Protection and Features:
• Electronic thermal motor protection against overload.
• Temperature monitoring of the heatsink ensures that the frequency converter trips if the temperature reaches 95 °C ± 5°C. An overload tem-
perature cannot be reset until the temperature of the heatsink is below 70 °C ± 5°C (Guideline - these temperatures may vary for different
power sizes, enclosures etc.). The frequency converter has an auto derating function to avoid it's heatsink reaching 95 deg C.
• The frequency converter is protected against short-circuits on motor terminals U, V, W.
• If a mains phase is missing, the frequency converter trips or issues a warning (depending on the load).
• Monitoring of the intermediate circuit voltage ensures that the frequency converter trips if the intermediate circuit voltage is too low or too high.
• The frequency converter is protected against earth faults on motor terminals U, V, W.
8.3 Efficiency
Efficiency of the frequency converter (η
VLT
)
The load on the frequency converter has little effect on its efficiency. In general, the efficiency is the same at the rated motor frequency f
M,N
, even if the
motor supplies 100% of the rated shaft torque or only 75%, i.e. in case of part loads.
This also means that the efficiency of the frequency converter does not change even if other U/f characteristics are chosen.
However, the U/f characteristics influence the efficiency of the motor.
The efficiency declines a little when the switching frequency is set to a value of above 5 kHz. The efficiency will also be slightly reduced if the mains
voltage is 480 V, or if the motor cable is longer than 30 m.
Frequency converter efficiency calculation
Calculate the efficiency of the frequency converter at different loads based on the graph below. The factor in this graph must be multiplied with the
specific efficiency factor listed in the specification tables:
Illustration 8.1: Typical efficiency curves
Example: Assume a 55 kW, 380-480 VAC frequency converter at 25% load at 50% speed. The graph is showing 0,97 - rated efficiency for a 55 kW FC
is 0.98. The actual efficiency is then: 0,97x0,98=0,95.
Efficiency of the motor (η
MOTOR
)
The efficiency of a motor connected to the frequency converter depends on magnetizing level. In general, the efficiency is just as good as with mains
operation. The efficiency of the motor depends on the type of motor.
In the range of 75-100% of the rated torque, the efficiency of the motor is practically constant, both when it is controlled by the frequency converter
and when it runs directly on mains.
VLT
®
HVAC Drive Design Guide 8 General Specifications and Troubleshooting
MG.11.B9.02 - VLT
®
is a registered Danfoss trademark
173
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