Danfoss VLT HVAC Drive in Fan Application Application guide
VLT HVAC Drive in Fan Appli... Application note
1 VLT HVAC Drive in Fan Application with Resonance vibrations in the
Fan
1.1 Application
In applications where Motor with fan mounted directly on
the drive end shaft and where the running point is in field
weakening area, a small number of customers have
experienced vibrations when applying FC102 drives in the
power range (7,5 [kW]-18 [kW]). This Application note
answers the question of the causes for these Vibrations
and give solutions to solve it
1.2 Definitions
Over modulation: Over modulation is a way to
•
increase the motor voltage delivered by the drive
between f
Hz and < 65Hz
The advantage of over modulation is:
•
- Lower currents and higher efficiency at
- That the drive can give nominal Grid
- If the mains voltage sometimes goes
Over modulation means that a voltage is
•
modulated above the possible level given by the
DC-link voltage.
- The result is non-sinusoidal output
The non-sinusoidal voltages increases the
•
harmonics of the voltages, thereby generating
harmonic currents which again generates noise
and torque ripple
The degree of over modulation is depending on
•
the voltage reference from the VVC+ control,
which depends on the motor frequency, torque
characteristic, load compensation, mains-voltage,
etc.
depends on the motor frequency, torque characteristic,
load compensation, mains-voltage, etc. Over modulation
begins when the required output voltage is higher than 95
% of the input voltage, in VT mode this means from
approx. 47 Hz and up.
> 45Hz and f
mot
over field weakening area. Speed wise?
voltage at nominal at nominal grid
frequency
below the correct motor voltage at e.g.
43 Hz over-modulation can again add
the needed motor voltage.
voltages, because of the shape of the
sinus wave
< 55Hz or f
mot
mot
< 55
1.3 Cause to the vibrations
I expect that this is just an example with one motor
frequency.
The vibrations occur at frequencies corresponding to the
6th harmonic of the drive output frequency, fout.
The system vibrates when the motor is supplied by a
output frequency of approximately 55 Hz (6 multiplied fout
= 6*55=330 Hz) (Depends of the resonance frequency of
the fan) which is approximately 330 [Hz] on the fan wheel .
This 6th harmonics is generated only by the over
modulation. If the resonance frequency of the fan is close
to this 330Hz, The resonance will cause vibrations. Those
vibrations can in the end cause cracks in the Fan and in
worst cases a broken fan or /and broken Bearings
1.4 Solutions
1. The best solution is disabling the overmodulation
By disabling over-modulation, vibrations will be
reduced to a minimum but this solution might
also cause a derating of the applied motor in the
range of 5-10% because of the missing voltage
applied by the over modulation.
2. Skipping a small frequency band of the output
frequencies may also be a solution
In some applications it is not possible to disable
the over modulation If the motor is designed to
the limit of the fan application the voltage losses
in the FC will cause missing torque. In these
situations the problem of vibration might be
reduced significantly by skipping a small
frequency band around the mechanical resonance
1
1
MN.11.B1.02 - VLT® is a registered Danfoss trademark 1
VLT HVAC Drive in Fan Appli... Application note
1
frequency approximately 330Hz. This can be done
by setting parameters (p4-6*) or by using the
Semi auto bypass setup p4-46. However, there is
no general design rule for making an optimal
skip of frequency bands as this is highly
dependent on the width of the resonance peak.
In most situations it is possible to hear the
resonance
2 MN.11.B1.02 - VLT® is a registered Danfoss trademark
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