OutdoorKing Repair Forum Forums General Discussion Forum Off Topics Making a generator

Hi all, I want to make a generator from a small engine and other parts, just as a proof of concept. Ultimately I want to consult with someone to get a design right for a portable unit driven by about a 50hp engine, but I want to assemble a few myself first so I'm familiar with the concepts.

I'm aware of risks relating to electricity, moving parts and fuels, I don't take them lightly and will be observing safe practice at all times.

I'd really appreciate some general information, if it's a welcome topic for discussion.

Hi John
Welcome here where we try to help.
Firstly I assume you are talking about using a 5 hp motor not a 50hp?
Secondly don't bother with the utube rubbish using car alternators, remember Mr Honda worked out many years ago that you require 13 hp to generate 10kva
I wish it was easier to generate power because the world would be a better place but we are stuck with the physics of it

Hi Norm, and thanks so much for replying so quickly.

I did mean 50hp because I overestimated the startup load on a soft serve machine. I don't need that much, but the LRA is pretty high for these things so I guess if I'm making something from scratch then I might as well get a small car engine. I've seen suggestions for engines from skid steers and so on, but that's going to be pricey, and other small engines are meant for high RPM and that's not suitable - I reckon a 1300cc Yaris donk or something like that. Smaller than that would be better, anything from a small car is going to be well and truly overspec for my needs but I do prefer that.

For the generator head, am I scrounging copper and winding my own poles, or should I be repurposing a large motor from something? Or am I barking mad and neither of these?

Hi John,

First, it's important to understand the power needs of a soft-serve machine. While they vary, a typical 240V single-phase soft-serve machine can draw anywhere from 1,500 to 5,000 watts (1.5 kW to 5 kW), with a starting surge that can be significantly higher. For example, one specification sheet indicated 2500W and 16 Amps at 220-240V. An electrician often recommends at least 20% more power capacity to handle these surges

A 50 HP car engine (approximately 37 kW) is quite powerful. This amount of horsepower is generally more than sufficient to run a soft-serve machine, which might only need around 2-7 HP (1.5-5 kW) for operation, even accounting for inefficiencies in power conversion. The challenge with using a car engine is often in managing its RPM for consistent power output and ensuring proper cooling for continuous high-load operation, as car engines are designed for variable loads and speeds, not constant generator duty. You would likely need a gearbox to match the engine's most efficient RPM to the generator's required RPM (often 1800 or 3600 RPM for 60Hz)

Repurposing a large motor: This is generally the more feasible and recommended option for a DIY 240V generator.

Motors already have the windings, stator, and rotor in place, saving immense time and effort in fabrication. Certain types of motors, especially those with permanent magnets (like some large DC motors from industrial equipment or even some washing machine motors), can be converted into generators with relatively less modification. Three-phase AC induction motors can also be made to generate, though they often require "excitation" (a starting capacitor bank) to build up a magnetic field, and their voltage regulation can be more complex.

Challenges: Finding a motor with suitable characteristics for 240V output and the desired wattage can be tricky. You'll need to research specific motor types and their suitability for generation. Car alternators, while common, typically produce 12-14V DC, so converting them to 240V AC for a soft-serve machine would require a large and efficient inverter, which adds cost and complexity. It's often better to start with a generator head designed to produce AC.

Scrounging copper and winding your own poles (building from scratch): While theoretically possible, this is a highly complex and difficult endeavor for a 240V generator capable of powering a significant load like a soft-serve machine.

Challenges:

Electrical Engineering Expertise: You'd need a deep understanding of magnetic fields, winding configurations (number of turns, wire gauge, coil arrangement), insulation, and the principles of AC generation (especially for producing a stable 240V at 60Hz). Achieving the correct voltage and frequency stability is paramount for sensitive equipment like a soft-serve machine.

Precision Manufacturing: Building a stator and rotor with the necessary tight tolerances, proper air gaps, and balanced rotation is extremely difficult without specialized tools and knowledge.

Efficiency and Heat Management: A poorly designed homemade generator will be very inefficient, converting a large portion of the mechanical energy into heat rather than electricity. This leads to wasted fuel and potential overheating.

Safety: Dealing with high voltage (240V) and a powerful rotating assembly requires rigorous safety considerations in design and construction.

Cost vs. Benefit: The time, specialized materials, and potential for failure often far outweigh the cost of buying a purpose-built generator head or a complete used generator.

repurposing a suitable large AC motor (or a dedicated generator head) is by far the most practical and realistic approach. Building a generator head from scratch to meet these specifications is a monumental task best left to electrical engineers with access to specialized equipment and materials.

You'll need to consider how to couple the engine to the generator head (likely with pulleys and belts), add a robust voltage regulator to maintain a stable 240V output, and implement a governor on the engine to keep the RPM constant for a stable 60Hz frequency.

If you repurpose an electric motor to generate 240V and drive it with your 50 HP car engine to run a soft-serve machine, whether or not you need an inverter depends on the type of motor you repurpose and the type of power your soft-serve machine requires.



If you repurpose an AC motor (like a washing machine motor) to generate AC power:

Ideal Scenario: Synchronous AC Generator: If you manage to convert an AC motor into a true synchronous AC generator (which means it produces a stable AC voltage and frequency, typically 60Hz , directly at 240V), you generally would not need an inverter. The soft-serve machine, being an AC appliance, could theoretically run directly off this generated AC power.

Challenges with Repurposed AC Motors:

Voltage Regulation: Maintaining a stable 240V output under varying load (like the soft-serve machine's compressor cycling on and off) is critical. Without proper voltage regulation, dips or spikes could damage the soft-serve machine.

Frequency Stability (Hz): The frequency (60Hz) is directly tied to the speed at which your car engine spins the generator. Maintaining a precise and constant RPM (Revolutions Per Minute) from the car engine is crucial for stable frequency. Soft-serve machines are sensitive to frequency fluctuations.

Induction Motors as Generators: Many common AC motors are induction motors. To make them generate, they need to be "excited" (often with capacitors) and spun faster than their synchronous speed. The output from such a setup can be less stable in voltage and frequency compared to a purpose-built generator or a synchronous motor converted to a generator.

Waveform Quality: The "cleanliness" of the AC waveform (pure sine wave vs. modified sine wave) is important for sensitive electronics. A repurposed motor might not produce a perfectly clean sine wave, which could be an issue for the soft-serve machine's control electronics.

If you repurpose a DC motor (like a large treadmill motor) to generate DC power:

In this case, you would be generating DC voltage (e.g., 12V, 24V, 48V, or higher depending on the motor). To power a 240V AC soft-serve machine, you would absolutely need a large and efficient inverter. The inverter would convert the DC power to 240V AC.

Advantages of DC + Inverter: The DC generation can be more straightforward to regulate, and a good quality inverter can produce a clean sine wave, which is excellent for sensitive electronics.

Disadvantages: A large 240V inverter capable of handling the soft-serve machine's wattage and surge currents can be quite expensive. There are also efficiency losses in the conversion process.

What about a car alternator?

A standard car alternator produces 12-14V DC power. To run a 240V AC soft-serve machine from a car alternator, you would definitely need a very large and high-quality inverter to step up the voltage and convert it to AC. This is generally not the most efficient or cost-effective solution for such a high-power AC appliance.

Recommendation:

For a 240V soft-serve machine, the ideal scenario if repurposing is to find a large three-phase AC motor that can be converted into an induction generator (with appropriate capacitor banks for excitation) or, even better, a synchronous AC generator that outputs 240V directly. However, controlling the voltage and frequency stability of such a DIY setup to a level acceptable for sensitive equipment like a soft-serve machine is very challenging.

Many commercial generator sets (which do not use inverters for their primary 240V AC output unless they are "inverter generators" designed for sensitive electronics) achieve stable voltage and frequency through precise engine governors and dedicated voltage regulators (AVRs).

Given the sensitivity and power requirements of a soft-serve machine, a more reliable approach, if not buying a complete generator, would be to obtain a purpose-built 240V AC generator head (also called an alternator) and couple it to your 50 HP engine. These are designed to produce stable AC power directly.

It's easier just to buy a cheap second hand generator ,I've seen 4 cylinder trailer welders that are also 240 volt generators
that have sold at Auctions and on Facebook marketplace for under $500. ,I've also seen abandoned side valve V8 generators
behind old factories.



Cheers
Max.

There is also a number of running big KVA gensets on Marketplace in Melb for a few thousand dollars

Thanks lads.