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  #1  
Old 10-04-2011, 10:08 AM
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Default 3 phase induction motor Lab

Hey guys

I'm having some trouble with what I need to determine from the measurements made in my lab.

I'll copy all of my lab procedure and parameters to avoid as much confusion as possible.

The objective of this experiment is to investigate various methods of induction motor speed control. How the standard motor characteristics are affected by the different methods of speed control, so as to allow informed decisions on the best type of control for a given application The Experiment includes:
Variable stator voltage control
Rotor resistance control
Frequency control


VARIABLE STATOR SUPPLY VOLT AGE (DEL T A CONNECTED)
1) Ensure the cage induction motor is Delta connected and the dynamometer is cumulatively compounded with minimum excitation.

2) Set the panel autotransformer to the 440 V tap. Switch the motor on and record the no-load speed and line current.

3) Set the dynamometer load to 10 N and record the induction motor line current and speed. (Note: Torque arm length is 0.25m)

4) Switch off the motor. Set the panel autotransformer to the next voltage level tap down and repeat steps 1 & 2

5) Repeat the above steps for successive voltage level taps down to the 220V tap.

6) Plot on a common graph Speed V Voltage for Delta connections Loaded and on No Load.

MY RESULTS


VARIABLE STATOR SUPPLY VOLTAGE (STAR CONNECTED)
1)Ensure the cage induction motor is Star connected and the dynamometer is cumulatively compounded with minimum excitation.

2) Set the panel autotransformer to the 440 V tap. Switch the motor on and record the no-load speed and line current.

3) Set the dynamometer load to 10 N and record the induction motor line current and speed. (Note: Torque arm length is 0.25m)

4) Switch off the motor. Set the panel autotransformer to the next voltage level tap down and repeat steps 1 and 2

5) Repeat the above three steps for successive voltage level taps down to the 220V tap.

6) Plot on a the same common graph Speed V Voltage for Star connections Loaded and on No Load so that a comparison can be made with the Delta connection.

My results

note- the results in red i think my be incorrect... maybe

VARIABLE ROTOR RESISTANCE. (DELTA CONNECTED)
1) Ensure the Wound Rotor Induction motor is Delta connected and the dynamometer is cumulatively compounded with minimum excitation.

2) Adjust the rotor resistance set to the first position. Switch the motor on and record the no-load speed and line current. Repeat for all rotor resistance positions.

3) Set the dynamometer load to 20 N and record the induction motor line current and speed. Repeat for all rotor resistance positions.

4) Plot a graph of resistor position Vs Speed for both no load and load and comment on the characteristic produced.

My results


VARIABLE VOLTAGE VARIABLE FREQUENCY CONTROL
1) Ensure the cage induction motor is Star connected.

2) Connect the inverter input terminals to the three phase supply on the panel and its output terminals to the motor with the power analyser set to measure
frequency. This is on voltage current and hertz range with the voltage inputs set on motor line to line voltage.

3) Vary the inverter frequency from 25 Hz to 60 Hz in steps of 5 Hz. Record frequency, speed, voltage and line current for each step.

4) Calculate the synchronous speed and slip for each test.

5) Plot a graph of Frequency Vs Speed of the motor and comment on the result.


So here is my confusion. Can I really determine anything from plotting Voltage against speed? Should I be tying to determine the equivalent circuit???
I am guessing I need to determine the torque induced for each test, then plot that agains Ns to determine the motor class. Then I can determine the motor application right?

How can I calculate the slip without the rated speed???

Let me know if you want to see my graphs.

thanks guys
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Old 10-04-2011, 10:10 AM
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My results from VARIABLE VOLTAGE VARIABLE FREQUENCY CONTROL

note - sync speed was calculated from Ns = (120*f)/4
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Old 10-04-2011, 11:04 AM
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Quote:
Originally Posted by Jess_88 View Post
4) Calculate the synchronous speed and slip for each test.

5) Plot a graph of Frequency Vs Speed of the motor and comment on the result.


How can I calculate the slip without the rated speed???

Let me know if you want to see my graphs.

thanks guys
rated speed is sychronous speed minus slip (no load). the fact that your motor always runs at or above sychronous speed in your variable frequency test makes me beleive that either (1) your rpm measurements are off (2) your frequency output is not being regulated correctly or is being measured incorrectly or (3) you are not dealing with an induction motor. One more thing that seems strange, looking at all your tests @ 440V and (assumed)60Hz; table 1.1 (delta, no load, 440V, 60Hz), table 1.2 (delta, load, 440V, 60Hz), table 1.3 (star, no load, 440V, 60Hz), your RPM are all within 12 RPM of eachother with an average of 1485RPM. Your RPM should double when you go from delta to star. And why did your measured RPM go from 1485 in tests 1.1, 1.2, and 1.3 to 1800 in test 3.1? One thing I notice is that you aren't getting 440V out of your transformer for the 440V tap; more like 415V. So proportionally @ 440V vise 415 you would get 1580RPM vise 1485RPM. Motor rated speed should probably be (assuming you were running at 60hz for all the tests prior to the variable frequency test) 1580RPM, and therefore slip would be 1800-1580 = 220. If slip is 220 and you are measuring 1800RPM, I calculate you would have to be running @ 67Hz. So, something is wrong...
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Old 10-04-2011, 12:05 PM
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Keep in mind that in Australia the mains frequency is 50Hz.
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Old 10-04-2011, 12:41 PM
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Quote:
Originally Posted by t_n_k View Post
Keep in mind that in Australia the mains frequency is 50Hz.
doh! missed that

EDIT: despite that oversight, he's still running above synchronous speed at 50Hz in variable frequency test, so something still doesn't add up.

EDIT 2; should still double RPM when in star
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Old 10-04-2011, 01:57 PM
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Is difference in expected results maybe because the torque arm length is 0.25m for all tests but the variable voltage variable frequency control test???
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Old 10-04-2011, 02:17 PM
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Quote:
Originally Posted by Jess_88 View Post
Is difference in expected results maybe because the torque arm length is 0.25m for all tests but the variable voltage variable frequency control test???
No, I thought 1.1, 1.3, and 3.1 were no-load tests. if 1.1 & 1.3 were no-load, and 3.1 was with a load, then that means the error is even greater than I thought.
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Old 10-04-2011, 02:26 PM
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sorry, my mistake (I feel silly). Yeah data from table 1.1, 1.3 and 2.1 are no load. As for Table 3.1... I'm a bit confused (I can't remember applying a load). Shouldn't the variable voltage variable frequency control test be done without a load?
goodness me... I feel like I could be in trouble. I was sure I was making my measurements correctly
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Old 10-04-2011, 03:16 PM
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Quote:
Originally Posted by Jess_88 View Post
sorry, my mistake (I feel silly). Yeah data from table 1.1, 1.3 and 2.1 are no load. As for Table 3.1... I'm a bit confused (I can't remember applying a load). Shouldn't the variable voltage variable frequency control test be done without a load?
goodness me... I feel like I could be in trouble. I was sure I was making my measurements correctly
well if you did all the other tests with & without a load, I would expect you would do the variable voltage variable frequency test both with and without a load as well. So are you missing a table of data?

I'm slightly confused myself now; I think the key (or part of it) lies in the voltage output of the transformer. I was using the numbers from the variable frequency variable voltage test (444V @ 60Hz) to determine what your RPM would be in the other tests, but then learned that those were 50Hz tests, so now I don't have much to go on. I strongly suspect that you flubbed some measurements in the variable frequency variable voltage test.
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  #10  
Old 10-04-2011, 04:05 PM
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I personally don't see any problems with your results. Yes, the graphs are a quick visual reference to the spectrum.

why would rpm double going from delta to star?

I agree that a loaded test should have been performed for variable frequency.

If I'm not mistaken, the intent of the exercise is to realize speed regulation over the varying methods.
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