You really don't care about frequency. All you care about is staying below the maximum allowed battery voltage (or staying below whatever the signal on the BMS says is "to high".)

Thus, the generator speed (and the engine speed) can be whatever you wish it to be. Of course, you have to stay below the design limits for the generator rotor speed, but that is typically 3600 rpm or something close to that.

Large generators have some sort of field control, so you run the rpm up to the desired amount, then dial up the field current until the generator is either putting out the max current it is designed for, or the voltage has reach the max battery voltage.

Conversely, you can set the field current to some fixed value, and run the rpm up until you reach one of the above same limits.

Bill D.

On 3/6/2018 11:04 PM, George Tyler via EV wrote:
He only wants 30 kw, many engine could do this at lower rpm. Low rpm is not a 
problem as long as it is high enough to keep the oil pressure up and not cause 
vibration that may cause crankshaft failure.

-----Original Message-----
From: "Lee Hart via EV" <>
Sent: ‎5/‎03/‎2018 04:55 p.m.
To: "R. Sparks Scott" <>; "Electric Vehicle Discussion List" 
Cc: "Lee Hart" <>
Subject: Re: [EVDL] Genset range extender

R. Sparks Scott <> asked me to post this for him, as he's
not subscribed. He wrote:

Aside from the worn-out idea that impromptu series hybrids are
anything worth bothering with, John needs to know that his *diesel*
genset head is intended to be driven by a diesel engine, not a
gasoline engine. Why this matters is that all directly-driven
generator heads for diesels will be optimized for 1800 RPM at rated
output (for 60Hz), while gasoline (and LPG) heads will expect 3600

Obviously, if he is intending on rectifying the output to DC, it
doesn't matter that the head won't need to be on-frequency of 60Hz,
but forcing a gas engine to lug along at lower RPMs to make the
voltage more-or-less correct is going to be a strain on the ICE and
probably give him a big hit in available peak voltage, and by
association, available current into his battery pack/motor.

My inclination would be to stop encouraging him until he understand
the dynamics of what he is proposing. Series hybrids don't work well
in the best of designs, and he's throwing together a collection of
incompatible components that are sure to disappoint.
That's a good point. While I've seen gasoline gensets that ran at 1800
RPM, most do indeed run at 3600 RPM. This makes them smaller (but
noisier). Gasoline engines need to run fast to generate their rated

But if he's driving it with a car engine, most of them will happily run
at 1800 RPM for hours on end. You just can't get anything near its rated
horsepower. For instance, it might require a "100 HP" engine to produce
30 HP continuously.

Projects like this are always a trade-off between the "right" way, and
the "expedient" way to do it. Using what you have, in unconventional
ways isn't perfect, but may be good enough to get by.

Please discuss EV drag racing at NEDRA (

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