Motor Power Curve

Generally power curves you may find on line are really not very helpful in defining how the motors will perform in an application. They are generated by running a motor at a constant voltage and varying the load on a dynomometer. This is useful for company a like Electric Yacht, when designing a system, but not very useful for determining how it will perform in the end application.

The Electric Yacht systems are set up with a motor controller that effectively varies the voltage applied to the motor. By varying voltage, speed and torque can be controlled. The curves in the graphs do not directly apply under these conditions.

The controllers are set up with a current limit which effectively controls the maximum torque the motor can apply. From zero speed to a maximum determined by the battery voltage, the torque “curve” is a flat line limited by the amount of current the controller will supply to the motor.

This also is not very useful, except it will determine the maximum available torque. In use, the motor is driving a propeller. Therefore, it is much more useful to plot current or power against shaft speed or boat speed using a propeller’s torque absorption curve. We can provide these plots. We use data from your vessel to estimate performance. The most important are LWL, displacement, and general hull type.

We use several different Motenergy motors and set the torque limits as follows:

ME1118: 24 Nm Approximate maximum shaft speed is 2000 RPM

ME1114: 35 Nm Approximate maximum shaft speed is 3000 RPM

ME1115: 48 Nm Approximate maximum shaft speed is 2000 RPM

ME1616: 78 Nm Approximate maximum shaft speed is 2000 RPM

Maximum shaft speed will depend primarily on actual battery voltage and how well the propeller is matched to the motor. The propeller shaft torque is multiplied by a reduction ratio in a range from 1.42:1 to 2.22:1 for our standard frame and up to 3.33:1 for a wide frame we use with certain 48Vdc liquid cooled, 72Vdc and 96Vdc systems. Also, the QT 20.0, 30.0 and 40.0 motors use two of the ME1114 or ME1115 motors, so torque is double.  QT 30.0 LC, QT 45.0 LC and QT 60.0 LC use two ME1616 motors, so torque is doubled.

Measured efficiency under high load is in the 80% to 85% range. This is total system efficiency and not just the peak motor efficiency often quoted by our competitors. They conveniently leave out motor controller losses, bearing losses and wiring losses that occur in any real system. In practice, a well-designed system will have similar efficiency to ours. A poorly designed one, or one with older technology components, will be less.

The Electric Yacht Engineering Team

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