It's a KT controller. The white wire is where it gets the speed signal from. It should be connected to the motor, which should have a speed sensor inside it. If there's no speed sensor in the motor, then you should use an external wheel magnet type.
The spee limit is set in the software, and in most cases can be adjusted by through an LCD. Without a speed sensor, it will get the speed signal from the motor anyway, so disconnecting the white wire won't work.
You need an LCD if you want to change something. Any of the KT ones should work: LCD1, LCD3, LCD4, LCD5, LCD8, etc.