AC power creates a Sine Wave. It's the "alternating up and down wave you see when we "Draw" AC. The sine wave is pure from the power company, but when the brownouts happen the current drops and they cause damage.
Many UPS' - the budget ones especially - create a "simulated" sine wave, these ones are not as good as a pure sine wave UPS. That said, the cost of a pure sine wave UPS is prohibitive in a non-commercial environment, and a sewing machine is not typically sensitive enough to know the difference. The Simulated sine wave vs the pure sine wave, is like the difference between a digital photo and one taken on a film camera. The film camera is a purer representation, but the digital image looks the same at a regular distance, but if you get in really close you can see it's different, but it's close enough for most of us.
There's a bit of an explanation here:
http://www.tomshardware.com/forum/28...-wave-inverter - They're discussing computers, but the science is the same.
All UPS' buffer brownouts. That's part of the "Uninterruptible Power Supply" portion.
The higher end ones will kick in faster than the lower cost ones but even the lower cost ones provide much better protection than a surge protector.