Start-up the synchronous motor with brushless excitation

[neluvel]

We have 6 synchronous motors (5000kW) with brushless excitation.
We have used a scheme with 6 diodes (for normal operation), 2 thyristors, 1 diode, 1 power resistor and 1 electronic module, which control the rotor frequency (for start-up in asynchronous regime ~ 5 sec.). In the 5 sec. the resistor is connected through 1 thyristor and 1 diode. When the frequency is 2.5Hz the electronic module disconnect the resistor and connect the bridge with 6 diodes.
This is a normal operation, but sometimes the resistor is connected at D.C. voltage from diodes bridge and it become very hot.
We have interesting about a method to prevent this abnormal situation.

 

[Shortstub]

How about a little more info!

Has problem persisted since commissioning?

Motor parameters including manufacturer?

Supply parameters?

Load characteristics?

 

[swgrmfg]

neluvel:

You said that this is a brushless exciter.... therefore, all you need is a full wave, single phase bridge rectifier (4 diodes) and a manually operated autotransformer type variac. You can apply the field of the brushless exciter as soon as you start the motor. The internal optimum phase angle application module installed on the rotor does the rest. You then vary the field via the variac according to the desired pf objective. Unles your motor is not equipped with an internal field application module, you don't need none of the hardware you have described. Simplicity is the key! (100+ units installed this way by author).

 

[neluvel]

I give you more information about the principal scheme which I have used.

At start-up the field has an alternating voltage causing to flow first through SCR-T2 and the discharge resistor. On the next half cycle, current flows through the diode D7 and discharge resistor. The control circuit waits until the frequency drops to the preset value (1.5Hz,...2.5Hz), indicate the rotor is at an adequate speed. Then, after a North pole on the stator is in the right position to be attracted to what will be a South pole on the rotor, it triggers SCR-T1 to apply excitation (negative half wave applied at control circuit, 1-4 terminals). If the rotor does not synchronize, it will slip a pole; the induced field voltage will oppose the exciter voltage causing the current to go to zero, turning SCR-T1 off SCR-T2 is turned on only at a voltage higher than the exciter voltage so it will not be on when SCR-T1 is on.
This works well but sometimes, I don't know when, the SCR-T1 and SCR-T2 became conductive and the discharge resistor become very hot (it is powered from DC voltage). I have seen in another scheme a supplementary module which senses a continuous voltage across the discharge resistor and another thyristor provides a turn-off circuit for SCR-T1 in case where a transient disturbance turns on the discharge resistor during normal operation.
I haven't this module and I don't know how can be it constructed.
An idea is the continuous control of the resistor temperature with infrared Non contact temperature measurement Sensors.
Another idea is for utilization slip rigs without brushes (perhaps with mercury) for control the resistor voltage. I know there is a firm which produce these slip rings but I don't remember who can be.
After these observations I want know your opinion.
I thank you very much and I apologize to my english.

 

[jbartos]

Suggestion: Please, would you justify your outlined solutions? If you have a resistor that is designed in a circuit with the resistance R to perform its function, then what is the use of controlling the voltage across the resistor? It appears that the proper solution would be the correct resistor power rating in Watts. The possibility of something improperly designed or malfunctions elsewhere exist.

 

[Shortstub]

Typically there are two resistor/thyristor configurations: the first is where the are in series; the other where they are in parallel.

The first is used for the field-discharge function; the second adds additional resistance to the field circuit, during startup. During the run-mode the first resistor is disconnected by its thyristor, while for the second, the resistor is short-circuited by its thyristor. In neither case should current flow thru the resistor after having reached full-speed. Are you sure the thyristors are operating as intended?

 

[neluvel]

Thank you all.