Pardon me if this question covers too many elements; also, originally I posted this in Engineering stack-exchange but it may be more relevant here.

Basically I do not fully comprehend what would be the important parameters which must be matched for a system to work which is based on: an internal combustion petrol engine, whose output shaft drives a BLDC generator, which in turn generates power for a BLDC motor

Essentially, what is confusing is that I find information regarding BLDCs that they are actually (perhaps only some of them; but the ones I was looking at) 3-phase AC motors which then have rectifiers to change AC to DC (if a generator), or a converter to transform DC input power to 3-phase AC, if used as a motor. Also, as far as I understood, to power the correct sequence of coils, if using a DC power from a battery, an ESC electronic speed controller is necessary.

So, would a Petrol>Generator>Motor setup be possible without such power conversion (directly 3-phase AC from the generator to the motor) and without electronic speed controllers?

[That is, if the petrol engine has a throttle, would that be sufficient as a means of controlling how much power the generator is providing to the electrical motor, therefore directly controlling the electrical motor power output, by the speed of the petrol engine? Or do the generator and motor have to be identical for that to work, that is, which parameters need to be matched between them? The kV of the electric generator and motor (RPM/V)? or maybe their rated RPM, or pole numbers, need to be the same?

I was trying to understand this setup with the following example parameters:

Engine: 2-stroke petrol • 35kW @ 6000 RPM

Generator: BLDC permanent magnet synchronous motor (PMSM), axial flux, sinusoidal three-phase, out-runner • 33kW @ 6000 RPM, 120V

Motor: BLDC PMSM, in-runner • 33kW @ 8400 RPM, 120V

the generator efficiency at the mentioned RPM is 90-96% depending on torque, but let’s ignore the efficiency for the sake of clarity.

So, what I would be very grateful to understand, is what are the requirements for this system to work properly when starting the petrol engine & changing its speed/power?

Thank you

  • \$\begingroup\$ Not practical. Outside a relatively narrow speed range, petrol engines are wimps. Hence gearboxes. Electric (like steam) can produce full torque from 0 rpm, simplifying transmission. You'd have to find a way to deal with this. DC and an AC inverter really is the simplest way. \$\endgroup\$
    – user16324
    Dec 7, 2020 at 11:58
  • \$\begingroup\$ The question does not ask about the practicality. A powerplant system includes an energy source and the motor using it: the petrol engine uses fuel with an energy density multiple times better than the current battery technology, therefore it is often used as an energy mediator for generating the electrical power for the el. motor in applications and situations where such a setup has benefits. Thanks for the comment, though \$\endgroup\$
    – ivanantuns
    Dec 7, 2020 at 20:26

2 Answers 2


You are missing the frequency of the AC voltage of the synchronous machines. The frequency and voltage ratings must match. If they match, the generator can power the motor directly. When the ICE is running at 6000 RPM, the generator will generate 120 V at its rated frequency. If that is the rated frequency and voltage of the motor, it will run at its rated speed. Adjusting the engine throttle will adjust the speed of the motor, but the torque necessary to drive the mechanical load connected to the motor will determine the throttle setting required for any given speed.

As mentioned by @Andy aka, the setup seems pointless. However it is somewhat similar to the setup that was long used for diesel-electric railroad locomotives. In that case, the electric machines act as the clutch and transmission. However the electric machines in that case are commutator motors that have field excitation adjustment. Without electrical controls, your setup will not run very well at low speed and may have some difficulty with sudden load changes. It will be particularly difficult to start with a mechanical load connected. Even without a mechanical load, starting may be problematic.

Re comment:

The frequency is the nominal or rated frequency rating for operating at the rated voltage. The rated frequency gives the rated mechanical frequency or speed when divided by the number of poles: RPM = 120f/p. The rated voltage also corresponds to the rated frequency. As a generator. both the output frequency and the output voltage of the machine are proportional to speed. As a motor, the proper operating voltage is proportional to the applied frequency. The magnetic circuit will saturate if the ratio of V/f is too high. If the V/f is too low, the motor will not produce full torque.

The setup describe has a pretty good chance of working if the load is a propellor. The torque to drive a propellor in either water or air is very low at low speed. The starting motor would probably be able to crank the engine with everything connected, and the system would probably start successfully and perform properly under load.

  • \$\begingroup\$ Hi, thanks - that is something which I overlooked, regarding the Hz, can you please elaborate on this? I suppose by frequency you mean the nominal frequency, which when multiplied by pole number gives the mechanical frequency of the rotor? Also, the setup only seems pointless; in any case the question was not about the practicality (even though there are obvious advantages, such as the transmission-less drivetrain, and not so obvious ones which are here besides the point) \$\endgroup\$
    – ivanantuns
    Dec 7, 2020 at 20:18
  • \$\begingroup\$ See addition to my answer. \$\endgroup\$
    – user80875
    Dec 7, 2020 at 20:42
  • \$\begingroup\$ Thanks that clears up a lot, then. A match of the AC f. That is what I had in mind when thinking about this power system - an application for which this setup is suited is a propeller, or a ventilator - that is, where a low-torque load is driven by a mechanism which operates constantly more or less at its rated RPM, except at starting or at idling. But I was unaware of what was required for proper connection of the machines. Cheers \$\endgroup\$
    – ivanantuns
    Dec 8, 2020 at 0:52

So, would a Petrol>Generator>Motor setup be possible without such power conversion (directly 3-phase AC from the generator to the motor) and without electronic speed controllers?

Your idea doesn't make sense. If you are trying to control the speed of an AC motor by controlling the speed of a petrol engine then you might just as well connect the petrol engine shaft directly to the load and save yourself a lot of pointless messing around.

The point about converting to DC and driving an AC motor via a motor speed controller is that you can run the petrol engine at its most power efficient speed and use a control-circuit to produce a massive range of AC drive voltages/frequencies and, get far more dynamic speed range without the need for a gearbox.

  • \$\begingroup\$ What I was trying to figure out is how such a system would be properly set up; the question does not explore the practicality aspect. However, to illustrate this for future reference: this idea does make sense, in a situation where structural or shape restraints remove the possibility of a direct drive; the engine, generator and motor only need to operate at their rated speeds or idle and a dynamic speed range is not required; the mechanical load requires a speed much different than the rated speed of the ICE; & very little torque is necessary for startup or acceleration to the rated speed. \$\endgroup\$
    – ivanantuns
    Dec 8, 2020 at 1:30

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