2MVA generator

[Emad Shaaban]

We’ve just completed a bearing replacement for a 4-pole, 50 Hz, 2 MVA synchronous generator. Before full commissioning, we urgently need clarification on the following points:

1. Minimum Speed for Voltage Generation:
   What is the minimum rotor speed at which we can expect the generator to start generating measurable voltage at the terminals?
2. Testing Generator Without Prime Mover (Engine):
   If the engine is not available, is there any alternative method to test the generator’s excitation and basic voltage generation capability?
3. Polarity of Rotor Excitation Wires:
   If we accidentally reversed the polarity of the wires connecting the rotor winding to the rotating rectifier, how can we test or verify the correct polarity before starting the unit?

 

[waross]

If we accidentally reversed the polarity of the wires connecting the rotor winding to the rotating rectifier, how can we test or verify the correct polarity before starting the unit?

Attach a small power supply to the rotor leads.
Check the residual magnetism of the field poles.
Slowly apply a small voltage to the field.
I would start at less than one Volt.
Slowly increase the voltage in small steps.
At each step, check the residual with a hacksaw blade or something similar.
If you find the residual magnetism reducing, your polarity is reversed.
But, I wouldn't bother.
If the polarity is reversed you may have to "flash" the exciter field to start generating.
But, after a rotor has been removed from the stator you may need to flash the exciter even with the correct polarity.
I don't see a problem with reversed polarity.

Minimum Speed for Voltage Generation:
What is the minimum rotor speed at which we can expect the generator to start generating measurable voltage at the terminals?

Not much.
Anecdotally, a rotation indicator for an induction motor may excite one winding with one or two dry cells. (1.5V or 3V)
The rotation indicator is connected to the motor and the rotor is given a spin by hand.
This will induce enough voltage in the other windings to detect the direction of rotation.
Residual magnetism will generate a small voltage as soon as the rotor starts to turn.
With the exciter field energized, the exciter will start to generate as soon as it is not stationary.
It shouldn't take much rotational speed to generate the first detectable voltage.
A quick and dirty linear estimate of an effect that may not be linear, but serves as an illustration.
1500 RPM = 100% voltage
150 RPM = 10% voltage
15 RPM = 1% voltage
1.5 RPM = 0.1% voltage.
The relationship may not be linear and depending on the current in the exciter field may be greater than the quick and dirty estimate.
This does give you a range to expect, give or take a factor of two.
eg: 15 RPM may develop less than 1% voltage and as much as 2% voltage give or take.

If the engine is not available, is there any alternative method to test the generator’s excitation and basic voltage generation capability?

I hope that this is a two bearing machine.
Use a pony motor to spin it up to 1500 RPM.
How large a pony motor?
Enough to supply the no load losses.
As a rough guess, a machine that size with 4% loss may take over 100 HP, but find your own figures and do your own calculations.
You may will need extra HP to accelerate the large machine.
A VFD on the pony motor is highly recommended for starting.

 

[edison123]

Changing the polarity of rotor field winding has zero effect on the synch generator/motor output.

If you changed only the bearings, why do you need to test the generator without the prime mover?

 

[waross]

Changing the polarity of rotor field winding has zero effect on the synch generator/motor output.

I agree Muthu. No difference electrically.
However it may be that someone in the chain of responsibility, possibly an MBA, is erring on the side of caution.
At times it may be quicker and easier, and a better career move, to just do a test, however needless.

 

[wcaseyharman]

A couple of thoughts:
1. Since V = d_phi/dt, voltage should be linear with speed with a fixed magnetic field. What voltage do you need?

2. That sounds complicated since you’d generally need to rotate the machine - shiv and belt drive with a pony motor? Is this a bust fed excitation system? We’ve tested firing of SCR bridges with lower voltage and a dummy resistor. You can also use the field if you keep the current down and the time short.

3. Field polarity generally doesn’t matter, unless it is at the exciter and the field ground relay is affected.
I’ve been told that some plants regularly swap field polarity - some brushes and slip ring wear more with one polarity versus another, or so it has been told me. Most of our machines don’t see a significant difference. I once helped specify a polarity swapping transfer switch, but the company sold the plant soon after and I never got to follow up on whether it made a difference.

Now if you have 3 phase voltage sensing on your voltage regulator, getting the phases right can be very important as we found out once.

 

[edison123]

In the rotor DC circuit, the negative slipring will wear out faster than the positive one due to electrochemical reaction. So, to maintain even wear of both sliprings, the polarities are swapped. This has zero effect on the synch generator output / synch motor direction of rotor.

For the same reason, in a DC machine a pair of brushes are staggered to avoid grooving in the commutator.

This OP just posts here without any engagement. Not worth it anymore.