×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Current ripple on DC Motor fed by Synchronous Genset with 6-pulse SCRs

Current ripple on DC Motor fed by Synchronous Genset with 6-pulse SCRs

Current ripple on DC Motor fed by Synchronous Genset with 6-pulse SCRs

(OP)
All,

We are looking at upgrading an old Ward Leonard loop with DC gens to AC synchronous generators.  The DC motors involved would be retained.  We are a remote operation with utility power an impossibility.

I am trying to estimate what levels of ripple current one would expect from ~1600kW machine with a 6-pulse thyristor drive solely feeding a DC motor (pump application).

It would seem to me now that, somewhat similar to determining the depth of notching on the AC side, that the determination of ripple current should use the subtransient reactance (direct-axis) of the machine, not the transient, or synchronous, for the commutating inductance of the source.

Just want to be sure.  The use of the subtransient in my attempted calculations give me perhaps acceptable though not ideal ripple current.  But, any higher levels of source reactance lead to much higher unacceptable levels of ripple current.

I'm sorry, I know this question may seem a bit obtuse.  But somehow, similar to notching, my mistaken impression at first was that since the system was not dealing with a sudden load shift (from the motor's perspective, re transient reactance) much less a serious short circuit (ie subtransient used), that the synchronous might be the value to use.  Much of the material I have researched uses examples involving utility power and transformer reactances, neither at play for us.

Finally, my calculations are only meant to give me a better feel for whether specifications given the contractor should require current ripple testing post-installation, 12-pulse SCR's or PWM (we have 6-pulse SCR's throughout our installed base thus the preference here), a calculation prior to manufacturing showing the calculated ripple current, etc.  So, any input along those lines would be greatly appreciated.

Thanks for any and all help.

RE: Current ripple on DC Motor fed by Synchronous Genset with 6-pulse SCRs

I am not quite clear as to what ripple current we are discussing. Is it the ripple contents of the armature current in the DC motor?

If so, the DC motor's armature inductance plays a major role while the AC side reactance mainly determines the commutation angle of the rectifier, and the notching, as you say. You do not mention armature inductance at all, which confuses me somewhat and makes me believe that you are after some other ripple current than that of the DC motor.

 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Current ripple on DC Motor fed by Synchronous Genset with 6-pulse SCRs

(OP)
Thanks, Skogsgurra.  Yes, ripple current on the DC motor's armature.

The motor in question has .13mH of inductance (and .0014ohm resistance) on the armature.  This is what I have been using in my attempted calculations.  It is the AC gen side that is a bit in question to me.  It is also the side that we can control, ie shifting to 12 pulse, specifying a certain max Xd", etc.

RE: Current ripple on DC Motor fed by Synchronous Genset with 6-pulse SCRs

If you keep the subtransient reactance in the same order of magnitude as you have in normal coummtation reactors, usually meaning a voltage drop at rated current being around 4 percent, then you can estimate the ripple current from the 300 (or 360) Hz voltage divided by the reactance of the armature inductance.

No exact calculation but good enough for practical purposes.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Current ripple on DC Motor fed by Synchronous Genset with 6-pulse SCRs

"commutation" - of course

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Current ripple on DC Motor fed by Synchronous Genset with 6-pulse SCRs

(OP)
To be more specific, I've been using a datasheet for a similarly sized alternator we have that has a subtransient (direct-axis) reactance of 0.19PU and a transient of 0.28PU.
Both values are quite a bit above the PU commutating reactances I've seen in textbook example with transformers and a utility upstream of the drive.

In this posting thread238-47512: Commutating reactance jbartos said that the commutating reactance would be approximately the subtransient.

But, in this posting thread238-80563: Commutating Reactance hvcad (Dr K S Smith)said it would be approximately Xc = (Xd" + Xd')/2.  Not sure if this would be for a HVDC transmission system with much much larger synchronous machines than what I'm dealing with?

Thanks again!  

RE: Current ripple on DC Motor fed by Synchronous Genset with 6-pulse SCRs

The transient reactance (x') has very little influence (I would say none) on commutation or DC ripple current. The commutation (overlap) is a sub-millisecond or millisecond affair (while x' is a several periods happening) and is governed by the total reactance and, to some extent, resistance in the supplying grid. It doesn't matter if that is a very strong grid with commutating reactors, a transformer with the 'right' impedance or a generator.
 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources