Softstart

Induction motors don't like to be treated that way. One of the reasons that a triac (or a couple of SCR's) won't act right is that the voltage and current are out of phase with each other due to the inductance of the motor. Also, induction motors would like to run at a speed dictated by the number of poles and the power line frequency, and they virtually have no torque at out-of-sync speeds.

You can buy an off the shelf Soft Starter.Allen-Bradley has the Bulletin 154 STC Starting Torque Controller, but there are many other vendors in the market.

Click http://www.ab.com/en/epub/catalogs/12768/229240/229262/3073021/229583/

Regards, Delmar Schmidt
Melfi Technologies Houston
www.melfitechnologies.com

Delmar, thanks for the information from me too! I worked on this very problem for low HP induction motors around 20 years ago, but not for power line surges.

You can use the triac method stated above, this will give a motor speed variance of about +/- 10 %.

Basic operation begins with an R-C circuit, as volt swings between pos. and neg, the cap charges positive during positive voltage vice versa. When the voltage across the cap reaches the breakover voltage of the diac, the diac triggers the triac to conduction(thus recieving a positive gate pulse). Once its triggered, the triac acts as a short for the rest of the cycle and the motor recieves the remaing line voltage.

The firing angle of the triac can be varied to deliver either more or less line voltage by varing a rheostat so the cap take either more or less time to charge.

Due to the high inductance, you would need a snubber across the triac to insure shut off as polarity changes during each half cycle.

This will give you voltage variation, so the torque will also vary, along with efficiency, so take care to insure a reasonable balance.

Since thisis a class, this would be a great use of fundementals.

Needed:

AC source, Cap, Rheostat, Diac, triac, and a snubber.

Good luck

I excuse myself, I wanted to answer earlier. I include/understand your answers very well. It should be understood here that I seek a small functional assembly which cuts primarily the sine to a variable angle. I am conscious of the phenomena of couple, of the ratio v/f, of cooling etc. I thus seek a small diagram which reduces the tension ac for approximately 2 seconds and thereafter 100%. I do not seek to buy an industrial softstart, we have many whom one uses with the students of more advanced level. My goal is to make a laboratory with a small assembly and a true engine (when there is movement, the student have always more interest)

Ok, I have to apologize, but you lost me here.

What do you mean by tension AC?

Tension = voltage or power.

Old timers used to call overhead power transmission wires "high tension cables".

How about a "buck" transformer and DPDT time-delay relay (rated, of course, for proper current), for a simple way to accomplish what you want.

Otherwise, here's a good website to go to on the subject: http://www.lmphotonics.com/sstart.htm

First you have to differenciate between permanent capacitor motors and starting capacitor motors. The first one may work with a reduced voltage starter considering the motor is driving a quardarture load (fan or centrifugal pump). In the other case the motor may not start at all since the centrifugal switch needs a certain speed to open up the staring circuit. that may not happen under load. In a lab, at no load condition you may start the motor but in real life , I doubt it.

Phase angel controller may be a better solution than an triac. I have one somewheer in my shop. If I find it you can have it for free for the lab. call me at 905 845 3666.

Joe

I design an sofstart for 2HP 240V single-phase using two SCR connected anti paralell causes effective cut of conduction at the end of semicycle. Gates are maintained at -1V when not triggered making false trigger almost impossible. Using analog opamp generate linear ramp from 0 to 5V synchroniced to line. Other opamp generate 5 to 0V ramp in 10 seconds. An third opamp compares ramps and generate square pulse for trigger the SCR (I use TIC126M). When the trigger angle reach 180 degrees, an bypass electromecanical rele shortcircuit SCR's supplying direct AC line to motor. An second rele is connected serial with line for security purposes. This circuit has been used for five years in paint mixers in Argentina and Brasil. If you like schematic diagram and PC board layout don't forget email me at eduardoreimundo@gmail.com. I don't know if is possible make attachmets here.

Thank you , I communicate with you Langdom