Motor Caps
Scarr, the fact that you have to give the motor a spin confirms that you don't have an operating start circuit. (By the way, hand starting a motor by pulling on the belt is an excellent way to lose fingers.)
The fact that when you spin the motor up it runs normally just means that you are doing manually what the start circuit should do automatically, i.e., get the rotor turning fast enough to induce phase shifts between the reversing magnetic field of the stator (which does not normally have any rotational component) and the rotating magnetic field of the rotor. With no start circuit in operation, there is no tendancy for the rotor to start turning. In fact, you can manually start and run the motor in either directioin the absence of a start circuit.
An induction motor is a lot like a transformer with a shorted secondary. If you look at the rotor, it consists of steel magnetic laminations with aluminum bars inset into slots running the length of the lamination stack and with solid aluminum rings at each end. The bars and end rings form shorted turns of extremely low resistance. With the rotor not turning (including at the instant of switching on), the transformer action of the motor causes very high currents in the rotor shorted turns. This is why there is a spike in the motor current at the instant of switching on.
With a normal start, the start circuit gets the rotor turning very quickly by creating a magnetic field that is displaced 90 degrees from the main magnetic field AND SHIFTED IN TIME BY THE ACTION OF THE WINDING INDUCTANCE AND THE CAPACITOR. This time and angularly displaced magnetic field adds a rotational component to the magnetic field of the stator, dragging the rotor along with it and starting rotation. When the rotor reaches about 1/6 to 1/5 of synchronous speed (3600 RPM for a 2-pole motor, 1800 RPM for a 4-pole motor), the reaction between the rotating rotor and the oscillating magnetic field of the main windings causes both a reduction in the currents circulating in the shorted turns of the rotor and a phase shift in those currents sufficient to cause torque that accelerates the motor to running speed and drives the load. That is the point (about 1/5 synchronous speed) at which the properly working centrifugal switch mounted on the rotor (or sometimes an external current-sensing relay) switches the start winding out of the circuit because at that point the start circuit is a burden, not an aid, to motor operation. Additionally, the start winding and capacitor are not designed for continuous operation and will overheat, often with disastrous consequences, if left in the circuit.
Scarr, the reason your motor trips the fuse if you don't get it running quickly is that it is acting like a big transformer with a shorted secondary. As I told SCMR2242, you should check the start capacitor and the centrifugal starting switch mounted on the rotor shaft inside the case. You can check the capacitor most easily because it is outside the case under that metal shell (assuming we're not talking about a split-phase motor that has no capacitor). But, if you find that replacing the capacitor without opening up the case restores normal starting, be sure to also check the functioning of the centrifugal switch to avoid the prompt failure of the new capacitor that SCMR2242 experienced. After all, something caused the capacitor to fail.
Good luck.
awright