This invention relates to electrically powered outboard motors, particularly to such motors that include electronic control for varying the speed of the motor. More particularly, the invention relates to an improved arrangement for cooling a power switching semiconductor of an electronic control for such motors, and a mounting structure therefor which is readily adaptable and usable with a variety of electric outboard motors.
Electronic control systems for controlling the speed of an electric motor are well known. Such systems generally incorporate a manually controlled potentiometer for varying the control signal to a semiconductor switch. In one form of control the emitter and collector of a power switching transistor are connected in series with the motor and the potentiometer is connected to the base of the transistor to regulate current flow through the transistor to the motor. In another form of control, a thyristor switching device such as a silicon controlled rectifier has its anode and cathode connected in series circuit with the motor and the potentiometer is connected in a resistor-capacitor (RC) timing circuit to vary the conduction period of the silicon controlled rectifier thereby to control the voltage applied to the motor windings. In either circuit embodiment, the power switching device generates significant heat which must be dissipated to protect the control components.
Control schemes of the aforementioned type utilized in electric outboard motors commonly locate the power switching semiconductor switch within the watertight housing of the lower drive unit of the outboard motor wherein heat given off by the semiconductor switch is transferred to the housing. When the motor is operating in a submersed condition, heat is conducted away from the housing by the water. When the motor is operated out of the water, the housing serves as a heat sink which distributes the heat over a larger surface and dissipates it to the air.
While the aforementioned embodiments are suitable for their intended purpose, there are disadvantages associated therewith. One disadvantage is that the interior of the motor housing must be structured to provide space for mounting the semiconductor switch therein. Another disadvantage is that heat generated by the semiconductor switch within the sealed housing of the lower unit is additive to heat generated by the motor. If the motor is operated for a substantial period of time in a non-submersed condition, the motor housing may not be an adequate heat sink to dissipate the combined heat to air, thereby entrapping potentially damaging heat within the housing.