| Drive topology | Multilevel voltage-source inverter (VSI) | Uses multiple switching levels to synthesize a stepped motor-voltage waveform. Common implementations include neutral-point-clamped and cascaded-cell arrangements. | Lower voltage step size reduces motor insulation stress, common-mode voltage, and output-filter requirements. Typical drive efficiency is approximately 97–99%, depending on voltage, power, switching frequency, and transformer losses. | General-purpose medium-voltage motors, pumps, fans, compressors, conveyors, and process equipment. |
| Drive topology | Neutral-point-clamped multilevel VSI | Uses a shared DC link and clamping devices to create multiple output-voltage levels. It can provide good waveform quality with a relatively compact power stage. | Provides low output-voltage distortion when properly modulated. Capacitor-voltage balancing and semiconductor switching losses must be managed carefully. | Large industrial drives where a common DC-link architecture and centralized power conversion are preferred. |
| Drive topology | Cascaded H-bridge multilevel VSI | Builds the motor voltage from series-connected power cells. The modular structure supports serviceability and scalability across different voltage classes. | Usually offers excellent output waveform quality and low motor-current distortion. Input phase-shifting transformers and multiple power cells can add size, cost, and standby losses. | High-power pumps, fans, mills, compressors, and installations requiring modular maintenance or high output-voltage quality. |
| Drive topology | Current-source inverter (CSI) | Uses a controlled DC-link current and an input-side converter. It is inherently suited to applications with high motor power and can provide robust short-circuit behavior. | Can support regenerative operation without a separate braking resistor, but motor suitability, minimum-speed operation, input power factor, and filtering requirements must be checked. | Very high-power applications, long motor cables, and processes where regeneration or robust current control is important. |
| Input arrangement | Diode or passive-front-end rectifier | Simple and robust input conversion with limited control of input current. A braking resistor or separate regenerative unit is normally required for sustained energy return. | Typically provides high reliability and low maintenance. It can produce line-current harmonics and cannot normally return braking energy to the supply. | Loads with predominantly motoring operation, such as pumps, fans, and conveyors without frequent braking. |
| Input arrangement | Active-front-end rectifier | Uses actively switched devices to control the input current and DC-link voltage. It can provide near-unity displacement power factor over a broad operating range. | Can achieve low input-current distortion with suitable filtering and control, and can return regenerative energy to the grid. Additional switching devices may increase capital cost and losses. | Four-quadrant drives, test stands, hoists, elevators, centrifuges, winders, and applications with frequent deceleration. |
| Control method | Scalar volts-per-hertz (V/f) control | Maintains an approximate relationship between motor voltage and frequency. It is relatively simple and does not require detailed motor-parameter estimation. | Usually provides lower dynamic accuracy and torque response than vector-based methods. It can be energy-effective for stable-speed variable-torque loads when correctly tuned. | Fans, pumps, and other loads with modest acceleration, speed-regulation, and torque-response requirements. |
| Control method | Sensorless vector control | Separately regulates flux-producing and torque-producing current components using a motor model rather than a physical speed sensor. | Improves speed regulation, low-speed torque, and transient response compared with basic V/f control. Performance depends on motor data, parameter identification, and operating conditions. | Most general industrial applications where good dynamic performance is required without installing a shaft encoder. |
| Control method | Closed-loop vector control | Uses a speed or position feedback device to regulate motor torque and speed precisely, particularly at low speed. | Provides excellent torque control and repeatability but adds sensor installation, wiring, commissioning, and maintenance requirements. | Hoists, elevators, winders, extruders, high-performance conveyors, and applications requiring accurate low-speed operation. |
| Control method | Direct torque control (DTC) | Controls motor torque and flux directly using rapid switching decisions and estimated motor states. | Can provide fast torque response and strong disturbance rejection. Torque ripple, acoustic noise, and switching-frequency variation depend on the implementation. | Applications requiring rapid torque response, including compressors, test systems, mills, and demanding process lines. |
| Load profile | Variable-torque load | Torque generally falls as speed decreases; fan and centrifugal-pump power approximately follows the cube of speed under comparable system conditions. | Speed reduction can produce substantial energy savings compared with throttling or mechanical control. Actual savings depend on static head, system curve, efficiency, and operating hours. | Fans, cooling-water pumps, boiler-feed pumps, ventilation systems, and chilled-water systems. |
| Load profile | Constant-torque load | Requires approximately constant torque over the normal speed range, although acceleration and overload requirements may vary. | Energy savings primarily result from eliminating mechanical losses and matching speed to production demand. The drive must be sized for continuous torque and overload duty. | Conveyors, positive-displacement pumps, mixers, extruders, and compressors with substantially constant torque. |
| Energy performance | Motor and drive efficiency at rated load | Efficiency varies with motor design, drive topology, switching frequency, cooling system, transformer, cable length, and operating point. | For a complete medium-voltage drive system, evaluate motor, converter, input transformer, auxiliary cooling, and harmonic-filter losses together. Do not compare converter efficiency alone. | All applications, especially continuous-duty processes where small efficiency differences accumulate over many operating hours. |
| Energy performance | Part-load efficiency | Drive and motor losses do not decrease in direct proportion to load. Fixed control, magnetic, cooling, and auxiliary losses become more significant at light load. | Request efficiency curves at the actual operating points rather than relying only on rated-load efficiency. A correctly sized drive can avoid unnecessary oversizing losses. | Processes that operate for long periods below rated capacity or across a wide speed range. |
| Power quality | Input harmonics and power factor | Rectifier type, pulse arrangement, phase-shifting transformers, active-front-end control, and line impedance influence current distortion and displacement power factor. | Lower harmonic current can reduce transformer and cable heating and may help meet facility power-quality limits. Harmonic compliance should be verified at the point of common coupling. | Facilities with weak grids, large numbers of converters, sensitive electrical loads, or strict power-quality requirements. |
| Motor interface | Output waveform and cable distance | Fast voltage transitions can increase motor insulation stress, reflected-wave effects, bearing currents, and electromagnetic interference, especially with long motor cables. | Multilevel output waveforms, suitable cable design, common-mode mitigation, shaft-grounding practices, and output filters can improve motor reliability and reduce losses. | Remote motors, existing motors, submersible installations, and plants with long cable runs. |
| Regenerative capability | Two-quadrant operation | Provides motoring in one direction and normally dissipates braking energy through a braking resistor or mechanical system. | Suitable when deceleration is infrequent or short. Resistor capacity must be checked against braking power, duty cycle, enclosure temperature, and fault requirements. | Fans, pumps, conveyors, and other loads with limited regenerative energy. |
| Regenerative capability | Four-quadrant operation | Supports motoring and regenerative braking in both directions of rotation when the complete converter and control system are designed for it. | Can recover braking energy and reduce resistor losses. The application must account for regenerated power, grid acceptance, DC-link control, and protective coordination. | Hoists, elevators, test benches, centrifuges, winders, downhill conveyors, and rapidly cycling machinery. |
| Selection criterion | Voltage and power rating | Confirm motor rated voltage, rated current, service factor, starting torque, overload duration, short-circuit level, and available medium-voltage supply. | Correct current and overload sizing prevents excessive thermal stress and avoids selecting a drive that is unnecessarily large and inefficient at the normal operating point. | Every project; sizing should be based on the motor nameplate, load torque curve, acceleration profile, and site electrical study. |
| Selection criterion | Reliability and maintainability | Assess semiconductor redundancy, modular power cells, bypass arrangements, cooling design, diagnostic functions, spare-parts strategy, and safe access requirements. | Higher availability can reduce production losses, while modular replacement can shorten maintenance time. Reliability claims should be supported by operating conditions and maintenance assumptions. | Continuous-process plants, remote installations, critical pumping systems, and applications with high cost of downtime. |
