ELEVATOR PROTECTION

[Remy]

Hi there fellas. I am new and I got here due to almost going crazy with a problem which I will explain....

I am an electronic engineer and I am also the superintendent of a building using a 30 year old Otis elevator. This elevator gets its motor burned an average of every 1.5 to 2 years. It gets repaired and there it goes again for the same period of time.

Maintenance people are so lousy that they have gave me full access to the euipment and I have been learning for several years and I have even made some repairs in emergency situations.

I want to provide protection to the motor so that it stops when a potentially wrongful situation is about to happen....

For those not familiar with elevators, this system uses an induction motor with two windings, one for high speed and one for low speed. A relay system consisting of 6 triphase relays (poorly maintained) make the changes in direction and speed.

This motor uses 220VAC thri phase energy. Now that the motor is being rewinded I will try to provide nominal currents for you to suggest any help, but you could suggest percentages of nominal to calculate values of protection means anyway.

I am not an expert in this area, and as I see that many of you are, I request your kind help in providing suggestions to protect this sucker!

I think that the motor should be protected by a current overload system plus perhaps a temperature sensor that shuts down the motor in case temperature gets too high. Obviously, I need to install two paralell systems, one for high speed and the other for low speed.

PLEASE HELP ME OUT ON THIS ONE !!

[marke]

Hello Remy

 

Welcome to the forum.

 

First, there are usually rules and regulations covering what can be done with the likes of elevators. Perhaps you need to check the rules and also ask Otis for their input on the problem.

 

Secondly, if you can give us more detail on the mode of failure, we may be able to offer more assistance. There are several areas that can fail. I assume that it is not a rotor failure, but the rotor is put under a lot of stress in this type of application due to the amount of time the motor operates under high slip conditions.

If a motor is overloaded, it will usually not fail immediately, the life expectation is usually shortened. One thing to look at immedidately, is what is the insulation that they are using. If the motor is wound with class B insulation, it should be increased to a higher temperature insulation. This will make it last longer.

Please confirm that you are experiencing a stator failure rather than a rotor failure.

Please advise, if a stator failure, what is the type of stator failure. shorted turn, burnt coil, all coils burnt etc.

 

Best regards,

[Remy]

Thanks very much for your fast answer (I am aware that I am not the only one with insomnia). I have been in contact with Otis, however we are located in Venezuela and as I told you, service is really lousy, son they always suggest absurd things (for their proffit a complete replacement of the whole system). Obviously this is stupid and out of the question. I would not be against a partial modernization BUT they have to start with good maintenance practices and they simply dont care, so I have decided to take the matter in my hands partially as they are messing with our pockets. Can you believe that they are ISO 9000 certified? I can provide pictures that will knock your socks out!

You are right, the failure is always in the stator as the rotor does not have any windings. Most of the time, low speed windings burn as these are the ones that operate just a few seconds prior to stopping so that the brake unit can level the cart with the floors, so they have to deal with higher currents for the final stage of the trip (I think)

I need some sort of protection that stops the machine for the time needed and avoid permanent damage. Some stuff can be also provided so that for example the motor is not tripped off during its up/down path and passengers are stuck in the cart, however this would even be prefered instead of paying another rewinding.

Thanks again....

[Remy]

OK Sorry Marke, I screed up in my last comment. Current at low speed should be also lower than at high speed....

Another thing, connections at the motor input are DELTA-DELTA and the control does not change from these connections for speed change, rather what it does is switch between windings.

[marke]

Hi Remy

 

I would suggest that you get rtds installed in the low speed winding and use a protection relay to prevent a restart if the temperature is above a set temperature. Obviously, you do not want to trip the elevator once it is running as it could stop between floors. If you can restrict the restarts when the windings are hot, you will extend the operating life of the windings. Also consider rewinding with higher temperature insulation.

A rule of thumb is that for every ten degrees C you increase the temperature of the insulation, you halve the life of that insulation.

 

With a rtd, you could measure the maximum temperature that the winding getts up to and set the restart lockout for ten degrees below that.

 

Best regards,

[Guest]

Hi there guys. Not much information but OK.

We have rewinded the motor and it is running fine. I measured with my digital Fluke and current clamp the following:

- Starting current on High speed 148 Amps, for a second or so.

- Running current on High speed 31 amps per phase.

- Running current on Low speed 32 Amps.

 

Al phases are closely balanced.

 

I found a great bargain for a GE Multilin 269 motor protection system that enables RTD use, so I am planning to get it. On the other hand, I am buying also a Telemecanique motor Overload relay for the low windings as they do not have any protection right now (I also suspect of the other protector on the High speed windings as it is very very old).

 

THANKS. I WOULD APPRECCIATE SOME MORE ADVICE

[marke]

Hi Remy

 

From the information that you have given, it would apear that there is a problem.

I would expect that low speed current to be considerably less than the high speed current.

 

Is the motor wound with two totally separate windings for high and low speed, or is it a tapped winding arranged as a dahlander starter (delta double star configuration or similar)?

 

If it is two separate windings, (essentially two motors on one shaft), I would question the winding of the low speed winding.

What is the rated current in low speed and high speed?

 

Best regards,

[Remy]

We do not have any access to the original rating data of the motor, even the dataplate is totally ruined, so I am relying on the measured currents I made myself and the fact that the motor has been running smoothly for one Week (tomorroW). The Windings are tWo separate Windings as they receive separate tri phase cables to run them and the contacts feeding the motor are separate also.

To me the cause of the failure Was really bad damage in the relay contacts Which I detected after the problem and that the maintenance jerks quickly changed to erase all proof of their lousy Work.

I Will install the multilin control and monitor closely the temperature to avoid operation of the motor While really hot. Can someone provide stator Wining usual temperatures to adjust the curves?

I think that perhaps currents could be simmilar as the Windigns surely are not the same lenght and the same diameter, so a smaller Winiding could be consuming the same current even though it Would be producing a smaller electromotive force due to its smaller size or length, thus generating the sloWer speed....

I do not think there is much margin for error for Wining scre up. This guy that reWininded the motor has done it several times and the motor runs for up to 3 years OK, What I think is causing the problems is the crappy maintenance, specially in the poWer contact relay areas, as these require careful and periodic checks and contact adjustment to avoid loss of phases or high resistance.

 

Thanks people!

[marke]

Hi Remy

 

If there are totally separate windings for the two speeds, then you do not have issues with preheating of the low speed from high speed current etc, but it is not normal to have the same ratings for high speed and low speed unless the high speed is derated because the cooling at low speed is much less than at high speed. The maximum rating of the motor is depedant on the losses in the motor and the cooling of the motor. If the two speed have the same ratings, then the losses will be similar. If the cooling is a quarter, due to the low speed, then the temperature rise would be four times!! This can be overcome by using cooling for the low speed and not uprating the highspeed for the higher cooling.

The motor rewinder should be able to tell you the insulation rating he has used, and recommend a maximum operating temperature. It is the insulation that determines what the maximum temperature rise should be.

 

Best regards,