Help: Transformer winding problem, please help me

sywlmzl

Help: Transformer winding problem, please help me [Copy link]

 

The situation is this:

I have a 200W transformer with a primary of 220V and a secondary of 27V.

Now I want to change to 4V 50A to make a small spot welder.

As a result, I removed the secondary and wound it with 16 square millimeters of wire for 7 turns. When I measured it, it was only 3 volts and the resistance was 0.8 ohms. Even at 4 volts, it was only 3 to 4 amps.

Excuse me:

What should I do next? Should I rewind the primary? How thick should the wire be and how many turns should it be wound? (The calculations for transformers are too complicated to learn in a short time)

wangerxian

Is it true that 200W can be achieved without changing the secondary coil?

maychang

"As a result, I removed the secondary and wound it with 16 square millimeters of wire seven times. I measured it and it was only 3 volts."

When you removed the original secondary, did you count the number of turns? Did you measure the wire diameter?

sywlmzl

wangerxian posted on 2022-10-19 19:26 Can it really reach 200W without changing the secondary coil?

The transformer is rated at 200W, so it should be fine.

The secondary wire of the original transformer is no longer usable. If I use the primary wire, I want to remove it, cut it, and combine it into 5 or 6 strands. How many strands are appropriate? How many turns should I rewind? Please advise.

sywlmzl

maychang posted on 2022-10-19 19:27 "As a result, I removed the secondary and wound it with 16 square millimeter wire for 7 turns. When I measured it, it was only 3 volts." When I removed the original secondary, did you count the number of turns? Did you measure the wire diameter?

Remove the secondary time gauge wire diameter, 2mm (including the paint). Removed 53 times. That's right

maychang

sywlmzl posted on 2022-10-19 20:12 Remove the secondary time measurement wire diameter, 2mm (including paint leather). Removed 53 turns. That's right

53 turns 27V, 1.963t/V, 1.963 turns per V. So 7 turns should be 3.566V. You actually measured 3V, which is about right.

maychang

sywlmzl posted on 2022-10-19 20:12 Remove the secondary time measurement wire diameter, 2mm (including paint leather). Removed 53 turns. That's right

You have wound 7 turns. If you wind 8 or 9 turns, the 4V requirement should be met.

maychang

What is questionable is that the resistance of the new winding is 0.8 ohms, which is not normal in any way.

sywlmzl

maychang posted on 2022-10-19 20:28 You wound 7 turns. If you wound 8 or 9 turns, it should meet the 4V requirement.

The space is full and can't accommodate one more turn. Even if the area of the wire is reduced to 8 or 9 turns, the voltage can reach 4 volts, and the resistance is already 0.8 ohms. The current is only about 5 amps. In fact, if the area of the wire is reduced, the resistance will also increase, and the short-circuit current will not reach 5 amps.

sywlmzl

maychang posted on 2022-10-19 20:27 53 turns of 27V, 1.963t/V, 1.963 turns per V. So 7 turns should be 3.566V. You actually measured 3V, which is about right.

It’s almost there, but what’s the next step?

maychang

sywlmzl posted on 2022-10-19 20:45 It is almost the same, but what is the next step?

"But what's the next step?"

Next, let's solve the 0.8 ohm resistance problem. A 16 square millimeter wire is wound 7 times, so the resistance cannot be 0.8 ohms.

maychang

sywlmzl posted on 2022-10-19 20:45 It is almost the same, but what is the next step?

You use 16 square millimeter wire to wind the secondary. Is the 16 square millimeter wire not a single strand? It is probably a multi-strand wire, otherwise the wire is too hard to wind. For the 0.8 ohm resistor, are some of the multi-strand wires broken, leaving only one or two strands for conductivity?

wangerxian

sywlmzl posted on 2022-10-19 20:09 wangerxian posted on 2022-10-19 19:26 Can it really reach 200W without changing the secondary coil? The transformer is rated at 200W, so it should...

According to what the big guy above said, solve the problem of 0.8 ohm resistor first.

chunyang

For multi-strand wires, make sure the joints are well soldered, and use copper wire. If the resistance is high, it may be that the multi-strand wires are not all connected, or the wrong wire material may be used.

chunyang

When making a welding transformer, it is best to use copper tape to wind the secondary, which is not as difficult to wind as thick copper wire. When winding multiple strands of wire, pay attention to controlling the length of each wire. You need to calculate the length in advance, then cut the wires and carefully wind them side by side, similar to copper tape. Do not wind them separately, otherwise the voltage of each strand will be different.

chunyang

There is no need to rewind the transformer primary, as long as its power is real, its wire size is appropriate.

sywlmzl

wangerxian posted on 2022-10-20 09:12 According to what the boss above said, solve the problem of 0.8 ohm resistor first.

Thank you moderator, thank you master for your advice. The 0.8 ohm I measured first was wrong. I measured it again, and the probe was 0.2 ohm, and the measuring wire was also 0.2 ohm. It can be seen that the actual resistance should be within the error of the multimeter. This error is that I have no intuitive understanding of the resistance of multi-strand cables. I checked it online and found the following explanation: "

Generally speaking, resistance value = line length / line cross section * line conductivity.

For example: the length of the copper wire is 20 meters, the cross-section is 2.5 square millimeters, and the conductivity of the copper wire is 0.0217, then the resistance value of this section of wire = 20/2.5*0.0217 = 0.1736 ohms.

Follow this method. My winding: 3 meters/16 square millimeters*0.0217=0.004 ohms. Then 3 volts/0.004=750 amps. This current is enough. I will continue to do it. If I have any questions, I will ask you guys for help. Thank you again.