Is this 3 phase motor suitable for delta/VFD?

Driving electric motors made before 1967 with a VFD is a bit risky.

I know the forum likes to believe ancient British electric motors are far better than any modern foreign rubbish. But get real! Although old British motors were well-made they were limited by the technology of the time, and don't forget old age and damp storage does them no favours.

Two reasons a VFD might damage an old motor:

• Inferior Insulation. The enamel used to insulate magnet wire has improved considerably over the years. Old enamels aren't as electrically, physically or thermally robust as the later ones.
  • The first enamels were only good for about 80°C, soon improved to 100°C rising to 120°C around WW2, then 140°C during the 1950's. Modern enamels can take at least 180°C and some motors run at about 220°C.
  • Cracks and bubbles were a serious problem in older enamels. Liquid enamel has to be carefully applied to bare copper wire because the dried layer can be flawed by bubbles, thin areas, and cracks caused by drying and winding the wire. Modern enamels are formulated to reduce the risk. Old magnet wire suffers because age and heat cycles tend to worsen the minor insulation flaws introduced by imperfect manufacturing methods. Damp makes flaws physically worse and much reduces the insulation's effectiveness overall. Much damage is caused when flawed insulation flashes over: magic smoke and the motor has to be rewired.
• Bearings. For many decades, bearing makers received persistent complaints that the bearings used in electric motors often wore out quickly. The cause was eventually traced to leakage currents earthing through the bearings. The current caused tiny sparks that eroded the bearing surface. Once identified, the problem was easily fixed by slightly altering the motor design. Motors made after 1967 are highly unlikely to have this fault. However, if an old motor is discharging through the bearing, the sparks are made much worse by spikey VFD waveforms.

1967 is the year after which the bearing problem was universally fixed and all motors were wound with modern enamel magnet wire.

Does it matter? Maybe not. Weak insulation and tiny sparks inside the bearings are unlikely to cause an instant bleeding obvious failure.

Bearing damage might never show up in a home-workshop and there's a reasonable chance even ancient insulation will be 'good enough' for a VFD. Problems are more likely to show up after many hours of operation than as an instant spectacular fail.

I wouldn't go out of my way to fit an old electric motor, but if I had one to hand I'd dry it out carefully before using it. The worst that can happen is having to replace the old banger with a new one!

Dave

A 415V Star connected motor running on a UK light industrial 3 phase supply will have 240 V between each supply terminal and ground and 415 V between phases. (the ratio between the voltages is the square root of the number of phases, sqrt3 or 1.732 for 3 phase). The supply transformer will be connected Star with the common point connected to ground and the neutral conductor. This allows both single and 3 phase loads.
If you re-wire motor to Delta to run on a VFD the voltage between terminals will be 240 V (at 100% speed. At lower speeds it will be lower and higher speeds higher voltage).
The voltage to ground is harder to define.
There is normally no transformer at the output of a VFD, just semconductors switches to a DC supply of around 400 V (derived from the incoming single phase supply) so no defined AC path to ground. If everything is balanced phase phase and phase to ground the voltage should be 240 / sqr3 = 138 V. In reality it is likely to be a combination of DC voltage and "spikes" up to several hundred Volts.
The output of a VFD is almost always more hazardous vlotage wise than the output. Even using normal Thmultimeters on them is not recommended as the voltage waveform, spikes and harmonics can damage them.
These spikes and harmonics are other charateristics that can cause a VFD to damage older motors.

In the past 3 or 4 years some one on this forum published am excellent article, with good clear pictures on how he located the Star point and brought out 3 extra wires into his motor terminal box. I have done the conversion for my friends, all on older motors, and as yet to hear of a failure, they bought their inverters via ebay and again not heard of a failure, could be they are just lucky ! I have an insulation tester up 1000volts which I used to give the motors a test before use. Have a go, you have not much to lose, you need to look out for a lump in the windings area which needs to be carefully opened up, and three wires attached. I used three different coloured wires for identification purposes, fibre glass insulation tube and heat shrink and a small amount of resin is readily available via ebay. Best of luck and let us know how you get on. John

I was thinking in terms of mechanical output power:

Power = torque x angular velocity

Assuming both voltages are at 50Hz the angular velocity is constant. The voltage reduced by the square root of 3 leads to phase currents reduced by the same. To a first approximation the torque is proportional to the phase current. So the torque, and hence power, is reduced by the same factor, ie, the square root of 3.

Andrew