Sunday, March 15, 2015

Smog Alert

The worldwide push to increase electric motor efficiency that I spoke of in my last blog has some major implications for OEMs that incorporate motors in their products. For OEM's using induction motors, achieving the required IE3 efficiency requirements may mean adding electronics where none had been employed before. All the major semiconductor manufacturers have jumped into this space with components and processors to implement V/F or Field Oriented control inverters. Drive manufacturers, too have off the shelf drives to offer.

Texas Instruments
ST Microelectronics

However, the OEM that is incorporating inverters in their product for the first time must address a problem they may not have had before; electromagnetic interference or EMI. All conventional inverter technologies control the motor by chopping the voltage to the motor stator into a 4Khz to 20Khz pulse width modulated square wave (PWM). One side effect of PWM is that the voltage to and on the stator now has high frequency harmonics that extend well in the radio spectrum. We can think of this as a radio frequency smog raising the noise level of the radio spectrum and making it more difficult for radio based communication to occur.

                          Motor Voltage Spectrum: Conventional Drive

If not constrained, this energy can appear on the motor leads, the power leads into the inverter and even on nearby conductive or magnetic structures that resonate at the harmonic frequencies. Industrial drive manufacturers and system integrators have dealt with this problem for years and have developed a portfolio of tools, techniques and products to solve EMI problems, typically on a case by case basis. However, for the OEM new to inverter development, these solutions may be too costly or too application specific. While the basics of EMI mitigation are the same for the industrial drive and a dishwasher, the business constraints of cost, size and repeat-ability are worlds apart.

At Digital Power Engineering, we had EMI in mind when we developed our Resonant Field Exciter technology (patent pending). Our EMI mitigation approach is to eliminate the problem, or at least make it a lot easier, by not producing the high frequency harmonics in the first place. By using a wound field synchronous motor with a Resonant Field Exciter providing the rotor field energy, the stator voltage need not be pulse width modulated, resulting in a much cleaner EMI excitation spectrum.

                              Motor Voltage Spectrum: WSM with RFE

For applications that turn at a constant, grid frequency related speed, using RFE technology means there is no inverter at all and the EMI footprint is little different than the original induction motor. For applications that are either driven from a DC source or need to be variable speed, the electronics between the source and the motor stator only steer, or commutate, the source power into the motor leads in sync with the motor rotation with no PWM. There is still power switching occurring, but it is at the rotor pole speed, which is typically less than 400Hz and usually 50 or 60Hz.

As a result, EMI mitigation, if required at all, is not only much simpler, smaller and less costly, but more effective over a broader range of installations and environments.

For more information on DPE's Resonant Field Exciter technology drop me an email at or visit our website at

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