Yuhei Okazaki

A Speed-Sensorless Start-Up Method of an Induction Motor Driven by a Modular Multilevel Cascade Inverter (MMCI-DSCC)

This paper presents theoretical and experimental discussions on a practical speed-sensorless start-up method for an induction motor driven by a modular multilevel cascade inverter based on double-star chopper cells (MMCI-DSCC) from standstill to middle speed. This motor drive is suitable, particularly for a large-capacity fan- or blower-like load. The load torque is proportional to a square of the motor mechanical speed. The start-up method is characterized by combining capacitor-voltage control with motor-speed control. The motor-speed control with the minimal stator current plays a crucial role in eliminating a speed sensor from the drive system and in reducing an ac-voltage fluctuation occurring across each dc capacitor. Experimental results obtained from the 400-V 15-kW downscaled system with no speed sensor verify that the motor-speed control proposed for the DSCC-based drive system can enhance the start-up torque by a factor of three under the same ac-voltage fluctuation. Several start-up waveforms show stable performance from standstill to middle speed with different load torques.

Experimental Comparisons Between Modular Multilevel DSCC Inverters and TSBC Converters for Medium-Voltage Motor Drives

This paper makes an intensive comparison in operating performance between a modular multilevel double-star chopper-cells (DSCC) inverter and a modular multilevel triple-star bridge-cells (TSBC) converter. Both inverter and converter are intended to drive medium-voltage motors in industrial applications. First, it makes numerical comparisons, thus, resulting in revealing that the torque and frequency of a driven motor produce a significant effect on capacitor-voltage fluctuation and arm or cluster current in the individual DSCC inverter and TSBC converter. Next, a three-phase DSCC inverter and a three-phase TSBC converter with the same rating as 400 V and 15 kW are designed and compared to drive the following two general purposes and specially-designed induction motors; one is rated at the 380-V, 15-kW, 50-Hz four-pole motor, and the other is at the 320-V, 15-kW, 38-Hz six-pole motor. This experimental comparison based on the two downscaled drive systems confirms the validity of the numerical comparison. Finally, this paper concludes that the DSCC inverter is more suitable for driving medium-voltage high-speed motors loaded with quadratic-torque-to-speed profiles like fans, blowers, pumps, and centrifugal compressors. On the other hand, the TSBC converter is more suitable for driving medium-voltage low-speed high-torque motors like mills, kilns, conveyors, and extruders.

Which is more suitable for MMCC-based medium-voltage motor drives, a DSCC inverter or a TSBC converter?

This paper provides theoretical, numerical, and experimental comparisons in electrical-drive performance between a double-star chopper-cells (DSCC) inverter and a triple-star bridge-cells (TSBC) converter. The inverter and converter are two of the most promising members of the modular multilevel cascade converter (MMCC) family. Two sets of downscaled electrical drives using the DSCC inverter and the TSBC converter are designed, constructed, and tested, along with the common three-phase four-pole induction motor rated at 380 V, 15 kW, and 50 Hz. This paper presents experimental waveforms of the electrical drives loaded with a quadratic torque-to-speed profile and at the rated torque.

Research trends of modular multilevel cascade inverter (MMCI-DSCC)-based medium-voltage motor drives in a low-speed range

A modular multilevel cascade inverter based on double-star chopper cells (MMCI-DSCC) has been expected as one of the next-generation multilevel PWM inverters for medium-voltage high-power motor drives. This inverter consists of cascaded bidirectional chopper cells with dc capacitors. One of concerns for the motor drive is how to achieve stable operation in a low-speed range. Some kind of solution should be taken to achieve stable operation in a low-speed range, because ac-voltage fluctuations in the dc capacitors are inversely proportional to an inverter frequency. This paper describes state-of-the-art research trends in MMCI-DSCC-based motor drives, especially focusing on the mitigation of the ac-voltage fluctuations. Four kinds of mitigation methods including the proposed ones are summarized, compared, and verified by experiments using a 400-V 15-kW downscaled system. The steady-state waveforms show the validity of the theoretical analysis on the peak current in IGBTs, and start-up waveforms show the stable operation from a standstill to 740 min-1 with 60% load torque.

Capacitor-Voltage Balancing for a Modular Multilevel DSCC Inverter Driving a Medium-Voltage Synchronous Motor

This paper provides a comprehensive discussion on voltage balancing of all the floating capacitors in a modular multilevel cascade inverter based on double-star chopper cells (DSCC) or a modular multilevel DSCC inverter. It intends to use the inverter for driving a medium-voltage synchronous motor. Theoretical and numerical design considerations on both amplitude and phase of the three-phase motor currents achieve better capacitor-voltage balancing in a low-power low-frequency range. A three-phase DSCC inverter with phase-shifted-carrier pulse width modulation is designed, constructed, and tested to drive the 370-V 15-kW 75-Hz six-pole interior-permanent-magnet synchronous motor loaded with a hypothetical centrifugal compressor. Experimental results verify the validity of the theoretical analysis, as well as the effectiveness and viability of the DSCC inverter for such a specific electrical drive.

Enhancement on capacitor-voltage-balancing capability of a modular multilevel cascade inverter for medium-voltage synchronous-motor drives

This paper discusses both dc-voltage balancing and ac-voltage minimizing of all the floating capacitors in a modular multilevel cascade inverter (MMCI-DSCC) intended for medium-voltage high-power synchronous-motor drives. It presents theoretical and numerical design considerations on both amplitude and phase of three-phase motor currents. A three-phase DSCC inverter with phase-shifted-carrier pulsewidth modulation (PWM) is designed and constructed to drive a 370-V, 15-kW, 75-Hz, six-pole, interior-permanent-magnet synchronous motor loaded with a hypothetical centrifugal compressor. Experimental results show satisfactory performance in dc-voltage balancing and ac-voltage minimizing of all the capacitors.

Design considerations on the DC capacitor of each chopper cell in a modular multilevel cascade inverters (MMCI-DSCC) for medium-voltage motor drives

This paper describes an adjustable-speed motor with a quadratic-torque load such as fans, blowers, pumps, and compressors. The motor is characterized by being driven by the modular multilevel cascaded inverter based on doublestar chopper cells (MMCI-DSCC) A design for both mean dc-capacitor voltage and ac-voltage fluctuation in each dc capacitor used within DSCC is carried out to provide a design consideration on capacitance values of each dc capacitor. Experimental waveforms obtained from a 400-V, 15-kW downscaled system verify the validity of the theoretical analysis, and stable driving performance from a standstill to a motor mechanical speed of 1334 min-1 with quadratic-torque load.

Multiple medium-voltage motor drives using modular multilevel cascade converters with medium-frequency transformers

This paper provides a multiple motor drive using modular multilevel cascade converters (MMCCs) with galvanic isolation achieved by medium-frequency transformers. The motor drive is suitable for large-capacity pump and conveyor applications where multiple motors are required. A mitigating control method designed for the motor drive can reduce inherent capacitor-voltage fluctuation in each floating dc capacitor. As a result, the capacitance values can be dramatically reduced, compared to the already-existing MMCCs. Simulated waveforms verify the effectiveness of the multiple motor drives and its mitigating control method. These results are obtained from two 6-kV 2.5-MW loads fed by a converter system consisting of a single supply-side MMCC, two load-side MMCCs, and two 450-Hz single-phase transformers.

A speed-sensorless startup method of an induction motor driven by a modular multilevel cascade inverter (MMCI-DSCC)

The modular multilevel cascade inverter based on double-star chopper-cells (MMCI-DSCC) has been expected as one of the next-generation multilevel PWM inverters for mediumvoltage motor drives. This paper has theoretical and experimental discussions on a practical speed-sensorless startup method for an induction motor driven by the MMCI-DSCC from the standstill to a middle speed. This motor drive is suitable, especially for large-capacity fan-/blower-like loads, the torque of which is proportional to a square of the motor mechanical speed. Unlike the so-called “voltz-per-heltz” or “slip-frequency” controls, three-phase stator currents are based on “feedback” control, whereas their amplitude and frequency are based on “feedforward” control. Although the motor drive has no speed sensor attached to the motor shaft, this method makes a slow startup stable with the help of a stator-current feedback loop. Experimental results obtained from a 400-V, 15-kW downscaled system verify stable operating performance from the standstill to a middle speed of 588 min-1 loaded with 60%.

A speed-sensorless startup of an induction motor driven by a modular multilevel cascade inverter (MMCI-DSCC)-Applications to quadratic-torque loads like fans, blowers, and compressors

A modular multilevel cascade inverter based on double-star chopper-cells (MMCI-DSCC) has been expected as one of the next-generation multilevel PWM inverters for medium-voltage high-power motor drives. This paper has theoretical and experimental discussions on a speed-sensorless startup of an induction motor driven by the MMCI-DSCC, especially when this motor drive is applied to fans, blowers, and compressors, the torque of which is proportional to a square of a motor mechanical speed. A theoretical analysis focusing on ac-voltage fluctuation of each dc capacitor indicates that the ac-voltage fluctuation can be kept within an acceptable level over a wide range of motor mechanical speeds by controlling the three-phase stator currents. Experimental steady-state waveforms obtained from a downscaled motor drive verify operating performance with a quadratic-torque load.