bridge rectifier filter

[scrudman]

I am working on a boat with a 120 vdc anchor windlass. I believe the motor to be 3 HP and the 120 vdc source is from a full wave bridge (120vac input). The output of the bridge has no capacitor. Should I install a capacitor and how do I determine the proper size?

 

[ScottyUK]

Hi scrudman,

If you add a large capacitor you will increase the output voltage of the power supply. This alone is probably enough to warn against adding the cap without other more involved mods such as changing the transformer. The rectifier will also see higher peak currents if the capacitor is installed, which may or may not be within the rating of the diodes.

Why do you want to do add a capacitor? Is the winch under-performing? Or does it just look 'wrong' without the capacitor?



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If we learn from our mistakes,
I'm getting a great education!

 

[scrudman]

Thanks Scotty, The motor runs but the switching relays are failing every other month. I believe that this is due to inadequate relays for this app and I have already spec'd and ordered replacements, but I wanted to make sure that the lack of a cap wasn't a contributing factor, and that the motor would not be harmed by running on a straight bridge output. I'm inferring from your post that it should be fine.
thanks again.
Scrudman

 

[ScottyUK]

The motor would perform better with a pure DC supply - a battery or well-smoothed DC source - but not at the expense of raising the voltage beyond the design level. This will cause other problems.

Don't forget that the relays switch a lot of current when the motor is at standstill because there is no back EMF from the armature. If the motor is separately excited or has a shunt field, there will be a powerful inductive arc at switch-off caused by the energy stored in the field circuit which will burn relay pole faces.

You say 'relays' rather than 'contactor'. For this sort of duty it would be normal to spec a contactor rather than a relay. In Europe at least, 120V DC is a fairly common control voltage and contactors are available suited to motor switching duty. If this is an industrial application look for DC3 duty contactors or maybe DC5 duty if the motor is particularly inductive.

On re-reading your post, I have assumed that the relays are failing through contact damage, not coil failure. Have I mis-assumed?





------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[OperaHouse]

Switching DC is no easy task for contacts. Chances are the mfg is fudging a little bit, hoping that the pulsed voltage will extignuish the arc that is created. With filtered DC the contacts would probably burn out when they opened. A RC network or large MOV across the contacts might help extend the life.

 

[busbar]

Would traditional diode protection across the contact(s) or motor be affected by the unfiltered-DC source?

 

[rbulsara]

Could you consider switching on AC side of the rectifire. As mentioned above switching DC requires much heavy duty contactor (not relay)than AC.

 

[ruggedscot]

Switching dc could be carried out by using multiple pole contactors and putting some of the contacts in series. Giving a better break. Also as mentioned switching the AC side would be better what with AC being somewhat self extinguishing.

If its the relay coils that are burning out Id use dc coils fed from a rectified that was connected to the supply with the output smoothed to try and reduce the ripple effect.

Rugged

 

[scrudman]

Thanks to all those posting. The motor was switched with 30 amp rated power relays, that the the windlass manufacturer insists theyve been using without problems-I am very doubtful. The failures were complete meltdown of contacts, and I immediately suspected improper switchgear for the app. but the owner of the vessel trusted the manufacturers claims, so I simply replaced the first failed relays. Then it failed again in the islands and I sent him more of the same. Now he has asked me to fix it permanantly and I have ordered 4-pole contactors which I will series (2-pos/2-neg) contacts. I cannot switch the AC because the motor is reversible => I am also reversing polarity with the switching. Based on all of your input I will leave the bridge output without a cap. Thanks again, Scrudman.