I'll relate a story about Vacuum Cleaner Motors:
When I was in high school electronics shop class 3/4 of a lifetime ago, my buddy brought an old vacuum to school and took it apart. He had it on the work bench, and rigged a 120V suicide cord to power it. He plugged it in. It began accelerating and began sounding like a siren or screaming banshee...
Mr Allen, the shop teacher was clear across the room, and I remember to this day how he looked up, had a puzzled look on his face, his eyes got wide, and he began running toward my buddy. He only got about 1/2 way across the room in the time that the motor reached (we figure) about 35000 rpm. With no impeller attached, there is nothing but windage in the core gap to limit the speed of a series-wound motor...
My buddy realized something bad was about to happen, so fortunately he ducked under the table. I turned my face away. I still remember that time had slowed and Mr Allen looked like he was moving in slow motion...
Just as the sound became piercing, the motor exploded. Pieces flew radially at right angles from the shaft axis, and fortunately no one was directly in line with it. It punched holes in the suspended ceiling tile above it, broke the florescent light fixture, broke out a window about 15' away, and threw pieces to both ends of the room. Made a hell of a bang.
It was a real physics lesson! First, the speed of series wound motors is limited only by friction; take off the impeller, and there is only a slight load due to windage. Second, when you spin a wound-rotor at ~35000 rpm, the centrifugal force is enough to cause the copper wiring to come out of the slots in the rotor. Third, when the copper wire comes out, you instantly transfer the momentum of the spinning rotor to the stationary stator, which causes an explosion equivalent to about 1/4stick of dynamite.
Bottom line: Do not plug in a vacuum cleaner motor with no load on it!!!