Two voltage sources at motor terminals

[Elecme]

Consider that we have a large 415V induction motor fed from a utility power supply and backed up by a generator. Also an ATS is available for transferring the motor to be connected to the generator supply in case of normal power supply failed.
Suppose that the normal power supply failed and the motor is still rotating because of its inertia, which in turns provides voltage at its terminals; at the same time, the generator is working supplying power to the motor terminal without any delay; so we are having two sources of voltage at the motor terminal of the motor, which may be a case of out of synchronization. Do we add, vectorially, the two voltages or what?
I know this is not an actual case, as the generator will take some time until starting up and picking the load.
Elecme

 

[Bung]

Yes, you would add the voltages vectorially. And yes, a scenario such as you describe is possible, and if the timing is wrong, potentially damaging. It is probably more of an issue with a synchronous motor, as it will maintain its terminal voltage longer than a squirrel cage motor. Best is to have very fast (<1 cycle) change over, or wait a couple of seconds then change over. but that is on the electrical side - you need to pay some attention to the mechanical properties of the loads - inertia, transient torques on shafts, etc also so you don't do any damage there too.

Bung

 

[electricpete]

This has come up before. As bung said, damage can be prevented by ensuring the duration of the interruption of power is short or long (assuming that both transferred sources of power are initially in phase).

Here are my notes on that subject of the typical scenario where both sources of power are initially in phase:

NEMA MG-1-1998R1, for large motors - section 20.34.1

&quot;Slow Transfer Reclosing - power interrupted for >1.5 times the &quot;open circuit ac time constant&quot;. This is the recommended approach since it allows residual flux/voltage to decay to acceptable levels.

&quot;Fast Transfer Reclosing - power interrupted for <0.5 times the open circuit ac time constant&quot;. Voltage should not have drifted that far apart by this time.

&quot;A fast transfer or reclosing is one which occurs within a time period shorter than 0.5 open circuit ac time constants. In such cases transfer or reclosure should be timed to occur when the difference in motor residual voltage and frequency and the incoming system voltage and frequency will not result in damaging transients...... it is recommended that the electromechanical interactions of the motor, driven equipment, and power system be studied for any system where fast transfer or reclosure is used&quot;

NEMA MG1 Section 1.60.1 - &quot;Open Circuit AC Time constant
= (Xm+X2)/(2pi*f*r2) where Xm, X2, r2 are equivalent circuit parameters (per-phase). f =power frequency.” [My note – this is the L/R time constant that you would expect if you looked at the equivalent circuit loop through R2, X2, Xm)]

EPRI recommendations suggest typical induction motor time constant is approx 6 cycles (i.e. preferred interruption duration for that typical motor would be <3 cycles or > 10 cycles).

Connected capacitance will significantly lengthen the time constants.

A generally accepted criterion (ref ANSI C50.41-2000 section 14.1) is that the vector difference between incoming and outgoing voltage should not exceed 1.33 p.u.

Time delay relay, undervoltage relay, or sync check relay might be applied for protection.

In your particular case with a generator starting up completely unsynced to the original power supply, than the fast-transfer method is not an option.