SCR versus PWM DC speed controller

Discussion in 'General Electronics Chat' started by T Werner, Feb 17, 2012.

  1. T Werner

    Thread Starter New Member

    Feb 16, 2012
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    I've been researching DC drives for a 90V motor I have, and today I spoke to a technical person at KB who said that some motors are "rated" for PWM controllers with Form Factors close to 1, and some are rated for SCR controllers with Form Factors of >1.3 or 1.37. He said using a PWM controller could actually cause problems with the bearings on a motor designed to be used with an SCR drive.

    So I checked with Granger who also said using a PWM controller with my motor might cause problems with the bearings. The motor is form factor rated at 1.3.

    I don't understand how a "smoother" DC supply would cause problems. Can anyone explain that to me?
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I've heard that harmonic current flow in the bearings can be a source of failure.
     
  3. T Werner

    Thread Starter New Member

    Feb 16, 2012
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    That is the language they used, but it seems like that would be a bigger issue with an SCR drive. The PWM is supposed to be almost pure DC compared to the SCR.
     
  4. pilko

    Active Member

    Dec 8, 2008
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    PWM is not pure DC, it is a square wave with variable pulse width (on to off ratio).Hence PWM (pulse width modulation).

    pilko
     
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Never heard of it in DC drives, but it is supposed to be a problem in three phase VFD drives. And before someone says that has nothing to do with DC, a VFD drive supplies DC to the three phase motor. :)
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    You can not use a SCR with DC, unless the DC is pulsating. Think a power supply without the filter cap. The current must go to 0 amps for the SCR to turn off, otherwise the SCR stays latched.

    I remember studying a base ball pitching machine that used this principle. There was no speed control involved, strictly on/off action. Even the slightest capacitance would have killed the circuit.

    SCRs can vary AC values, it is what they were designed to do.
     
  7. jimkeith

    Active Member

    Oct 26, 2011
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    AC motors are constructed differently--with the frame grounded, the majority of the harmonic ground current caused by capacitive coupling is shunted to ground. On the other hand, the armature of DC motors is grounded only through the bearings.

    While PWM ground currents may tend to degrade bearings in DC motors, I have never heard that it was a problem. I, myself, would not worry about it.

    Form factor has absolutely nothing to do with bearing problems. Form factor is simply the ratio of the RMS to the average armature current--with pure DC, it is 1.0--with DC phase control, it varies from about 1.2 to 1.5 depending upon armature inductance. If a DC motor is designed for a form factor of 1.2, and applied with a PWM drive, the temperature rise will be significantly lower.
     
    Last edited: Feb 17, 2012
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    While they may be different, it is a problem with a VFD, enough of one that at least one company has made a work-around for it.

    http://www.ien.com/article/preventing-vfdac-drive/7666

    http://www.est-aegis.com/
     
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  9. T Werner

    Thread Starter New Member

    Feb 16, 2012
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    So I'm thinking about this, and wondering. If I get an SCR Power supply that can put 90V across the motor, and the motor draw is about 1.7 amps, then the resistance is about 53 ohms.

    So if I were to build a Pi filter, with say 3300uf, a 2ohm resistor, and another 3300mf across the leads, and then the motor, wouldn't that decrease the Form Factor to closer to 1? If it's resistance is only 2 ohms, that doesn't seem like it would make much difference in series with the 50+ ohm load the motor represents.

    I have those components just sitting around. Is there a downside to adding that type of filtering to an SCR source?

    Thanks,
    Todd
     
  10. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I've recollect seeing the attached technical paper some time ago which looks at the likely link to bearing problems in DC machines with AC sourced electronic drives.
     
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