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Basic Softstart For Single Phase 700w Centrifigul Pump

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#1 ianz


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Posted 05 March 2008 - 10:23 PM


I have been reading many topics from this forum and guess this might be a good place to ask some questions. I am hoping to build a softstart using a Mega128 microcontroller but seriously wonder after reading comments in this forum if it is even feasible..given my lack of experience with motors. So I will outline some details below and hope that I will become better informed from the replies.

My machine is an 230Vac, single phase, centrifugal water pump that has a tested' running current of 3.2A. As far as I know it is capacitor start..rather than induction start..but need to confirm that with the manufacturers.

My design has the output pin of the Mega128 micro controller driving a optocoupled triac driver which controls the gate of an Alternistor. The alternistor is on the high side and motor on the low side. From what I can make out an Alternistor is a style of Triac that is designed for improved phase symmetry and therefore lower harmonc output. Please correct me if i am wrong.

The alternistor I have chosen is Treccor Q8025LF rated at 25A 800V so should be up to the job, bearing in mind that the locked rotor current of this pump is in the order of 15-20A and that, in theory, this level of current would not occur with the softstart active. see http://www.teccor.co...nistorTriac.pdf

The optocoupler triac driver is a random phase style driver. Fairchild MOC3052M rated at 600V. see http://www.fairchild...O/MOC3052-M.pdf

I have a current transformer in series with the motor. It is CRMagnetics 8300 type and can be viewed at http://www.crmagneti...ProdName=CR8349

The secondary winding of the CT feeds a Precision rectifier circuit kindly provided by the CT manufacturer and can be seen at http://www.crmagneti...df/ancrct-4.pdf This circuit provides a DC voltage that is in proportion to the motor current and can be monitored using the ADC input of the Mega128.

The purpose of the CT is not only for its potential use with the soft start function, but also to monitor the long term motor current in order to detect filter blockages, overloads etc.

Another hardware feature that I built in is a snubber circuit on the triac to allow it to turn off cleanly, considering the inductive nature of the load.

I also have a seperate mains monitoring transformer that is monitored by the microcontroller and its purpose is to protect against mains brown out, but also allows me to detect the zero cross point of the mains supply.

So thats my hardware. If anyone wants, I can post the circuit diagram.

My initial plan is NOT to use the Current Transformer in a feedback control loop, and to simply to feed the optocoupler with a set of timed pulses, syncronised to a zero crossing reference, as in a simple light dimmer circuit. By reducing the delay of successive pulses, the voltage to the motor will gradually increase, from 50% to 100% and the motor will speed up. My planned ramp time is in the order of 2-5 seconds so is quite quick. Of course I can also watch the motor current for overloads etc, and I guess it makes sense to use it in the ramp determination eventually. But I have never been down this road before so am going for the simplest idea first.

Having said all of the above..and outlined my design, I would like to hear comments from other forums users, in particular with regard to issues that I might not have considered. What other gotcha's should I be aware of before I actually fire this thing up?

Thanks in advance for any comments or replies

Ianz..(yes another kiwi DIY tinkerer)

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