Bob Engelhardt wrote:
RoyJ wrote:
The usual rule of thumb for VFD's is that you have rated torque up to
the nameplate RPM (ie less than your 5 hp), and rated HP above
nameplate RPM (this is a function of the amount of power the motor can
handle without overheating. )

That's what I've heard (on RCM), but I don't recall seeing it in my VFD
documentation. And I've never understood it. Keeping the HP constant
above the rated RPM means that the torque/current is being
proportionally reduced. Why would you want to reduce the current above
the rated speed? It can't be to avoid overheating - that's just a
matter of current, not power. In fact, higher speeds would generally
mean more air flow & more cooling, allowing *more* current. Maybe it's
a matter of the VFD's limitations.
No, it is the motor. The motor can deliver rated
torque as long as the voltage is proportional to
frequency. So, if the motor is rated for 220 V at
60 Hz, it would need 440 V at 120 Hz, and would
then deliver twice rated power output. But, the
higher combined voltage and frequency will
increase losses in the motor. Even though the
current would be the same, both increasing voltage
AND increasing frequency will deliver more heat to
the iron laminations, mostly, in the motor. Note
that this is NOT changing the motor wiring for the
high-voltage setting on a dual voltage motor, just
upping the voltage with frequency. So, the motor
is not rated for this service, and will overheat
pretty quickly. If you allow the voltage to level
off, while delivering rated current at higher
frequency, you give the iron a break, by reducing
at least the magnitude of the magnetic flux peaks.
This allows the heat loss in the motor to stay
within reason, and the speed-up of the integral
cooling fan mostly keeps it in check.

(Hmmm, this doesn't sound right, it seems as long
as the motor is drawing rated current, the Bmax
will still be the same, no matter the voltage.....
Anybody know?
Now, then, when the voltage no longer follows
frequency as you go up, the ROTOR magnetization
falls off, maybe that is what reduces the heating.)

Jon