I've finally finished my electrical motor speed controller box to use with line winder. It is based on a chinese PWM controller bought for about 12$ on aliexpress.com. It can deliver up to 15A with voltage range of 12-30V.
I've added voltage meter, amp meter and a rotary switch. Switch is used along with electrical relay to connect and disconnect PWM controller to the motor. In the future I'll add additional PWM controller and some artificial resistance load to create electrodynamic motor brake to control speed at which the line will be unwound (usefull especially with high wind speeds). What is really neat, when potentiometer is set to "0" PWM controller shorts both motor wires together - force needed then to rotate the engine manually is so big it prevents line from unwinding when pulled by kite.
Electrical relay switch coil circuit is rated for 12V so I added additional resistors to prevent it from damaging when 24V is applied.
