Regenerative power units save energy
When an electric motor is driven by a variable frequency drive (VFD), electric power delivered to the motor is regenerated while the motor decelerates by applying negative torque to the motor shaft. Usually energy storage capacity inside the VFD is very limited so regenerative energy should be returned to the grid or quickly dissipated by a braking resistor. Otherwise, the dc bus will be overcharged and an over-voltage fault can occur. Dynamic braking resistors have been widely used to convert regenerated energy into heat loss because of simplicity and low installation cost, as illustrated in Figure 1. But a regenerative power unit provides a significant energy cost saving opportunity, especially in applications that require frequent run and stop, deceleration with high inertia load, and overhauling torque. Such applications include spindle drives, decanter centrifuges, hoists, cranes, elevators, and torque dynamometer test rigs. Electricity cost per kilowatt hour is getting more expensive. So it will be worthwhile to review the basics of regenerative units and understand estimated energy cost saving. Three types of regenerative power units are available in the U.S. market. Basic features and power topologies are explained and compared.
1. Regenerative converter
The regenerative converter is a cost-effective solution that can replace the dynamic braking transistor and resistor network. It absorbs excess regenerative energy from the VFD and returns it to the ac power source. Figure 2 illustrates the regenerative energy flow from a motor to the grid using a regenerative converter on the left, and detailed power circuit schematic with power switches on the right. During motoring, the VFD delivers power without the regenerative converter in the main power flow. So there is no conduction loss in the regenerative converter during motoring. The regenerative converter is activated when regenerative energy charges dc link capacitors of the VFD. The regenerative converter returns stored energy in the dc capacitors to the grid.
The size of the regenerative converter is determined by the size of the VFD, regen power, and duty cycle. The 6-step pulse control method is used to keep the switching loss of power devices very low. For applications that require high duty cycle braking, the regenerative converter can significantly improve the system operating efficiency and reduce the cost of electricity [1-2].
2. Sinusoidal PWM converter
The sinusoidal pulse-width modulation (PWM) converter is a high-performance solution designed to regulate dc bus voltage under both motoring and regenerative power conditions. The grid side current waveform of the sinusoidal PWM converter is sinusoidal with very low PWM harmonic distortion, approximately 5% total harmonic distortion (THD). It is designed to meet the IEEE-519 standard. Input current is also controlled to synchronize with input grid voltage, which enables achieving unity input power factor. It is connected in series between the incoming power line and the VFD. An ac filter, such as an LCL filter [inductor (L)-capacitor (C)-inductor (L)], is used as an external component to reduce ac current harmonics.
The sinusoidal PWM converter in Figure 3 is ideal for applications that require very low current harmonic distortion to meet the IEEE-519 Harmonic Limits standard and have large overhauling loads or those that make frequent stops such as elevators, centrifuges, test stands, and winders. This can result in significantly reduced system operating costs of machinery by “re-cycling” the excess energy back to the power grid. The sinusoidal PWM converter package is highly efficient without the heat loss and mounting location problems associated with braking resistors. For reference, the unidirectional regenerative converter is a lower cost solution with a six-step waveform that does not provide IEEE-519 compliant current waveform.
(See additional explanations, more graphics and links on the next page.)
3. Matrix converter
A matrix converter is a direct ac-to-ac power converter that has motoring and regeneration capability. Recent technology advances in power semiconductor and CPU computing power have made commercial products possible. Basic features of the matrix converter are:
– All-in-one, where ac input is directly converted into variable frequency ac output by nine bidirectional switches. So the functions of the sinusoidal regen converter and VFD are combined. This feature enables the motors’ operation in motoring and regeneration modes without an additional VFD.
– No electrolytic capacitor: Matrix converter needs less maintenance because there is no diode rectifier and dc electrolytic capacitor in the main power flow. Generally, an electrolytic capacitor is bulky and has a shorter lifetime than other components.
– Low input current harmonics: Input current control capability makes it possible to reduce input current harmonics significantly. Input current harmonic distortion is in the range of 5% to 10% under full load condition. Neither phase-shifting transformer nor external line reactor is required to reduce harmonic currents, which are usually bulky.
– Compact size: Physical size of matrix converter is smaller than the sinusoidal PWM converter-inverter system, which has motoring and regenerative power capabilities that are the same as the matrix converter. [5-7]
4. Utility cost saving by regeneration power unit
Figure 5 demonstrates the energy costs of operating a 45 kW elevator system with and without a regenerative power unit. Average motoring power consumed per cycle is 8.8 kW while regenerative power is 4.78 kW. If a dynamic braking unit is used to produce braking torque, 11,300 kWh will be converted into heat loss. By installing a regenerative power unit, 54% of power can be saved and annual utility cost savings is $1017 assuming average electricity costs of 9 cents per kWh.
5. Regeneration power unit selection
All three regeneration power units described have a good energy cost savings capability under regenerative load conditions. The regenerative converter described in section 2 is the best low-cost solution. But if extra advantages such as low input current harmonics and unity power factor are also important, the best solution may differ. The table compares various aspects of three regeneration power units.
Table: Comparison of Regeneration Power Units
– Jun Kang is research and application manager, Yaskawa America Inc. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering and Plant Engineering, email@example.com.
Certain applications benefit from regenerative technologies that convert kinetic energy back into electricity instead of dissipating at wasted heat. Select the best regen unit for the application.
Have you looked at your processes for regenerative opportunities? After return on investment, it prints money, as they say.
This article is part of the April 2013 CFE Media supplement, Industrial Energy Management. See other articles linked below.