Electrician Talk banner
1 - 15 of 15 Posts

·
Registered
Scada Supervisor
Joined
·
3,621 Posts
Discussion Starter · #1 ·
We just got a heat tunnel from one of our overseas plants. It is a 380v 3 phase machine, the controls and motors have a xformer to step down. My question is the heat elements are 380v 16kw x3, If I only supply 240v to them will I get a direct heat loss of approximately 1/3. The unit will heat to over 400F and I only need 240F so at 1/3 loss it should work I think. I am going to test it but was just curious what others say. I do have control of the air flow also.
 

·
Banned
Joined
·
1,486 Posts
We just got a heat tunnel from one of our overseas plants. It is a 380v 3 phase machine, the controls and motors have a xformer to step down. My question is the heat elements are 380v 16kw x3, If I only supply 240v to them will I get a direct heat loss of approximately 1/3. The unit will heat to over 400F and I only need 240F so at 1/3 loss it should work I think. I am going to test it but was just curious what others say. I do have control of the air flow also.


You wan to put 380v 50hz Heater elements on 240v60hz ?


pete
 

·
Registered
Joined
·
5,522 Posts
Resistive heater elements don't care about frequency, they DO care about voltage.

All you know is voltage and watts so far, but it's enough.

P = E^2/R. The R is fixed by the heater elements, but you don't know what it is yet.

So...

R = E^2/P and you do know that. So R = 380^2/16,000. = 9.025ohms

So the new P at the lower voltage is 240^2 / 9.025 = 6.382kW

Shortcut for future use:
Take the difference in the voltages as a percent, square it, and multiply by the original wattage.

So (240/380)^2 * 16000 = 6382!

Now, all that said, if you are using a transformer for all those motors and controls, and they are not rated for 50/60Hz (which is unlikely), they will burn up.
 

·
Registered
Scada Supervisor
Joined
·
3,621 Posts
Discussion Starter · #7 ·
Yes the controls are ok. The machine is actually made in US.
Thanks

They are in connected in delta

Thanks for the input guys,I did not think there would be that much KW loss
 

·
Registered
Joined
·
5,522 Posts
Yes the controls are ok. The machine is actually made in US.
Thanks

They are in connected in delta

Thanks for the input guys,I did not think there would be that much KW loss
Keep in mind though that it may not effect the ultimate temperature you get to, but it will affect how fast you get there.
 

·
Premium Member
Joined
·
20,983 Posts
We just got a heat tunnel from one of our overseas plants. It is a 380v 3 phase machine, the controls and motors have a xformer to step down. My question is the heat elements are 380v 16kw x3, If I only supply 240v to them will I get a direct heat loss of approximately 1/3. The unit will heat to over 400F and I only need 240F so at 1/3 loss it should work I think. I am going to test it but was just curious what others say. I do have control of the air flow also.
You SURE you're gonna be able to get 240v to them in an industrial setting? That would be an odd voltage unless you have a delta service.
 

·
Registered
Scada Supervisor
Joined
·
3,621 Posts
Discussion Starter · #10 ·
You SURE you're gonna be able to get 240v to them in an industrial setting? That would be an odd voltage unless you have a delta service.

Yea we have a 240v High leg delta as well as 480v and 208v. we run all our heat tunnels at 240v that’s the way they come in.

Funny every place I worked for as an employee had a delta system, the navy yard had a corner grounded system. That was odd taking reading.
Phase A to Grd 240V.
B to Grd 240V.
C to Grd 0V.
That is where I served my apprenticeship, when I went for another job and they asked me what voltages do you get to ground on a delta system and I gave them that answer they just looked at me funny because they never heard of it, they had a high leg system and wanted that answer.




 

·
Registered
Joined
·
1,437 Posts
Resistive heater elements don't care about frequency, they DO care about voltage.

All you know is voltage and watts so far, but it's enough.

P = E^2/R. The R is fixed by the heater elements, but you don't know what it is yet.

So...

R = E^2/P and you do know that. So R = 380^2/16,000. = 9.025ohms

So the new P at the lower voltage is 240^2 / 9.025 = 6.382kW

Shortcut for future use:
Take the difference in the voltages as a percent, square it, and multiply by the original wattage.

So (240/380)^2 * 16000 = 6382!

Now, all that said, if you are using a transformer for all those motors and controls, and they are not rated for 50/60Hz (which is unlikely), they will burn up.
Why would the motors burn up. They should just spin faster. I've done this before and never had an issue.
 

·
Registered
Joined
·
5,522 Posts
Why would the motors burn up. They should just spin faster. I've done this before and never had an issue.
Motor output torque is a function of the ratio of voltage and frequency. A higher V/Hz ratio creates unnecessary magnetic flux in the motor that does no work, it just adds heat. A lower V/Hz ratio causes a loss of torque, the slip increases, the motor pulls MORE current for the same amount of work and heats up more, assuming the same load.

A motor designed for 220V 50Hz has a V/Hz ratio of 4.4:1. If you give it 220V 60Hz, the V/Hz ratio is 3.67:1, 17% lower. Motor torque varies by the SQUARE of the change in that ratio, so that means it is reduced to roughly 70% of what it should be (.8333 x .8333), meaning what the motor was DESIGNED in the machine to provide. If the motor is over sized by 30% it may never be a problem, but now days, who over sizes a motor by 30%?

Maybe you found some...

PS: To expand on the concept as it relates to this thread more...

In the beginning, he said the machine was designed for 380V 50Hz, he has 480V 60Hz and used a transformer for the heaters. My comment was meant to highlight that doing so for the heaters was fine, but to leave the motors alone.

380V 50Hz is a V/Hz ratio of 7.6:1, 480V 60Hz is a V/Hz ratio of 8:1, if you are +-10% that's fine, this is only 5%. It's no accident that you can run a 380V 50Hz designed motor without using a transformer, in fact a lot of motors you get from the EU that have weird sounding HP ratings are like that because they are really 380/50 designs re-rated for 460/60. The speed will be 20% higher so the nameplate HP is higher, but the torque will be within spec and most machines are designed around torque.

Centrifugal pumps and fans become a problem, but that's another issue.
 
  • Like
Reactions: Ultrafault

·
Registered
Scada Supervisor
Joined
·
3,621 Posts
Discussion Starter · #15 ·
Results

JRaef
You were right on with the numbers, 8.89 Ohms and 16amps. It was plenty to run the heat tunnel for the test we need to do commissioning another piece of packaging equipment.


Thanks for the input guys its been along time since theory
 
1 - 15 of 15 Posts
Top