How does DTC contribute to superior energy efficiency?
Direct Torque Control (DTC) is well-known for its superior dynamic performance. Therefore, it is successfully used in applications requiring rapid change of torque in the range of few milliseconds. However, DTC has something to offer for less dynamic applications as well. In this post you will learn how DTC contributes to superior energy efficiency of VFD.
Majority of VFD losses are generated in the semiconductor switching devices (thyristors, IGBTs, IGCTs etc). Semiconductor losses can be further divided into conduction losses, switching losses and losses in blocking state. The last portion of losses is so small that it can usually be neglected.
Therefore, the dominant contributors are the conduction losses and switching losses. Reducing those loss components contributes to an improvement of overall drive energy efficiency.
Conduction losses are given by the properties of the semiconductor device, specifically the on-state voltage. Thyristor based semionductors, including IGCTs, have low on-state voltage and consequently low conduction losses. That is a great advantage, especially in high-power range where cooling is more demanding.
Switching losses also depend on the properties of the semiconductor device plus they are a function of the switching frequency. And here is where DTC comes into the picture. DTC controls the motor flux and torque within a defined band. This hysteresis band defines the ripple of torque and flux. Due to the nature of the hysteresis controllers used in DTC schemes, the switching frequency of DTC is inherently lower than switching frequency of PWM modulators for the same ripple of the controlled quantity.
Such behavior is illustrated in Figure 1. DTC switches “when necessary”, i.e. when the controlled quantity would drive out of the tolerance band. In contrast, PWM switchings are given by the intersections of carrier and modulation signals.
Combining the information in last two paragraphs we can conclude that IGCT based medium voltage VFDs with DTC control algorithms achieve superior drive energy efficiency. Such fact brings several benefits:
- Increased VFD efficiency → Reduced operational cost & shorter payback
- Lower VFD losses → Smaller cooling unit & reduced investment cost
- Lower specific losses → Higher power density & reduced footprint
Summary
DTC was primarily developed to achieve extraordinary dynamic performance of the electric drive. As a side effect, DTC also contributes to superior VFD energy efficiency and thus saves energy, reduced operational cost and allows design with higher power density. Combination of the IGCT technology with low conduction losses and hysteresis based modulation such as DTC further strenghtens the efficiency benefit.
References
[1] Energy efficiency series, https://mb-drive-services.com/category/energyefficiency/
[2] Power semiconductors: Comparison of IGBT and IGCT, https://mb-drive-services.com/power-semiconductors-comparison-of-igbt-and-igct/
[3] ABB drives, Technical guide No.1: Direct torque control, https://library.e.abb.com/public/14f3a3ad8f3362bac12578a70041e728/ABB_Technical_guide_No_1_REVC.pdf
