Focus on ELECTROMAGNETIC INTERFERENCE: Reducing EMI from 3-phase drives

Drive current variations due to PWM switching are the root cause of EMI from 3-phase drives. Reducing them reduces interference.

12/04/2008


Electromagnetic interference (EMI) consists of unwanted electrical signals emanating from a piece of electrical or electronic equipment that interferes with the operation of other equipment. EMI can be conducted or radiated. Generally, EMI appears at frequencies higher than the normal frequencies of currents involved in the radiating equipment’s operation (baseband).
For example, the normal frequencies involved in operation of a 3-phase motor/drive set are in the tens or hundreds of Hertz. Within the drive, however, switching frequencies of pulse-width-modulated (PWM) inverters are likely to be in the tens of kiloHertz or higher. In addition, inductive and capacitive effects can lead to ringing at radio frequencies induced by rapid changes in current as IGBTs in inverter final stages rapidly switch on and off. While the 3-phase baseband currents carry power between the inverter and motor, switching transients should really stay within the electromagnetically shielded drive enclosure, and radio frequency ringing should ideally not occur at all.
Yet, these unwanted signals can and do escape from the drive enclosure. Unless drive designers take steps to suppress these signals, they will escape at levels high enough to disturb nearby equipment, and become an EMI nuisance.
Conducted EMI consists of interfering signals conducted along cables and wires into equipment directly connected to the drive. Conducted EMI signals can travel along high-voltage cables carrying phased voltages, along neutral return lines, through ostensibly grounded cable and chassis shields, and even through ground buses themselves.
Radiated EMI travels as an electromagnetic wave similar to a radio wave from the EMI source to circuits and equipment that may be completely disconnected electrically.
In all cases, an EMI signal’s strength relates directly to the rate of change (first time derivative) of the inducing current. For sinusoidal currents, it is proportional to peak current and frequency by Faraday’s Law . Furthermore, Fourier’s Theorem says that any periodic signal can be decomposed into a series of sinusoids at harmonics of the base frequency, so higher harmonics of rapidly changing currents (such as the switching currents in PWM inverters) cause the most problem.
Conventional inverter technology, where large currents must be switched very rapidly, generates large EMI signals. To suppress them, drive designers incorporate expensive suppression measures, such as heavy shielding and EMI suppression filters.
This paper shows how the G7 Drive technology by Yaskawa mitigates EMI issues associated with industrial and commercial application of PWM inverters for high-power motor drives. It does so by drastically reducing the strength of EMI signals at the generating source: the inverter final stage.
The 480V G7 Drive adopts a Neutral-Point Clamped (NPC) 3-level inverter technology. The basic behavior of the NPC technology has significant advantages over a conventional 2-level inverter. In particular, 3-level inverters have smaller output voltage steps that reduce problems related to surge voltages at the motor terminals, motor shaft voltage and bearing current, leakage current, as well as others. Reducing these currents at the source reduces the need for bulky and expensive EMI suppression later on.
When the cable between the inverter and motor is long, voltages at the motor terminals are higher than those at the inverter terminals due to the steep voltage transient and distributed inductance-capacitance combination of the cable. Since the voltage step of the 3-level inverter is one-half that of the two-level inverter, the peak voltage at the motor terminal is significantly lower than for a 2-level inverter.
As Figure 1a shows, voltage in an inverter final stage can swing up to twice the input voltage when a step voltage is applied to an L-C resonant circuit. This is exactly the situation when fast-acting IGBTs in a drive final drive an inductive motor load.

2-level inverters produce large spikes
3-level inverters produce smaller spikes
Figure 1: Incorporating 3-level invertors in a drive’s final stage significantly reduces voltage spikes.


The overshoot magnitude of E adds to the original voltage E , making the peak value as high as 2 E .
Contrast this with the situation in Figure 1b, the voltage jump in a 3-level NPC inverter is 0.5 E , which is added to the original voltage of E , resulting in the peak value of 1.5 E .
High dv/dt of the common-mode voltage causes leakage currents to flow from the conductors of the cable and motor windings to the ground through the parasitic capacitances in these components. These leakage currents create noise problems to equipment installed near the inverter. It is also strongly related to the radiated EMI noise level.
Thanks to the smaller common-mode voltage steps, 3-level inverter leakage current is much smaller than that in a 2-level inverter. Smaller steps mean proportionately reduced EMI generation. Reduced EMI generation makes it possible to achieve better EMI performance with less radical suppression efforts. Coupled with additional benefits of improved insulation life and reduced bearing wear (see “

How switching transients reduce motor life

” elsewhere in this newsletter) make the G7 Drive Series a superior solution for high-power motor installations.
For more on EMI issues relating to variable speed drives, see “ Silence of the Drives ” by consulting editor Frank J. Bartos, P.E. in the June 2008 issue of Control Engineering . You can also download this article in podcast form here .





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