AC Resistance Starting Calculation

[GYSpark]

I wonder If anyone can help me.

I have a 3PH 3 wire motor using Primary Resistance Starting which keeps tripping on overload. It is used to drive a Gearbox to turn a Large Steam Turbine when the turbine is Shutdown. AC Primary Resistance Starting on three wire motor is not concept that I am familiar with.

The motor IR and winding resistance balance ok. When the turbine is cool the motor is able to start, it is just will not work when the turbine has come off load and every thing is hot.

Can anyone point me in the direction of any formulae or information that I can use to check if my start up resistor is correctly rated and In circuit for the correct amount of time.

The only details I have are :-

415V Delta
Motor rated FLC 10.6 A
Speed 1430 RPM
Winding R 2 Ohms
Starting resistance 12.1 Ohms
Time resistance in circuit 3 Seconds
Power factor 0.85

Many Thanks.

 

[jbartos]

Suggestion: If the startup is automatic, then the timing may need to be reviewed and adjusted first. If this does not help, then the troubleshooting will be needed. The resistors may require having a finer grading/steps. Similarly, for the manual start-up except for timing that can then be adjusted manually.

 

[Barry1961]

This sounds like it may be a mechanical problem. Has it ever worked or is something that just started happening?

Barry

 

[rbulsara]

It appears that there is a timer interlock (very wisely though), preventing jogging of the motor or restarting when the motor winding is still hot.

 

[gsimson]

Turbine being hot or cold should not matter as the GD2 value almost same. There may be some mechanical problem which is likely to load the turbine when hot and the motor is not able to start. May please check.
Stator resistance reduces the voltage and the torque is directly proportional to the square of voltage.In your case the resistance is included only for smooth starting. You can comfortably reduce the resistance by 10% provided the mechanical side is OK
In the long run you can opt for Vector drive which can give higher starting torque as well as smooth acceleration
Simson

 

[edison123]

Was your bearing oil pump (for turbine and generator)running when this motor tripped ? If it was not running, then the breakaway torque could have been beyond the motor capacity resulting higher starting amps and overload trip.

 

[jbartos]

Suggestion to the original posting: Please, would you provide more background about what is going on there. Namely, have those resistors been incorporated and never worked properly or have those starting resistors worked properly with the gear box or were there no starting resistors when the gear box was there, etc.? Typically, the more info is provided, the sooner satisfactory postings appear.

 

[GYSpark]

Thanks for all of you replies to my post.
Sorry I have not been able to reply sooner

To answer some of your questions
I suspect there is a little more mechanical resistance when the turbine is hot rather than cold.
The start up of the motor is initiated from a DCS system when the turbine shuts down Jacking oil pumps start up to give high-pressure oil at the white metal bearings, which float the shaft on a slight film of oil. As the shaft speed comes to zero a turning gear program is initiated to start the motor, which is 5.5Kw.
The start up resistor is in circuit for three seconds then is shorted out for the rest of the time the motor is running by using an extra contactor.

We have checked the function of the starter with all contactor resistances being ok.

The motor will sometimes go straight into service when the Turbine runs down but more often than not the motor will not start first time.
We end up firstly hand turn the turbine via a drive nut attached to the motor shaft then installing a wind tool to the hand turning point. The motor is retried every two hours and typically will go in service after about seven hours.

The motor normally does not trip straight away it usually runs for 2-4 seconds which makes me wonder if the timer is not set correctly.


I am installing current monitoring this weekend so we will be able to plot current against turbine and bearing temperatures, which may help indicate if major mechanical problems exsist.

Reading your posts the other thing that has occurred to me would be to hand bar the machine using a torque wrench both hot and cold to see if the force required is different.

With my limited knowledge I do not think I have enough information to do any meaningful electrical calculations to work out maximum starting up and running torques for this motor Or optiumum start up resistance times.

The other difficulty is that Turbine normally only shuts down twice a year so any trial and error techniques for resistance or timer changes could take some time.

Many Thanks for all of your comments so far.

 

[jbartos]

Suggestion: Let a qualified electrical engineer or licensed electrical contractor to inspect/analyze/test the motor power and control circuits.