Siemens LMV5 Quick Start Manual page 186

For example, a real life 2 pole three phase AC induction motor fed 60 Hz 3 phase power
under no load will turn very close to 3600 RPM, most likely in the neighborhood of 3585
RPM.
As load is applied to the motor shaft, the RPM will drop (slip will increase),
and the amperage on all three phases will increase.
The same 2 pole three phase AC induction motor that was turning 3585 RPM @ 60 Hz,
when subjected to heavy loads, may drop down to 3510 RPM or lower,
even though it is still being fed power @ 60 Hz.
For a fixed motor input frequency, slip can be thought of as a spring that compresses when
loaded and the height of the spring can be thought of as the RPM of the motor.
In summary, a VFD can control the amplitude and frequency of the simulated three phase
power (pulsed, square wave DC) that they produce very accurately.
Unfortunately, this accuracy does not translate into accurate control of the motor RPM due
to induction motor slip.
To further distort the situation, these AC induction motors typically drive centrifugal blowers
which have highly un-linear characteristics.
The horsepower requirement of centrifugal blowers increases by the cube (x
ratio, so the load on the motor and therefore the slip increase dramatically with increasing
blower RPM.
As one may conclude these factors can lead to inaccuracies in blower speed and therefore
air delivery to the burner.
For these reasons, as well as the element of safety that verifying the speed of the blower
ensures, are why the LMV52 has a closed loop VSD control.
Another point worth mentioning is the effect that accelerating and de-accelerating the blower
wheel has on the VFD.
Most blower wheels, especially on larger burners tend to be heavy and have a large
moment of inertia.
Since this is quite typically the case:
faster VFD and LMV52 ramp rates require:
more VFD power and
more braking capability,
just like heavier trucks require:
more powerful engines and
more braking capability
to achieve the same acceleration and deceleration as lighter trucks.
Setting ramp rates will be discussed later in this section.
LMV5 QSG Rev 4 Sec 7 Pg 2
3
) of the speed
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