Focus on PRODUCTS: Three-level inverter improves motor/drive performance

Unique architecture reduces electromagnetic interference, extends insulation life and reduces bearing damage.

12/04/2008


Yaskawa’s 480V G7 incorporates the world’s first 3-level control topology in a mass-produced commercial drive. In this article, we hope to show why that simple fact makes the 480V G7 the best drive choice for motor’s rated from
Benefits of the 3-level control architecture include:
• Reduced surge voltage that reduces motor insulation deterioration;
• Reduced leakage currents (common mode current);
• Reduced bearing currents that greatly extend motor life;
• Reduced PWM-generated audible noise that improves factory safety.
Basic 3-level control architecture
Figure 1 shows the circuit configuration of the 3-level inverter. Each phase has four switching devices (IGBTs) connected in series. The applied voltage on its power switching devices is one-half of the conventional 2-level inverter. This topology was traditionally used for medium voltage drives both in industrial and traction applications. In addition to the capability of handling higher voltages, the 3-level inverter has the following favorable features; lower line-to-line and common-mode voltage steps, more frequent voltage steps in one carrier frequency cycle, and a lower ripple component in the output current for the same carrier frequency. These features lead to significant advantages for motor drives over conventional 2-level inverters in the form of lower voltage stresses on the motor windings and bearings along with less influence (electrical noise) to the adjacent equipment.


Figure 1: The G7 drive improves on conventional 3-phase drives by using separate 3-level inverters for each phase.


Figure 2 details the 3-level inverter’s behavior taking phase U as an example. The dc bus capacitors are connected in series to establish the mid-point that provides the zero voltage at the output. Series connection of dc capacitors for this purpose is common practice in all general-purpose 480 V inverters due to the unavailability of high voltage electrolytic capacitors. In NPC 3-level inverters, maintaining the voltage balance between the capacitors is important and influences the control strategy. In the G7 product, a unique technology is used to achieve balancing of the dc bus capacitor voltages.


Figure 2: Controlled switching of four IGBTs places the output level at any of three possible voltage levels.


When IGBTs QU1 and QU2 are turned on, output U is connected to the positive rail (P) of the dc bus. When QU2 and QU3 are on, it is connected to the mid-point (M) of the dc bus, and when QU3 and QU4 are on, it is connected to the negative rail (N). Thus, the output can produce three voltage values compared to two values for the conventional 2-level topology. The relationship between the switching states of the IGBTs and the resulting output voltage with respect to the dc bus mid-point is summarized in Table 1.

Table 1: Switching states for a single-phase 3-level inverter


Applications
The G7 Drive is designed for worldwide application. The G7 Drive has UL/cUL approval for the North American market as well as CE certification for the European market. It is available from0 V and 480 V) from 25 to 500 hp has a built-in dc link choke (reactor). This dc link choke reduces input harmonic current distortion and improves power factor. In addition, these units are equipped with a dual input rectifier bridge to facilitate twelve-pulse rectification. This can be achieved using a delta-deltawye isolation transformer for phase shifting. The input current harmonic distortion (THD) can be reduced to approximately 12% using the twelve-pulse method.
These 3-level-topology features and benefits, and the architecture’s dynamic, precise speed, and torque control open many potential markets and applications.
Applications requiring precise motor control capability include winders, spindle drives, elevators, crane hoists, sectionalized systems, etc.
Applications requiring long motor leads appear in most large industrial plants and in the application of fans and pumps. Lead lengths of 300 m or more are common in the pumping of water and oil. Such applications benefit from the 3-level topology’s lower surge voltage and minimal need for output filters.
Modernization projects needing to keep existing motors also benefit from the use of the G7 Drive.
Applications with large motors where the motor manufacturer recommends insulated bearings, it is possible to use motors with standard bearings when a G7 Drive is employed, reducing costs.
Building-automation applications where acoustic noise needs to be kept down benefit from the G7 Drive’s low audible noise capability.
Fan applications in building-automation, as well as pumps for air conditioning and heating systems where motor shafts are often electrically isolated, resulting in high shaft voltages and bearing currents, are helped by the G7 3-level topology.

Conclusion
The 3-level topology benefits result in reduced motor surge voltages and bearing currents, lower leakage current (common-mode noise), and reduced audible noise. These all reduce motor-drive installation issues. The low motor surge voltage reduces the stress on motor insulation, and provides an advantage for modernization projects where the user would prefer to use an existing motor.
The G7 output waveform can provide benefits for voltage step-up applications by reducing the size and cost of required output-filter components. Lower bearing currents avoid the need to employ costly mitigation methods, such as insulated bearings, shaft brushes, or common-mode filters to minimize bearing failure.
In addition, the 3-level topology’s higher frequency spectrum reduces electrical and acoustic noise. The noise level is comparable to noise levels of the motors being supplied by line power.
Thus, the G7 drive is the ultimate performance solution with increased speed and torque response to provide servo-like performance from an induction motor. As the world’s first 480 V 3-level inverter topology, it eliminates or minimizes installation problems associated with IGBT switching and protects the entire motor-drive system.





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