×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*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.

Motor Excitation

Motor Excitation

(OP)
Why do we apply DC excitation on Synchronous Motor at nearly 95% of the sync speed, why not at 60 or 70%?

RE: Motor Excitation

High likelihood of either tripping off the supply circuit on overcurrent or causing major equipment damage due to overtorquing.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Motor Excitation

Because a synchronous motor doesn't slip which means it can only work as a synchronous motor at full speed.

RE: Motor Excitation

It is a similar process to synchronising a generator set to the grid, and for much the same reasons.

With a generator you want to match the grid frequency and voltage, then close the generator breaker.

With a synchronous motor, you close the motor breaker and the motor is accelerated to speed via a squirrel cage winding built into the rotor pole faces (other starting systems are possible). The rotor field breaker is then closed to apply the field when the rotor is at maximum speed (could be 90% as you say). The motor then accelerates to synchronous speed and full load can be applied. Larger motors have to be started unloaded, as the squirrel cage often has relatively poor torque.

If you apply the field at too low a speed the motor will try and draw a high current from the supply and will trip on over current.

RE: Motor Excitation

As soon as the DC is applied to the field on rotor, the rotor is pulled in to synchronism with the power supply. This causes a jerk on the rotor shaft that amounts to torsional stress which can reduce the life of the shaft.
Higher the deviation from the synchronous speed, higher the stress.
This is the reason why one would like to interlock Field DC application with motor speed (not less than 95% rated).

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.

Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a partâ€™s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!