The other day I was sawing some wood on my mitre saw. I have a single light bulb in that room and it was on. The moment I press the button for the saw to turn on, the lights dim a little and go back to full brightness while the saw is running. The saw was connected to the same room. Why does this happen only when the saw is started? The saw has supposedly slightly less than 2 kW power and uses a universal electric motor.
The wires in your home have resistance. Current going through those wires causes a voltage drop. The startup (inrush) current of electric motors can be several times their normal operating current. It's impossible to know exactly what the startup current is for your motor without knowing specifics supplied by its manufacturer. The high startup current occurs because the wires in the armature windings appear almost as a short until they set up a magnetic field (become energized). The momentary high current doesn't last long enough to cause the breaker to trip or fuse to blow. There's a pretty good explanation here.
So...the momentary high current causes a like momentarily high voltage drop which in turn causes the light to dim.