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If you are looking to save money, consider salvaging the caps you need from discarded units. Check with applicance and HVAC dealers. Even though a washing machine or air conditioner is in the trash pile, the caps inside may be fine. Motor start capacitors are used on virtually every commercial motor application out there. Good Luck!
When power is applied to a larger motor, it must overcome the 'at rest' state to get running. A large (usually oil filled) cap can give the motor a 'hot-foot' to get it running. Once it has attained ( a majority of) its rated RPM, the power suplied can keep it running.
Induction Motors are available in different types for different purposes...
If a motor is required to have a predictable direction then it will have a start capacitor (a cap-start motor), the motor has two windings with one being the full-time 'running' winding and the other (in series with the capacitor AND a centrifugial switch) is only used to start the motor and accelerate it up to around half-speed when the centrifugial switch will open and the motor runs on the single 'running' winding.
These motors take about SEVEN times as much power to start as they do to run due to the low impedance of this 'start' winding.
During the start they are effectively two-phase motors, the cap giving some phase-shift for the second winding.
A second design (the cap-run motor) has a much higher 'start' winding impedance so that its current is much lower and there is no centrifugial switch. The 'start' winding now stays in circuit all the time and the motor will cope with bigger loads but will be poorer on starting.
Another design adds a third winding and another capacitor. There is now a 'start' winding (with centrifugial switch) and the two 'run' windings. These cap-start, cap-run motors are more complex and so cost more.
OK, so the capacitors ...
start capacitors are NOT capable of sustained operation - don't use them!
run capacitors expect to see continuous duty so are the type you require.
If you have only a few uF I guess they are not motor caps.
As you live in the UK I will suggest RadioSpares (http://rswww.com) for new caps (or info anyway!)
A 50uF motor-run cap is £12.98 + VAT (stock code 377-8879)
### IMPORTANT ###
The website assumes 120vAC mains at 60Hz, the UK (as you know) has 240v at 50Hz.
The website talks vaguely of speeds and having to 'overspeed' the motor. This is a simple issue of 'magnetic slip' in the motor. The rotor has to move slower than the magnetic field in order to induce into the rotor's windings. You are going to reverse this effect and have to rotate faster than the field you hope to generate.
The motor's dataplate will give a speed (probably 1480rpm or 2940rpm), notice these are both just slower than the UK mains 3000rpm (or 1500rpm for a four-pole device), you need to exceed the mains speed by about the same amount.
Exceeding this speed significantly could cause the motor to 'shatter' with centrifugial forces. Too slow and it won't generate?
The frequency you generate will be a function of this speed and the load on it.
DO NOT GET CASUAL WITH 240v It Can KILL !
The capacitors WILL BITE if you touch them while the 'generator' is working.