Discharge resistors 3

[reactive]

Our client has a 1650kW 6,6kV synchronous motor (on a minewinder ward leonard system). There are what appear to be discharge resistors mounted on top of the control panels (air cooled, wire wound, housing is approx. 1m3). Are these resistors definitely discharge resistors and will they make much difference to the starting current.

How can I tell if the rotor has short circuit bars for starting. Will they be visible?

 

[ScottyUK]

Hi Reactive,

Most of my synchronous machines experience is on generating plant of varying sizes, so I may be wide of the mark here, but a possibility is that the resistors are field dump resistors, connected to the synchronous machine's field terminals via a contactor working in antiphase to the main field contactor. That said, the enclosure sounds pretty big for a field dump resistor on a relatively small machine. But in the 'old days' things were built bigger and tougher, so maybe the old adage that size isn't everything applies here!

 

[jschwei314]

Mr. Reactive,

The resistor is most likely the starting/discharge resistor. The value of the resistor is normaly between 3 to 6 times the field resistance. This resistor has some relationship to the starting torque of the motor, thro small. The starting or squirrel cage winding are normally part of the field poles and they are visible.

Sparky

 

[jbartos]

Suggestion: Try to trace "discharge resistor" wiring and observe conductor sizes. If the conductor sizes are large and conductors lead to the motor stator, e.g. terminal box, the resistor are probably the starting resistors. Also, depending on their connection, if they are disconnected the motor would not start.
The motor starting bars may be addressed in the documentation or somewhere on the nameplate as an amortisseur or amortisseur winding.

 

[reactive]

If these were starting resistors would they not be much more substantial i.e. liquid resistors ?

I am not in a position to do any tests at site but from memory the cable from the resistor looks to be approx. 16mm2.

My understanding is that even if they are discharge resistors they will be in series with the field winding, meaning they will carry all of the rotor current during starting. What am I not getting here i.e. if they are in series what is the difference between starting resistors and discharge resistors?

 

[TomG33]

Hello Mr Reactive.

It sounds to me like you are describing one of our machines. )

I think from your description that these resistors are definitely what I call "field Discharge Resistors" Other Posts here have given other names but that is the nature of an international forum such as this.

If they were resistors in series with the stator, They would have to be located in a high voltage enclosure and not simply placed on top of the control panels.

During DOL Starting of a synchronous machine, such as used on these MG Sets, Most of the starting Torque Comes form the Amortisseur windings, which is a relatively simple looking winding consisting of two rings, one on each end of the rotor, with a few bars ( perhaps 19mm diameter )going axially through the extremity of each of the salient poles.
THe ones I deal with look very much like the so called "Squirell Cage"

When Power is first applied and the rotor is stationery, The rotating magnetic field produced by the stator induces some pretty large currents in the salient pole windings at line frequency. The behaviour is much the same as a Current Transformer, so we MUST provide a path for it otherwise we can get some dangereously high voltages.

DC Field excitation is not on at this time, so we need to provide an alternative path for these currents. Field discharge resistors are used to short out the two ends of the Field supply wires until such times as the DC field excitation is applied to the field.

As the motor accelerates, the frequency falls (with the relative frequency between the rotor and the stator) until it reaches 0 Frequency at synchronous speed. The amplitude of the current drops off during the last 10 or 15 % of the speed range.

Usually the DC field is applied at about 95 to 98% synchronous speed.

These resistors are sized to handle the large induced currents, and the resistance value is selected which will keep the voltage safely below the insulation rating of the windings. Obviously, higher values of resistance will give higher voltages.

During a normal MG Set Start These resistors get very hot, Thus the large physical size.

To answer your question, the magnitude of the Starting Line current is not affected by changes to the value of these resistors, Just the resistor terminal voltage generated by the induced currents and the Total power dissipated by the resistors ( Power = I^2 R ) Where "I" is fixed. There is probably some minimum value, but I do not know the criteria for this.

The Starting current is governed by the design of the Motor Stator windings. The duration of the high starting currents is dependent on the inertia of the rotating part of the MG Set assembly, The mine winders I have dealt with, usually only have two generators, Compared to up to 6 in other applications, So, if my memory serves me right, The starting time should be less 5 or 6 seconds.

on Other Jobs, the starting current has been reduced by inserting Reactors in series with the motor stator windings, and shorted out once the speed gets up above some set speed. As mentioned above, All Of this sort of equipment would have to be located in a High VOltage enclosure and switched by High voltage circuit breakers.

Hope this long winded description helps you understand the starting process.

Best Regards
Tom

 

[jraef]

reactive,
It appears from your response about "liquid resistors" that you may be confusing a Synchronous motor with a Wound Rotor Induction motor. Both are "slip ring" motors, but beyond that are not the same thing. Other than that comment, I can't improve on TomG33's response, for which I gave him a star.

Quando Omni Flunkus Moritati

 

[aolalde]

I agree with TomG33, these resistors bank has to be the field discharging path while the motor is accelerating. Look for a single phase circuit since the field is a low voltage DC circuit.

The rotor amortisseur winding is pretty visible inserted in the rotor pole shoes.

 

[jbartos]

Suggestion: The mentioned physical dimensions of the resistors, 1mx1mx1m or so, suggest a large size of the control panel. The motor starting resistors at 6.6kV would have to be controlled in some steps to accomplish the smooth start of the synchronous machine.

 

[reactive]

Thanks TomG33, that helps a lot.

I never intended to suggest that the resistors were in the stator circuit.

I am not confused with an induction machine. Is an alternative method of starting the synchronous machine to do away with the amortisseur winding and have a larger resistor (liquid) in the rotor circuit? This would surely soft start the machine.

 

[edison123]

reactive,

Synchronous Induction motors having normal wrim windings in the pole faces conencted to external reistors and salient poles for synchronous running are also used in starting heavy inertia loads.

 

[TomG33]

Dear Mr Reactive
Your alternative starting method is completely wrong.
The salient poles will not produce any useable torque to accelerate the motor rotor, but rather a series of positive and negative torque pulses as the stator magnetic field goes flying past.

The only starting torque you will get comes from the amortissuer winding. do away with this and the motor will not start DOL.

Once the motor gets near synchronous speed the Salient poles will contribute a little Accelerating torque but not a lot until DC Excitation is applied to them.

In Australia we have been "Soft Starting" the larger MG Sets by motorizing one of the Generators with a DC Drive and running it up to speed, Putting field on the Synchronous motor and then actively synchronizing it to the incoming line. This works like a charm with no real disturbance to the power system.

Obviousely you are talking more then a few slabs of beer here )

A less fancy way is to motorize one of the generators and run it up somewhere near sync speed and then DOL it. There is still the instaneous short on the 6.6Kv line until the windings develop back EMF but the long drawn out high starting current is eliminated.

Other then this your only option is a reduced voltage method of some sort. Reactors in line being probably the easiest, followed by the liquid resistance mentioned already in a few places.

Tom

 

[TomG33]

Jbartos,
Your point about the size of these resistors is quite valid. They do seem rather large.

The equipment mentioned, Seems very similar to a few jobs I have been on, Where the Synchronous motor starting and field control is done in one cubical and the winder drive Generator field control done in a second cubical located right beside the first.

Usually Resistors are also included in the line feeding the generator field exciter packages and in some cases in series or parallel with the field exciter outputs.

So it is possible for the total control to include quite a few fairly large resistors.

Putting them on the roof in a large enclosure is common.

Only a close look at the wiring lables and comparing them to the schematics will give a definite answer to this.

Usually starting the Synchronous motors on these smaller MG Sets is done without the niceity of any form of soft start. we simply close the motor contactor and a measured time later we bang on the DC field. Usualy the power system on the industrial sites is not sensitive the voltage dips during the starts which usually only happen after a maintainence day. Otherwise they run all the time.

The fact that Mr Reactive is considering limiting the starting current in some way, suggests that something has changed from the originally designed installation or that the voltage dips have now become un-acceptable for some reason.

Tom