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toroidal transformer voltage drop

yoosefheidari

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hi guys
i make a toroidal transformer to run my 12dvc motor
dc stall current at 12v is 6 amps.
my problem is when i put my motor under load in stall condition output voltage of transformer after rectifier drop to about 6vdc and stall current is about 3/5amps.
how can i fix this problem?
primary winding resistance is 31ohms
secondary winding resistance is 0/5 ohms.
secondary dc voltage at no-load is 12/8 volts.but under load it drop to 6v and 3/5 amps.
20210227_170014.jpg
 

betwixt

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What is the rating of the transformer, it looks to be quite a small one and probably can't provide enough current. I would be concerned about the bridge rectifier overheating too.

Brian.
 

yoosefheidari

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What is the rating of the transformer, it looks to be quite a small one and probably can't provide enough current. I would be concerned about the bridge rectifier overheating too.

Brian.
there is no problem about rectifier temp.
core size is 85mm OD and 40mm ID and 40mm height and 1300 grams weight and saturation magnetic field is about 1.7 tesla.
desired output power is about 60VA and i think this core is suitable for that.
but what causes this big voltage drop?
 

schmitt trigger

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The magnetic saturation is 1.7 T for magnetic steel, this is correct. But have you calculated the actual operating flux density for this transformer?

If your secondary winding is only the red exterior wire which covers about 2/3 of the toroid surface, it is very loosely coupled.

To verify my statement, you may want to perform a transformer characterization via the open and short circuit tests, for instance:
 

yoosefheidari

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The magnetic saturation is 1.7 T for magnetic steel, this is correct. But have you calculated the actual operating flux density for this transformer?

If your secondary winding is only the red exterior wire which covers about 2/3 of the toroid surface, it is very loosely coupled.

To verify my statement, you may want to perform a transformer characterization via the open and short circuit tests, for instance:
operating flux is about 1 tesla due to this formula: V=4.44NFBA where V=220vac , N=1050 , F=50hz A=90mm2
there are 5 more turns of secondary under yellow tape and secondary winding covers abou 3/4 or 7/8 of the toroid surface.( you want to say may it causes about 7 volts drop in secondary?!)plus i use 4 magnet wires in strands for secondary in order higher current capability(i didn't have thicker wire for secondary so using 4 wire in strands)
for oc-sc test i need a wattmeter that can measure power factor which i havn't.
does measuring inductance of primary and secondary with LC meter helps?
 
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dick_freebird

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I don't see where you have measured all
of the voltage drops around the secondary
and primary current loops. This, if organized,
would tell you where the losses are and then
you'd know what to attack.
 

KlausST

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Hi,

At low load the capacitor voltage will be about the peak of the sine.
But with increasing load there will be ripple voltage ... making average voltage to drop.
Quite usual and not avoidable. You may reduce it by using bigger capacitors.

But be aware that the current in the transformer is pulsed, thus the RMS transformer current is much higher than the average motor current.
For 70W motor power you maybe need to use a 100VA rated transformer.

Or use a switch mode power supply.

Klaus
 

Easy peasy

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this is a common problem for newbie Tx designers, you say you want 12V 6A, ostensibly 72 watts, but not 72VA as with a cap input filter on the bridge rectifier the power factor is 0.6, so the current in the sec will be 10A rms,

the pri current will therefore be 12/230 * 10 = 520mA, x 31 ohms ( cold ) = 16.2 volts lost in the primary R, 7%

Sec R is given as 0.5 ohms, giving 5 volts lost in IR drop ( 42% )

230 - 16.2 = 213.8, *12/230 = 11.15 Vac sec, - 5V = 6.15V, which is roughly what you have on the sec DC side ( 1.8V diode drops )

So you can see pretty quickly you do not have enough copper area in the windings.
 

yoosefheidari

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this is a common problem for newbie Tx designers, you say you want 12V 6A, ostensibly 72 watts, but not 72VA as with a cap input filter on the bridge rectifier the power factor is 0.6, so the current in the sec will be 10A rms,

the pri current will therefore be 12/230 * 10 = 520mA, x 31 ohms ( cold ) = 16.2 volts lost in the primary R, 7%

Sec R is given as 0.5 ohms, giving 5 volts lost in IR drop ( 42% )

230 - 16.2 = 213.8, *12/230 = 11.15 Vac sec, - 5V = 6.15V, which is roughly what you have on the sec DC side ( 1.8V diode drops )

So you can see pretty quickly you do not have enough copper area in the windings.
so a big problem here is poor power factor.is there any way to imporve power factor?and raise it up above 0/8?

i change my secondary winding and use a wire with 0.1 ohm total resistance to see what happens.on paper output voltage should raise up about 4volts.
 

Easy peasy

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There is no easy way to improve your power factor.
--- Updated ---

you could follow the bridge rectifier with a boost converter to step the volts up to 13V say - thus when the o/p of the Tx sags under load - the boost converter will draw more current to try to keep the o/p at 13V, if you condition the boost section to draw a (rectified ) sine wave from the Tx, then the power factor will be high with the lowest RMS currents in the windings, kind regards ...
 
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yoosefheidari

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There is no easy way to improve your power factor.
--- Updated ---

you could follow the bridge rectifier with a boost converter to step the volts up to 13V say - thus when the o/p of the Tx sags under load - the boost converter will draw more current to try to keep the o/p at 13V, if you condition the boost section to draw a (rectified ) sine wave from the Tx, then the power factor will be high with the lowest RMS currents in the windings, kind regards ...
i change the secondary winding and now it's resistance is 0.055ohms
i put the transformer under load again and motor stall current was 5.5amps and voltage was 9volts.that was big improvment.now voltage drop is about 3.8volts instead 6.8volts in previous winding.
now by improve power factor voltage drop can be lower.is that right?which is not so easy
 

crutschow

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so a big problem here is poor power factor.is there any way to imporve power factor?and raise it up above 0/8?
The problem is the high and short duration peak current drawn from the transformer by the rectifier-capacitor supply to generate the average current drawn by the motor.
Can you operate the motor from the rectified output without the filter?
 

yoosefheidari

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The problem is the high and short duration peak current drawn from the transformer by the rectifier-capacitor supply to generate the average current drawn by the motor.
Can you operate the motor from the rectified output without the filter?
i remove filter and run motor from rectified output but voltage drop became much much worse.so i put it back
 

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