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Old August 13th, 2019, 02:32 PM   #16
kalabdel
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Thanks Scott. I clarified my first post, it does not trip immediately buy after it lowers 2-3 feet.
I haven't receives the proper wiring diagram yet
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Old August 13th, 2019, 08:29 PM   #17
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I know of no way that a braking resistor works with contactors alone to brake a 3 phase AC induction motor. AC induction motors that are overhauled do not turn into generators without having voltage on them to create flux. When you have a VFD, the VFD lowers the output frequency so that the rotational speed is higher than the electrical speed, so the motor does become a generator and the excess energy goes back into the VFD, then the VFD dumps it into the resistor. Just putting a resistor across the windings does nothing more than just opening the circuit, i.e. the motor coasts to a stop. So whatever someone thought they were doing with that resistor and contactors is not doing anything useful, and if done wrong, could be tripping your breaker. This needs thorough vetting.


From the sounds of it, the OEM made the machine with a VFD and braking resistor. Someone may have decided after the fact to ditch the VFD, then tried to figure out how to keep the braking, not understanding how it worked, so they cobbled together something that ended up just making a mess.
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Old August 13th, 2019, 08:38 PM   #18
kalabdel
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Quote:
Originally Posted by jraef View Post
AC induction motors that are overhauled do not turn into generators without having voltage on them to create flux.



Thanks. I'm not a fan of remote troubleshooting and don't trust second hand information that much. I wanted to be prepared for when I get there which looks like tomorrow.


Could you please expound on the above statement.



If a motor is "overhauled" that is being rotated by means of a load while it is not powered will not generate.


A motor that is being "overhauled" while powered (having voltage on it) will generate.


Do I understand correctly?



Thanks a lot.
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Old August 13th, 2019, 08:41 PM   #19
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I’ve kept my mouth shut to not be the guy to give bad advice. However I was waiting on some that are more experienced such as jreaf to chime in. I asked to see the schematics just for the simple fact that during my short 15 years I’ve never seen any type of vertical motion on an ac induction motor require a breaking resistor with contactors.

With that said you’re really down to two issues in my mind. You are fighting the electrically controlled mechanical break, lots to be desired here if the system was not engineered correctly. Most manufacturers can supply a number of brake configurations for fast acting, across the line, vfd or simple contactor control.

Or, you have mechanical interference causing the motor to overload.
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Old August 13th, 2019, 08:47 PM   #20
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Thanks Dock. In hind sight I should've asked for explanation of motors and and generation in a contactor scenarios without mentioning the actual situation because as I just mentioned I do not do remote troubleshooting and don't expect anyone to. I knew I lacked certain knowledge and wanted help with that.



Thanks
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Old August 13th, 2019, 08:52 PM   #21
_Dock_
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Kalabdel

Read this, don’t get lost in the weeds. I have a bad habit of that as well, start simple first.

If you need help on site pm me your contact info. I’m just in the office tomorrow if you’d like to chat.

http://cosmos.phy.tufts.edu/mhonarc/.../msg00253.html
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Old August 13th, 2019, 09:02 PM   #22
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Thanks. That is the information I'm looking for. So back again to my main question. Reading the quote below if the motor (rotor) turns faster than the magnetic field produced by the 60hZ line power the motor will generate. Is that extra current not as critical when it is fed back to the lines? I mean for as long as the wiring and circuit protection is rated for it then it just runs problem free?






"(You can also move the rotor faster than the stator magnetic field rotation and turn the thing into a generator of AC power.) The faster the speed difference, the higher the induced rotor current, the higher it's magnetic field, and the higher the torque the motor produces. In truth, an induction's motor torque is proportional to slip rpm, Nslip, or the speed difference between synchronous and actual speeds."
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Old August 14th, 2019, 06:29 AM   #23
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The power grid which is absorbing the generated power is capable of handling plenty more power than the motor and overloads can provide (assuming, of course there isn't a small transformer, wiring, etc which is improperly sized).

The overload relay, fuses/cb/MCP will still protect the motor and wiring whether its motoring or generating.

As the quote above states, the motor would have to be rotating pretty fast before it would cause an overload trip... Say it's a 1750 rpm motor, full load generating would be 1850 rpm. If its rotating at 1900rpm, its ~ 2x fla, so its going to trip.

I, too was sitting back waiting to understand what DB resistors are doing on an induction motor. Perhaps it was a wound-rotor motor originally, and they were used for speed/torque control by adjusting the secondary (rotor) current?

In theory, series resistors could be used with an induction motor to starve the current, so the motor would produce lower torque, and thus speed, but it's uncommon. It was used in some reduced voltage starters way back, but not cost effective, so you'll rarely see it.

I'll bet when you see it in person, the problem will eventually show up as something simple.
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Old August 14th, 2019, 06:46 AM   #24
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Quote:
Originally Posted by Gene Bond View Post
As the quote above states, the motor would have to be rotating pretty fast before it would cause an overload trip... Say it's a 1750 rpm motor, full load generating would be 1850 rpm. If its rotating at 1900rpm, its ~ 2x fla, so its going to trip.
Not instantanously. For 2x FLA it would take several minutes - depending on the overload.

And Kalabdel specifically says "the breaker not the overload" and also "immediately trips".
I assume he means an MCCB for short-circuit protection only.
Quote:
Originally Posted by kalabdel View Post
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.
Such an MCCB should not trip at all at 2x FLA. The MCCB should ignore for example the starting current, which usually is 7x FLA for the typical asyncronous motor.
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Old August 14th, 2019, 07:30 AM   #25
Gene Bond
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Agreed. I meant it as just an example to explain the function of generating vs motoring.

I assume there is a short circuit of some sort which is happening after the the motor is running in the down direction.

Something like this could be due to a wye/delta starter wired wrong, or similar.
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Old August 14th, 2019, 07:46 AM   #26
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It would be nice to get the circuit diagram, as mentioned several times before.

It blows my mind to have a maschine, but not having the circuit diagram for it.
I wouldnt try to figure out what goes wrong without the circuit diagrams.
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Old August 14th, 2019, 07:54 AM   #27
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Quote:
Originally Posted by JesperMP View Post
It blows my mind to have a maschine, but not having the circuit diagram for it.
Welcome to the US! It's almost the standard to be asked to troubleshoot a machine without any documentation (other than a purchase order)... It's extra-fun when the machine is from another country, not in english, and there is something like an obscure brand safety controller involved!

But, remote troubleshooting is always like shooting skeet in the dark.
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Old August 14th, 2019, 08:34 AM   #28
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Some other things to consider. If in fact that the load is overrunning your 3hp motor then this isn’t really your problem. Someone in the mechanical design phase has severely undersized the motor for the load.
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Old August 14th, 2019, 08:52 PM   #29
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Thanks so much gentlemen. The replies in this page are exactly what I was looking for and the exchange between Gene and Jesper made things that much clearer for me.

With regard to lack of circuit diagrams,I frequently get machines with generic/standard wiring diagrams that do not reflect customization. On machines that have been there a while I sometimes find them on the floor in a dirty corner beside the machine with torn and missing pages and often they're gone altogether. Occasionally they find them days later filed somewhere in their offices.

I couldn't make out there today as it's four hours away.
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Old August 15th, 2019, 12:18 PM   #30
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The braking resister or chopper is not what i was referring to. I use these on VFD DC buss to control the voltage. We actually use a three phase resistive load bank on the supply side to dissipate (by adding load) any regen. We have used these on self elevating jack-up barges, while driving the legs down we experience regen from the motors.
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