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Help with smoothing DC power supply

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Joshua cank

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Hello everyone, I'm after some advice if possible. Thanks in advance for any suggestions.

I am trying to build a 12v DC power supply for a friend to run a permanent magnet motor. The motor is currently run off a 12v car battery but they would like it to be 240v mains fed. I have a 240/12 step down transformer with a 60amp secondary rating. Am I correct in thinking if I rectify the 12vac via a bridge rectifier I will need a capacitor of the correct value to smooth the rippled DC output?.

I don't have any specific details of the motor as of yet, im sure it is a two pole permanent magnet type DC motor. I expect the motor rating to be around 40/50 amp.

Thanks again, josh.
 
You will get 12v x 1.414 for a DC out put if using a capacitor bank.
It is not generally necessary for a DC motor.
Are you using across line starting or any kind of controller?
Max.
 
The Mechanical rotational Inertia of the motor armature + load will make it such that the motor doesn't care if it gets full-wave rectified AC or filtered DC. To provide a modicum of filtering it will take a huge capacitor. I*t=q=C*ΔV, so at 50Hz line frequency to get 3V of ripple, C = I*t/ΔV = 50A*(2/50)s/3V = 2/3F, yes that is 2/3 of a Farad!
 
You would need a seriously large capacitor to smooth that much current. If you aim for a 2 V drop in a half-cycle, 1/100th of a second, the charge will be 50/100 = 0.5 C. A capacitor that drop by 2 V with that charge is 0.25 F.

Something like this:-
https://uk.farnell.com/vishay/mal210127224e3/cap-alu-elec-0-22f-40v-screw/dp/8820775

On the other hand, it may not make a lot of difference if you don't smooth the DC. The motor won't speed up and slow down much in 1/100th of second
 
Perhaps I am over complicating things. I have nearly completed my HNC in electrical engineering and since starting it i find myself delving into the technical side of everything, just to gain more knowledge! So in reality the motor won't be two fussy about the rectified DC waveform it is receiving
 
So in reality the motor won't be two fussy about the rectified DC waveform it is receiving
It is often used this way in industrial environments.
If you needed a soft start, you could do it with a triac followed by a bridge rectifier.
The initial current will be due to the armature resistance, make that a fraction of an ohm.
I would have thought a DC rated contactor would be more appropriate, these also have some kind of arc blow out on the contacts when switching direct.
Max.
 
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You will need a serious heatsink for the bridge rectifier that you use.

As for:

" transformer with a 60amp secondary rating. /////I expect the motor rating to be around 40/50 amp."

Then you will need a transformer with a secondary rated for about 80 - 100 A (RMS). The transformer secondary RMS current is roughly 1.7X the DC current when using a bridge rectifier design.
 
Okay thanks for the advice, I'll use it wisely when I actually get to see what motors I've got to work with. Hopefully they are smaller than I initially thought
 
You will need a serious heatsink for the bridge rectifier that you use.

As for:

" transformer with a 60amp secondary rating. /////I expect the motor rating to be around 40/50 amp."

Then you will need a transformer with a secondary rated for about 80 - 100 A (RMS). The transformer secondary RMS current is roughly 1.7X the DC current when using a bridge rectifier design.

That scaling factor is true if you are going into a rectifier/capacitor filter, and are necessary for two reasons:
1) The capacitor DC voltage (and therefore the load voltage) will be the peak AC voltage, 1.414 x RMS
2) Current only flows in the transformer secondary when the voltage is near that peak.

If the load is purely resistive, then the transformer and load currents are the same.

In this case, (assuming that no filter capacitor is used) the motor will be an inductive load. Normally that would just shift the phase of the load current. But, since there is a rectifier in the current path, it becomes a non-linear load. There might need to be some scaling factor used for the transformer current, but I'm not sure how to put a number on it.
 
Take some advice from somebody who designed power supplies for 25 years: when you need 600W ballpark, you REALLY do not want to try to do it with a 60 Hz transformer design. A single phase Xformer has a power factor of about 0.6, which means to deliver 600W you need more than 1000 V-A transformer (actually probably 30% more than that because of efficiency losses). That transformer will be the size of a bowling ball. You'll need large, expensive stud mount shottky diodes for rectifiers because the power dissipation in them will exceed 100W. You'll need a massive heatsink and maybe a fan. Those components alone will probably cost several hundred bucks or more.

If I needed 12V @ 50A, I would run the net for a used lab supply (like we used to sell from Power Ten) that has a switching converter in it. It will be small and light and be short circuit and overload protected.

Just some free advice.
 
In this case, (assuming that no filter capacitor is used) the motor will be an inductive load.
I'm not sure that large DC motor would run off a transformer/rectifier alone as the cap might be necessary to provide current peaks. I am not 100% sure, but I would not assume it will run straight off a rectified transformer.
 
I guess it would be easier for me to report back when I know the exact loads I'm dealing with, I am also from the UK so my single phase supply will be 50hz. The machine is designed to run directly off of a 12volt automotive battery switched direct on line by a relay.
 
I guess it would be easier for me to report back when I know the exact loads I'm dealing with, I am also from the UK so my single phase supply will be 50hz. The machine is designed to run directly off of a 12volt automotive battery switched direct on line by a relay.
I would retain the car battery, and use a 1A "smart charger" to maintain the battery in a fully-charged condition. I use a similar system to crank-up/down my 20m telescoping radio antenna tower.

The 500 lb tower/antenna array is lowed/raised by a reversible 12V winch such as you would see on the front bumper of a off-road vehicle. It draws about 50A start-up and over 25A for running current. On average, I raise/lower the tower once a week (don't like to leave it up if we are expecting high winds). The 1A charger has plenty of time to restore the battery to full charge in between uses... I figure one round trip takes less than 2Ah out of a 50Ah battery.

With several vehicles around here for which I am buying new starting batteries, there is always a used battery floating around that is not good enough to start a car (500A), but is more than good enough to crank the tower up and down (25A) ....

An old car battery and a $25 charger is way cheaper than obtaining a 50A transformer and 100A rectifier.
 
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That's that the machine currently runs on mate. It is a DTL clay pigeon trap, one motor runs constantly to rotate the trap from left to right. And the other motor is switched when the shooter calls pull and runs for about 5 seconds to re arm the trap.

The trickle charger works fine on slow days, but when competitions are held they drain fast and the shooting is interupted. I intend to try and build this power supply to enable us to use the existing 12v trap, rather than paying thousands for a new one. I have been given a 60a transformer it is now up to me to see if it is possible to rectify it to run the two small DC motors
 
From what you describe, I would not have expected that large a motors necessary for that application?
Especially in an application like that I would expect some gearing in place to lower the size requirement of the motor.
Max.
 
From what you describe, I would not have expected that large a motors necessary for that application?
Especially in an application like that I would expect some gearing in place to lower the size requirement of the motor.
Max.

Correct buddy, there is a gearbox on both motors. I will know more when I can get the ratings of the motors, I may have to put a clamp Meter on them as I doubt they'll be marked up.
 
If there is no plate on the motor then the clamp-on will not tell you the continuous rated torque, just what the motor experiences in this application, but it should give you an over all idea of what you would need for power.
Max.
 
I guess it would be easier for me to report back when I know the exact loads I'm dealing with, I am also from the UK so my single phase supply will be 50hz. The machine is designed to run directly off of a 12volt automotive battery switched direct on line by a relay.
My question is why AC driven supply?
It would be better to use a battery charger and use a "good" battery as the Farad smoothing capacitor as the load current will be 8x the rated motor current. If it takes 50A under load then the surge current will be 8x or 400A peak and with 13.5V this means an acceptable drop of 1 or 2V implies a source impedance of 1/400 Ohms which becomes excessively expensive for RdsOn values near 1~2 mΩ compared to a 10A to 40A battery charger which can use power semi's in the range of 10~20mΩ or simply Triac primary controlled with full diode bridge output using very cheap parts.
 
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