Electric Motor vs. Diesel Engine

Hello,

On a boat there are two diesel engines, each 110 Hp, for the main propulsion.

Because the boat is only going to be used for inland navigation and the speed of the boat is limited to 60 km/h, the two diesel engines have too much power and consume too much of petrol.

We are considering exchanging the 2 diesel engines with

2 electric motors, 2 frequency converters, a transformer and a diesel generator set.

We estimate we only need 75 Hp for the main propulsion. The propeller needs to turn at a shaft speed of 4500 RPM

We are thinking about the following solution, please give us advise if we are working in the right way.

We use a 3 phase motor 18.5 kW at 50Hz 230/400V. The motor is connected in delta and takes 56.3A at 230V We are going to supply the motor with a VFD.

The VFD output voltage changes linear form 50Hz/230V to 86 Hz/400V

• At 75Hz the output voltage equals 345V
• At 75Hz we have an output speed from 75/50 x 3000 = 4500 RPM
• At 75Hz we have an output power from 75/50 x 18.5 = 22.7 kW

The two motors together deliver an output from 2 x 27.75 = 55.5 kW (75.4 Hp)

• The VFD delivers a current from 58 A continuous, each motor consumes 56.3 A
• The VFD overload capacity for current is 150% for 60 sec. and 200% for 3 sec.
• The VFD uses sensor less vector control to control the motor speed

The input voltage of the VFD is increased to 450V. If the output voltage from the generator decreases, the VFD will "see" enough voltage, because the capacitors of the VFD are only reloaded if the instantaneous voltage of the generator set is higher than the actual voltage of the capacitor.

A three phase separation transformer is placed between the 2 VFD's and de generator for

• increasing the voltage form 400 to 450 Volt
• to have a time delay from 30 electrical degrees (the low voltage side are delta and triangle connected)

Because of the time delay, the current spikes the generator "sees" are not at the same moment and the harmonic distortion will be lower. The generator current will be more sinus shape.
The problem I fear is that the generator will give an output voltage that is round off at the top. The RMS value of the voltage will be 400V but the top value will be less then SQRT 2 of it.

Please give us your remarks about this solution.

Is it necessary to use a separation transformer will an auto transformer also fit ?

Perhaps only one VFD on a transformer ?