High Frequency Generator 5

[BoyNaruto]

Gentlemen,

Do anybody here uses a High Frequency Generator (combination of a driver motor and a generator)? The mains as described in the motor plate is 380V/50Hz 33A, Exciter Field is 380V 15A, and the output rotor voltage is 135V 200Hz, 22KVA. This high freq. generator is used to power our Bosch High Frequency Angle Grinders in our Fabrication shop. The problem is we have 60Hz here in our location and the output freq. is reaching up to 230Hz. This makes our grinders hotter and overheating at prolonged time of use. Do you have any suggestions on how can I control the output frequency down to 200Hz? Thanks guys.

 

[BoyNaruto]

Another Photo showing motor-stator-rotor slip rings.....

 

[edison123]

Bill, may be you could into those treasure troves of yours and post those schemes here ? This sure is interesting.

Muthu

http://www.edison.co.in

 

[BoyNaruto]

Hi edison123 and Bill,

Attached is the old documentation which has guided me to design the control gear for the generator....

 

[djs]

Supply both the motor and the excitation from the VFD.

 

[waross]

Hi Edison;
I did not find the exact circuit but I something close enough to make a good guess at the operation. If you use another motor to turn a wound rotor motor at synchronous speed and then apply an alternating voltage to the rotor, you will either get zero volts or a voltage at 2X frequency from the main leads of the wound rotor motor. A two pole motor driving a six pole WRIM should develop 200 Hz.
I'll look for more info this evening.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jcarrr]

How many of these grinders are involved? Maybe smaller individual drives are the best solution.

I would think again about using one drive. We use often use a single drive to power more than motor. Some drive manuals speak to this by instructing you to start the largest loads first and then the smaller ones.

 

[BoyNaruto]

Hi jcarr,

We have two sets of this HF generator, and each one should handle at least 18pcs for small angle grinders and at least 10 for the big ones. My boss preferred to use this HF grinders because there are no consumables since they are brushless.

But I am faced with this problem operating the HF gen on a 60Hz line. My output frequency goes as high as 230Hz, 30Hz higher than the designed specifications for the grinders.

Resins inside the field windings sometimes melt, and the only solution I have is to open up the grinders affected, take out the melted pieces of resin, clean a bit and then back to business. Winding insulations are a bit tough though and the bearings are superbly strong compared to our metabo grinders.

 

[waross]

It does look like this is an AC excited machine. djs's suggestion to run both the drive motor and the excitation from the VFD is starting to look good. Hopefully when the VFD does its self tuning it will see the combined motor/field as a motor taking a little more than usual magnetizing current.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

BoyNaruto

Looks like AC exciter. I don't profess to know how that system works (Bill will educate us, hopefully). I will go back to my books on that.

Since both the motor stator and generator exciter stator are 380 V, 50 Hz, as djs noted above, the same VFD can feed both the motor and generator. Just size the VFD to handle the 33+15 = 48 Amps. A 60 Amps VFD should do fine.

And yes, both the motor and the generator can be fed simultaneously so that both the motor speed and the generator output voltage will raise together. You may need two different overcurrent protections though - one for the motor and one for the generator exciter. Each one of them should trip the VFD in case of overcurrent.

Muthu

http://www.edison.co.in

 

[edison123]

Bill, talk about great minds thinking alike at the same time. 2thumbsup

Muthu

http://www.edison.co.in

 

[edison123]

BoyNaruto - A LPS for posting an interesting problem.

Bill - You probably don't need one, but the what the heck. I am going back to my books to understand how this damn machine works.

Muthu

http://www.edison.co.in

 

[waross]

A star also for djs.
Look for the term "concatenation".
Do you have a test bench edison123? Can you spin up a wound rotor induction motor to 2x or 3x rated speed and apply AC to the rotor and measure the frequency on the slip rings? A normal WRIM wil not be optimzed forconcatenation so keep the applied voltage low. We just want to see the frequency at the slip rings.
Yours
Bill
ps: Is Muthu your name or a salutation in a language that I don't speak? grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BoyNaruto]

Hi Edison123 and Bill,

Thanks a lot for sharing your expertise. That was my first idea before, to put a VFD on the motor and exciter, but I was also in doubt, maybe there are other cheaper solution. Thanks to this HF Gen, that I have known masters in this field.

I agree, this generator is a very interesting one. With just a little less than 60A input, I can power up to 22KVA capacity of 3-phase angle grinders for our whole fabrication shop. Good thing also is that there are no carbon brush replacements.

One thing though is that I am here in a 60Hz area. Thanks again gurus.

 

[edison123]

Bill - Yes, Muthu is my name (you don't wanna know my full name) and yes, it's the 2nd oldest language in the world.

I am planning to do the set-up you suggested in my shop though it might take some time. What I am guessing is this motor is 2 pole and the generator (rather its' an induction motor) stator & rotor are wound for 8 pole to get the 200 Hz. I would use a 2 pole motor and 4 pole "generator" to see if I get 100 Hz.

djs - my bad. A lps for you too.



Muthu

http://www.edison.co.in

 

[Compositepro]

Has anyone considered the possibility that the grinders are simply being overloaded by the operators? This can occur at any speed. In hand-held tools the operator can tell how hot they are and must be responsible for not overloading them.

 

[BoyNaruto]

Hi Compositepro,

That is also one possibility, but we are using 2 types here in the fabrication shop, one is metabo equipped with overheat and speed control, one is these bosch high frequency grinders. The specification is limited to 200Hz only, but the result of the operation via 60Hz causes 230Hz++, this causes the overheating.

Most of the operators uses thick safety gloves, before they could feel the heat on the grinders, the internal too hot melting the resins. On this part, I cannot really blame the operators.

Hi Edison, in my pursuit to look for other types of HF grinders, I found one working with 300Hz, but the operating voltage is very low, only 72 volts? Surely there are also HF gen for this....another mind boggling pole combination.


I was hoping to find at least 140V/250Hz grinders in the product of Bosch but no where near.

 

[edison123]

BoyNaruto

The generator rotor has 4 sliprings. Are they for 3 phase and neutral ? Are your grinders single phase or three phase ?

Muthu

http://www.edison.co.in

 

[BoyNaruto]

Hi edison123,

Yes the rotor side has 4 slip rings, 3P+N. For the Bosch grinders yes, they are also 3P+E.

 

[panter]

Hello everyone,
I have similar frequency converter but , luckily, I live in 50Hz land and I did not have problem with higher frequency than 200Hz. This generator is situated in one old testing station and it is used for testing transformers of higher voltages and power consumption and rotating machines .
BoyNaruto maybe you can provide slipring motor with 4 pole and then you should have * only* 180Hz .
Good luck .

 

[BoyNaruto]

Hi Panter,

Thanks, good to know I'm not the only one using this HFG... Guess I am stuck with this problem if I cannot justify enough to buy a 20KVA VFD....