Running a 3-phase 50hz motor on single-phase 60hz power

[richie8mann]

I'm working on a project where I'm transforming a Honda Civic into a hybrid vehicle (where the generator will run on wood-gas) and I'm having some concerns with the electric motor's power. A Baldor 7.5 hp, 3-phase, 50 hz, 400v motor was donated to my project which I really appreciate, but it's obviously incompatible with the 6500 watt, 110v, 60 hz generator that I got prior to receiving the motor (that I plan on running on wood-gas from a gasifier). The hp is adequate (you need 6 to 8 hp per 1000Lbs of weight) and I've discovered various ways of transforming single phase power to 3-phase power (using another electric motor). I know how to step up the voltage and I read the FAQ about running a 50 hz motor on 60 hz power (faq237-1224). What I would like to know before I go to all this work creating the converter is am I trying to do too much by converting 115v single phase power to 400v 3-phase power while at the same time running it at 60hz? Will this tax the motor or generator too much?
I appreciate any advice or help you can provide.
Thank you!

 

[iop995]

As most vehicle, speed need to be variable, so motor/engine must have variable speed. To have variable speed on 3 phase motor, it must be feed from an inverter with variable frequency. Compatiblity frequency between generator and motor is not an issues here. Generator may feed a rectifier and so obtain a DC bus of about 155Vdc; from DC bus by a step-up converter feed inverter that supply motor. All these power coverters may be made or may choose commercial products to build up such system; all these blocks and car itself need a control system to assure proper functions.

 

[jraef]

7.5HP motor at 90% electrical efficiency = 6200W, you have a 6500W generator. Don't drive this at night, you will have trouble running the headlights and the motor at the same time...

You have two problems to solve: converting from 115V to 400V, and converting 1 phase to 3 phase.

There is no way around the 115-400V conversion, you are going to have to add a transformer somewhere. That is not only going to be expensive (compared to the donated stuff you have now), but also heavy and for vehicles, weight takes HP.

1 Phase to 3 phase is easier, but if you are already going to have to add a transformer, adding another motor to make a Rotary Phase Converter is going to add even MORE weight. You would be better off using a VFD (Variable Frequency Drive), which is capable of being fed single phase power and putting out 3 phase power. You would need to buy a 15HP VFD however, because the current through the input side of the VFD increases by the sq. rt. of 3 and the components must be capable of that higher current. But the VFD cannot create voltage that is not already there, so you still have the transformer to boost the input to the VFD to 400VAC minimum.

You might want to check on the motor to see if it is capable of being connected at a lower voltage like 230V, many are. That would help with the cost of the VFD. You will still need a transformer though, 115V is just too low to work with for anything that is going to need 3 phase. There are some VFDs you might see advertised that will take 115V input and run 230V 3 phase motors, but what they do is use what is called a "voltage doubler" board on the front-end at the DC conversion side of the VFD. That's fine, but those are limited to 1HP; nobody makes one for anything as big as 7-1/2HP. If you were an electronics wiz (or know one that likes a challenge) you could roll your own, but the parts would be expensive.

I would try for a different generator donation that puts out a higher voltage.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Your cheapest and lightest transformer will be a 120:240 Volt auto-transformer. You may be able to get away with a 5 KVA auto transformer or a 2.5 KVA or 3 KVA conventional transformer connected as an auto-transformer. You won't be running at 100% output 100$ of the time (hopefully). Mounting the transformer in the air flow to help cooling will allow some overload.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iop995]

Using standard transformer it's sure cheapest solution but increase weight. Maybe better is to find a 380V generator; if not possible, step-up converter may be a solution, not cheap but much less weight. 6-7kW power it's not a problem for power switchs available today (maybe IGBT it's best choice). Step-up converter may be with variable output voltage and togheter with inverter frequency control can be do in V/f=ct formula to obtain variable speed of 3 phase motor although V/f it's not recomanded for vehicle because low torque at low speed (vehicle need the opposite - high torque at zero / low speeds) but for experimental design may be a first step.

 

[ornerynorsk]

Why not convert the original engine itself to run on wood-gas? You got a chain of inefficiencies every time you add another component to the mix.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[akselj]

Is 7.5 Hp enough for the car, as far as i remember the weakest Civic was 55Hp.
In my opinion you are wasting time and money on the project.

 

[roydm]

I'm with Ornerynorsk on this one. I'm not old enough to have seen them but heard stories about cars running on charcoal.

There was a discussion here about a week ago someone wanted to run a VFD on a battery as a backup, If I recall correctly it
would run at full torque up to a limited speed because of the V/F curve
If you were to rectify the 110 Volt would it not run to 110/400 x full speed?

Roy

 

[akselj]

Your project has a lot of problems.
1: Your generator is too small, 6500W, propably much less if it is converted to run on wood-gas.
2: Different voltages, can be overcome, but at great cost.
3: Too low power, you must have at least 20 KW.
4: AC motor is not suited to car operation, low starting torque, expensive regulation.