You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

---------->>>>>Get FREE PLC Programming Tips

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

PLC training tools sale

Reply
 
Thread Tools Display Modes
Old August 12th, 2019, 08:06 AM   #1
kalabdel
Member
Canada

kalabdel is offline
 
Join Date: Feb 2015
Location: Ontario
Posts: 441
Motor tripping breaker when generating.

I have very little information thus far and would like some general understanding to prepare for site visit in a couple of days.



So we have a machine that lifts a container (load) up and down about feet. The weight doesn't change much.
The machine is very basic, 3HP motor, gear box, reversing motor starter and a braking resistor. There's a circuit breaker supplying the machine with 600VAC located about 20' before the machine. There's no drive and it's a simple one speed up and down operation.

On commissioning the machine the technician reported that the machine lifts up the load with no problem at all but lowering form any position will immediately trip the breaker not the overload. We will have an electrician on site when I go there in case the breaker needs to be replaced.



I would like to understand how the generated current affects the circuit. Let's assume there's no resistor at all would the breaker trip first or the overload? (assume both are sized and working properly).




Thanks
  Reply With Quote
Old August 12th, 2019, 08:57 AM   #2
James Mcquade
Member
United States

James Mcquade is online now
 
Join Date: Oct 2007
Location: Tennessee
Posts: 2,533
does the motor starters have a manual interlock to prevent both starters from being energized at the same time?

it could be that the up motor starter has a phase on the contactor welded together thus creating the problem when you try to lower the lift.

double check the wiring to the motor starters.

if everything checks ok, raise the lift and then disconnect the power to the lift contactor and then try to lower the system. If the breaker doesn't trip, you are energizing both contactors at the same time.


this is what I would check first.

james
  Reply With Quote
Old August 12th, 2019, 09:04 AM   #3
JesperMP
Lifetime Supporting Member + Moderator
Denmark

JesperMP is offline
 
JesperMP's Avatar
 
Join Date: Feb 2003
Location: Copenhagen.
Posts: 13,803
Quote:
Originally Posted by James Mcquade View Post
if everything checks ok, raise the lift and then disconnect the power to the lift contactor and then try to lower the system.
Be careful. One or more of the wires you disconnect in order to perform this test may become live. Dont let these wires dangle freely. Connect them to some terminals so they dont cause any problems.

Quote:
Originally Posted by James Mcquade View Post
If the breaker doesn't trip, you are energizing both contactors at the same time.
Or there is a welded contact in the contactor that is disconnected.
__________________
Jesper
See my profile interests for Q&A
  Reply With Quote
Old August 12th, 2019, 09:26 AM   #4
_Dock_
Member
United States

_Dock_ is offline
 
Join Date: Sep 2015
Location: KY
Posts: 79
You said braking resistor? Where do you have a braking resistor while using a contactor?

If the circuit breaker trips immediately then your not generating. As the others have said I would look at the contactor
  Reply With Quote
Old August 12th, 2019, 10:30 AM   #5
kalabdel
Member
Canada

kalabdel is offline
 
Join Date: Feb 2015
Location: Ontario
Posts: 441
Quote:
Originally Posted by _Dock_ View Post
You said braking resistor? Where do you have a braking resistor while using a contactor?
Thanks everyone. The above quote interests me. Where should the resistor be connected when using a contactor?

I should clarify that the motor runs to lower the load a few feet (2 or 3) then the breaker trips.


What I gather from the replies that the overload should trip in case of over current and the fact that tje brwaker is tripping indicates a direct short (very fast over current). Am I correct?
  Reply With Quote
Old August 12th, 2019, 11:00 AM   #6
Aabeck
Member
United States

Aabeck is offline
 
Aabeck's Avatar
 
Join Date: Feb 2013
Location: Detroit
Posts: 1,339
Some motor overloads are not instantaneous. They may let the overload happen for a few seconds. Some are, and will trip immediately. Also, the circuit breaker may be sized closer to the motor spec's than the overload.

As for a braking resistor on a 3 phase motor- that would require another contactor that is energized when the motor is off. This would short the 3 motor leads across the resistor assembly. Braking resistors only help slowing a moving motor quicker than it would stop spinning on its own, and extend the life of a physical brake.

If the hoist has a physical brake, see if it is released in both directions. It could be that only the lift contactor releases the brake and dropping is driving against the brake.
__________________
Never underestimate the quality of idiots that will be running your machines
http://aabeck.com
  Reply With Quote
Old August 12th, 2019, 11:23 AM   #7
JesperMP
Lifetime Supporting Member + Moderator
Denmark

JesperMP is offline
 
JesperMP's Avatar
 
Join Date: Feb 2003
Location: Copenhagen.
Posts: 13,803
A circuit diagram of the current setup would be nice. Inclm dimensions on motor circuitbreaker settings etc.
__________________
Jesper
See my profile interests for Q&A
  Reply With Quote
Old August 12th, 2019, 11:40 AM   #8
kalabdel
Member
Canada

kalabdel is offline
 
Join Date: Feb 2015
Location: Ontario
Posts: 441
Quote:
Originally Posted by JesperMP View Post
A circuit diagram of the current setup would be nice. Inclm dimensions on motor circuitbreaker settings etc.
Thanks Jesper. I just emailed them for one as the machine came with one for a different model that includes a VFD
  Reply With Quote
Old August 12th, 2019, 11:55 AM   #9
kalabdel
Member
Canada

kalabdel is offline
 
Join Date: Feb 2015
Location: Ontario
Posts: 441
Quote:
Originally Posted by Aabeck View Post
Some motor overloads are not instantaneous. They may let the overload happen for a few seconds. Some are, and will trip immediately. Also, the circuit breaker may be sized closer to the motor spec's than the overload.

As for a braking resistor on a 3 phase motor- that would require another contactor that is energized when the motor is off. This would short the 3 motor leads across the resistor assembly. Braking resistors only help slowing a moving motor quicker than it would stop spinning on its own, and extend the life of a physical brake.

If the hoist has a physical brake, see if it is released in both directions. It could be that only the lift contactor releases the brake and dropping is driving against the brake.
Thanks a lot. My questions go beyond troubleshooting as I'd like to understand how motors on a crane, winch or similar applications work when powered across the lone without a VFD.

Let's say we have a crane or winch lifting and lowering a load that does not change. As the load is being lowered the motor will generate excess current as it is rotating, doesn't that current have to go through a resistor to "burn it off"? If the resistor is used only after the motor is stopped where does that excess current go?
  Reply With Quote
Old August 12th, 2019, 01:45 PM   #10
Aabeck
Member
United States

Aabeck is offline
 
Aabeck's Avatar
 
Join Date: Feb 2013
Location: Detroit
Posts: 1,339
Kalabdel,

The resistors are only used when the contactor to the motor is off, or a VFD has shut off the output.

A load pulling on a motor would only generate when the motor wasn't powered. The load pulling should actually reduce the power consumption of the motor, as it is getting an assist.

The more I think about it I am thinking there is a brake on the hoist that is not releasing during a drop, causing the motor to fight the brake.
__________________
Never underestimate the quality of idiots that will be running your machines
http://aabeck.com
  Reply With Quote
Old August 12th, 2019, 03:08 PM   #11
nswu1
Member
United States

nswu1 is offline
 
nswu1's Avatar
 
Join Date: Oct 2005
Location: louisiana
Posts: 274
What is happening is the weight of the object being lowered is causing the motor to regen back to the supply breaker. Usually you would use a three phase resistive load bank to dissipate the regen power.
  Reply With Quote
Old August 12th, 2019, 04:14 PM   #12
kalabdel
Member
Canada

kalabdel is offline
 
Join Date: Feb 2015
Location: Ontario
Posts: 441
Quote:
Originally Posted by nswu1 View Post
What is happening is the weight of the object being lowered is causing the motor to regen back to the supply breaker. Usually you would use a three phase resistive load bank to dissipate the regen power.
Thanks you. I thought I had completely lost my mind. The load is causing the motor to rotate faster that ist's magnetic field is faster than the line frequency. Did I get the theory right?

If so where would the resistive load bank go, between the motor and the overload?
And, calling that resistive load bank as I did "braking resistor" technically incorrect?

Thanks a lot.
  Reply With Quote
Old August 12th, 2019, 09:00 PM   #13
_Dock_
Member
United States

_Dock_ is offline
 
Join Date: Sep 2015
Location: KY
Posts: 79
Can you post the schematic for the motor?
  Reply With Quote
Old August 12th, 2019, 09:02 PM   #14
kalabdel
Member
Canada

kalabdel is offline
 
Join Date: Feb 2015
Location: Ontario
Posts: 441
I will as soon as I get it.
  Reply With Quote
Old August 13th, 2019, 01:02 PM   #15
ScottTheEngineer
Member
United States

ScottTheEngineer is offline
 
ScottTheEngineer's Avatar
 
Join Date: Sep 2006
Location: SE Michigan/NW Ohio
Posts: 144
Sounds like a direct short. Measure across the motor contacts, all 3. Should get 0 ohms when not moving. If one of them is different then you have a welded contact.
Overloads work by opening due to generated heat from excess current in the wires. If a mechanical brake was really good and motor is fighting it then it should trip overloads. If not then overloads are set too high. Use clamp on current meter to check current at any point on all 3 phases.
The Cont. current draw on the motor should be on the nameplate. If not you'll have to calculate it.

Only time I've seen the disconnect trip is when we had a motor poor wire connection in the peckerhead heat up, burn through the electrical tape and short out to another of the phases.

Last edited by ScottTheEngineer; August 13th, 2019 at 01:07 PM.
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
OT: Circuit breaker + shunt trip = one-time isolation contactor ? Ken Roach LIVE PLC Questions And Answers 12 September 16th, 2015 09:14 AM
OT: Breaker Sizing For Control Panel Tim Ganz LIVE PLC Questions And Answers 27 April 19th, 2012 02:37 AM
120V circuit breaker lockouts milldrone LIVE PLC Questions And Answers 21 January 26th, 2012 09:15 AM
How to decide breaker spec for circuit? zai_jnr LIVE PLC Questions And Answers 1 January 13th, 2009 09:23 PM
OT Tripping pump breaker rta53 LIVE PLC Questions And Answers 19 December 8th, 2005 07:49 PM


All times are GMT -5. The time now is 01:20 PM.


.