Electric Propulsion

The most important factor regarding an electric drive installation is the efficiency of the components. Overall efficiency of the system is calculated by multiplying the individual efficiency of each of the components. Our system utilizes a Mars brushless, permanent magnet motor (93% efficient), a Sevcon controller (93% efficient) and a Edison gearbox (97% efficient). System efficiency is .93 x .93 x .97 or 83% efficient. What this means is that 17% of the power that is consumed generates heat. So when the motor is drawing 4.7 KW, 17% of that or 800 watts is being converted into heat.

Electricity does not like heat, all conductors become less conductive when they are hotter. If the engine room is unable to dissipate 800 watts of heat in this example, the system will overheat. When the system overheats, the controller either reduces power output or shuts it off completely, something you do not want to happen. When you select efficient components you create less heat. All systems create heat because nothing is 100 percent efficient. Dissipating the heat ensures that the system stays cooler. Heat sinks, fans, adequate ventilation and water cooling are several ways the heat can be dissipated.

A DC motor accepts direct current, while an AC motor accepts alternating current. Here's the interesting part though - all electric motors are AC motors. DC motors convert DC to AC mechanically through the armature and brushes. The Mars brushless motor is a 3 phase AC motor. The controller is where the DC is changed into AC. AC motors are desireable because they do not require brushes. Brushes require maintenance and also spark so are not ignition proof.

A motor shaft turns because the magnet that is attached to the shaft ( the rotor), is inside a moving magnetic field created by the stator. At least one of these two magnets must be an electromagnet. Most motors use two electromagnets. An electromagnet is a created by wrapping a wire around a metallic object and then applying electrical power to that wire. Effectively this is a short circuit which creates heat. A Mars permanent magnet motor employs rare earth permanent magnets in its rotor, thus eliminating the need for the second electromagnet, thus increasing efficiency.

Brushless, Permanent Magnet motors are the most efficient motors available today.

The Sevcon Permanent Magnet AC controller which we use for the Mars motor is a variable frequency inverter which converts DC power into 3 phase AC power.

Higher voltage is desirable because it requires less current, however higher voltage is also dangerous. You have DC and AC voltages on a boat. DC voltage is considered dangerous over 50 volts while AC voltage is commonly employed and considered safe up to 120 volts. Our electric drives are DC between the batteries and the controller and then AC between the controller and the motor. We are currently using a 48 volt system but are going to be introducing a 72 volt system soon. With respect to the design, anything in excess of 50 volts must be handled carefully and fully insulated with a secondary sheath enclosing the high voltage DC cables. My recommendation is to pick a voltage that allows you to not exceed 100 amps in current. If you need 4.8KW, use a 48 volt system. If you need 7.2 KW, use a 72 volt system.

Amperage creates heat. Lower voltage systems require higher amperage to create the same power. Remember that Power is Voltage multiplied by Amperage. Reducing amperage is always desireable. However lower voltage is safer. You are going to have to deal with amperage and minimize its effects on the system. Use high quality, large cables for your main power cables. I use 2/0 cable on my DC battery cables. They have an ampacity of 330 amps so are well within their designed use. Larger cable has less voltage drop so the engine uses a higher voltage. This is especially important when the electric engine is functioning at full power.

Above the Waterline has solved the gear ratio problem associated with electric drive installations by using Browning Gearboxes. We have 10 different gear ratios between 1.25:1 and 5:1. We offer an exchange program for our customers to allow them to optimize to their particular sailboat. All propellors are different and all packing glands have different resistances so this service is very important. No other electric drive manufacturer offers this service as part of the purchase.

If the motor does not reach its peak operating current it will not develop its peak power.

If the motor exceeds its peak power it will cost efficiency. However, it may be desireable to be able to develop more than its peak power. If it is geared to develop more than its peak power, beware of the intermittent rating. For instance, Sevcon's controller is capable of producing 95 amps at 48 volts continuously, however it is capable of producing up to 200 amps for 2 minutes. If pressed to exceed the time limit, either the controller will overheat or a time out circuit is activated, then the controller either substantially reduces power or shuts off completely.

Gearing it to be able to develop the intermittent rating allows incredible acceleration and deceleration. One test showed achieving full boat speed within two boat lengths when geared in this fashion. We recommend if a customer wants to gear their boat this way, that they do so at their own risk, and install an ammeter with a warning of exceeding the continous ratings.

CONCLUSION

Properly installed, an electric motor can give years of reliable service, all at the touch of a switch. Be careful and play close attention to the details of the installation for safety and efficiency. Higher efficiency translates into higher power and longer range. Use quality components and don't rush the installation. Electricity is powerful and when harnessed safely can be the most enjoyable means of transportation known to man.