Motors

Motor selection is a complex subject. First there are a wide variety of motors available. Brushed, brushless, PM or Permanent Magnet, and Series-wound and A/C. All have their pluses and minuses, but as a general trend, PM motors, whether brushed or un-brushed have efficiency, size, and weight advantages that make them ideal for propulsion.

I opted for a brushless MARS system with a SEVCON controller - partly because it was packaged as a kit, and saved me from having to make a decision. A brushed system has some maintenance requirements for the brushes, will probably be somewhat louder, and may need a mechanical transmission for forward/reverse. But they are generally somewhat cheaper than a brushless of the same specifications. However, price should not really be weighed too heavily here - the motor is only a fraction of the overall cost of the system - but it forms the basis for almost every other component.

One advantage of a brushed motor is that you can, in theory, operate it without a controller in an emergency. In practice, this could be very challenging and possibly hazardous. Brushed motors have much simpler wiring - brushless systems have an additional wiring harness for a control circuit which communicates with the brushless controller. This makes troubleshooting much more straightforward - on a brushless motor it can be difficult to determine if the fault lies in the motor or the controller since both have integrated electronics.

Brushed series-wound motors and controllers, particularly 36V systems, are widely available on the used equipment market due to the number of owners upgrading golf-carts to 48V for higher speed and range. You may be able to find brushed PM systems as well.

Brushless PM systems are the "hot commodity" and will probably only be available new.

E-Motion Marine has a very good discussion of how brushless, PWM controlled motors and controller work.

One of the contributors to the ElectricBoats group is developing a purpose-built e-motor specifically for boat installations. Watch the video...