Induction welding has many important industrial applications. One example is the manufacture of airtight tubes, represented in Figure Q31.19. A sheet of metal is rolled into a cylinder and forced between compression rollers to bring its edges into contact. The tube then enters a coil carrying a time-varying current. The seam is welded when induced currents around the tube raise its temperature. Typically, a sinusoidal current with a frequency of 10 kHz is used.
(a) What causes a current in the tube?
(b) Why is a high frequency like 10 kHz chosen, rather than the 120 Hz commonly used for power transmission?
(c) Why do the induced currents raise the temperature mainly of the seam, rather than all of the metal of the tube?
(d) Why is it necessary to bring the edges of the sheet together with the compression rollers before the seam can be welded?