Meaningful performance indicators
Torqeedo provides meaningful performance indicators for boat drives. Basically, the power of a drive is measured according to how much propulsion it actually delivers for moving the boat. This power is referred to as the output or propulsive power. It is calculated by the power times the speed and can be expressed in watts or horsepower.
Although the propulsive power forms an extremely informative index for boat drives, most manufacturers don’t provide it.
Suppliers of electric motors generally use the input power of their motors as the power rating. However, only a fraction of this value is actually available to the boat drive as propulsive power. The rest is lost – for example in the motor or propeller – as inefficiency. Manufacturers of combustion engines, on the other hand, usually give the power at the motor shaft as their index. Also here, only a fraction of this value is available as propulsive power.
Propellers on combustion engines of a lower power class cause some 70% of the motor shaft power to be lost as inefficiency that cannot be converted to propulsion.
The second important index is the overall efficiency. It describes the efficiency with which the drive system converts the available energy sources into output power and is calculated by dividing the propulsive power by the input power.
Torqeedo builds the most efficient outboards on the market by far, by means of consistent orientation towards propulsive power and overall efficiency. As the overall efficiency of a drive system is given by the product of the partial efficiency levels of all components, a single poor partial-efficiency level can have a significant negative influence on overall efficiency. For this reason, Torqeedo takes great care in the comprehensive optimization and interaction of all components. Another commonly used motor index is the static thrust expressed in kilogram force (kf) or Newton (N). In comparison to the propulsive power and overall efficiency, this index is less meaningful, as it only measures propulsion in association with the static bollard pull experiment. In this case, as the speed is zero, the effective power (power times speed) is also zero. As a consequence to this, the static thrust does not provide any information on the propulsive power that can be actually achieved and is therefore not a meaningful index when taken in isolation. It merely serves as an indication for the maximum boat size that the motor can be used on. Before purchasing a motor, it is recommendable to test the static thrust specifications
personally. In Torqeedo’s tests of competitor products, the manufacturers’ specified static thrust values could generally not be reproduced.
High-performance and safety: Torqeedo LIMA cells
Lithium-based battery systems are by far the most powerful energy carriers currently available. On the one hand they are characterized by a high specific energy density. This means that they are able to store a large amount of energy per kilogram of battery weight. In addition, lithium batteries can withstand high current: in other words, they are able to deliver their capacity even under high loads. Both of these characteristics are of great importance for applications in boat drives: on the one hand, the battery weight and volume on board is reduced. On the other hand, the lithium-based battery systems ensure that the power supply does not collapse even when the boat‘s electro-motors temporarily draw high currents from the batteries.
An additional advantage: lithium batteries do not display a memory effect and are cyclically stable. Even when stored for many months, almost no charge is lost, in contrast to conventional lead batteries.
Torqeedo batteries are also extremely robust and are secured against incorrect handling. In addition, they are protected against short-circuiting, overvoltage and excessive discharge.
The high energy density of lithium cells demands effective safety technology. For this reason, Torqeedo uses exclusively lithium-manganese safety cells. These so-called LIMA cells offer the highest safety standard of all lithium-based batteries: Only LIMA cells are able to master the necessary safety tests even when the safety electronics are switched off. These safety tests include in particular:
•Crash tests: a fully-charged battery is crushed with a ten-ton hydraulic press
•Nail tests: a nail is hammered right through the case of a charged battery until it comes out of the other side. This leads to the maximum possible number of battery cells being short-circuited.
•Overcharge tests: the battery packs are overcharged with 42 V at 7.5 A until the cells are destroyed.
•Short-circuit tests: the main contacts are short-circuited without the safety circuit.
•High-temperature tests: the aforementioned tests are undertaken on a 150° C pre-heated battery.
Alternative lithium battery concepts in round cell format or in lithium polymer packs (“Li poly”) do not fulfill these criteria due to the chemical composition of the battery (cobalt or nickel cathode). Additionally, in case of a fault such as, e.g. short-circuit, overload, mechanical damage, lithium polymer packs react in a highly critical manner due to the missing safety mechanisms immanent within the cells.
Background knowledge on battery capacity:
The stored energy quantity – in other words, the capacity of a battery – is measured in watt-hours (Wh). For historical reasons, the battery capacity is sometimes indicated by the nominal voltage in volts (V) and the stored charge in ampere-hours (Ah).
In this case, the stored energy is calculated as voltage times charge.
Motor and power electronics
The motor is the core of a boat drive. With its torque motors, Torqeedo sets new standards in the areas of torque, efficiency and power per weight and volume. The motor which equips the Travel 1003 with the effective power of a 3 HP combustion engine only weighs 500 g and is no larger than a packet of cigarettes.
In the case of a torque motor, the objective of the design is to use as many factors as possible to maximize the torque. Torqeedo has uncompromisingly optimized the torque motor and, with the introduction of synchronous motors, which come as permanent excited magnet, electronically commutated and designed as external rotor motors, has created true torque giants.
This motor equips the Travel 1003 model with the effective power of a 3 HP combustion engine – with a considerably higher thrust. The motor has a stator diameter of 43 mm and weighs 500 g. As a comparison: an equivalent-type motor the size of a matchbox and the weight of a letter (22 g) would suffice as the drive of a bicycle (70 W peak power).
Clever brain – the new digital Torqeedo power electronics:
The electronic commutation of electric motors described above can generally be either analog or digital. While most providers of electric motors continue to work mechanically using carbon brushes for commutation, Torqeedo has gone two steps further and uses digital power electronics in its new motor models. In contrast to analog-based electronic commutation, digital electronics has a more intelligent power control and handling. This provides more power, more stability and more comfort.
The intelligence of the power control is in its combination of propeller-speed control and control of the power intake. The propeller-speed control regulates the rpm of the propeller, i.e. the motor keeps tightly within the speed specifications and draws whatever power it requires to reach the defined propeller-speed. If, on the other hand, the power consumption is controlled then the drive processes the power made available to it as good as it can and the resultant force is then the speed of the propeller.
Compact power – the Travel 1003 motor:
However, that is not all: The use of high-tech materials further improves the performance parameters of the Torqeedo torque motors: a Torqeedo motor, such as that used in the Travel 1003 model, uses rare earth magnets instead of hexaferrites.
Altogether, it exceeds a conventional internal rotor motor by 24-fold torque – although it is the same size. Combined with a 1:14 step-down gear, the 1000-watt motor can easily drive a propeller typically used by a 20 HP combustion engine.
The Torqeedo motors also take on a new dimension with regard to efficiency. They experience no excitation current and brush losses: and are equipped with additional, patented control mechanisms, reducing losses to a minimum. The efficiency not only ensures the efficient use of the available battery capacity – it also prevents thermal problems. In this way, Torqeedo can combine high performance with a small structural shape.