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motor #71559
11/01/06 06:48 PM
11/01/06 06:48 PM
Sandro  Offline OP
Joined: Dec 2001
Posts: 444
Stoney Creek, ON, Canada
3 phase 208V 5hp(approx 30 years old), each time we reset the o/l, upon start up, motor buzzes/hums for up to 30 secs, then trips the o/l.

didn't have time to check the voltage at the starter to see if it was dirty contacts.

Spend more time and energy diagnosing this, or just replace the motor as the customer insists?

edit: spelling

[This message has been edited by Sandro (edited 11-01-2006).]

Work Gear for Electricians and the Trades
Re: motor #71560
11/01/06 07:36 PM
11/01/06 07:36 PM
gfretwell  Online Content

Joined: Jul 2004
Posts: 9,338
I would check the voltages across all 3 legs and also make sure the load is not siezed up.

Greg Fretwell
Re: motor #71561
11/01/06 07:41 PM
11/01/06 07:41 PM
frank  Offline
Joined: May 2001
Posts: 376
windsor ontario canada
You have to determine if it's a mechanical problem with the motor(bearings) or it's mechanical load(locked up) if not single phasing.Then determine if it is a short in the motor windings or the circuit that feeds it.
What type of starter electronic(SCR)controlled or other.
How you go about trouble shooting depens on whether you have a meger or not.
Could be anything including a short in the run from the starter to the motor.If everything else checks out disconect the wires at the motor then the load side of the starter and see if it pulls-in and holds each time.Disconnect the motor from its mechanical load before you order the new motor for sure.

[This message has been edited by frank (edited 11-01-2006).]

Re: motor #71562
11/01/06 09:18 PM
11/01/06 09:18 PM
jraef  Offline
Joined: Jul 2004
Posts: 101
San francisco, CA, USA
Could be as simple as having a burned contact on the motor starter. if you gave it only 2 out of 3 phases it would not start, it would just sit there humming until the OL tripped. Sound familiar?

By the way, 1 blown fuse, one open contact on the circuit breaker, 1 broken motor lead, 1 bad connection in the pecker head... all would have the same effect.

[This message has been edited by jraef (edited 11-01-2006).]

Re: motor #71563
11/01/06 10:14 PM
11/01/06 10:14 PM
Dave T  Offline
Joined: Jan 2005
Posts: 155
Waukesha, WI, USA
I would like to understand that the obvious was already shecked, that is in the motor turned freely.
I would conclude from your description that is a simply across the line starter since you did not describe anything specific. If so, I would "ditto" that of te motor has been single phase as previously stated due to a blown fuse or a contact in the contactor that is defective.

Re: motor #71564
11/01/06 11:57 PM
11/01/06 11:57 PM
Rewired  Offline
Joined: Jan 2006
Posts: 558
Hamilton, Ontario, Canada
I "ditto" that as well. Screams "single phasing" to me.


Re: motor #71565
11/02/06 03:05 AM
11/02/06 03:05 AM
Trumpy  Offline

Joined: Jul 2002
Posts: 8,260
SI,New Zealand
Single phasing sounds like it.
You can lose a phase during operation of a motor and it will carry on quite happily.
Try and start a 3 phase motor Direct-On-Line with a phase missing, just isn't the same.
Or even with A/Transformer start.
You can to a degree, wreck starting gear and the motor with the imbalance.
I've seen it before.

Re: motor #71566
11/02/06 05:39 AM
11/02/06 05:39 AM
RODALCO  Offline
Joined: Dec 2005
Posts: 857
Titirangi, Akld, New Zealand
Check for single phasing !!
Check voltages at your supply point first,
could even be a blown fuse.
Also perhaps a high resistance contact on one of the phases of the contactor.

Take the load of the motor and see if it will start with no load.

Do an isulation test and continuity test on the windings and see if equal values are obtained.

If it is star delta, check all six wires if connected correctly or poor contacts somewhere.

edited for typo's RF.

[This message has been edited by RODALCO (edited 11-02-2006).]

The product of rotation, excitation and flux produces electricty.
Re: motor #71567
11/02/06 06:18 AM
11/02/06 06:18 AM
Sandro  Offline OP
Joined: Dec 2001
Posts: 444
Stoney Creek, ON, Canada
Thanks for the posts. Will return next week and do more diagnosing.

Solution will be posted.

Re: motor #71568
11/03/06 01:51 AM
11/03/06 01:51 AM
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,717
Anaheim, CA. USA
This thing is SCREAMING Single Phasing!!!

If the Bearings are so worn out / frozen, they will not allow the Rotor to spin, that would be the culprite - so verify the Rotor may spin freely _PRIOR TO_ taking any additional steps.

If the Rotor spins freely, verify that there indeed is 3 Phase available _AT THE MOTOR'S TERMINALS_, and there is no "pseudo 3 phase" being created elsewhere in the facility (more on this later).

If Rotor spins freely, and we know that a "Real" and 'Solid" 3 Phase circuit is available at the Motor Terminals, verify the Motor has been connected properly - and that no Winding Lead(s) have come loose or Ground faulted.
I am _Assuming_ this Motor uses an "Across-The-Line Starter", not any Wye-Delta Starting Contactor arrangements.
If Contactors perform any type of Winding Re-Configuration, verify these guys are working properly prior to going on to the next step.
*** Along these lines, if there are any reduced Voltage Starting techniques involved, verify these guys are working correctly before proceeding.

If the Starting techniques check out good, and/or the Motor Leads are terminated properly + are not faulting out to Ground, find out what the Voltage is at the Motor's Terminals (if you can test first with the Motor leads not connected, that would be great).
Voltage needs to be within tolerance of the Motor's _NAMEPLATE RATING_.

If this checks out good, time to search for barbecued Stator Winding(s).

Just about forgot this one!!! (D'Ohh!!!):
Is the Motor loaded down during start?
Could the Motor be driving a Compressor, which does not "Dump" the head pressure before starting, resulting in the Motor stuck in Locked Rotor.

Another scenario: Is this Motor driven from a transformer on-site, and this transformer may be insufficiently sized for the Motor?

Check into the Single Phasing thing if you get odd Voltage results on one or two Lines - and especially if you notice very odd behavior from Lighting on the same system.

If the Single Phasing problem is "Masked" by already operating 3 Phase Induction Equipment (such as 3 phase motors running under load), you will find Voltage on the "Dead Phase", which will be measurable L-G, L-N and L-L.
The Voltages may be similar to what's normal, however when something changes on the Motors' load (one is turned off, or loads increase/decrease), the "Dead Phase" will have a noticably different Voltage reading.

Things become very interesting when there are two different systems - like the system from the PoCo is 480Y/277VAC 3Ø 4 Wire, and there may be one or more SDS's included - such as 208Y/120VAC 3p 4W - and possibly in addition, a 240VAC 3p 3W Delta.

Loss of phase on the SDS system's Secondary side may be quite a handfull to trace out - but when the loss of phase occurs ahead of these guys, things become a major search!

At least when a Primary Fuse on the PoCo's side blows, it makes Lighting Fixtures react very strangely, and is kind of a no-brainer when you go outside to see the fuse cartridge dangling downwards in the fuse holder!

Good luck!


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!


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