Improving motor drive efficiency with power factor control
14 June 2011
Mark Steinmetz reminds us that power factor control can also achieve energy efficiency gains for inverter driven motors. In this article he compares two approaches – single boost and interleaved – and discusses where these are best implemented to suit the application
Governments around the world are using regulatory pressures to improve the efficiency of electric motors. In the United States, the EISA (Energy Independence & Security Act) became effective on December 19, 2010 and ruled that motor manufacturers may not sell motors built after this date with a lower nameplate efficiency than that stipulated by the EISA legislation. In March 2009, the European Union passed Minimum Efficiency Performance Standards (MEPS) for motors. Brazil and China, too, have current or planned mandatory electrical motor standards.
Many current motor applications are oversized and run continually at full speed regardless of demand, or operate at light loads (one study claims 44% of industrial motors run at 40% or less of full load). These practices waste energy. Savings can be achieved by using an inverter based drive, which reduces motor current by controlling motor speed. Managing motor speeds, ramps, and available torque translates directly into managing power consumption. But what further steps can be taken, beyond the use of inverters and high-efficiency motors, to maximise energy savings?
Power factor and power factor control
Power factor (PF) is defined by the relationship between the instantaneous voltage and current waveform being applied – in this case, to our motor. For purely resistive loads, PF=1 (maximum), both the voltage and current are completely in phase with one another. But in the case of a motor, the current and voltage are out of phase, and the power factor is less than one. The motor load is inductive, which causes this out-of-phase condition; the power applied will not be used optimally, and the result is wasted energy.
Since the voltage at the motor input is fixed, the current increases to compensate for this phase shift to satisfy the mechanical load demands. Not only does this increase operating costs, it also impacts on infrastructure costs as bigger currents require larger conductors and larger circuit breakers. Moreover, the motor runs hotter, resulting in a shortened operating life.
Implementing power factor control (PFC) in the drive (which, as has already been stated, saves energy over fixed speed applications) will further improve the efficiency of the drive by correcting the out-of-phase voltage and current condition.
Classic PFC circuits used in many drive applications have been of the single boost topology type; more recently, the ‘interleaved’ PFC topology has gained in popularity. The single boost PFC requires a single boost inductor and power switch. However, in high power motor applications (3hp or greater), the boost inductor becomes quite large and therefore more costly.
Interleaved PFC involves two parallel boost stages working 180 degrees out-of-phase with one another, so it is possible to use smaller boost inductors. Two small current sensing transformers are also required for feedback purposes. The technique reduces both the input and output ripple current, the results of which reduces total inductor boost volume and the size of the EMI filter - lowering systems costs.
Depending on the allowable motor ripple current, the single boost inductor will require a large number of capacitors to smooth the output from the PFC. The Interleaved PFC has roughly 50% less high-frequency output capacitor current requirements compared with the single-stage topology. This reduction in current can result in savings of up to 25% in boost capacitor volume reduction.
Single boost PFC circuits are supported by a wide variety of controllers. It is a mature technology, and advanced chipsets have reduced the amount of support circuitry required for easy integration into the drive. Interleaved PFC circuits have hitherto been rather complex, as they involve a lot of analogue circuit design.
However, the chip makers have helped reduce the complexity of the topology -Texas Instruments’ UCC28070 controller being a good example. While more discrete component support is needed, compared with that for a single boost PFC, the two out-of-phase boost circuits are identical so some design rationalisation is possible.
Both the single boost and interleaved PFC will increase the drive’s efficiency over the ±10% input voltage range. The UCC28070 controller has additional provisions to improve efficiency under light load conditions by turning off a phase. Smaller sized boost inductors and reduced capacitor requirements give the interleaved PFC a clear advantage over the single boost topology, which will have an impact on the size and weight of the drive, and advance the technical feasibility of integrating both drive and motor as a single unit.
Mark Steinmetz is a field applications engineer with Vincotech GmbH
Module or discrete? Single boost or interleaved?
Vincotech GmbH offers a variety of PFC modules for the motor drive designer, whether his choice is single boost or interleaved. Hitherto, designers have preferred to use low-cost discrete packaged power switches and have generally ignored the module approach. From a design standpoint, modules offer a number of advantages, not least being easier assembly (since a single component package replaces numerous discrete components), speeding design and improving reliability.
Vincotech’s modules provide both single boost and interleaved PFC topologies, and if the designer wishes to take a more integrated approach, such as incorporating a six pack inverter with PFC, Vincotech has a broad range of modules to meet these types of applications as well.
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