3ph motor running slow

[Wout MoermanParticipant @woutmoerman25063]

Fellow ME's,

I bought a 2nd hand woodturning lathe a year ago and am trying to get it to run. I removed from the lathe for wiring and testing and it runs extremely slow at about 60 rpm instead of 2160 rpm (1 turn by second) and can be stopped by hand. It is self starting from all positions, I tried 8 positions 45 degrees apart.

It was wired for 380V 3phase, I rewired the motor for 230V 2 phase according to the schematic on the motor and use a run capicitor to generate the extra phase. It is an old 1/3 hp synchronous motor which runs freely when turned by hand.

I tried a 20 muFd and a 10 muFd run capacitor, both with the same effect: reliable start but very slow running.

Can anyone help me to find out the problem? I can send pictures if needed.

Kind regards from the Netherlands.

[Wout MoermanParticipant @woutmoerman25063]

[John HaineParticipant @johnhaine32865]

To be pedantic, I assume it's an induction motor not synchronous?

I think you will need a proper 230V 3 phase supply rather than a capacitor kluge, which probably isn't giving anything like the right phase angles so you are not getting much torque.

I assume that the schematic on the motor was for a star (380V) to delta (230V) conversion? That is still 3 phase.

Best bet is to buy a low cost VFD which will generate true 3 phase and give you variable speed. Newton Tesla are a good supplier and I have two IMO low power inverters, one on my mill running a half horsepower motor and the other a smaller motor on a Unimat. IMO aren't the cheapest but NT's support is great. Many people here have bought cheaper inverters from eBay and a reliable brand seems to be Huanyang though the instructions are not easy to follow I believe, but plenty of people who can advise.

[John HaineParticipant @johnhaine32865]

Seeing you are in the Netherlands then maybe you could find a local supplier to avoid the Brexit nightmare, but NT could advise perhaps.

[Clive FosterParticipant @clivefoster55965]

The static converter "capacitor trick" to self generate a pseudo third phase "wild leg" does not work on synchronous motors. As you have found, although you can make them self start they are unable to run up to speed.

Actually the whole capacitor trick three phase to single phase motor conversion is generally rather less reliable than the pundits would have it. Unless you have the instrumentation to set it up properly by measuring the relative phase of the current in each winding and fairly stable loads such conversions usually result in the motor running mostly on single phase with some assistance from the other windings.

Before getting a VFD its best to check that it is capable of bringing a synchronous motor up to speed under load. Synchronous motors are a bit different to ordinary three phase motors in that they don't change speed under normal loads so the phase slip behaviour is different.

Clive

Edited By Clive Foster on 22/08/2021 12:27:02

[Wout MoermanParticipant @woutmoerman25063]

Maybe it is indeed an induction motor instead of synchronous. Is there a way to check? It is certainly not an universal motor as it has no brushes. I have a VFD still unused. So if that is the best option I can use that one. But I really hoped the capacitor would do the trick as it seemed a lot simpler to mount and use.

Thanks for all replies!

[John HaineParticipant @johnhaine32865]

Synchronous motors in large sizes are very rare. The easiest way to check is to look at the rotor. A synchronous motor will either have pronounced "poles" on the rotor, or have a plain magnetic steel rotor if it's a hysteresis type motor. This Wikipedia article has photos of squirrel cage rotors. One frequent giveaway is to look at the end of the rotor, which you may be able to see if the end caps have cooling ports in them. You usually see the riveted ends of the conductor bars in the rotor. Another giveaway is the speed – you said it should be running at 2160 rpm but a synchronous motor would run at 3000 rpm on 50 Hz! Capacitors work with true 2-phase motors designed for capacitor run but are unlikely to work with 3 phase as the windings are at 120* rather than 90*, and all the windings are identical. One thing to watch out for if you have rewired the motor is that the winding senses are correct – sometimes people get one winding reversed and the motor runs rough and torque is low. If the internal wires were colour coded and the diagram showed the colours this is hard to get wrong though.

If you have a VFD then just use it!

[Clive FosterParticipant @clivefoster55965]

+1 for Johns advice to just use the VFD.

Although the capacitor trick or, to give it its proper name, Steinmetz connection can work very well indeed when properly balanced in this day and age sorting it falls into the life is too short category.

Back in the day I built and sold a number of such devices with proper automatic cut out of the extra starter capacitor so they just worked. But even being in practice getting good balance was more work than I really cared for and some motors just point blank would not behave. OK when there was no remotely affordable alternative but that ended 25 years ago.

In pre-WW2 days you could buy 3 phase motors either configured for Steinmetz connection only or with the necessary capacitor data on the plate to use that method if access to proper 3 phase was lacking. Generally such motors were of somewhat inefficient design and rather lossy but they could deliver full nameplate power however connected. Some inefficiency appears to be essential for decent operation. Trying to so connect a modern, high efficiency, "energy star" motor is said to be at least disappointing and frequently frustrating.

Clive

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