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Frequency Conversion Soft Starter


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#1 MaVericK

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Posted 28 November 2007 - 09:55 AM

Hello all,

I have just heard of a chinese firm which is using the principle of frequency conversion in their HV soft starters. Had never heard of it before and am very curious to know more. It seems that they have patented this technology and are planning to enter the International markets soon.

I could not get much information about this technology as there isnt any structured documentation.

I will share with what I have learnt so far:

The firm is Rong Xin Power Elelctronics Corp., Ltd. based in Liaoning, P.R. China.
Website: www.rxpe.com

The Soft Starter is called RHSS. And they call it as a ''3-stage frequency-conversion stepless voltage regulation soft starter for AC motor''.

The basic power circuitry is same as traditional SCR based starters with the complexity lying in the control circuit. The frequency is increased in steps from 12.5 Hz to 25 Hz and finally 50 Hz.

The procedure is: "first trigger 1/4f of the frequency waveform by means of of control method to realize 12.5 Hz Voltage regulation start, keep maximum starting torque at low starting current effectively.

After reaching 1/4 of full frequency repeat the steps for 1/2f and then for full triggering signal to reach full rotating speed."


Nothing is available as to how this is achieved. But I can deduce that they reduce frequency by blanking the SCR firing for suitable time periods to obtain 12.5 and 25 Hz.

Using this technology they claim to reduce the starting current to as low as 1.6 of full load current. They also claim to be able to realize "full starting torque which shall be 10 times of other starting ways and meet the working requirement of heavy loads."

Some product Specs which I got hold of:
Power: 130-20000kW
Rated Voltage: 3kV-10kV
Rotating direction: both forward and reverse
Protection: over current, starting over time etc.


After going through all this I still cant figure out why doesnt anyone else use this technology? The only reaon that comes to my mind is the harmonics problem. But since the starter will be bypassed during RUN, the starting harmonics might me acceptable to many customers.

Has anyone else heard of this technology? Please give your inputs and explain, if any, limitations of this method.

Thanks!



#2 marke

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Posted 28 November 2007 - 06:20 PM

Hello MaVericK

This is using a form of cycloconverter control to give subharmonic frequencies.
You will not get a "variable speed drive" type performance even though it initially sounds like you might.

The current drawn by the motor is a function of the slip and as you change to the next sub harmonic, there is still considerable slip, so the curent required to develop the torque is still very high.
For example if you are operating at 25Hz and have the rotor spinning at very close to 25 Hz, when you stp through to 50Hz, you have 25 Hz of slip and the current drawn by the motor is the same as with a standard soft starter when the rotor is at half speed. - this can not be avoided, so you will have the same current draw as the motor accelerates from 50% speed to 80% speed as you would normally have with a standard soft starter.
There may be some advantage in developing a high breakaway torque from zero speed, and in some instances, such as loaded conveyors, this may reduce the breakaway current, in most applications the current will be the same as with a normal soft starter.

One of the major problems with this form of cycloconverter technology, is that there are only 6 SCR used and at reduced frequencies, the current is a series of pulses which causes subharmonic flicker problems on the supply and the peaky currents can cause interference with the operation of other equipment and protection systems.

A number of manufacturers have offered this technique for low jog speeds etc on soft starters for many years.

Best regards,

#3 MaVericK

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Posted 09 December 2007 - 02:14 PM

QUOTE (marke @ Nov 28 2007, 11:50 PM) <{POST_SNAPBACK}>
Hello MaVericK

This is using a form of cycloconverter control to give subharmonic frequencies.

There may be some advantage in developing a high breakaway torque from zero speed, and in some instances, such as loaded conveyors, this may reduce the breakaway current, in most applications the current will be the same as with a normal soft starter.

One of the major problems with this form of cycloconverter technology, is that there are only 6 SCR used and at reduced frequencies, the current is a series of pulses which causes subharmonic flicker problems on the supply and the peaky currents can cause interference with the operation of other equipment and protection systems.

A number of manufacturers have offered this technique for low jog speeds etc on soft starters for many years.

Best regards,


Hello Marke,
Thanks for the detailed reply and sorry for the late response. I am not able to access this forum through my office network. It seems to be a proxy server problem and I do not see to any solution for it as of now.

Back to the topic, I would like to know how severe is this subharmonic filter problem? Is it bad enough for the product to be rejected outright? Because I feel that this system can pose a good challenge to the conventional thyristor based starters. And being a chinese make, I am sure that the prices will be very competitive. (atleast in the Indian market)

If a number of manufacturers have already been employing this technique then I do not understand how can the chinese people apply for a patent for this technology? They have given a reference no. on their website with other patent application details. http://www.rxpe.com/...amp;Nclassid=34
Excerpt:
The patent of utility model of Our Company –“Frequency-doubled step frequency conversion stepless voltage regulating soft starter” has been officially authorized by National Intellectual Property Bureau. The patent No.: ZL200620092263.0,The authorized announcement date: August 1, 2007.

Perhaps they have made slight modifications to the original technology? I am not too conversant with cyclo converters so forgive me if this question sounds lame.

Best regards,
MK


#4 jraef

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Posted 10 December 2007 - 06:38 PM

If you Google the term Cycloconverter Motor Starter you will see that several people have taken out US patents on the technology over the years, but nobody has ended up with a product on the market. The reason is, it's advantages don't really overcome the challenges to using it. The fqact that a Chinese company has applied for a Chinese patent means next to nothing, they can do that for ANY technology already in existance because from what I've heard, nobody else in the world recognizes Chinese patents anyway (at least not yet).

Allen Bradley uses the technology not only for it's slow-speed function on soft starters, but also as a form of braking (their "Smart Motor Braking" feature). I once witnessed a demonstration of it at an OEM for woodworking machinery. After engaging the brake, the noise from the motor was horrific. Then the Engineer looked at his watch and laughed about how many seconds it would be before his Accounting Manager came storming into the shop to complain. It seems that whenever he tested it, the extreme voltage disturbance it caused would trigger all of their UPS systems to start an automatic shutdown of their accounting computers as a protective precaution!
"He's not dead, he's just pinin' for the fjords!"




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