There is widespread use of alternating current (AC) motors because it is more cost effective and efficient where speed control and significant power output is required. Variable frequency Danfoss drives accurately control the speed of a standard AC induction motor. Through the Danfoss drives, speed control with full torque can be achieved from 0 RPM to the maximum rated speed that is above the rated speed at reduced torque. Variable frequency drives manipulate the frequency of output by rectifying incoming AC current into direct current (DC). A voltage pulse width modulation recreates the AC current and voltage output waveform.
Types of variable frequency drives
The three common types of variable frequency drives include:
- Current source inversion (CSI) – is the type of variable frequency drive that has regenerative power capability. In simple terms, the CSI VFD can absorb power flow back from the motor to the power supply. CSI Danfoss drives have been successfully used in signal processing as well as industrial power applications.
- Pulse-width modulation (PWM) – has excellent input power factor due to its fixed DC bus voltage. PWM variable frequency drives are widely used in industries because they do not cause motor clogging at below 6 Hz which is the frequent problem with voltage source inversion drives. PWMs are also more efficient and cost effective.
- Voltage source inversion (VSI) – is not widely used by industries because of the poor power factor.
One of the most common problems with variable frequency drives is heating due to system harmonics. Harmonics are irregularities in the sine wave. In an ideal power circuit world, there should be no harmonics; however, there are ways to mitigate harmonics. The simplest method is by using a sine wave filter on either side of the Danfloss drives. Another proven tactic that can be used on the line side of variable frequency drives is to place a capacitor at the common bus. The impedance of a capacitor that is inversely proportional to the frequency of the signal will allow harmonics to see a short through the capacitor and travel through it and ignore other loads on the bus. If harmonics are not properly mitigated on the line side of the Danfoss drives, issues on cross talk and heating may be encountered. Cross talk means the signal from one circuit is interfering with the other and this is a larger issue than overheating. Overheating can increase cooling costs but cross talk can be extremely annoying in telecommunications. In power circuitry, cross talk can cause overheating and frequency relay trips.
Harmonics can also cause problems on the load side because of its nature as waves. Electromagnetic waves can act like a high amplitude sine wave meaning a small amount of reactance can result in large voltage spikes. On the other hand, the capacitor connected on the load side of the variable frequency drive can also create additional problems like destroying a drive. Although the methods work well, it is important to be properly informed to avoid further issues from cropping up.