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bob14

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Hi,

 

I am having a very hard time starting a fan, motor rating 132 k W 400 V, on a 635 kVA 900 Amps Genset. The motor is a squirrel cage one and the fan is started with resistance in series with the stator. The statoric resistance is a very old one with a series of buckets filled with electrolyte. Thre is no means to test the concentration of the electrolyte.

When the fan is started, the voltage on the genset pulled down to around 230 V before the genset is shutdown.But before shutting down, the current on the fan went from 500 Amps to 250 Amps in a few seconds and before ramping further down, the genset shut down.

The motor ran well with the fan uncoupled.

I am of opinion that the genset should be able to start a fan of this size under this mode of starting. In fact it was used to start under the same genset and under the same mode of starting.

I presume that the starting current, after the first few cycles, should be around 3-4 time the nominal current and this should not be a problem with the genset. I am of opinion that there is a problem with the genset thought the people on site do not agree with me.

Any advice.

 

Bob

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Hello Bob

 

If the genset is operating correctly, then I would expect that it should be able to start the motor with fan attached.

 

One of the negative issues, is that you are using a primary resistance starter.

The Genset has two major components, the engine and the alternator. The engine is loaded by power (KW) and the alternator is loaded by current (amps or KVA).

The major disadvantage of the primary resistance starter, is that it dissipates a lot of power, during start, in the primary resistors and this will load the engine and could cause it to slow down.

 

To start a large fan with an older motor, I would expect that you need to draw around 400% to 450% FLC for the duration required for the motor to accelerate to around 80%. The starter should then begin to step through to lower resistance values and then short circuit.

 

If the starter steps through to quicky, the current will be too high and the output voltage could drop.

 

If there is a problem with the excitation of the alternator, the voltage could begin to drop at less than rated current.

I assume that the alternator is self excited rather than externally excited. If one of the excitation diodes is faulty, the output current will be limited. Similarly, if there is a problem with the AVR, you could also have a limited output current.

 

From your description, it would appear that the alternator is current limiting at around 500 Amps rather than the 900 amp rating. This strongly suggests an alternator problem except that you need to be sure that the current is not folding back to 500Amps once it goes into a current limit mode. (this can be induced by a collapse in the excitation)

If the resistors are of the correct value, I would expect that the current should be in the order of 600 - 800 amps, rising as the electrolyte heats up. If the current is higher than this, add more dilution to the electrolyte or reduce the area in contact with the electrolyte.

If the volume of the resistors is too small, the start current wil rise too quickly and you will have a very high start current part way through the start.

 

Best regards,

Mark.

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Hello Mark,

 

Thanks for your expert academic reply. I did manage to start the fan by increasing the concentration of the electrolyte. However, I learnt from site that they did replace the genset two years ago because the engine shaft was broken. I presume the open transistion could be a serious cause of this breakdown.

 

Kindest regards.

 

Bob

 

 

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