Re: [CAD_CAM_EDM_DRO] Re: VFD's

doug98105 wrote:

>Dave,
>
>You say a 1hp VFD will run a 25hp motor up the max amps of the
>drive. That's kind of extreme, but I get your point.
>
>

Well, it won't. the problem is the inductance of the windings, and the
imaginary current
required to excite the rotor field. You should know that the current
draw of integral-HP
motors is almost constant, but the phase angle changes drastically from
no load to full
load. The current rarely increases more than 15%. So, a 25 Hp
(3-phase) motor will draw current
of about 75 A/Phase at 220 V, 3-phase. A 1 Hp VFD definitely will not
like such a load!

>This is a question I've never been able to get a straight answer to.
>I was concerned about running a 5hp motor off a 3hp drive. None of
>the engineeers at the various motor or drive manufacturers could give
>a definitive answer or were willing to venture a guess as to whether
>there was any downside to doing this, other than reduced hp from the
>5hp motor.
>
>

It is generally the case that you can't run larger motors on smaller
drives, due to the above
conditions. A highly efficient motor, run perhaps at less than rated
Hz, might work.
One way to help it a bit is to program the VFD for 200 V @ 60 Hz,
instead of 208 or 240 V
@ 60 Hz. This will reduce idling current and improve the power factor a
bit, but at the
loss of motor torque.

I have just barely gotten by with this on occasion, but I strongly
recommend you don't try
this. It could cost you a burned-up VFD, in fact. That happened to me
when I ran a 1/2 Hp
VFD with a 1 Hp motor. It worked for a while, but eventually fried the
VFD. It also
tripped a lot, because the motor's idling current was near the limit for
the VFD.
You might also need series inductors to keep the PWM waveform from
saturating the
lower inductance of the larger motor.

Jon