a 1000 HP propeller electrical motor 2

[181273]

Hello, I got a problem on a propeller electrical motor, the name plates of the problem are the next:
-1000 HP
- Vertical motor, it´s posible to turn it by hand.
- 780 rpm 60 hz 440 V 1260 A full load.
- The propeller is a pitch control one, but when we start the motor is on the 0 position.

Now i´ll explain the way it start, the customer used a generator runing on its nominal speed, but change the excitation of the generator doing a soft start on the motor.
It have been working till one month ago, and now when you start the motor you see:
- with a voltage of 80 V a current of 800 A, if you increase the voltage to 90 V the current go to 1180, and the speed is 180 rpm.
What do you think, I was thinking about the squirrel cage of the rotor.... but... i discoupled the flanges of the rotors and try to start and the motor start in two seconds aprox. with a peak current of 1450 A but it decrease very fast ultil 92 A. and the voltage on the generator rise from 80 to 150 V, i think due to the less current the generator have to feed.
I think if you put 1180 A during 3 minutes on a motor, with the nominal frecuency its speed will grow up more or less fast depending on the load, but always grow up, ...
please, i wait for your always nice commentaries
thanks in advance

 

[waross]

Check your gear box and your variable pitch settings. This could be an indication that the load on the motor during starting has increased. It may be the gearbox or bearings but I doubt it. If there was enough drag to have this affect on starting there may shortly be catastrophic failure.
Most likely your variable pitch has a problem and the propeller is not returning to the proper position for starting. Has the variable pitch been worked on lately?
respectfully

 

[jraef]

You state what the reduced voltage starting current looks like now when it appears bad, but you don't say what it was when it worked correctly. Do you have that information? Also, can we assume you have done all the other basic electrical tests as well, such as putting a megger on the windings and ensuring that all 3 phases are puling equal current? One concern I have is that you were starting the voltage off too low. When reduced voltage starting is used, starting at too low of a voltage is actually harmfull to the motor. Between zero and some voltage level, the motor will not create enough torque to even begin rotating, so it will remain in a "locked rotor" condition. Without rotation and the resultant impedence, current will rise rapidly yet do no useful work, but will result in heating the windings. That can cause the insulation to break down faster than you would expect. A typical initial voltage applied to bow thruster motors that I have done with solid state soft starters is 40% of nominal, so in your case, 176V. The problem now may be that the damage has been done and you have a short in your windings, which will give you higher current with less torque (up until the point of catastrophic failure of course). A megger test would find that.

Make sure you test everything electrical first. If so, I would tend to agree with waross. If you eliminate electrical problems it leaves only mechanical.

 

[aolalde]

The motor torque drops with the squared ratio of voltage reduction and for 1000 HP 8 poles the starting torque at full voltage is only 60% of the full load torque.
Based on that the torque at 80 volts is only 2% of the full load torque and 2.5% with 90 Volts.
More Voltage is required after the motor starts moving to achieve acceleration to full speed,in spite of the propeller pitch at 0 position, some water is being desplazed increasing the resistant (load) torque with the squared ratio of the speed increase. The voltage needs gradual increase as the speed of the motor is progressing.

 

[181273]

thanks you all of you, about the basic measurements as megger and equality of the currents, all this have been done....
I have to say something, in example for jraef, i agree if you don´t use enough voltage, the rotor stay in the locked position, but...the rotor start to turn.... going from 0 rpm to 180 rpm, if you could start the rotation the worst situation, i think, you could go more or less slowly to nominal speed.. don´t you think?.
I don´t know exactly how it works before this problem, but I think same as i do without coupling to the shaft of the propeller...I takes a high current during less than one second and go to nominal speed, and the current remains then in 92 A per phase(the same on three phases) with a voltage of 150 V, that means that the generator internal voltage is 150 V but cause there´s low excitation it falls to 80 V.
Aoalde, i´m not so sure about what do you say, that in a 1000 hp the torque is 60 %, normally is more than 100% starting torque at full voltage, isn´t it?, could you explain this?. And yes, it´s true the voltage rise when the speed go ups as the current go down..(when you start without propeller) but the problem is that speed don´t go far away of 180 rpm, and the current is high....
Could the probem be on the squirrel cage? could this squirrel cage be damaged by a low voltage startings working?
my best to all of you..... i wait you here on spain to drink one cold beer.... thanks