Entering a new era

The concept of the Danfoss APP is – interesting enough – very similar to the Karcher. Unlike a usual high pressure plunger pump, as a CAT pump or so, the pistons (plungers) are mounted co-axial to the main shaft of the pump. They are driven by some sort of wobbling disk instead of a crankshaft, resulting in lower noise, a bit less power consumption and no need of oil lubrication. The cheap Karcher that we used was that type of pump, too (although with oil lubrication). You need to couple the Danfoss APP directly onto an electric motor, either DC or AC, of about 1.5 kW (better a bit more) or some 2 HP. On youtube you may find a nice presentation on how those pumps work: https://youtu.be/_WLtj_fzPAg Should the link not work search there for Danfoss Axial Pump.

I decided to use an AC motor, due to above mentioned reasons: DC motors of that size are expensive (2 HP continuous duty @ 3600 rpm are somewhat around USD 500), and they are exposed to wear because they use brushes. A comparable AC motor here costs about USD 180. It is an ordinary industrial standard product.

But those ordinary AC motors show one problem: When started the need a very high starting current (about five times the nominal current), which is also true for the Karcher, but the Karcher may be regulated by the above mentioned little control device. With this device, you may “soft start” them and the rpms are easily controlled. The industrial AC motors – basically 3-phase-motors – may not be controlled by that sort of device. However, you may get a “soft start device” for 3-phase-motors for little money. But I decided to use a so called VFD, which gives you full control over the AC motor.

About AC motors

Basically, there are two sorts of AC motors: the so called Universal Motor and the so called Asynchronous Motor. The Universal Motor is easy to control and cheap. You will find it in an electric drill, inside of a vacuum cleaner, or, e.g. inside of the Karcher. It uses brushes and there are practically no motors available that come in a housing. They are rather OEM-products to be integrated into various machine concepts.

The Asynchronous Motor is a standard industrial motor that comes in a certain number of housings, with standard measures, standard ratings etc. They work without brushes and at better efficiency than Universal Motors. They have a fixed rate of rpm. Depending on the mains frequency (60 Hertz in the US, 50 Hertz in Europe) they run at 3600, 1800, 900, 450 rpm (60 Hz) or 3000, 1500, 750, 375 rpm (50 Hz). This depends on the number of “poles”: a 2-pole motor runs at 3600, a 4-pole motor at 1800 and so on. If you want e.g. 2200 rpm you need a gearbox. If you simply reduce Voltage you reduce the kW resp. horsepower, but the rpm remain the same.

All asynchronous motors are basically 3-phase-motors. But you may buy also single-phase-motors, where the missing phases are replaced by a capacitor (further details in Wikipedia). Nonetheless you need some sort of three phases.

The rpm of an asynchronous motor depends on the frequency of the AC. So, if you change the frequency you change the rpm. Not too long ago such devices were developed at affordable prices. A VFD is such a device, a Variable Frequency Drive. What it does sounds simple, is, however, sophisticated: You plug it into the wall socket where you get 115V/60Hz or 230V/50 Hz or whatever. On the output side, you decide what you get there: 0 to 400 Hz at a chosen Voltage. Which means 0 to 24,000 rpm, if the motor could do this (usually the bearings won’t). Now you have full control over your AC asynchronous motor: you start it as slowly as you want (which keeps initial current low) and go up until you think it is fine. Such a device is not cheap. For our purpose, you may calculate ca. USD 300.

So, I bought a standard AC asynchronous motor and a VFD.

The test run (without the Danfoss pump, because it must not run dry) went well. So I mounted everything into Dido’s hull. And then we went for our summer vacation. For security reasons I also took the Karcher with us.