how to do fast fourier transform of a 50Hz motor

[Drizvi]

hi

i've been monitoring performance of a 50Hz induction motor. I have to monitor its instantaneous supply voltage and frequency spectrum.

i'm using data acquisition card PCI 6025E and CB 50LP as my input board.

i'm an amateur in using labview. i'm giving my input to pin 5. the output voltage levels are shown correctly on the labview software but i dont know how to do fourier analysis on this signal.

So please if anyone can help me with this?

Regards,

Danish

[altenbach]

Well, any "instantaneous" function has all frequencies, you need to decide on a realistic sampling interval for the FFT. At least a couple of cycles of your primary frequency.

What are you interested in? (Main frequency, magnitude spectrum, harmonics, phase, etc.)

Have a look at the shipping examples and help. Maybe windowed_fft_ptbypt could be useful?

[Drizvi]

i'm interested in a  graph of magnitude vs. frequency. i tried it real fft

 

as the frequency of my input signal is 50Hz, i reckon that the magnitude should be maximum at 50 hz. but its not coming that way.

What should i do??

[johnsold]

Drizvi,

1. The standard FFT VI does not know anything about the actual frequencies because it has no data about the sample timing.  Did you calculate the frequency of the peak(s) correctly from dt or fs and N?

2. Usually currents tell more about the behavior of a motor than the voltage.  The voltage waveform is strongly affected by the impedance of the power line feeding the motor and the user may not have any good way of measuring that.

3. Please post your entire VI with some typical data saved as default so the we can see what you have tried.  Also includes some notes about what is not working and what you expect to see.

Lynn

[Drizvi]

THE BLOCK DIAGRAM THAT I'VE MADE IS THIS

[altenbach]

That's not a block diagram, just a picture of one. For better help, I recommend you attach the actual VI instead 🙂

[Drizvi]

view of the two graphs at a particular instant is found to be as follows... note that the voltage levels shown are correct and the frequency is also 50Hz.

changing the rate and number of samples per channel effects my frequency spectrum also along with the Voltage vs. Time graph!!... is this because of the wrong values that i'm not getting the right frequency spectrum graph(peak at 50 hz)??

the actual picture that i should get should look similar to this one:- a peak at 50 hz-

[Drizvi]

these are the statistical results of the two graphs which i saved in spreadsheet form

frequency - Dev1/ai1	Power - Dev1/ai1		Time - Dev1/ai1	Voltage - Dev1/ai1
0	1.29883		0	-2.22168
1	2.09414		0.001	-4.21875
2	1.13731		0.002	-5.32715
3	5.43666		0.003	-5.58594
4	5.82516		0.004	-5.38574
5	-30.983		0.005	-5.48828
6	1.41211		0.006	-5.06348
7	7.42809		0.007	-3.50098
8	-2.53653		0.008	-1.61133
9	-0.909421		0.009	0.092773
10	3.42202		0.01	1.85059
11	1.55612		0.011	3.62793
12	1.80721		0.012	5.11719
13	0.889882		0.013	5.63965
14	2.6722		0.014	5.53223
15	-3.30237		0.015	5.71289
16	0.447473		0.016	5.06348
17	4.00947		0.017	3.76465
18	-0.0827129		0.018	1.94824
19	1.02037		0.019	0.375977
20	-0.779498		0.02	-1.25977
21	-0.137275		0.021	-3.10059
22	-1.02245		0.022	-4.81445
23	1.707		0.023	-5.58105
24	-0.926762		0.024	-5.56641
25	5.74219		0.025	-5.81055

[Drizvi]

data continued.....

26	2.13652		0.026	-5.43457
27	2.68125		0.027	-4.27734
28	1.56023		0.028	-2.64648
29	1.23211		0.029	-1.08398
30	1.58775		0.03	0.732422
31	1.50831		0.031	2.86621
32	1.47299		0.032	4.74121
33	1.36161		0.033	5.64453
34	1.6734		0.034	5.69336
35	0.831643		0.035	5.77148
36	0.946556		0.036	5.41992
37	0.362077		0.037	4.63379
38	1.37709		0.038	2.91504
39	0.9127		0.039	0.986328
40	1.5461		0.04	-0.9668
41	1.79433		0.041	-2.9834
42	1.5145		0.042	-4.73633
43	1.57445		0.043	-5.49316
44	0.833135		0.044	-5.49805
45	1.27115		0.045	-5.4248
46	2.42666		0.046	-5.37598
47	1.40348		0.047	-4.57031
48	1.30052		0.048	-2.85156
49	1.62348		0.049	-1.01074
50	1.45508		0.05	0.820312
51	1.62348		0.051	2.53418
52	1.30052		0.052	4.34082
53	1.40348		0.053	5.29785
54	2.42666		0.054	5.60547
55	1.27115		0.055	5.4834
56	0.833135		0.056	5.43945
57	1.57445		0.057	4.60938
58	1.5145		0.058	3.06641
59	1.79433		0.059	1.40137
60	1.5461		0.06	-0.23438

[Drizvi]

oh sure sir!!