Advise on running a 20kW bldc system off AC genset

[Jeffery]

Hello,
I am looking to create a 20kw bldc boat propulsion system that will be a hybrid with a generator to power the unit if battery power does out. Generator pushes out ac power. What do you suggest to run dc system off an ac generator?

[Bikemad]

Hi Jeffery andto the forum.

You would require a generator with a lot more than 12kW output to produce enough power to run the motor at anywhere near 12kW output after losses due to the inevitable inefficiencies of the voltage conversion.

As you haven't mentioned the power output of the generator (voltage and current) or the nominal voltage, capacity and chemistry of the battery pack, it is difficult to give a definitive answer to your question.

Below you can see what the minimum current draw would be @ 90% efficiency to produce 12kW power output at different voltages:

• 12kW @  24V requires 555 Amps
• 12kW @  36V requires 370 Amps
• 12kW @  48V requires 278 Amps
• 12kW @  60V requires 222 Amps
• 12kW @  72V requires 185 Amps
• 12kW @  84V requires 158 Amps
• 12kW @  96V requires 139 Amps
• 12kW @ 108V requires 124 Amps
• 12kW @ 120V requires 111 Amps

If you have a generator with a 110V regulated output, it might be possible to connect it directly to a 120V lead acid battery pack through a suitable high current rectifier to provide additional power when the batteries are low, as I seem to recall that a rectified 110V mains output could be in excess of 130V depending upon the AC waveform and the capacitance of the battery pack. 

Alan
 

[Jeffery]

Hello Alan

Everything being discussed is for future planning, no major purchases have been made as yet. Thank you for taking the time to iron the wrinkles.

The battery bank that seems to be the best suit for this application is a lithium po4. The bank would put out 120v dc, the design and amount of units to be determined. The thought was to have the bank on a switch that could simply be turned off to prevent stray current from going into the bank and over charging the batteries, or causing the batteries damage. This would allow the full current from the genset to go to the bldc to power the vessel. Being a sailboat the vessel would not need full power to move at a comfortable speed.
Would a full bridge rectifier work in this situation? Could the lithium iron phosphate batteries still be connected or would the current cause damage? Also, where would I look to find a 120vac rectifier that puts out enough amperage to power a system like this?
 

[Bikemad]

If you are only going to use the generator when the battery pack has been exhausted, it would make more sense to leave the battery pack connected so that it can be recharged by any surplus energy being generated. This would be beneficial if the generator ran out of fuel or broke down, as you would then have more battery power available to use.

Here is a rectifier that should be able to cope with up to 200 Amps @ 138V (27.6kW) which is a lot more than most affordable generators will be able to produce:


Click here or on the above picture for more details.

I presume that the supplied heatsink will be suitable for continuous use under high current load. 

The maximum permissible charging current will depend upon the total battery capacity and the charge rate of the battery cells being used.
A LiFePO₄ battery pack that is capable of 160 Amp continuous discharge may well be able to accept a similar charging current, but you would need to check with chosen supplier.

Assuming the battery has a 1~2C charge rating:

• A 160Ah LiFePO4 battery pack (121.6V x 160A = 19.5kWh), should be able to accept a continuous 160 Amps of charging current with a 1C charge rating.
• A 80Ah LiFePO4 battery pack (121.6V x 80A = 9.75kWh), should also be able to accept a continuous charging current of 160 Amps if it has a 2C charge rating.

A discharged battery pack will typically ensure that the charging voltage is at an acceptable limit, as the maximum power output of the generator is likely to be the limited by the current being consumed by the battery and motor. But if the battery approaches a fully charged state, the generated voltage output must not be allowed to exceed the maximum charging voltage of the battery (i.e. 138V for a 38s 121.6V LiFePO4 pack).

Whilst Googling, I came across this picture:


117.2VAC @ 60Hz actually becomes 159VDC

So it looks like you might even need to reduce the generators maximum rpm to prevent overcharging a 38s 121.6V LiFePO₄ battery pack (max charging voltage = 136.8V @ 3.65V per cell ). 

Alan
 

[Jeffery]

That is great information Alan. Thank you again.
Do you think a voltage regulator that did not exceed 137 or 138 volts would work?

[Bikemad]

As long as it prevents the battery voltage from exceeding ~138V it should be fine.
Ideally, your battery should be equipped with a suitable Battery Management System (BMS) to help balance the individual cells, and also to prevent the battery from being overloaded, overcharged or overdischarged.

The LiFePO₄ cells seem to be much more tolerant of slight overcharging than other Lithium cells (i.e. LiMn and LiPo), so a charging voltage of up to 140V could probably be tolerated for a short amount of time if no BMS is fitted.

The information on the 96V HPC700H controller states an input voltage range of 90~140V, so a regulated 137~138V should also keep the voltage within the controller's acceptable working voltage range. 

When they were first introduced, LiFePO₄ cells used to be charged to 4.2V max but it was discovered the this decomposed/oxidized the electrolyte.
Nowadays, the manufacturers' maximum recommended charging voltage seem to vary slightly, and 3.5V, 3.6V or 3.65V are commonly used for LiFePO4 cells (i.e. 133V, 136.8V or 138.7V for a 38S pack).

Although the following article is not about electric propulsion batteries, the LiFePO4 Batteries On Boats section on marinehowto.com is definitely worth checking out. 

Alan