Transient

[bob]

Hi,

 

I have been through Mark`s writings on the torque transient during the star to delta transition. This is now well understood and star/delta starter is seldom used these days. However, I would like to raise a point for tandem motors applications.It is a common practice to start the first using a convenient starting methods e.g liquid starter, soft starter and then DOL the second motor after the first one has picked up speed. Does the same analogy not apply here ?

 

Regards.

 

Bob

[marke]

Hello Bob

 

Not really, as you are not refluxing an out of synch field. When the motor is driven up to speed by some other means (such as another motor) then you are DOL switching at close to full speed.

The will be an inrush current for a very short time (cycles) to establish the magnetic field and then the current will drop down to the run current at full speed.

The inrush current will be similar to the inrush current when you DOL start at zero speed, and will be much less that the transient when you "open transition" switch the motor as is the case with the star delta starter.

 

Best regards,

Mark.

[bob]

Hello Mark,

 

Thank you. Got it ! But, I believe it`s true to state that running motors, particularly large ones, should be brought to rest before starting again, to allow the motor to "de-flux" before bringing back to operation.

 

Regards.

 

Bob

[marke]

Hello Bob

 

There is no need to wait for the motor to stop before re-energising it.

The generated voltage falls away as the flux in the rotor decays and although the rate of deceleration does seem to affect the rotor flux decay, it is essentially a function of the rotor time constant and this is commonly in the order of parts of a second rather than seconds. The high efficiency motors have a lower rotor resistance and so the rotor time constant will be longer, but a reclose delay of around a second will result in a transient not to different from a normal Direct On Line transient.

 

Best regards,

Mark.

[bob]

Dear Mark,

 

Ok. What do you think of very high inertia load and a 6,0 MW motor rating ?

 

Best regrds.

 

Bob

[marke]

Hello Bob

 

I would think that there would be little voltage generated after 1 - 2 seconds.

You could always monitor the voltage at the terminals and see how quickly it decays and that would give you the answer.

Note, the voltage does not need to decay to zero, just a significant drop from line voltage. If you reclose when the voltage is at 20%, there will be a small increase in inrush current relative to DOL but not significant.

 

Best regards,

Mark.

[bob]

Dear Mark,

 

OK.I was thinking that so long that the rotor current continues to flow and it thereby continues to create flux which induces voltage in the stator winding. I understand that if this voltage is low enough, it won`t be a problem to re-start the motor back again.

Now I come to my point. I have an application where I have two 6,0 MW motors running in tandem i.e the two motors are connected to the end of the shaft and share the common load. They are two slip ring motors. A similar application on another site had three motors breakdowns and it was stated that there is requirement to phase the two rotors so that the two motors have the same rotor phase alaignment. I believe that for synchronous motors, it stands right as there is no slip. This argument is questionable as far as asynchronous motors are concerned. Your opinion, please ?

 

Bob

[bob]

Hi Mark,

 

It`s just a remark that my last reply was not updated on the main page.

Best regards.

 

Bob

[marke]

Hello Bob

 

As you are using slip ring motors, the time constant of the rotor is determined by the rotor resistance.

You may want to do some experiments to see how quickly the rotor current decays with different rotor resistances in circuit.

 

there is requirement to phase the two rotors so that the two motors have the same rotor phase alaignment

This is true for synchronous motors, but not for induction motors. The synchronous motor operates at the synchronous speed and is sensitive to the phase angle between the rotating stator field and the rotor field.

The induction motor relies on slip to induce a current in the rotor so the phase ange relative to the rotor is constantly changing.

 

Best regards,

Mark.

[bob]

I can no other answer make, but, thanks, and thanks. ~William Shakespeare.

Thank you Mark.

 

Bob