Detecting inter-turn short circuits in rotor on wind turbine

[BigBill53]

Hello,
I have a wind turbine that is overheating on the rotor. The rotor consists of 60 separate electromagnetic coils, all connected in series. There is a separate DC circuit used to excite the coils.

I have other machines that don't get hot, and they are using much less current to excite the coils at similar wind speeds and current outputs.

To me it looks like there are inter-turn short circuits in the coils in the machine that is overheating. Measurements taken of the entire coil circuit when stationary show the resistance is about 7ohms compared to 10ohms on the good machines.

I want to know which coils are damaged and which ones are not.

I thought about getting a micro-ohmeter and measuring each coil. A normal coil at DC should be about 0.17ohms. But who knows when the thing is being rotated and heats up? A colleague got hold of a henry meter and used this to measure the henry's of the coils, but most of the coils came back as zero. Measuring the whole coil didn't add up to the sum of the individual coils. The data just wasn't conclusive and I suspect something was wrong.

Does anybody have any advice? I'm really scratching my head here.

 

[waross]

Put a Dc voltage across the poles and measure the voltage drop across each pole.
If you are still not sure put AC across the poles and repeat the test.
On DC impedance equals resistance.
On AC the impedance will change much more than the resistance with shorted turns.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[beyond86]

Try to measure the excitation curve (dependence U from I) of each coil. Put AC current at each coil and then rise this current and measure voltage. The excitation curve of the damaged coil will be lower.
The same principle uses for CT and it's a good way to detect inter-tern short circuits.

 

[Gr8blu]

BigBill53: Are you SURE the rotor uses DC excitation, and that ALL the coils are connected in series?
Far more common to have an AC stator winding (connected via a frequency converter to the grid), with a power take-off on the grid side of the converter to supply AC to another (smaller) frequency converter that feeds AC to the rotor winding to maintain optimum power transfer at the grid side. This allows the generator to produce usable power over a very wide blade speed range (or wind speed, if you prefer) - typical ranges are 5-20 m/s wind speed as an example with "synchronous" speed occurring around 14 m/s.
If you do actually have a DC rotor circuit, the thing must be a fairly large diameter. 60 coils = 60 poles, which would typically result in roughly 10 feet (3 m) at the rotor OD. If it is a DC rotor winding, try the DC drop (and then the AC drop to "fine tune") test proposed by waross. If things are working normally, all individual pole drops should be within about 5% of the (voltage applied / number poles tested).
Just further thinking - you did check that there wasn't something mechanically wrong with the coolant circuit, causing you not to remove enough heat? Like a blocked filter or duct, or maybe a fan running in reverse? Or a blocked tubes on an air (or water) heat exchanger so the secondary coolant cannot flow?

Converting energy to motion for more than half a century

 

[edison123]

Lower resistance, higher current, hotter machine = Interturn short in the field coils.

You may have to bring down the machine from heaven to a repair shop/OEM on earth to do further tests and repair.

Muthu

http://www.edison.co.in

 

[petronila]

Hi Bill,

If your rotor is drawing higher amps than the rated current my suggestion is to check first the excitation values, high current may come from overexcitation. With 60 coils in the rotor (field), this is one is a very-low speed machine (120 rpm at 60 Hz ) and a poor refrigeration may cause this overheating so adding vent blades to the rotor may help. Another factor may be synchronicity loss.

If you suspect the issue is overcoming from short-circuited coils my suggestion is to test the rotor using an AC Voltage Drop Test. To do this, the machine should be idle with the rotor disconnected. Apply the rated field voltage between F1 and F2 (AC Voltage) and take readings of the voltage drop in each coil, thus mind that you may need to remove the insulation between each coil connected in series to accomplish this test (do not forget to reinsulate if all results are OK). When you have completed all readings calculate the average (60 readings/60) and compare each reading with that average.

Any deviation of 10 % should be investigated.

Petronila