×
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

primary resistance starters
3

primary resistance starters

primary resistance starters

(OP)
how do they operate

RE: primary resistance starters

The simple answer would be
1) Appropriate Resistance is introduced in series with each winding to start.
2) After a suitable time delay the resistance are bypssed and the motor reaches full speed.
3) Alternatively the series resitance is gradually reduced (like a reheostat) and finally the full voltage is across the winding.
This way the primary winding current is limited till the full speed is reached and back emf is prodcued.
marvin

RE: primary resistance starters

There is a simpler version - the so called KUSA starter (Kurzschluss Sanft Anlauf, which means Short circuit Soft Start). It works with just one resistor connected in series with one of the windings. If the motor is wye connected, one can imagine the star point being moved out of center of the voltage star. That reduces all three winding voltages which, in turn, reduces starting torque. The torque can be adjusted with the resistor and a contactor shorts the resistor out when motor has got up in speed.

RE: primary resistance starters

Hello tomhill56

NYedison is correct in his description, but I woulld like to add the following:
1. the voltage across the motor terminals is able to be calculated by ohms law and is a function of the value of the resistance and the motor impedance.
2. As the motor accelerates, it's impedance changes. The intirial power factor is very low and the effective resistance is also low. As the motor acelerates, the impedance gradually increases and the power factor improves.
3. The changing impedance of the motor during aceleration, results in an increasing voltage on the terminals of the motor, giving an increasing torque relative to an constant voltage starter such as an auto transformer starter.
4. The power disipated in the primary resistors during start is very high and is of particular problems for generator starting.

skogsgurra. The single phase control method also introduces a significant twist in the current vectors and a resultant increase in negative sequence currents. Not to be used on higher inertia or frequent starting loads.

Best regards

Mark Empson
http://www.lmphotonics.com

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