Synchronous motor starting

[hec64]

Does anyone knows where can I get information regarding how the synchronous motors starters works?

We have a 700 HP, 2300 volts synchronous motor connected to a water pump. We used to have a ball valve to open the valve after the motor started. A mechanical engineer came and remove the ball valve and put a hydraulic check valve that move slowly during start up and shutdown, thinking that this will be better than the ball valve, but he did not thought of the consequences on the starter, since the starter was not set up for starting under load. We call the manufacturer (Toshiba starter) but I was not there and the electricians just told me that they only aqdjusted a timer. But I believe that the excitation current needs to be increased. Is there any books or web site or anything that talk about this?

 

[waross]

A synchronous motor has a squirrel cage type damper winding to reduce speed oscilations around syncronous speed. It uses this squirrel cage winding to start as an induction motor.
A synchronous motor field is energised by a polorized field frequency relay or an electronic equivalent. This device monitors both the slip frequency and the phase angle between the power supply and the revolving field position. It applies the field when the slip frequency is low enough to indicate that the motor is close to operating speed and the rotor is in the correct rotational position relative to the rotating field of the main stator winding. Starting a synchronous motor is not a function of time.

The excitation current does not need to be increased.
If the motor is now starting under load don't worry about it.
Most of the starting acceleration is done on the damper winding and the field excitation is not connected until the motor is almost up to speed.
The timer is most likely a "failure to start" timer for motor protection. If so it would have to be adjusted to allow for the longer starting time under load.
respectfully

 

[aolalde]

The difference now is the load torque.
The pull-in torque, which is the maximum constant torque under which the motor pull into synchronism when the rated field is applied, is being reduced by the load demanded torque.
I suggest that the ball-valve be reinstalled behind the check valve and it will be opened only after the motor has synchronized.

 

[jraef]

The timer is most likely a "failure to start" timer for motor protection. If so it would have to be adjusted to allow for the longer starting time under load.

While this is true, I would be concerned that the additional load may not allow the motor to attain pull-in speed. If that is the case, extending the timer may run the risk of having it on the amortisseur winding for too long. Is there a problem with just waiting until the fie;d is applied before beginning to open the valve?

 

[waross]

Hi "J"
I am under the impression that an adjustment was made and now the motor starts. I may be wrong. If it is a "fail to start" relay, it will not apply the field, rather it will terminate the start cycle if the field is not applied within the time setting. If so, it may have to be adjusted to allow a longer starting time.

Is there a problem with just waiting until the field is applied before beginning to open the valve?
Yes there is. The original valve was changed to a check valve that opens automatically as the motor starts.
Respectfully

 

[jraef]

Yes I interpreted that it had worked as well, but I thought it a risky move, one that I would not have undertaken so lightly as it appeared it had been.

I still think it would not have been terribly difficult to just start the valve opening sequence AFTER the FA relay had been pulled in. That would ensure that there was less risk of incomplete sequence. However if it was a purely hydraulic valve, as opposed to one initiated via electric solenoid, then I suppose it could not be adapted as such.

Another problem to my mind in just extending the incomplete sequence timer is that although it might work today, it might not under some future change in conditions, i.e. a change in the dynamic head.

 

[LionelHutz]

If there is any adjustment that you can make to slow the opening of the valve down it may help depending on how the motor is starting. Try to make it open slow enough that the motor sychronizes before much of the load is applied.

To answer your other question - no, you don't need to adjust the field since the field current would have already been set to the level required when the motor is loaded.

 

[hec64]

Thanks everybody for your response.

Lionhutz, you are right, the field current is not the problem. The problem is that the MOVs are blowing up, because during start up the motor is under load.We will change the MOV to a larger size. We cannot put back the ball valve. That will be more costly. The "hydro-pneumatic check valve" was installed due to a surge analysis study provided by the hydraulic engineer since these ball valve were creating a very large surge problem in the system, when there is a power lost at the pump station. That is why they were changed in the first place.

 

[LionelHutz]

What MOVs? There should not be any MOVs in the circuit that would be overloaded by an extended start. The first component that should fail is the motor rotor.

 

[oftenlost]

There are motor exciters from the sixties (we used to manufacture them) which utilize thyrites to shunt the induced rotor currents during the start cycle to protect the rotor. Now, we monitor phase angle, frequency and the like but our offering after the knife switch was the thyrite.