Further going in recently during a shut down, the operation run the DC ELOP. then after some time it got burnt completely with reasons unknown. so we have instantly isolated the circuitry and checked for the resistance and meggered the motor, which is showing body we have disconnected and placed a spare motor. but then again the replaced motor also after running for few min, we saw smoke coming from the motor so again the same we meggered and all. my question is

1.what would be possible reasons for the motor failure.
and to mention the second motor what we have replaced sent for overhauling where the tested the motor on test bench it was running fine. the same when we connected got burnt again. we have disconnected the terminals and checked for the incoming supply, ie field and armature. it was 220v dc. no abnormality noticed.

2. why is it twin speed motor (My interpretation on seeing the name plate) s2-15 min on high speed may be to reduce the load on the batteries and also when machine trips all the instantaneous oil pressure will be high and later it may get reduced.

3. for changing the direction is it like either the field windings or armature windings is sufficient or both should be changed.

Apologies of the length of the msg. I just want to be clear.

 

sudamadhav,

Please define how long the DC ELOP was running before "...it got burnt..."

Please describe where it was "burnt"--the field, the armature/rotor, the brushrigging/commutator? Just the leads in the motor's junction box?

When was the last time the ELOP was run? Is it started and run for a few seconds every time the unit is started from zero speed and/or turning gear?

When each DC motors were used to drive the ELOP, what was the discharge pressure of the ELOP?

Did anyone monitor the DC motor speed of the second motor when it was running? If so, what was the speed of the motor/pump?

Was any other L.O. pump (AC motor-driven, or gear driven) running when the ELOP was running?

Is the ELOP a separate pump--meaning the DC motor drives an individual pump, or is the DC motor "piggy-backed" on (coupled to) an AC motor driving a single pump producing L.O. pressure? (So, there is an AC and a DC motor coupled together also coupled to the same L.O. pump?)

If the DC motor is coupled to the same pump as an AC motor, there MUST be some kind of interlock, or permissive, to shut down the AC motor when the DC motor is energized--or there WILL BE damage to the DC motor.

Have you looked at the L.O. system P&ID to see if there is a check valve (non-return valve, or NRV) that might be stuck and allowing pressure from an AC or gear-driven pump to back-pressure the ELOP?

Is the ELOP a centrifugal pump, or is it a positive displacement type pump (gear pump)? If it's positive displacement pump is the discharge of the pump somehow blocked or restricted?

The most common reason for DC motors failing is incorrect connection during replacement (see below). If the motor is overloaded for some reason, there is usually a thermal overcurrent/overload relay to alarm the operator--but the overcurrent/overload relay is not used to trip/stop the motor; better to ruin a motor than turbine-generator bearings!

In general two-step DC starters are used to reduce the inrush current from the battery when the DC ELOP motor is started. So, the DC motor is started and run at a lower speed than that required for the ELOP to produce rated pressure, and then after a second or two, the DC ELOP motor starter increases the speed yet again to another intermediate speed between the first speed and ELOP rated speed, and then after another short time delay (another second or two) the speed is increased to the rating of the ELOP (which is not always exactly the same as the rated DC motor speed).

This is done by inserting resistance in the stationary DC motor field winding circuit, and then shorting-out the resistances over a couple of short time periods. (Strengthening the stationary field(s) of a DC motor to more than rated field strength (which a function of the voltage applied to the stationary coil windings) causes the speed of the DC motor to be higher than it would otherwise be, so by having voltage drops across resistor(s) in the field winding circuit the speed of the motor can be "stepped" up as the resistances are shorted out.

Many DC motors have LOTS of leads in the motor's junction box, and these have to be connected properly--and any unused leads <b>MUST BE SHORTED TOGETHER PROPERLY</b> or there will be motor damage. If the first motor was a replacement for another motor (changed out during the outage before the DC motor was started and run) then are you sure the motor leads were properly connected, and any unused leads were shorted together properly? If a similar motor was used for the replacement and it had more winding leads coming out of the motor and they weren't properly connected/shorted together and the same mistake was made when the second motor was installed, then problems can re-occur.

Many modern DC motors have lots of different possible configurations--shunt windings; commutation windings; and on, and on. Since DC motors aren't as common as they previously were (in decades past) most modern DC motors are built to suit a wide variety of applications which means there is the potential to not use some of the windings in the motor's junction box--which can dictate how the various windings are to be connected or possibly requiring shorting. It takes very great care when installing or replacing DC motors, especially with a "one-size-fits-all-applications" DC motor.

You haven't provided enough information about the conditions of the event for us to be of much help. More concrete information about the period(s) of operation, the pump discharge pressure, the type of pump, the burnt areas of the motor--all of this is very important information.

As for changing motor direction, usually only the polarity of the DC being applied to the stationary field windings is reversed--but sometimes if this is done it's also necessary to reconfigure other field windings (which can have armature (rotor) DC flowing in them when the motor is started and running!). But, if it's easier to change the polarity of the DC being applied to the rotor then that's done also--it all really depends on the configuration and application and the motor connections. Some, as was said, are very difficult to change appropriately, especially when there are unused windings.

It's interesting that the unit has a DC ELOP motor--because a lot of turbine manufacturers these days are switching to variable frequency AC drives powering AC induction motors for ELOP service. Any AC drive converts AC to DC and then chops up the AC to convert it back to "variable frequency" AC--so using a DC battery as the source for AC drive provides the DC battery back-up for an ELOP function. AC induction motors are cheaper, easier to hook up, and the variable frequency function of the DC-AC drive serves as a "soft-starter" to reduce the DC current inrush during starting of the ELOP motor. So, it has a LOT of benefits--though the cost of the DC-AC drive can be high. DC-AC drives have a good record of service and reliability, too, in this modern world. However, technology changes over time and keeping them running (spares; replacement parts) can cause more frequent purchases of new drives to be necessary--but that doesn't mean new motors have to be purchased when new drives are necessary. ("There's no such thing as a free meal," as a colleague was often heard to say! Meaning that nothing is perfect, like getting a free meal would be.)

Many sites with running turbines are finding it very difficult to find DC motors for replacement or spares, and are switching to AC motors with DC-AC drives. Some insurance companies are also recommending this change-out. Finding engineers and technicians knowledgeable about DC motor applications and proper connections is getting increasingly difficult, especially with these "one-size-fits-all-application motors. Sometimes even the motor manufacturers don't have application engineers any more.... They just keep making the same designs which don't require engineers and employing engineers for answering application questions costs money, decreasing profits and/or causing higher prices (to maintain profit margins!) which means fewer motors will eventually be sold.

Hope this helps!

 

Sudamadhav

I have plenty of experience with ALSTOM GTs.

One question that I have is, why was operations running the pump continuously? The ELOP is not meant to be run continuously for long periods of time because the motor will burn up. The ELOP is only meant to be used as emergency for controlled shutdowns in case the 2 main lube oil pumps have failed.If the ELOP ran continuously for a long period of time, that's why it burnt up

The reason it is a DC motor is so that it can run from the batteries in case there is a loss of all AC power.

The ELOP has 2 different speeds. There should be a key in the AA module to select low speed or high speed operation. If I remember correctly, the logic can also command the pump to run at low or high speed as required, we would have to check the logic to see under which conditions it runs at certain speed. Usually this is determined and set up during commissioning.

If you need further assistance, feel free to e-mail me directly.
[email protected]

 

Dear sir,

there was some operational checks going on before they (operation dept.) were going to take the machine in service, and I came to hear that there were no of start-ups on that day particularly. the oil pressure observed was 1.5 bar, and yes it has been started in manual mode with key selection at the LCR. I agree it could have got burnt due to repeated and long run of the motor it got burnt. but my question is we have replaced the motor with new one (spare). the new one during the trial run we observed smoke coming out. so we immediately stopped it. how come this can be with the new one again?

 

Dear CSA,

1.the pump was running for long hours on that day, couldn't able to collect the info about it.

2. the pressure observed is 1.5 bar and speed, no no one noticed it.

3. it has its own separate pump. (centrifugal type)

4. the rotor as well the conductor connecting the commutator to the armature is also completely burnt. I feel the SC current would have been very high, coz we observed some oil kind of thing came out (could be the varnish coz it stinked a lot)

I agree the first one which was there might have got burnt due to more no of working hours. but what about the one we have replaced.

 

sudhmadhav,

> 1.the pump was running for long hours on that day, couldn't able to
> collect the info about it.

Was the DC ELOP being used instead of the normal L.O. pump(s) to allow turbine operation (such as prolonged turning gear operation or fired operation)?

Running a DC motor operating off a battery for long periods of time can cause voltage to drop which can cause current to increase which can lead to damage. Since many DC ELOP motors draw more current than the battery charger can supply continuously it's possible the battery charger may have been damaged, also. Be sure to compare the DC motor nameplate current rating for the condition it was being operated in to the rated battery charger output. Many sites mistakenly believe if the battery charger is receiving AC the DC ELOP can be run continuously--and that's not true, without damage of some kind.

It would be extremely unusual for there NOT to have been an alarm about high DC ELOP motor current and/or low battery voltage while the first motor was operating and failed. Are you certain the alarm logs don't show anything for that period related to the DC ELOP and/or battery?

> I agree the first one which was there might have got burnt due to more no
> of working hours. but what about the one we have replaced.

Again, one of the most common causes of failure after replacement is improper connection.

Also, it's possible that something in the motor starter circuit was also damaged and if not caught before installation of the second motor caused premature damage to the second motor. (This happens a LOT with printed circuit cards--something in the field is damaged or failed causing the printed circuit card to fail. The printed circuit card is replaced without properly diagnosing the reason for the failure, meaning a perfectly fine card is connected to failed circuit(s) and/or devices, and the new printed circuit card is quickly damaged on power-up.)

Any time a failure like this occurs it's very important to understand the cause of the failure and investigate all related circuits and devices before returning a new component to service. High current devices like DC motors drawing excessive current can cause collateral damage to in-line devices and wiring which if not properly identified and repaired can lead to other failures.

Please write back to let us know what you find!

Thanks also to SBANOS for sharing his knowledge of 13E2 gas turbine systems and operations. A quick look at his company's website (www.saltek.ca) reveals a nice alternative to OEM ADVANT operator stations (clients and servers).

 

dear sudhmadhav,

S2-15 min

DC motor can run for only 15 min and after that a rest time required to get the winding cooled down.

another thing please try to note down is that, measure the DC current when the motor is running. it should be below its rated current in the name plate.

if it is higher please check the field circuit connection and shaft overload.

 

Dear All,

Apologies for the delay. but the wait was due to the time taken to identify the problem.

1. The first motor failed (burnt): frequent operation of motor for long hours and its a compound motor there is a key selection as mentioned in the above posts to cut in the shunt field after 15 min was not selected, so it was continuously running on only series field for a prolonged time. so it got burnt and since for DC ELOP there wont be any overload relay as you said, the contactor also got burnt but the MCC not tripped.

2. The second motor (replaced one) got burnt due to: the one which we replaced hot burnt previously and it has been sent for overhauling. which was not properly done(I came to hear that DC motors once failed, will not cater the same properties as the original new one and the replaced one also having some problem with the comutator circuit so that also failed.

3 finally we have replaced the contactor and replaced the motor with a brand new one, it worked out....!