Power Generator Conducted Emissions

[BlownUp]

I have recently been testing EMI on a permanent magnet generator and observed something I can not explain. I am not extremely familiar with PMGs so I thought I might see if any of you guys have seen anything like this before. I was operating the generator to provide 400Hz output and was monitoring conducted emissions. Of course I saw a large component at 400Hz and then smaller components at a couple of the harmonics. But I also saw a significant component (larger than any of the harmonics) at 350Hz and 450Hz and then another smaller bump at 300Hz and 500Hz. I have not been able to determine what might be causing the noise at +/-50Hz and +/-100Hz of the fundamental. Any ideas?

 

[itsmoked]

First question: were you running this thing into a load of any kind?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[BlownUp]

Yes. When doing the conducted emissions testing a resistive load of about 15kW was on the generator. To get 400Hz from the generator we had to run the diesel engine at 1000RPM and the engine could only handle about 20kW of load at that speed.

Later we did radiated emissions testing with the output at 820Hz and about 70kW resistive load and saw similar noise blips on either side of the fundamental. I only have plots of the results so it is difficult to tell for sure, but it looks like the radiated blips are offset about half as much as they were at 400Hz.

 

[waross]

What type of PMG are you working with?
Many of us here are familiar with a Permanent Magnet Generator as a small unit on the back end of a much larger generator.
The purpose is to provide energy to the Automatic Voltage Regulator to excite the stationary field of the brushless exciter of the main alternator.
Although PM generators are used with smaller wind turbines to charge batteries, there may be serious voltage control issues if a PMG is used to directly supply a varying load. A 15KW PMG, and an engine speed of 1000 rpm at 400 Hz. seem a little awkward. Not wrong, but begging clarification.
respectfully

 

[BlownUp]

Hi waross,

Yes it is a little bit of a "different" application. We are testing a standalone PMG being driven by a diesel engine. The voltage and frequency output of the generator in the end application are unregulated and we have a fairly wide tolerance to what we can feed to the next stage. Normally we would be operating closer to 800Hz. But for EMI testing the LISNs that were available could only handle 400Hz so we had to slow the generator down. I am including a portion of the plot to better show what we saw.

 

[ijl]

1) How do you get 400Hz from 1000rpm?
2) What is the number of poles in the rotor?
3) What is your net frequency?

 

[BlownUp]

I am not sure how many poles are in the generator but I do know it is a high pole count unit. The generator is designed to produce 500 to 1000 Hz, 3 phase Wye power when connected to the diesel prime mover we are using it with. I am not sure what you mean by net frequency. The unit is operated at variable frequency depending on load.

 

[ijl]

The plot looks very much like amplitude modulation of the 400Hz voltage. My guess (educated or not) is that there are several magnets in the rotor, and that the magnets have slightly different strengths. The amplitude of the induced voltage varies slightly, depending on which magnet is "in turn". This resembles amplitude modulation.

The question about the frequency of the net or mains voltage was due to the (far fetched?) idea, that the bumps at 350Hz and 450Hz result from some kind of an interference from the mains voltage in the measuring instrument.

 

[BlownUp]

Thanks ilj,

I have come to about the same conclusion. Something about the generator's construction (magnet strength, location, coil spacing, rotor alignment, etc...) is causing a slight modulation in the output. I was hoping this is something someone might have seen before and could confirm. Thanks for the input.

 

[itsmoked]

I wonder if you could just put a high power magnet somewhere safe and temporary and see if it modifies your result. This could show a relationship for you.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com