Loss distribution for low-voltage U-converters
A decrease in converter efficiency may lead to a reduced output voltage to the motor. This may prevent the motor from reaching top speed and/or require field weakening operation
which will reduce motor efficiency.
The power factor of DC-link converters is only dependent on the design of the converter input rectifier. Motor design or motor loading does not influence converter power factor.
Due to the harmonic content of the input current of frequency converters the total power factor λ has to be analysed:
PowerFactor λ =
ActivePower P ApparentPower S
The power factor can be adjusted nearly to unity by using a converter with an active line-side converter (active front-end).
The following typical examples can be given for the most common types of converters for low- voltage motors (indirect converter of the voltage source type with uncontrolled single or three
phase diode rectifier as line side converter):
- - - -
1-phase converter: 3-phase converter: 3-phase converter with line-choke:
3-phase converter with lean DC-link (small DC-cap.):
λ ≈ 0 , 5 8 ( f o r P N ≈ 0 , 5 k W ) λ ≈ 0 , 6 4 ( f o r P N ≈ 2 k W ) λ ≈ 0 , 9 2 ( f o r P N ≈ 2 k W ) λ ≈ 0 , 9 4 ( f o r P N ≈ 1 … 1 0 k W )
As frequency converters improve the power factor on the line-side only it is most energy efficient to install the converter as close to the motor as suitable (decentralized installation).
Energy efficient cage-induction motors are typically built with more active material, i.e. longer core length and/or higher core diameter in order to achieve the higher efficiency. For these reasons the starting performance of energy efficient motors differs somewhat from motors with a lower efficiency.
On average, the locked-rotor current increases by 10 to 15% for motors from one energy efficiency class compared to motors of the next higher class with the same output power. Individually, this difference is depending on the construction principle of the motor and should be checked with the manufacturer when replacing motors in an existing installation. Copper rotor motors typically have a higher locked-rotor current compared to aluminium rotor motors.
Typically, the average pull-up torque of energy efficient motors is also increased by about 10 to 20% per efficiency class for motors of the same rated output power.
Copper rotor motors typically have a lower pull-up torque compared to aluminium rotor motors.
The manufacturer has to ensure by appropriate design measures to meet the starting performance characteristics as defined in IEC 60034-12 (typically Design-N).