Hi,

I have a frequent problem of 160kw, 500vac, 1485rpm sq. cage motors installed in rota forging machine. The pair of two motors using DOL method for starting. Both motors are programmed to start alternatively that is in first production cycle motor#1 start folowed by #2 and in 2nd cycle #2 starts first followed by #1.

(motors and their control is of SIEMENS make)

The motor failure mostly occure during starting phase and either motor gets burnt off without any tripping .....The machine was installed in 1998 and since then 09 failures have been occured....

start delta starter can not be employed as the system requires high starting torque.......

How should I prevent motor from getting damaged so frequently?

Na

Hello nahmed

Welcome to the forum.

That is a very poor record!! nine failures in that short a time.

Can you give some details on the cause/mode of the motor failures.
I suspect that the failure may be due to rotor damage, but this needs to be confirmed.

Once we know the cause/mode of failure, we may be able to offer suggestions.

Best regards,

Thank you Marke for your reply. Yes indeed, this is a poor record....all this has already been informed to GFM-Germany but they could not provide us any proper rectification.


In the majority of cases rotor got demaged (cage motors with strips type rotor winding)...the motor takes normal running load current around 160Amps.....and starting currnt of about 230A. No sign of heating/overloading ....it is important to note that failure usually occures during motor starting period.

should I use softstarter to prevent these failure? like TELEMECHANEQU ATS 48 32Y?

Or you would suggest some thing else?


NA

Hello nahmed

Your starting current at 230 Amps seems to be very low. Do you mean 1230A??

It sounds to me as though you have the wrong motor for the application.

Rotor damage is invariably related to starting problems or motor/load selection.

When a motor is started, there are very high slip losses in the rotor of the motor. The total energy dissipated in the rotor is equal to the full speed kinetic energy of the driven load.
If you have a high inertia load, you must use a rotor that is capable of absorbing the high energy of the load.

Motors are usually rated two ways, either maximum load inertia as seen by the motor shaft, or maximum start time (assumeing full voltage starting).

If you exceed these figures, the rotor of the motor will fail early.
The maximum start times for motors vary considerably with some as low as 10 seconds and others as high as 40 seconds.

I would begin by investigating the thermal inertia of the rotor and checking it's suitability for the driven load.

If there is a problem of the rotor being unsuitable for the driven load, changing the start method will not overcome this.
Reduced voltage starting will reduce the start current and the instantaneous current and power in the rotor, but the start time will be extended resulting in the same energy or higher being dissipated in the rotor.

Best regards,

I agree and besides, you already mentioned that you cannot use Star Delta because you need high starting torque, so a soft starter is going to have the same issue. it's only advantage would be in that it can be adjusted to highER starting torque than a star-delta. But if as Marke says, the motor is unsuitable to begin with, a soft starter is only going to make it worse.

Hi nahmed,
if the inertia of your loads are too high, you have two solutions:
- electronic, using at least on inverter to start both motors, shifted in time as today.
- mechanically, installing one oil-type turbo-coupling on each motor. Motors will start soon, the energy losses during start will be transferrer externally, inside the oil of turbo coupling.

Regards
Mario

Hello nahmed,

I tend to agree with the information provided by Marke and would suggest the most economical solution would be to replace the motors with alternatives that have better thermal withstand capabilities.

The solutions offered by mariomaggi would also provide good results. At the end of the day, it all comes down to how much money you want to throw at this.

Regards,
GGOSS

Mario, currently the motor is coupled with load through 500mm dia cut wheel type coupling....should it be replaced with oil-type turbo-coupling as you mentioned in your reply?

The required inverter for 160KW motor would be extremely costly ... I am skeptical about using inverter as solution to this problem.

I will post more information for further problem assesment soon.

regards,

NA

nahmed,
I believe that you could replace the wheel with a fluid coupling, for example:
Transfluid
There are on the market also some fluid couplings that can avoid slip at the end of starting period.
Using a fluid coupling you will have also an improvement on starting torque, compared to actual situation.

About inverter cost, I don't believe that such inverter costs more that 9 motor repairs, included loss of money for loss of production during failure times.
I'm convinced that your machine don't ask 160 kW of power, but simply ask the starting torque offered by a 160 kW motor.


If my thinking is good, "probably" you machine need a medium power of 80 to 100 kW (can you confirm this?). If you could accept a longer acceleration time, if load inertia is not so high, you could accelerate one motor each time using a smaller inverter, i.e. an inverter for 110 kW motor, up to 40? Hz , switching to line frequency as soon as the overload limit for that inverter will be reached.
This application is possible, but it must be checked accurately to avoid undesired problems.
-----------
In another option using one inverter full-size for each motor, always connected, you will have an interesting energy saving and PF improvement during years, could you calculate it?


Bye bye
Mario

Thank you very much Mario for all your help. I am work on the issue with information you provided. I will get back to you as soon as we complete the work.

NA

Dear Mario:

After intense evalution and discussion with the turbo coupling suppliers we are ended with that due to strict space limitation turbo coupling couldn't be installed. Now we left the option to employ softstarter. Due your vast experience would you suggest any particular brand on the basis of reliability?

Nahmed

Hi Nahmed, Just to recap some comments;







The problem seems to very much be one of an incorrect motor selection. I take it from your initial post that the failures have been shared between the two different motors so it is not one paticular part of the machine that is causing the problem. Mario has offered as a potential solution the use of inverters which you assume are too expensive. Have you looked into replacing the motors? It would be very unfortunate to have the motor failures to continue after fitting soft starters to the machine.

Ken

Nahmed,
With all due respect, I really feel that regardless of brand, you will not be successful applying a soft starter to this application. As I said earlier if you ended up with insufficient torque to accelerate the motor using other means, the soft starter will be no different in that regard. The only method of starting that maintains full torque at the shaft without exceeding 150% current will be a VFD (inverter). I'm not sure why you seem to think Mario said to use a soft starter, he never did. He mentioned the possibility of an inverter instead of the turbo-coupling, but never a soft starter.

If you insist on pursing that option, the best soft starter is the one that is supported locally to you. If I were to recommend one that I know is well made, but you have no local support for it, then it really is not going to be a good choice. Maybe if you can list the brands available to you we can give opinions on them.

Dear Nahmed,
I'm just arrived at home after my summer holidays.

I confirm that I never suggested to you to use a soft starter for this application.

Do you asked Siemens to explain the reasons of such motor's failures?

At this point, before making other tentative actions, I would recommend to you to collect all info in only one document (you will lose some tens minutes). Degree of protection, ambient temperatures, duty cycles, overload sequences, insulation, protections, type of application and type of factory, etc. ), and some history on damages and repairs.
Such document will be useful to discuss possible solutions.

To change motor, to add soft starters or inverters or turbo couplings, all are expensive actions, you have to make the right thing now, you cannot make other tentatives in the darkness.

Regards
Mario

Hello Nahmed

As the failures appear to be with the rotor of the motor, it almost definite that there is a problem with the wrong motor selected. If the rotor of the motor is not suitable for starting the load inertia, then you must change the rotor or motor for a rotor with a higher thermal capacity.
A soft starter will only make things worse, not better.natively, use a VSD or a fluid coupling.

Best regards,

Thank you all for your replies. I am in process to prepare a complete document as asked by Mario....will get back soon.

Thanks again.