×
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

Increasing dc motor speed by armature control

Increasing dc motor speed by armature control

Increasing dc motor speed by armature control

(OP)
My client has a 550 V, 1500 HP, 1000 RPM DC motor for a rubber mill. Since the motor is not working well in field weakening mode (sparking issues), he wants to increase the motor speed to 1200 RPM by increasing the armature voltage to 660 V. The DC drive is rated for 700 V max.

Is this advisable ?

RE: Increasing dc motor speed by armature control


Edison123:

That will be 20% over speed (in the border limits), the motor can work with that, but the problem is; increasing the speed the power demand of the load will increase too. For a rubber mill probably at constant torque the HP demand will be 1200HP.
The combination over-speed and over temperature due to overload could soften the mechanical support of the armature winding, leading to a catastrophic failure.

Whoever, if the cooling volume can be improved, or the motor design and construction are conservative and the temperature rise is not over the insulation safety operation limits, it could work.

RE: Increasing dc motor speed by armature control

(OP)
Thx aoloade.

I forgot to mention that the motor was rated for 1500 RPM in the field weakened mode. But due to serious sparking in this mode, the motor could not run beyond the base speed of 1000 RPM. Inadequate field stength seems to be the problem with field weakening.

BTW, what about bar to bar voltage if the armature voltage is increased to 660 V ? The motor has 6 poles, 63 slots and 189 segments. I calculated the bar to bar voltage as 17.5 V at 550 V and 21 V at 660 V. Will this create sparking at 660 V?

RE: Increasing dc motor speed by armature control


   Edison123:

For a commutator with 1mm thick micas the maximum recommended voltage per bar is 40V.
The volts per bar you calculated are affected by the ratio of the pole arc to the pole pitch (around 0.7) assuming your machine has pole face compensating windings.

At 660 V armature:   Volts/bar = 660*6/(189*0.7) = 29.93 Volts.

RE: Increasing dc motor speed by armature control

(OP)
Thx aolalde. This motor has pole face comp. windings.

Is there any reason why volts/bar is affected by pole arc/pole pitch ratio ?

I had thought it was just a straight forward calculation of armature volts / Segments between two brushes since the voltage is applied between these brushes.

RE: Increasing dc motor speed by armature control

Edison123; you are right, since this is a motor, the voltage between brush arms is the line voltage V and then the maximum volts per bar are same as the average volts per bar.
          Vave = Vline*p / comm. bars
When the machine is a generator, only those conductors under the pole faces generate voltage and the peak voltage between those active bars is higher than the average.

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