×
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.

# Wound rotor motors

## Wound rotor motors

(OP)

Does the added resistance in these motors on start up improve power factor. And also how is the inrush current limited I know that the as the rotors EMF passes the motor windings a counter EMF is produced decreasing the line current but how is inrush current different in these compared to squirrel cage motors?

### RE: Wound rotor motors

Power factor will be better, on start up, but motors spend so little time spinning up it will not matter.

This fig does shows steady state torque. Inrush depends on the resistance inserted. Inrush will be approximately proportional to percent full loss torque at zero speed.

### RE: Wound rotor motors

Aren't Audel's books fantastic? One does have to use liberal quantities of salt when assessing the legitimacy and description of electrical medical devices, but other than that . . .

CR

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

### RE: Wound rotor motors

jK1996: In the big scheme of things developed torque (at any speed point) is dependent on the applied voltage, the current flowing in the winding, the power factor at that particular speed point, the resistance of the winding(s) involved, and how far away from synchronism the specific speed point is (i.e, the slip).

If the contacts close simultaneously
(Torque) = (Tlockrotor) + (lock rotor current)*(rotor leakage reactance/(stator leakage reactance + rotor leakage reactance)) * (factor) + sin(2*pi*line hz*time)
where
(factor) = e^((-1)*(2*pi*line hz)*(Stator cct resistance)*(rotor cct resistance) / (magnetizing reactance * (stator cct resistance + rotor cct resistance)))*time

Note: calculated torque, lock rotor torque, lock rotor current, leakage and magnetizing reactance, and all resistances are in PER UNIT.

As any added resistance (e.g. a liquid rheostat or a switched resistor bank) in the rotor circuit becomes part of the rotor circuit during the start, it can be seen that the addition of resistance will have an effect on both the starting torque and the lock rotor current. More resistance in the circuit reduces inrush current (because there's only so much voltage to go around) and consequently the torque developed will also be less than a full-voltage condition.

Converting energy to motion for more than half a century

### RE: Wound rotor motors

Gr8blu - are you trying to equate a wound rotor to a squirrel cage by saying what happens with a reduced applied voltage because your statement "the torque developed will also be less than a full-voltage condition" isn't making sense.

Torque doesn't go down, the breakdown peak just shifts around.

### RE: Wound rotor motors

#### Quote:

If the contacts close simultaneously
(Torque) = (Tlockrotor) + (lock rotor current)*(rotor leakage reactance/(stator leakage reactance + rotor leakage reactance)) * (factor) + sin(2*pi*line hz*time)
where
(factor) = e^((-1)*(2*pi*line hz)*(Stator cct resistance)*(rotor cct resistance) / (magnetizing reactance * (stator cct resistance + rotor cct resistance)))*time
I don't know about the + sin(2*pi*line hz*time). I do indeed recall a line frequency component of torque based on interaction product of ac component of current with a decaying dc component of current.... but that product decays with the decaying dc component whereas your line frequency torque component is shown as if it would continue undiminished (assuming rotor were locked in position and hadn't tripped). I think your sin(2*pi*line hz*time) term should be multiplied by a decaying term, maybe by that middle term that includes factor.

=====================================
(2B)+(2B)' ?

#### 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!