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High Frequency Generator
5

High Frequency Generator

High Frequency Generator

(OP)
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.

RE: High Frequency Generator

Use a VFD for the motor to run it at 50 Hz. I presume this is a vertical machine with both the motor rotor and the generator rotor mounted on the same shaft ?

Muthu
www.edison.co.in

RE: High Frequency Generator

You may want to try upping the voltage on the grinders to
135V x 60 Hz/50 Hz = 162 Volts. At 60 Hz and 135 Volts your Volts per Hertz ratio will be off.
But the best solution is probably edison's suggestion, VFD and run the motor/generator at 50 Hz.

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

RE: High Frequency Generator

Bill

I don't think it's a V/Hz issue. The generator voltage, with a given excitation, will go up by 20% at 60 Hz (hopefully OP can confirm that).

I think it's the over-speed issue with their grinders with higher rate of material removal resulting in overload too. So, it's a both mechanical and electrical problem.

Muthu
www.edison.co.in

RE: High Frequency Generator

You're probably correct, edison123.
Yours
Bill

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

RE: High Frequency Generator

Would it not be simpler to do away with the motor/altenator and power the grinders directly from the VFD at 200 Hz?

RE: High Frequency Generator

You may have issues if not all the grinders are used all the time. VFDs want to know the parameters of the motor that they are driving. A variable number of motors or a number of motors with individual varying loads may not be the best application of a VFD.

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

RE: High Frequency Generator

(OP)
Hi Edison123,

I tried to suggest to use a VFD on the motor side, but its gonna be a big and expensive 15KVA VFD, for just grinders. I was thinking maybe I can control the motor speed via the exciter? The exciter side is only 15A, I was thinking I can use a 6KVA VFD for this and run it with a 50Hz, I don't have any idea on what will be the output, as I cannot find any formula for this type of generator.  

RE: High Frequency Generator

Hi BoyNaruto

The only way to change ac motor speed is by changing the frequency in your case and hence the need for VFD.

A 25 KVA VFD shouldn't be that expensive. May the VFD aces here will chip in about the price.  

Muthu
www.edison.co.in

RE: High Frequency Generator

(OP)
Hi Edison123,

The motor side has delta to star starter, If I use VFD then I will just connect to a star directly and control the start parameters via the VFD?

How about the exciter side, if I will switch this one after the motor has been stabilized to 50Hz, will it affect my programmed parameters for the motor side loadings?

Thanks in advance

RE: High Frequency Generator

You need to connect the motor permanently in delta in your terminal box and then connect the motor directly to VFD.

Your VFD must be programmed to output 380 V at 50 Hz. The exciter is used to control the output voltage of the 200 Hz generator but not its frequency. The VFD can start the motor with or without the exciter.

Muthu
www.edison.co.in

RE: High Frequency Generator

(OP)
Hi Edison123,

If I start the motor using the VFD (with 380V/50Hz settings), where will I get the supply for the exciter? From the mains 380V/60Hz? What can possibly happen to my output? Wish I can find any formula / voltage relations on this one. Thanks

RE: High Frequency Generator

If your exciter is a three phase bridge rectifier originally fed directly from 380 V, 50 Hz, then it should not have any problem being fed directly from 380 V, 60 Hz supply. As a matter of fact, its dc output will be better at 60 Hz due to lesser ripple content.

Do not feed your exciter from the VFD.

Muthu
www.edison.co.in

RE: High Frequency Generator

(OP)
Hi edison123,

My exciter is not DC, its also an AC three phase 380V field.

RE: High Frequency Generator

Please recheck. I don't think the excitation voltage and current are AC. Do you have a manual for this MG set ?

Muthu
www.edison.co.in

RE: High Frequency Generator

(OP)
Hi edison123,

It is indeed a 3-Phase 380V supply for the exciter field. In this case the exciter is stationary and not a moving coil. You can check the attached photo.

In the motor plate, it is termed as the STATOR. And my load side is coming from a brush from the ROTOR. I have to supply 380V/50Hz 15A on the Stator for the rotor to supply me 135V/200Hz 22KVA.

The motor (prime mover) is 380V/50Hz 33A.

RE: High Frequency Generator

Do you have photos of the insides of the generator ?

Muthu
www.edison.co.in

RE: High Frequency Generator

I don't have time to dig out my old (very old) text books, but there are several frequency multiplying schemes that utilize an AC field. It cuts down on the number of stator poles.

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

RE: High Frequency Generator

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

Muthu
www.edison.co.in

RE: High Frequency Generator

2
Supply both the motor and the excitation from the VFD.

RE: High Frequency Generator

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

RE: High Frequency Generator

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.

RE: High Frequency Generator

(OP)
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.

RE: High Frequency Generator

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

RE: High Frequency Generator

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
www.edison.co.in

RE: High Frequency Generator

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

Muthu
www.edison.co.in

RE: High Frequency Generator

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
www.edison.co.in

RE: High Frequency Generator

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

RE: High Frequency Generator

(OP)
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.

RE: High Frequency Generator

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
www.edison.co.in

RE: High Frequency Generator

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.

RE: High Frequency Generator

(OP)
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. :(
 

RE: High Frequency Generator

BoyNaruto

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

Muthu
www.edison.co.in

RE: High Frequency Generator

(OP)
Hi edison123,

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

RE: High Frequency Generator

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 .
 

RE: High Frequency Generator

(OP)
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....

RE: High Frequency Generator

You will be able to justify buying a VFD (or solid state power supply for the grinders) if you replace the machine set by it.

Efficiency of rotating machinery for generating high frequency is quite poor and you will be able to save a lot of operating cost.

RE: High Frequency Generator

On the other hand rotating plant is much more tolerant of overloads and short circuits than the electronic equivalent.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

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