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.

 

[edison123]

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

http://www.edison.co.in

 

[waross]

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

 

[edison123]

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

http://www.edison.co.in

 

[waross]

You're probably correct, edison123.
Yours
Bill

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

 

[roydm]

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

 

[edison123]

Good question roydm.

Muthu

http://www.edison.co.in

 

[waross]

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

 

[BoyNaruto]

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.

 

[edison123]

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

http://www.edison.co.in

 

[BoyNaruto]

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

 

[edison123]

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

http://www.edison.co.in

 

[BoyNaruto]

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

 

[edison123]

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

http://www.edison.co.in

 

[BoyNaruto]

Hi edison123,

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

 

[edison123]

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

Muthu

http://www.edison.co.in

 

[BoyNaruto]

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.

 

[edison123]

Do you have photos of the insides of the generator ?

Muthu

http://www.edison.co.in

 

[waross]

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

 

[BoyNaruto]

Stator Assembly Photo attached.....