PolyMike
Electrical
- Nov 14, 2008
- 9
Synchronous motor 3800hp 6300V 259A 50hz 250rpm P.F.=1 brushless type. Exciter field=7.1Amps Exciter output =144.5V 135Amps. 2 different controlled rectifier bridges (for auto/hand PF control mode) feeding the exciter field.
The motor load is a reciprocating compressor. Suction and discharge are by-passed before starting up in order to carry out an almost no-load start.
The motor starts up and after 5-6 sec. and it seems to be synchronized, but with the line current too high and the power factor too low (0.6 lagging). Either hand or auto mode, even great changes in the exciter field current cause a negligible reaction in PF and line current. A few seconds later (20-30), trips the Out Of Step and the Locked Rotor Relays.
The exciter field circuit had been tested (insulation and resistance) and the values seems to be O.K. We have make sure all the current injected by the excitation panel (both auto/hand mode) reach the exciter field by means of an DC clamp meter on the exciter.
The insulation and resistance of the rotor has been tested. Amortisseur winding has been inspected. It has been tested all the diodes, and thyiristors and capacitors of the rotating bridge.
Even more, by means of a three phase autotransformer, 107Vac has been injected to the bridge, and the output measured is exactly 144.5Vcd (for three phase full wave rectifier bridges Vcd=1.35Vac_rms). Of course, previously the exciter and rotor connections were removed.
Resistance and insulation tests had been carried out in the exciter field circuit, the rotor circuit of the exciter, rotor and stator of the motor; all of these values seemed to be O.K.
The only suspect measure was the resistance unbalance of the exciter rotor winding: 0.69mohm, 0.39mohm and 0.42mohm.
Taking into account all the measures and tests, our best hypothesis is that a failure on the exciter rotor winding (probably short-circuit between coils) causes a voltage drop on the rotating rectifier input. So, after startup, the motor is so underexcited that causes the Out of Step Relay trip. But as I’m not pretty sure, I will truly appreciate all your comments or suggestions.
Thank you in advance,
Mike.
The motor load is a reciprocating compressor. Suction and discharge are by-passed before starting up in order to carry out an almost no-load start.
The motor starts up and after 5-6 sec. and it seems to be synchronized, but with the line current too high and the power factor too low (0.6 lagging). Either hand or auto mode, even great changes in the exciter field current cause a negligible reaction in PF and line current. A few seconds later (20-30), trips the Out Of Step and the Locked Rotor Relays.
The exciter field circuit had been tested (insulation and resistance) and the values seems to be O.K. We have make sure all the current injected by the excitation panel (both auto/hand mode) reach the exciter field by means of an DC clamp meter on the exciter.
The insulation and resistance of the rotor has been tested. Amortisseur winding has been inspected. It has been tested all the diodes, and thyiristors and capacitors of the rotating bridge.
Even more, by means of a three phase autotransformer, 107Vac has been injected to the bridge, and the output measured is exactly 144.5Vcd (for three phase full wave rectifier bridges Vcd=1.35Vac_rms). Of course, previously the exciter and rotor connections were removed.
Resistance and insulation tests had been carried out in the exciter field circuit, the rotor circuit of the exciter, rotor and stator of the motor; all of these values seemed to be O.K.
The only suspect measure was the resistance unbalance of the exciter rotor winding: 0.69mohm, 0.39mohm and 0.42mohm.
Taking into account all the measures and tests, our best hypothesis is that a failure on the exciter rotor winding (probably short-circuit between coils) causes a voltage drop on the rotating rectifier input. So, after startup, the motor is so underexcited that causes the Out of Step Relay trip. But as I’m not pretty sure, I will truly appreciate all your comments or suggestions.
Thank you in advance,
Mike.