induced voltage synchronous motor

[freetown]

Hi all,

A synchronous motor 11 kV and 2.8 MW (DOL contactor) employs a series impedance (inductor) to limit the starting current during startup. After a few seconds, a bypass contactor is activated to bypass the impedance. The DC excitation is applied following the bypass.

A manufacturer asks what is the induced voltage before startup/excitation and how this voltage is obtained.

In my case, is the induced voltage equal to the nominal voltage of 11 kV?

 

[waross]

By induced voltage do you mean Back EMF?
I would estimate the back EMF as line with frequency or speed.
Zero back EMF at zero speed and 100% back EMF, (Close to applied EMF) at 100% of synchronous speed.

 

[Gr8blu]

Voltage induced where:
In the stator winding?
In the rotor field winding?
In the rotor cage winding?

Which manufacturer is asking: the motor guy or the contactor guy? Or the excitation guy (who may also be the motor guy)?

 

[LionelHutz]

There won't be close to rated voltage back EMF even at full speed unless the field is applied. Then, the voltage can be higher than the rated voltage.

 

[waross]

Even acting as an induction motor on the damper winding? Not contradicting you, I am asking.

 

[freetown]

Voltage induced where:
In the stator winding?
In the rotor field winding?
In the rotor cage winding?

Which manufacturer is asking: the motor guy or the contactor guy? Or the excitation guy (who may also be the motor guy)?

The amplitude of the voltage on the stator winding before the bypass contactor closes.
Which manufacturer is asking: The motor guy

 

[edison123]

It's a 11 KV motor. When you bypass series impedance, the stator will see 11 KV. As a motor guy, he should know it. Even allowing for switching impulse voltage, a new 11 KV motor should be able to withstand 23 KV per standards.

 

[waross]

Which manufacturer is asking: The motor guy

If the motor guy does not know the decay rate of the residual magnetism in his own motor.........
On the other hand:

A manufacturer asks what is the induced voltage before startup

Before startup the induced voltage and the applied voltage are both zero.
BUT
If the manufacturer wants to know the voltage when the bypass contactor is about to be closed, you will have to measure it at the motor terminals.
The voltage across the series impedance and across the motor windings will not sum to the applied voltage due to different power factors.
You can not just subtract the inductor voltage from the applied voltage and get an accurate result.
That said, the error will be repeatable and the voltage across the impedance may be used for control of the bypass contactor.

 

[waross]

The amplitude of the voltage on the stator winding before the bypass contactor closes.
Which manufacturer is asking: The motor guy

To find that accurately it should be measured at the motor terminals.
Subtracting the voltage across the series impedance to find the motor voltage will not give an accurate value.
The motor and the impedance will be at different power factors and the voltages across each will not sum to the applied voltage.
However, once an operating point is determined for closing the bypass contactor, the voltage across the impedance may be used for the control of the bypass contactor.

 

[edison123]

The voltage across the series impedance and across the motor windings will not sum to the applied voltage due to different power factors.

V = IZ. Power factor does not play a role at all.

 

[freetown]

If the motor guy does not know the decay rate of the residual magnetism in his own motor.........
On the other hand:

The motor is somewhat old (from the 80s), and the manufacturer has limited information on it due to the departure of several staff members.
What method do you recommend as the best for measuring voltage at the motor terminals during start-up?

 

[waross]

V = IZ. Power factor does not play a role at all.

The impedance is in series with the motor windings.
If the power factor of the impedance is different than the power factor of the motor windings, the value of the series impedance can not be added arithmetically to the impedance of the motor windings.
The sum of the voltage drops across the starting impedance and across the motor impedance will not sum to the applied voltage.
The voltage at the motor windings must be measured. This does not mean that the measurement must take place at the motor junction box.
The measurement may be taken at the motor side of the shorting contactor.