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PWM into 6 amps using the VNH3SP30TR-E Hbridge IC. — Parallax Forums

PWM into 6 amps using the VNH3SP30TR-E Hbridge IC.

Digital JunkieDigital Junkie Posts: 11
edited 2009-09-30 23:37 in General Discussion
I am using the VNH3SP30TR-E and a propeller. I am using PWM object at 1Khz and sending this signal into the VNH3SP30TR-E. My problem is that the Power supply I am using begins to make a high pitched hum when running around 60%(duty cycle) and below (PWM object). My motor is a 6 amp motor @ 15 Volts

Any ideas of how I can over come this issue? Any discussion will be appreciated.

Mean well SP-200-15:
www.mouser.com/ProductDetail/Mean-Well/SP-200-15/?qs=6olcXJbp99SfiTvwqdXr1Q%3d%3d

VNH3SP30TR-E:
www.st.com/stonline/products/literature/ds/12688/vnh3sp30-e.pdf

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2009-09-29 16:42
    Hello,

    At first guess I would say it has something to do with the supply and it being a switched supply. Perhaps noise generated by the pulse width modulation of the H-Bridge is affecting the feedback circuit on your power supply? Are you also noticing voltage fluctuations when this happens? How about noise? You would need a scope for that.

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    Chris Savage

    Parallax Engineering
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  • Digital JunkieDigital Junkie Posts: 11
    edited 2009-09-29 16:52
    Chris,

    I have a scope and the only thing I notice is that the 15Volt pulse begins to grow smaller as the duty cycle is decreased. Its a great looking square wave on the scope. I also don't see any noise however I am inclination is that the Power Supply's transformer is the source of the "Hum". Do I place the scope on the output of the power supply to try and acquire a noise signal? Do you think that the power supply is under rated or that prolonged use may damage the supply? Also any test that you can think of that I can run to learn more on this issue?
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2009-09-29 17:05
    Hello,

    The power supply you’re using (or at least the one you linked) doesn’t usually have a transformer like a linear supply. Some do, but most have switching circuits that function without them. The switching supply can sometimes whine during peak current although it does say the unit you have has constant current limiting. What happens if you increase your PWM frequency to 4kHz?

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    Chris Savage

    Parallax Engineering
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  • Digital JunkieDigital Junkie Posts: 11
    edited 2009-09-29 17:15
    I can not go above 1Khz the VNH3SP30TR-E is rated at that limit. Hum...

    ...I think that I can let the Hum slide but its always on my mind and I cant let it go.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2009-09-29 17:18
    The H-Bridge IC you linked is rated for PWM operation at up to 10 kHz...it is the cousin to the one we use in our HB-25 Motor Controller. The difference is the version we use has a lower RDS.· The HB-25 runs at 9+ kHz.

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    Chris Savage

    Parallax Engineering
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  • Digital JunkieDigital Junkie Posts: 11
    edited 2009-09-29 17:26
    Well Ill be darned some reason I lost that in the shuffle.

    So this line of code using the PwmAsm object would run the IC at 80Hz? If so Ill calculate and run it at 4Khz.

    OBJ  
MosFet  : "PwmAsm" 

Pub Main
MosFet.setPeriod (1_000_000)




    P.S. Why are you suggesting 4Khz? Is there a reason behind that frequency or just an random test frequency?

    Post Edited By Moderator (Chris Savage (Parallax)) : 9/29/2009 8:41:06 PM GMT
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2009-09-29 20:38
    I was trying to see if the frequency was affecting the supply. Ideally you’d use closer to 10 kHz for PWN of motors when possible. Even then at some lower duty cycles you will still hear a high-pitched ringing from the motor briefly.

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    Chris Savage

    Parallax Engineering
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  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-09-29 22:20
    Another way to suppress noise on a motor is if the motor has a tachometer and you know the number of armature coils.

    If you dynamically match the PWM base frequency to the 'rpm times the number of armature coils', in most cases you won't hear any motor whine.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Digital JunkieDigital Junkie Posts: 11
    edited 2009-09-29 22:36
    Chris Savage (Parallax) said...
    I was trying to see if the frequency was affecting the supply. Ideally you’d use closer to 10 kHz for PWN of motors when possible. Even then at some lower duty cycles you will still hear a high-pitched ringing from the motor briefly.

    Yea I ran it at 10Khz and the noise was about 10Khz. I could hear the Power supply buzzing/ I am running it lower but lost some control of the motor.

    I am not sure about the PWM object it has conflicting comments posted in the period section. I cant decipher the assembly code to verify.
  • Digital JunkieDigital Junkie Posts: 11
    edited 2009-09-29 22:39
    Beau Schwabe (Parallax) said...
    Another way to suppress noise on a motor is if the motor has a tachometer and you know the number of armature coils.

    If you dynamically match the PWM base frequency to the 'rpm times the number of armature coils', in most cases you won't hear any motor whine.

    Its not the motor that is making the noise its the Power supply. I can hear it with my ears as if its not adequate or I am PWM too much current. At full speed no noticeable noise it present its when I run at 20% duty cycle etc..
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2009-09-30 04:28
    I have a machine here with multiple stepper motors and solenoids and a 24V @3A switch-mode supply (convenient but underrated) so I know the problems that can arise. The trouble with these switch-modes is that they are quick to react to any perceived overload, however brief. You can probably compensate by using a large low ESR capacitor across the supply. Many stepper motors are low-voltage and it's up to the stepper controller to regulate the current especially with the higher voltage used. The inductive load itself can cause these PSUs to whine or even hiccup (cutout and back in) not to mention the momentary peak currents which would not bother a conventional transformer style supply (but those are way too bulky).

    Changing the PWM frequency can help and even changing the frequency along with the pulse-width is a solution. I have some A3985 micro-stepper drivers hooked to some mosfets to handle these motors but the oscillator is driven directly from the Prop so I can change the PWM frequency at will.

    BTW, just reading your post again it sounds like you are trying to match the power-supply voltage and the coil voltage but that's not the usual way to do it as you are better off with a much higher power supply voltage relative to the coil voltage as the PWM controller will regulate the current anyway but the higher voltage increases the torque.

    Just try the big capacitor as a solution. As a rule of thumb around 4,700uf low ESR should be good. Smaller higher ESR caps can be used in parallel for a similar effect.

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    *Peter*
  • Digital JunkieDigital Junkie Posts: 11
    edited 2009-09-30 15:07
    Peter,

    I was thinking about using an inductor in series of the motor, wont this be a better choice than a cap? I would calculate using the XL = 2πfL.
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2009-09-30 23:37
    The motor coils are inductors so I'm not sure what you are trying to achieve then.

    Most of these SMPSUs are designed to drive nice clean electronics and not great big motors so don't be surprised when they whinge and whine. A capacitor is an easy thing to add and test and may be all that is required. Just an extra thought though, you could add a series inductor in the supply and then the filter capacitor to isolate the PSU (in this case) from the "ups and downs" of driving motors. The inductor needs to be fairly large though and handle a bit of current so I leave that to you as an area of experimentation.

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    *Peter*
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