I make .55kw to be just under 3/4 hp so might be OK for light use. (but I'm no expert)

Brian

You are replacing a 1 hp motor with one that is 3/4 hp, which as stated should be OK for anything except heavy cutting. But as you slow the motor speed down with the VFD, the power/torque of the motor will also reduce.

Paul.

Another factor is the rated rpm of the motors. I have changed a 1425 rpm single phase to a 935 rpm three phase one, as the minimum lower speeds available would have been too high for larger drills and cutters. With the vfd set as I want it, the three phase motor will reach nearly 1400 rpm and go down to 467 rpm, which in conjunction with the four belt positions gives a better overall spread of power and speed. Remember, that a three phase motor on a VFD running at 25Hz is not only half the speed, but also half the power, and the power drops rapidly if the frequency is lowered further.

Three phase motors for small applications have up to 8 poles, the more poles, the slower the running, so with equal horsepower, the torque increases the slower the motor runs natively. The motor I bought has 6 poles, the 8 pole one was 50% more expensive.

This is the one I bought:**LINK**

The motor should have a plate on it which has the voltage, frequency, amps and rpm. You will need this information to program a VFD. A visual inspection of the windings will only be of use to an electrical engineer. You will also have to make sure it is in delta configuration, not star.

Lynne, on my RDM 350 it was a four pole 1425 rpm and the motor was 3/4hp.

I made a completely new motor mounting plate and discarded the centre pulley arrangement, ( its in a cupboard) So, now have a longish link belt front to rear, it works well. The replacement motor is a 1 HP 4 pole which I already had, they are available second hand. If you are considering buying a second hand motor do check that the mounting are OK for your machine and get some one with an ohm meter and a Megger to test it for you. Some motors are very easy to change from Star to Delta, other take much Lynne,longer. There has been some excellent picture and explanations published on here in the past on the electrical change over. I made a control box which is mounted above the quill lock, with a speed control knob, stop/start and a jog button below. No reverse here.

If the mill is the same, that may be so. But we don’t actually know what the ‘current’ mill is. The current mill motor might be more than one horse power. Guessing doesn’t particularly help if the real details are not known. Is the mill directly driven by belt or does it have a gearbox to consider?

Lynne, rather than take the motor apart to sleeve the shaft, you could sleeve the pulley. When I fitted a 3 ph motor to my VMB I removed the intermediate pulley, which was almost exactly the same set of sizes as the spindle pulley, turned up a sleeve to fit closely in its bore, with a keyway for the motor key, and pressed / loctited it into the pulley. An excellent modification.

I think that the crux of the document is about saving energy and a VFD is a useful device to manage a motor to its load. As most of our home workshops do not have three phase and we either use a VFD to convert a three phase machine to run on single phase or we choose to convert for the improvement in control and smoothness of a machine then we comply with the regulation even if we may not be required to.

Mike

I was told that a single phase 3/4 hp motor would be equalled by a three phase 1/2 hp motor, because of the smother power delivery. Being a mechanical Engineer, rather than an Electrical one, I accepted what I was told.

My lathe would have come with a 2 hp single phase motor, but I opted for a VFD with a 1.5 hp three phase. In sixteen years, I have yet to manage to stall it! Belt slip, yes, but stall motor, NO

Howard

The 2007 EU Directive isn't designed to annoy Model Engineers, it's one of several measures being taken to deal with a gathering international storm. The USA acted first in 1992, but the rationale behind legislation of this type is to reduce the impact of rising energy costs as oil depletes. The negative effects of Climate Change and Pollution matter too!

"Based on U.S. Department of Energy data, it is estimated that the National Electrical Manufacturers Association (NEMA) premium-efficiency motor program would save 5.8 terawatts of electricity and prevent the release of nearly 80 million metric tons of carbon into the atmosphere over the next ten years. This is equivalent to keeping 16 million cars off the road.

Roughly 30 million new electric motors are sold each year for industrial purposes. Some 300 million motors are in use in industry, infrastructure and large buildings. These electric motors are responsible for 40% of global electricity used to drive pumps, fans, compressors and other mechanical traction equipment. "

VFD's aren't only handy for turning single phase domestic mains into 3-phase. These days 3-phase to 3-phase VFD's are likely fitted in any new 3-phase motor installation. Their purpose is to save money by matching motors better to their loads over a wide duty range. Busy motors benefit most obviously, think Lift Motors in tall buildings rather than home workshop tools. The VFD usually pays for itself within 3 years.

Dave