Thread: Suitable generator for pumps and refrigerator?

Unless specifically marked otherwise, your sump pumps and domestic fridges/freezers will be powered by mains induction motors, rather than DC motors. Basically pure economics, induction motors have a wide voltage tolerance and no voltage critical components to blow in a surge, so why go to a DC motor and controller/driver module that will have reliability issues eventually. Bit different with washing machines like the F&P because they want to get as many fuctions as possible from a single motor (very low speed for wash, very high speed for spin).

Old school alternators used to have a DC generator as an exciter, then the alternator. Voltage was regulated by varying the exiter output current. Motor speed controls frequency, and either you have to be willing to wear wild frequency swings as the motor and govenor adapts to load variations, or the motor is overpowered for the demand so speed is not impacted by load change. The alternator generally runs at 3000RPM or 1500 RPM for 50Hz output. The motor runs at the same speed unless there is a speed increase/decrease from a transmission between the motor and alternator. Recent gear tends to be direct coupled, so diesel motors need to run at 1500 and petrol at 3000 RPM because petrol produce less torque for similar capacity, physical size and RPM.

Modern day basic domestic units have scrapped the seperate exciter and control system, and just generate a nominal voltage then clip it to control the voltage, resulting in a square wave output rather than a sine wave output. The frequency is still dependant on motor speed but the motors often are struggling at rated output so there is little reserve for load variation and the frequency will swing with load changes. The square wave output is the prime cause of concern with sensitive equipment, although frequency variation can also be an issue.

Inverter units operate on a different principle. The motor operates at fast idle while the generator produces a high voltage DC output. This supplies the inverter stage which produces near sinewave output with fixed frequency and voltage. As the load applied increases, there will be a point where the inverter demands more power than the generator is producing, and the system will then open the motor throttle to increase motor speed and generator power, which the inverter then supplies to the load. Provided that the manufacturer gets the design right, you get voltage and frequency stable electricity at the lowest possible motor RPM, fuel consumpsion and noise.

Induction motors will run off the clipped output of the modern fixed excitation generators but the high level of harmonics in the clipped output will create additional current and heat in the motor which it cannot convert to usable motive power. The motor will attempt to run at mid to max speed with corresponding noise and fuel consumpsion. The inverter generator system will supply a more 'mains like' supply provided that it is not significantly overloaded, and will be more fuel efficient and quieter. Regardless of choice, you need to know that the motors will need a kick start surge 6 to 10 times their rated power to get started.

Thanks for the detailed response.

With induction motors I would have thought a clean sine wave output would be even more important - I think that is what you are saying. Induction motors don't really like clipped sine waves?

If you look at the second link (generac brand), do you think would that be a clipped output alternator? There are no specs which say anything. Would you therefore say it is unsuitable for a fridge?

Btw, I am very surprised the submersible sump pump would be an induction motor - it does not seem heavy enough. It's a model similar design to this: www.masters.com.au/product/900008966/einhell-dirty-water-pump-red-1050w

Next issue is that if all the motors are induction, I'm worried about the startup current using a small inverter generator.

I had no idea what you were using as a sump pump, but responded assuming it was something similar to the generic Chinese units around for the past few years. The link provided suggests that I guessed correctly. At over 1KW capacity, and at the prices that Aldi and others can sell them, I am very sure that the unit is a close coupled pump and induction motor potted to make the motor and cabling junctions waterproof. I tried to google the Masters Einhall unit for more details but there doesnt seem to anything more than available on the Masters page.

Re induction motors and clipped supply, it is not normally an issue unless the motor is operating right at its limits and the additional heat generated by the harmonic laden supply tips them over the edge. Motor speed will be determined by the predominant frequency of the supply which will be the 50Hz, the harmonics in total would be under 10% of the power drawn, but would be disipated as heat, and not transfered to useful work.

Any AVR (Automatic Voltage Regulation) style generator like the unit in your second link will produce a clipped output. Old school units with seperate exciters had a control panel monitoring exciter current and output current and voltage, and a big high power control pot to adjust exciter current. The modern day commercial units are basically similar but use a computerised module to control the exciter currents and hence mains outputs. The control modules and metering can cost way more than the hardware store style generator units.

Either of the generators you linked to should be able to start and run a domestic fridge or freezer without problems. With regard to the pump it is harder to guess from afar. The pump linked to is rated at over 1KW so oon the face of it you need a surge capacity of 5-6KW to start it. But if the unit is doing light duty, (low lift and short large diameter plumbing) it might well be running way under limits at say 200-300W. In that case either unit would have ample surge capacity to start the pump. So really, what it needs to start and run depends on what the pump has to do and how it is plumbed. For a definative answer, you probably need to have the start up and run currents measured over a few typical start run cycles. This requires access to some specialist gear, the wiring currently feeding the pump and the knowledge to make the measurements. A simpler approach might be to bot or hire a similar generator for a day and try a suck and see approach.

Thanks for the response.
My actual sump pump says 700W. It is just of similar design to the link I posted. It is just a motor above an impeller with a float switch. Needs to pump water up about 1m from the sump tank into the storm water.

I think the conclusion I am reaching is to get the second unit. It doesn't have the clean sine wave output of the first, but has better capacity for start up. e.g. during a storm the sump pump may be turning on an off once every couple of minutes.
I was also worried a clipped output may damage my fridge motor.

What about an evap cooler? I think it is also rated 700W. I think it would have two motors: the water pump, plus the fan motor.
If the fan motor is induction, does that mean the unit has some sort of automatic frequency controller? How else does it do variable fan speed? One advantage is once on it stays on - there is no cycling on and off.