Hi, a question that I read answered in a lot of forums, but each post says a different thing.

I have to control an universal motor in AC (230V, 50Hz) coming from a not so old washing machine.
At the spin velocity (7000 RPM) it adsorbs P=400W, I=3.0 A, at the washing velocity (500-1000 RPM) it adsorbs P=180W, I=4.0 A as for datasheet.
Note that the RPM values are "before" the pulley ratio.

I'm using a snuberless triac BTB12 BW in a circuit very similar to the follow. I have chosen R=100 ohm and C=100 nF, as I read for this kind of load. I omitted at the first stage the snubber thinking at the "snuberless" triac, but the motor had some velocity spike. For the moment I solved introducing the snubber. My next check will be use a less sensitive optocoupler as the FOD420, trying to avoid the (big) snubber.

What I would understand is the correct minimum wattage of the resistor and the voltage of the capacitor.
At the moment I'm using an R 2W and it works well but I would need to be sure that is a safe value with all the temperatures and condition. As capacitor I'm using a polypropylene type of 400 Volt.

Thanks a lot.

 

Can anyone help me? Thanks.

 

Calculating the exact power dissipation of the resistor is mathematically complex.

A pragmatist asks "does the resistor get really hot?" If the answer is yes, replace it with a higher rated one.

 

Hi if you say you have it all up and running then, you could put a scope across the snubber and have a look at the signal that is actually being snubbed. You should then be able to integrate under the trance and get a reaonable figure for the actual power being dissipated... and then.. over engineer it!!

 

Hi if you say you have it all up and running then, you could put a scope across the snubber and have a look at the signal that is actually being snubbed. You should then be able to integrate under the trance and get a reaonable figure for the actual power being dissipated... and then.. over engineer it!!

Yes but use extreme caution with this! MAINS VOLTAGE - Use an isolation transformer!

 

I'm thinking to buy a Rigol 1054Z or a Siglent SDS1102X (just understand if I need 2 or 4 channels), but at the moment unfortunately I don't have an oscilloscope...

 

Hmm maybe you could simulate it with LTSpice?