so, the vfd would stop and
show an error code. But wait, the motor can be connected for either
....

I had the same result trying a 5 hp. VFD on a 7.5 hp motor.

Karl

Big electric motors and little VFD's
 

I have run motors on smaller VFDs unloaded once or twice and it worked but I
don't remember how far out I was

wrote in message
...
I got a good deal on a couple of little VFD's. They take 120 volt
single phase and put out 240 volt 3 phase and are not very big, but
they are only rated for 1/4 hp. And me without any 1/4 hp three phase
motors.

But at the scrap yard, I ran across a 1/2 hp three phase motor that
had a key taped in place on the shaft. May not have ever been used.
There were no wire nuts on the power leads and it did not look like
they had ever had a wire nut on them.

So can you run an unloaded motor off a VFD that is only rated at half
the motors hp? Well the answer is that you can not at least with the
VFD that I was trying. After a second or so, the vfd would stop and
show an error code. But wait, the motor can be connected for either
240 or 480 volts. Meaning it has two sets of windings. Connect is
parallel for 240 volts and in series for 480 volts.

Or you can connect just one set of windings, and voila a 1/4 hp
motor. Works with the VFD just fine.


Dan

Big electric motors and little VFD's
 

" writes:


Or you can connect just one set of windings, and voila a 1/4 hp
motor. Works with the VFD just fine.

Do the VFD's have any provison to sync two together? If so, you could use
one on each winding set....
--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

Big electric motors and little VFD's
 

On Mar 20, 7:45*pm, David Lesher wrote:
" writes:
Or you can connect just one set of windings, and voila a 1/4 hp
motor. *Works with the VFD just fine.

Do the VFD's have any provison to sync two together? If so, you could use
one on each winding set....
--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

I will have to look at the manual again, but I do not think there is
any way to sync the VFD's.


Dan

Big electric motors and little VFD's
 

Under-powered VFDs cannot cope with the starting surge demands of a "too" large motor.

Bob Swinney
"Karl Townsend" wrote in message
anews.com...
So can you run an unloaded motor off a VFD that is only rated at half
the motors hp? Well the answer is that you can not at least with the
VFD that I was trying. After a second or so, the vfd would stop and
show an error code. But wait, the motor can be connected for either
....

I had the same result trying a 5 hp. VFD on a 7.5 hp motor.

Karl

Big electric motors and little VFD's
 

Robert Swinney wrote:
Under-powered VFDs cannot cope with the starting surge demands of a "too" large motor.
It is not the starting surge, it is the line current. The line current
of most motors doesn't change much from no-load to full load. What
changes is the angle between voltage and current. So, an undersize VFD
can't supply the line current the motor draws, even at no-load.

Jon

Big electric motors and little VFD's
 

David Lesher wrote:
" writes:


Or you can connect just one set of windings, and voila a 1/4 hp
motor. Works with the VFD just fine.

Do the VFD's have any provison to sync two together? If so, you could use
one on each winding set....
NEVER, EVER, try this, unless you like BIG sparks! There are modular
VFDs, usually in hundred HP sizes, that can be paralleled and synched
for larger motors. otherwise, they will not tolerate this AT ALL!

Jon

Big electric motors and little VFD's
 

"Jon Elson" wrote in message
...
Robert Swinney wrote:
Under-powered VFDs cannot cope with the starting surge demands of a "too"
large motor.
It is not the starting surge, it is the line current. The line current of
most motors doesn't change much from no-load to full load. What changes
is the angle between voltage and current. So, an undersize VFD can't
supply the line current the motor draws, even at no-load.

Jon

Most VFDs incorporate over current protection for the output transistors. A
large motor
will be seen as a overload and shutdown the output. The inrush current looks
like a short
circuit to the vfd and it shuts down.

Best Regards
Tom.

Big electric motors and little VFD's
 

Jon Elson writes:

Do the VFD's have any provison to sync two together? If so, you could use
one on each winding set....
NEVER, EVER, try this, unless you like BIG sparks! There are modular
VFDs, usually in hundred HP sizes, that can be paralleled and synched
for larger motors. otherwise, they will not tolerate this AT ALL!

Even when they say so? Note I was asking.,,,

(I used to work on low megawatt UPS's, built out of 10KW power
modules.)

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433