Seam welds are a series of spot welds in a straight line. They can be overlapping or separated. Overlapped welds are used in fluid tight application and spaced welds in normal structural weld situations. A washing machine drum would be a water tight application. If the welds are separated the process is sometimes referred to as roll spot welding.
Continuous Seam Weld
Roll Spot Weld
The spacing of the weld is determined by the wheel speed and the timing of the welds. Any seam welder is capable of either spacing as long as there is adequate power and cooling.
Reference: AWS C1.1 Recommended Practices for Resistance Welding
RWMA Manual Section 4