Mv Drive Used As A Soft Starter

Hello all,

I have a rather peculiar doubt. Someone asked me whether it was possible to use an MV drive instead of a soft starter and obtain a soft start? The basic reason for this query is that he wants to use a de-rated MV drive and save on the costs. (the drive will be bypassed once start is obtained)

Some background of the application:

The load is a positive displacement pump. And it will always be started in no load condition. the rating is 11kV, 7.5 MW
Motor GD2= 9856 kg M2
Pump GD2= 70254 KG M2 (I dont know how relevant this data is?)
4 pole motor at 50Hz

Basically, some one has claimed that if the drive acc. time is 60 sec, the motor and pump can be started with a 3.6MW drive.
How can he arrive at this figure? (he above mentioned data is all that is available) And more importantly, will the application work?? I am very skeptical that such a system will be a good option to go for. I think that since the drive will have to run at a lower frequency, this will reduce its efficiency.

Any comments???

P.S. the same person has also claimed that if we increase the acc. time of the rive, the rating can be further reduced to around 1.8 MW.

Frequency Conversion Soft Starter

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!

Under Voltage and Over Voltage Protection

Hi folks,

I am a marketing engineer with Larsen & Toubro Ltd., India. We are currently dealing a lot with Soft Starters. I have noticed that in some makes , under/over voltage protection is provided along with earth fault protection. However, many other notable manufactures do not have these features.

In some of our tenders, in the spec sheet for soft starters, under/over voltage as well as earth fault protection is asked.

I would like to know whether such protection is essential for a soft starter. Also what purpose does this protection solve.
Can under/over current prtection serve the same purpose?