In small motors, the influence from the U/f characteristic on efficiency is marginal. However, in motors from 11 kW and up, the advantages are significant.
In general, the switching frequency does not affect the efficiency of small motors. Motors from 11 kW and up have their efficiency improved (1-2%). This
is because the sine shape of the motor current is almost perfect at high switching frequency.
Efficiency of the system (η
SYSTEM
)
To calculate the system efficiency, the efficiency of the frequency converter (η
VLT
) is multiplied by the efficiency of the motor (η
MOTOR
):
η
SYSTEM
) = η
VLT
x η
MOTOR
8.4 Acoustic Noise
The acoustic noise from the frequency converter comes from three sources:
1. DC intermediate circuit coils.
2. Integral fan.
3. RFI filter choke.
The typical values measured at a distance of 1 m from the unit:
Enclosure
At reduced fan speed (50%) [dBA] *** Full fan speed [dBA]
A2 51 60
A3 51 60
A5 54 63
B1 61 67
B2 58 70
B3 59.4 70.5
B4 53 62.8
C1 52 62
C2 55 65
C3 56.4 67.3
C4 --
D1/D3 74 76
D2/D4 73 74
E1/E2* 73 74
** 82 83
F1/F2/F3/F4 78 80
* 315 kW, 380-480 VAC and 450-500 kW, 525-690 VAC only!
** Remaining E1/E2 power sizes.
*** For D, E and F sizes, reduced fan speed is at 87%, measured at 200 V.
8.5 Peak Voltage on Motor
When a transistor in the inverter bridge switches, the voltage across the motor increases by a du/dt ratio depending on:
- the motor cable (type, cross-section, length screened or unscreened)
- inductance
The natural induction causes an overshoot U
PEAK
in the motor voltage before it stabilizes itself at a level depending on the voltage in the intermediate
circuit. The rise time and the peak voltage U
PEAK
affect the service life of the motor. If the peak voltage is too high, especially motors without phase coil
insulation are affected. If the motor cable is short (a few metres), the rise time and peak voltage are lower.
If the motor cable is long (100 m), the rise time and peak voltage increases.
In motors without phase insulation paper or other insulation reinforcement suitable for operation with voltage supply (such as a frequency converter),
fit a sine-wave filter on the output of the frequency converter.
To obtain approximate values for cable lengths and voltages not mentioned below, use the following rules of thumb:
8 General Specifications and Troubleshooting VLT
®
HVAC Drive Design Guide
174
MG.11.B9.02 - VLT
®
is a registered Danfoss trademark
8
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