×
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

DC Motor will only run in 1 direction ?

DC Motor will only run in 1 direction ?

DC Motor will only run in 1 direction ?

(OP)
Hi all

Ok here is the problem, i have a small dc motor attatched to a gearbox, i have no nameplate so i will describe it in detail for you,

The field is made up of just 2 coils, connected top to bottom resistance is 1500 ohms, with 2 leads out to the terminal block, there are no interpoles fitted, there are just two leads coming from the brushgear, one from each side, im getting around 7 ohms from these leads

I fed the field 100v dc, and then fed the armature 100v dc and the motor ran ok, now i understood that if i switch over the + - leads form the field or the armature the motor should run in the opposite direction, but it dosnt it, always goes the same way, what am i doing wrong???

RE: DC Motor will only run in 1 direction ?

It's hard to say what is going wrong because the motor will reverse if you reverse the polarity of either the armature OR the field (but not both).

From your measured resistances it appears that you have a shunt wound motor (i.e. the field and armature circuits are in parallel), not that this is relevant here because the above applies to both series and shunt wound motors.  Are both circuits completely separate?

RE: DC Motor will only run in 1 direction ?

Measure the field current, maybe the shunt field is open and the motor is running with the residual flux.

RE: DC Motor will only run in 1 direction ?

(OP)
thanks for the replies

Both of the circuts are seperate, the field draws 1.7 amps, i also did a polarity test on the fiels, it had 1 north and 1 south pole so that seems to check out, its baffling me, could it possibly be the position of the brush gear?

RE: DC Motor will only run in 1 direction ?

(OP)
btw,i did only reverse one circut at a time, and it still kept going in the same direction

RE: DC Motor will only run in 1 direction ?

Possibly the terminals you have access to are not for the shunt. Maybe the wiring is for a set of windings in series with the armature for the purpose of speed control??

RE: DC Motor will only run in 1 direction ?

The field resistance 1500 ohms does not match the 1.7 field amps.

    If = 100 Volts/1500 Ohms = 0.066 amps?

RE: DC Motor will only run in 1 direction ?

It won't be the brushgear position.  The brushes should be positioned midway between the field poles i.e. at 90deg to the pole centres for a 2-pole motor, but even if you shifted them a considerable amount, you wouldn't get the effect you describe.  They would spark more though.

You could try energizing the field, spinning the motor by hand and measuring the induced brush voltage.  It should change polarity with a change in rotation direction.

RE: DC Motor will only run in 1 direction ?

(OP)
yes pete the brushgear is positioned as you described, ill try the test you mentioned, if i understand you correctly ill need to supply the field only, put an avo across the two brushboxes, then spin the armature by hand in either direction, to see if the polarity changes...

RE: DC Motor will only run in 1 direction ?

What sort of DC starter are you using?  Perhaps the starter is smart enough to know which way the motor is turning and is swapping the field over by itself?  Have you measured the polarity to the field with the motor running before and after you have swapped the field wiring?

RE: DC Motor will only run in 1 direction ?

(OP)
I did that test pete, and as you said, the polarity changed from + to - when i spun the armature in deifferent directions, fennell, ill double check everything agian monday and get back to you, thx for all the replies

RE: DC Motor will only run in 1 direction ?

jammyjim, aolalde has made a good point, there is a discrepancy in the information you have supplied on the field winding.

The test you have done on the armature shows that this is working as expected, it seems likely then that the field circuit is the cause.

RE: DC Motor will only run in 1 direction ?

(OP)
tbh the ampmeter im using probably needs calibrating, i doubt the 1.7 amps it was showing was accurate

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