×
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

reactors for slipring motor starting

reactors for slipring motor starting

reactors for slipring motor starting

(OP)
Can anyone recommend the best way to calculate a starting reactor? Can it be as simple as X = Rotor volts/root 3.Rotor amps for full load torque(assuming negligable resistance)

RE: reactors for slipring motor starting

Hello Tony
If you are looking to build a secondary starter, you will need to add resistance to the rotor circuit in order to develop torque. If you add pure inductance only, you wil reduce the start current, but you will also reduce the start torque. i.e. this will behave in a similar fashion to a standard reduced voltage starter with a slip ring motor and a shorted rotor.
Under shorted rotor conditions, the slip ring motor exhibits a very high Locked Rotor Current and a low locked rotor torque. Reducing the voltage reduces the start current, but also reduces the start torque by the square of the current reduction.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: reactors for slipring motor starting

Suggestion: The wound rotor induction motors normally do not use starting reactors for starting on the rotor side. They only use resistors to start the motor and in some instances to control motor speed. Where have you found that application?

RE: reactors for slipring motor starting

(OP)
The application is a compressor with an old resistor starter where the resistors burnt out. They tried shorting out the slipprings and starting via a soft starter. The motor stalled and they blew up the soft starter! A rotor reactor looks an elegant solution as the impedence falls with the rotor frequency. However it sounds like you need some resistance to move the pull-out torque point towards zero speed.

RE: reactors for slipring motor starting

Hello tony
You have it in a nutshell. You need rotor resistancde to develop torque. You would probably find that with the rings shorted, you would have a locked rotor current of 1400% or thereabouts, and a locked rotor torque that could be less than 50%. That will not start much at all. Primary side starting will reduce the voltage and current, and the torque by the square of the reduction. Secondary side starting with reactors will have a similar effect. Resistors in the rotor will shift the torque curve and will enable the machine to start.
You could slect resistors equal to the final stage, or with a maximum torque at about 90% speed, and then use a primary side starter. At this point, the motor would behave similar to a standard cage motor. Once up to speed, short out the resistors.
The start current will be higher than could be obtained with a properly designed secondary resistance starter.
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