Electric Motor: The flat wound electric motor used in the Jefa drive units is not a standard industrial motor, but specially designed and produced for this application. This solution is more expensive but there are multiple advantages over standard industrial motors.

• The collector (1) is extremely big so a large contact surface of the coal brushes is achieved. The result of this is a very high efficiency under heavy load and a very small drop in speed at the maximum torque. (Only 25% loss of speed at full load compared to a stall situation (zero speed) with other brands of autopilot drives).
• The central motor shaft runs in ball bearings in stead of gliding bearings. This results in a higher efficiency, less noise and a much longer life time.
• A large flat wound rotor to achieve a high starting toque and an immediate response to the autopilot speed control signal.
• Specially produced magnets with an extremely long lifetime and field strength. (Normal magnets can loose up to 50% field strength over a couple of years).
• Exchangeable coal brushes to guarantee an infinite lifetime of the drive.
• Aluminium motor housing in stead of sheet steel plate to avoid corrosion.

Planetary gearbox: To achieve a correct rudder travel speed (hard over time) the electric motor has to be reduced in speed with a factor 486:1. Some autopilot drive producers use a worm reduction box, but the efficiency is extremely low as the gears rub each other. Some producers use a combination of a planetary gearbox with a 2 stage spur gear set (one small gear and one big gear), but this creates a much bigger unit with an offset to the centre and much heavier loaded gears. The best way to achieve this high reduction in combination with a high torque is a 3 stage planetary gearbox. It's also the most expensive way as every gear step has 4 gears in stead of 2 gears but the huge advantages overrule the higher price:

• The highest possible efficiency compared to any other gearbox.
• All forces are equally spread over 3 gear teeth in stead of one allowing a much compacter and stronger solution.
• The forces and torques from the motor to the output shaft remain in the centre line of the drive unit, resulting in a higher efficiency and extremely reduces the loads on the housing and other internal parts.

Electro magnetic clutch: On the moment the transmission steering system on the yacht is manually operated, the autopilot drive has to be disconnected from the steering system. This is achieved with the electro-magnetic clutch (4) and controlled automatically by the autopilot junction box. Jefa has developed a unique and patented engagement clutch. The solution is based on two electrically operated spring loaded clutch pins that engage and disengage the outer gear ring of last planetary gear step. This solution has multiple advantages over the existing friction plate clutches:

• Less friction to back drive the unit.
• Lower power consumption (1.3 Amp. at 12 Volt, 0.7 Amp at 24 Volt). When the clutch isn't powered, it's disengaged and engaged when powered.
• The clutch doesn't wear in time.
• More compact than any friction clutch.
• When the autopilot is switched off, the helmsman isn't suddenly confronted with the full rudder torque, but has to put loading on the wheel to equalise the forces so the the clutch can disengage, making the manual take over much safer. 

Jefa transmission drive units

Part No.

Description

Price in Euro

DU-TS250-12

Transmission autopilot drive unit 250 Nm 12 Volts

DU-TS250-24

Transmission autopilot drive unit 250 Nm 24 Volts