What size capacitor should I use to smooth the 220V DC output from a full wave bridge rectifier? The Rectifier is to be used to supply a small 220/240V DC ex sowing machine motor via a speed controller which will run it at slow speeds. The reason for choosing a DC motor is that at speeds less than about 30 rpm the AC motor currently in use stutters.
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Yeah, most of us would have little knowledge of (presumably) automated devices for planting seeds and their power requirements.
A general rule of thumb for low voltage rectified AC is 1000uF per A of load current. However this simply gives an amount of role which should be easy enough to handle given normal design practices.
This may not apply in your case as e have no idea what tolerance the seed layer (sowing machine) has to ripple.
Note that this advice would equally apply to more common appliances (even to sewing machines) where we would likewise be unable to determine the requirements from just our experience.
A general rule of thumb for low voltage rectified AC is 1000uF per A of load current. However this simply gives an amount of role which should be easy enough to handle given normal design practices.
This may not apply in your case as e have no idea what tolerance the seed layer (sowing machine) has to ripple.
Note that this advice would equally apply to more common appliances (even to sewing machines) where we would likewise be unable to determine the requirements from just our experience.
Sorry my typo there - it is an ex sewing machine motor - not "sowing". There are no specs on the casing other than it is a Yamamoto model YM-295 220/240V DC only motor. I am assuming it is a brush-less motor as it has three wires: Blue, black and white. White and black go into one side of the case and the blue goes to the other side.
I intend to use it to drive a fishing rod lathe, hence very slow speed needed for wrapping bindings and drying epoxy coatings. Also high torque is not a priority here.
Bluejets: The mains power supply here is 240V AC 60Hz. I am looking at purchasing a full wave diode rectifier and adding the smoothing capacitor due to the low speeds that I need. I have not yet chosen a speed controller.
My first thought would be to use simple rectified (using a bridge rectifier) AC and no filtering.
Try using a lamp dimmer to control the speed.
If it runs way too fast then half wave rectification may be a better start (since you say you want it to run slowly).
The peak value of 240VAC is around 340V -- you will REALLY need to know how these were originally wired up in a sewing machine.
Check this out. Early speed controller is just a fancy light dimmer. You'll still have to figure out where the 3 wires go though...
Try using a lamp dimmer to control the speed.
If it runs way too fast then half wave rectification may be a better start (since you say you want it to run slowly).
The peak value of 240VAC is around 340V -- you will REALLY need to know how these were originally wired up in a sewing machine.
Check this out. Early speed controller is just a fancy light dimmer. You'll still have to figure out where the 3 wires go though...
Thank you for your thoughts everybody. Once I figure out the wiring on the motor I will go ahead and try it first without the capacitor using the light dimmer I already have set up for the existing AC motor. The vendor of the rectifier I am looking at tells me that 230vac in would give 210 vdc unfiltered out for that module.
Interesting. However it pretty much defies mathematics.
You'll get a series of pulses that reach a peak of around 330V but have the same heating effect as DC as 230VDC (actually a very small fraction less -- probably no more than 2V less)
Thanks Steve. I was trying to figure how he worked that one out too. His explanation is that the voltage drop is due to the fact that it is an un-smoothed output. I don't know if he has actually measured it. I guess his logic is that the loss is due to the pulse effect. Please excuse my ignorance on this as I am trying hard to learn something here. Perhaps his knowledge of such things is even less than mine.
So : Say this rectifier gives a peak output pulse of 330V but the effective the drop is < 2 volts giving an effective output of around 228V DC - is that correct? Would a 2000uF capacitor work as the filter?
So : Say this rectifier gives a peak output pulse of 330V but the effective the drop is < 2 volts giving an effective output of around 228V DC - is that correct? Would a 2000uF capacitor work as the filter?
OK Bob, in the absence of any info on the label other than make, model and voltage, and no history of the motor apart from that it is from an old sewing machine , I have to make some assumptions here.
It is a sewing machine motor. According to what I can search a typical sewing machine motor runs at 75W to 100W. Although the torque and speed requirements are much lower than for a sewing machine in my intended application, I will assume the upper limit of 100W, and so a current requirement of less than 0.5 amp given an averaged smoothed DC output of around 210V max.
Alec_t: Would a sewing machine need a tachometer output? I am assuming that the third wire is to energise the magnets in this brushless motor.
It is a sewing machine motor. According to what I can search a typical sewing machine motor runs at 75W to 100W. Although the torque and speed requirements are much lower than for a sewing machine in my intended application, I will assume the upper limit of 100W, and so a current requirement of less than 0.5 amp given an averaged smoothed DC output of around 210V max.
Alec_t: Would a sewing machine need a tachometer output? I am assuming that the third wire is to energise the magnets in this brushless motor.
Only if the machine was designed to have an accurate control of the speed. It would certainly be useful if you are aiming for a steady 30rpm (which is much lower than the speed my wif'e's sewing machine runs at, and I think will be a challenge to achieve). The wire is more likely a power supply one though.
Thank you Alec. I can get a little over 40 rpm with the AC sewing machine motor currently fitted. Which is just usable if a little unstable. But at 30rpm or less it stutters.
Actually I should clarify here that my required rpm is actually at the chuck of the fishing rod lathe, which is geared down so the motor itself is running somewhat faster - around 135 rpm when the chuck is at 30 rpm.
I get the feeling that with my limited electronics knowledge I am pissing against the wind here!
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Have you considered using a cordless drill or screwdriver for this purpose? It has some advantages over your AC powered setup:
regards,
Harald
- It is designed to run slow.
- A speed controlller is usually built into the machine.
- It is safe operating at typ. 12 V ...18 V. No risk of an electric shock.
- It comes with a chuck to grab material of different sizes (of course you will need an adapter for materila with a diameter considerably higher than ~ 10 mm).
- It is available with a built-in torque limiter which may come handy to avoid damage to the object being processed in case of a failure of the setup.
regards,
Harald
Hey Harold thanks so much for your input. Unfortunately I do not want to have to clamp a battery powered drill or screwdriver on to drive the the existing chuck set up, which is designed for the purpose of fishing rod building and has a pulley wheel to turn the chuck.. My existing AC powered setup incorporates a switch over to a pedal switch to drive the motor.
when u say this fast rule 1000uf per amp , do you mean in full bridge or half bridge , and for 1phase ? so can i consider half -500uf for 2ph and 1/3rd about 330uf for 3ph ?
and this is for about 10%ripple or to expect 5, 1 ?
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@jetnew : Are you aware that this is a 7 year old thread?
I'll close it. Please open a thread of your own if you have further questions.
I'll close it. Please open a thread of your own if you have further questions.
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