Transformer possibility
- Thread starter boptrop
- Start date
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Yes. But just curious, what the heck needs 60V?
Oh wait. If you need 2.5 kVA at 60V, you will need to use a 5kVA 120V transformer. The transformer rating is based on current, 2.5kVA at 60V is about 40A, 2.5kVA at 120V is about 20A.
Thanks for reply,
Where I work we use SCR's to control power to stretched wires to cut EPS. If the wire is long (15' or so) it takes approx 480V Max. shorter wires lower voltage. In this case, I have a 2' wire. The original machine (iso-trans blew up) had an adjustable transformer with an Iso-trans following. The input to the machine is 120V 20A max. So I know I can replace the adj. transformer with the SCR but I am not sure if I will have enough control over the temp of the wire...I can lower the voltage to obtain more control.
Guess I just wanted to see if anyone had used the voltage ratio to obtain oddball voltages
Where I work we use SCR's to control power to stretched wires to cut EPS. If the wire is long (15' or so) it takes approx 480V Max. shorter wires lower voltage. In this case, I have a 2' wire. The original machine (iso-trans blew up) had an adjustable transformer with an Iso-trans following. The input to the machine is 120V 20A max. So I know I can replace the adj. transformer with the SCR but I am not sure if I will have enough control over the temp of the wire...I can lower the voltage to obtain more control.
Guess I just wanted to see if anyone had used the voltage ratio to obtain oddball voltages
Last edited:
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
170202-2340 EST
boptrop:
You ask a good question, but I think it is the wrong question, and you did not provide enough information.
We really need more load information.
Using some of your data I can roughly conclude:
1. Maximum available power to the load is 20*120 = 2400 W.
2. Don't know if you have a single wire as a load, or multiple wires physically in parallel. If multiple wires physically in parallel, then electrically these could be either series or parallel.
3. 2400 W at 480 V is 5 A. This would require a load resistance of 96 ohms or about 100 ohms. Nichrome 60 is 6.75 ohms/ft and is 0.01" in diameter. 15' of 6.75 ohms/ft is 101 ohms. Looks like your load might be a single strand 15' long.
4. Reduce your length to 2' and the voltage required is 480*2/15 = 64 V to get about the same wire temperature. Current does not change, thus, 5 A.
5. You refer to a variable transformer followed by an isolation transformer. Apparently the variable transformer is OK, but the isolation transformer failed. You mentioned SCRs. I don't know why these were needed if you had a Variac (variable transformer). Placement of the SCRs could have been the cause of transformer failure.
6. For your 2' long 60 V wire you only need a secondary current capability of 5 A or a 300 VA transformer.
7. If the transformer you have is 2.5 kVA then the 120 V winding is good for about 20 A. Run in reverse with the 120 V winding as the secondary you could draw about 20 A on this secondary and be within rating. The transformer will run cool in your application, especially using a Variac at the input for voltage and thus temperature control.
Most Variacs are wired to produce about 140 V maximum with 120 V input. Thus, you can get more than 60 V out.
8. Need to know more about why SCRs are needed.
.
boptrop:
You ask a good question, but I think it is the wrong question, and you did not provide enough information.
We really need more load information.
Using some of your data I can roughly conclude:
1. Maximum available power to the load is 20*120 = 2400 W.
2. Don't know if you have a single wire as a load, or multiple wires physically in parallel. If multiple wires physically in parallel, then electrically these could be either series or parallel.
3. 2400 W at 480 V is 5 A. This would require a load resistance of 96 ohms or about 100 ohms. Nichrome 60 is 6.75 ohms/ft and is 0.01" in diameter. 15' of 6.75 ohms/ft is 101 ohms. Looks like your load might be a single strand 15' long.
4. Reduce your length to 2' and the voltage required is 480*2/15 = 64 V to get about the same wire temperature. Current does not change, thus, 5 A.
5. You refer to a variable transformer followed by an isolation transformer. Apparently the variable transformer is OK, but the isolation transformer failed. You mentioned SCRs. I don't know why these were needed if you had a Variac (variable transformer). Placement of the SCRs could have been the cause of transformer failure.
6. For your 2' long 60 V wire you only need a secondary current capability of 5 A or a 300 VA transformer.
7. If the transformer you have is 2.5 kVA then the 120 V winding is good for about 20 A. Run in reverse with the 120 V winding as the secondary you could draw about 20 A on this secondary and be within rating. The transformer will run cool in your application, especially using a Variac at the input for voltage and thus temperature control.
Most Variacs are wired to produce about 140 V maximum with 120 V input. Thus, you can get more than 60 V out.
8. Need to know more about why SCRs are needed.
.
Thanks for you reply Gar,
I am going to use the SCR because I prefer it over the Variac. The resistance of the wire can change as the shape of the wire can be changed depending on the shape desired to be cut, that is why I want to be able to deliver max power. I am not at the facility (8 hrs away). Just one wire in this case. Not using 480V in this case just giving it as example. We do have machines that have multiple wires, not this one.
In this case the Iso trans blew. no info on it is available..destroyed. The wire in this case is actually a more like a knife, thin & flat. I may not need a Iso trans.. Need to test it and see first. I do not have info on the material used. I Just asked the question of input output of the 240-120 trans arranged in 120-60V application if needed.
Your answer is correct, however, like you I am working off what I know and there is a lot of unknown. I find that the Variac's made now are junky and don't last unless you want to pay top dollar- SCR's are more reliable for the price in this application.
I am going to use the SCR because I prefer it over the Variac. The resistance of the wire can change as the shape of the wire can be changed depending on the shape desired to be cut, that is why I want to be able to deliver max power. I am not at the facility (8 hrs away). Just one wire in this case. Not using 480V in this case just giving it as example. We do have machines that have multiple wires, not this one.
In this case the Iso trans blew. no info on it is available..destroyed. The wire in this case is actually a more like a knife, thin & flat. I may not need a Iso trans.. Need to test it and see first. I do not have info on the material used. I Just asked the question of input output of the 240-120 trans arranged in 120-60V application if needed.
Your answer is correct, however, like you I am working off what I know and there is a lot of unknown. I find that the Variac's made now are junky and don't last unless you want to pay top dollar- SCR's are more reliable for the price in this application.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
170203-0838 EST
boptrop:
Apparently you prefer SCR control and that is fine, then there is no need for a Variac at all.
If you only need 60 V, then a a 2 to 1 transformer from a 120 V source is a good choice. This is better than just using 1/2 of a cycle to get an average of 60 V.
An isolation transformer is probably desirable no matter what output voltage you need so that neither input AC line is connected directly to your load.
If you use SCR control thru the transformer, either on the secondary or primary, then you need an SCR control that does not produce an excessive DC component. A DC component forces an unbalance of the magnetic core hysteresis curve and higher saturation current. Some dimmers are listed as suitable for dimming (average voltage adjustment) with a magnetic transformer. Ordinary phase shift dimmers likely have unbalanced turn on times between the positive and negative 1/2 cycles resulting in a DC component.
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boptrop:
Apparently you prefer SCR control and that is fine, then there is no need for a Variac at all.
If you only need 60 V, then a a 2 to 1 transformer from a 120 V source is a good choice. This is better than just using 1/2 of a cycle to get an average of 60 V.
An isolation transformer is probably desirable no matter what output voltage you need so that neither input AC line is connected directly to your load.
If you use SCR control thru the transformer, either on the secondary or primary, then you need an SCR control that does not produce an excessive DC component. A DC component forces an unbalance of the magnetic core hysteresis curve and higher saturation current. Some dimmers are listed as suitable for dimming (average voltage adjustment) with a magnetic transformer. Ordinary phase shift dimmers likely have unbalanced turn on times between the positive and negative 1/2 cycles resulting in a DC component.
.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
170204-1014 EST
Besoeker2:
http://www.lutron.com/en-US/Education-Training/Pages/LCE/DimmingBasics.aspx
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Besoeker2:
I agree. However, it is done and Lutron makes dimmers specifically for this purpose.
http://www.lutron.com/en-US/Education-Training/Pages/LCE/DimmingBasics.aspx
What do you mean by "bottom end of SCR module ratings"?
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Yes. The company I worked for had a lighting division that was in direct competion with Lutron. Different division to mine but I got dragged in to do power circuit analysis mainly on harmonics.
My background is mostly industrial power electronics. Variable speed drives and high current rectifiers.
At the lower end, typically for DC motor controlled shunt fields we used isolated base SCRs. The minimum rating we used was rated at 40A, 1600V. There were lower ratings but but they were the same physical size and there was little cost saving.
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