mechaniczach

I have a DW758 bench grinder that will not start. When turned on it hums and gets warm. It will only start if you give the wheel a spin by hand. I first thought it was caused by a bad start capacitor but after changing it found it not to be the problem. Has anyone ever experienced this before? I removed the rotor and it appears to be very worn out but can't seem to find a picture of a new rotor for comparison. Any ideas greatly appreciated.