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    BLDC motor delta winding

    Hi,
    I want to measure phase voltage of bldc motor which is having delta connected system. Since there is no ground terminal in delta. I am confused to measure the phase voltage of motor with oscilloscope. Can anyone tell me how to connect the channel probes to measure phase voltage of delta connected motor?

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    Re: BLDC motor delta winding

    phase voltage is line voltage in delta connection.



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    Re: BLDC motor delta winding

    Can you tell me how i can connect oscilloscope probe pins on motor phases?



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    Re: BLDC motor delta winding

    Create a virtual ground from 3 resistors (say 100k ohms).

    1) Join one end of all resistors, in a Y shape. At center is your oscilloscope ground.

    2) Connect the other ends to the 3 nodes of your system.

    Click image for larger version. 

Name:	3 resistors form virtual ground for scope readings of 3-phase delta.png 
Views:	2 
Size:	35.9 KB 
ID:	155785

    Phase differences can be observed (120 degrees) as displayed by scope traces. However waveform peaks do not coincide, since you're not directly reading generator voltages.


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    Re: BLDC motor delta winding

    I have to connect the coincident point of resistor to gnd pin of scope. But do I have to connect the other ends of resistors to three phases and three oscilloscope channel probes?



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    Re: BLDC motor delta winding

    Is your scope dual channel? The obvious (and direct) hookup is to ground the scope at one node of the delta, and probe the other two nodes. The scope ought to have an isolated power supply.

    As an alternative it might be possible to trigger a sweep externally at one node, while probing the other nodes.

    Consider making a lissajous figure via XY plot.

    Of course such tactics are less than ideal since you're only seeing two generators in reference to the third, yet you don't see the third generator. (That is why I suggested the idea of creating one reference from 3 resistors.)

    A 3-channel scope is best if you plan to work longterm with 3-phase.

    Or read from optoisolators across each of the 3 generators. This may be the safest method because such a circuit can have its own power supply, run on low voltage. To verify proper current direction in each generator, install 2 optoisolators anti-parallel (6 detectors total).

    Consider adding a channel to your scope. I turned my one-channel scope into a dual-channel, by admitting two signals one at a time, through a switching circuit made from a 4066 IC. Each signal has a potentiometer which attenuates and another potentiometer to level shift up or down. It's quirky to make it work right. 70kHz is about the maximum switching speed before the signals become hard to distinguish.



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    Re: BLDC motor delta winding

    Hi,

    Please tell us more.

    You say you want to measure phase voltage.
    --> Why?
    Do you expect some unsymmetries?
    * if yes, then the virtual star point method does not work. You need the true star point.
    * if no, then simply measure delta voltage and divide it by sqrt(3)

    Klaus
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    Re: BLDC motor delta winding

    Hi klaus,
    Yes, i am expecting some unsymmetries in output phase voltages. Since there is no gnd terminal in delta, i am confused to measure phase voltage and how to connect probes?



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    Re: BLDC motor delta winding

    Hi,

    Now you say "output" voltage.
    But for a motor the voltage is the input.

    Thus I assume you want to test your control electronics.
    If so: You should have told us earlier.
    --> Try to generate V_bus/2 as measurement reference.

    Btw: with "unsymmetries" I meant motor winding unsymmetries (before you only talked about motor)
    Generated voltage unsymmetries can be measured/seen "phase to phase", too.

    Klaus
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    Re: BLDC motor delta winding

    I meant i want to see the motor phase voltages for our delta connected motor. The three wires out from the motor, but there is no neutral wire. So i am asking how to see how the phases are reacting while motor runs.



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    Re: BLDC motor delta winding

    Hi,

    Still unclear.
    You want to see unsymmetries.

    What causes those unsymmetries? The motor or the control unit?

    Klaus
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    Re: BLDC motor delta winding

    So i am asking how to see how the phases are reacting while motor runs.
    I do not see any problem with the scheme outlined in post #4. Basically you will see waveforms of phase 1 and 2 and 3 (provided the phases do not change very rapidly).

    You need to see the waveforms phase 1,2 and 3 before and after connecting the load. If phase relations are off by more than 10-20deg (vs expected), you need to dig deeper.

    Once you get the results, and if they are not as expected, you need to worry. Phases are relative and if the relative phases are not offset by 120 and 240degs without load (and with load), there may be additional problems.

    The virtual ground approach (as seen in #4) should work in most cases if the phases are not changing rapidly (within 1/2 cycle).



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    Re: BLDC motor delta winding

    Quote Originally Posted by BradtheRad View Post
    Create a virtual ground from 3 resistors (say 100k ohms).

    1) Join one end of all resistors, in a Y shape. At center is your oscilloscope ground.

    2) Connect the other ends to the 3 nodes of your system.

    Click image for larger version. 

Name:	3 resistors form virtual ground for scope readings of 3-phase delta.png 
Views:	2 
Size:	35.9 KB 
ID:	155785

    Phase differences can be observed (120 degrees) as displayed by scope traces. However waveform peaks do not coincide, since you're not directly reading generator voltages.

    Can i connect power resistors in Y shape? Is it ok?



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    Re: BLDC motor delta winding

    Hi,

    Can i connect power resistors in Y shape? Is it ok?
    For sure.
    Your control unit is not able to decide whether the load is in delta or star connection (besides the magnitude of the current)

    Klaus
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