British Thomson-Houston AC motor (Smart and Brown lathe)

[6ply]

I recently acquired another lathe, this one fitted with a British Thomson-Houston 2-speed 3-phase motor. The motor is massive for its rated output (0.75 HP) and could easily be replaced with a different motor that is better suited for a cheap (220V) VFD. However, I'm inclined to keep this motor and have it permanently wired "high speed" and run from a 230V single phase to 380V 3-phase VFD. I already use this setup on my Arboga drill press. If nothing else, it's a cool looking motor that matches the vintage lathe. It doesn't bother me that it's not a "VFD rated" motor.

The lathe appears to have been played around with and I don't trust any of the wiring, so I'd like to start from scratch. I'm pretty sure the chances are nill, but if anyone has connection diagram Z810806 I would like to hear from you!

The motor has six terminals:

A2, B2, C2
A3, B3, C3

I have a whole heap of readings that I am currently trying to sort out before I post them here. I'm assuming it is a Dahlander 2-speed motor but there's something niggling me in the back of my mind suggesting that it may be something else, possibly due to the identical power on high/low speed?

 

[Brad93]

Two speed motor.

 

[6ply]

Yes, Brad. Figured that out already.

 

[6ply]

All measurements taken with no bridging in the terminal box.

18.5 Ohms each:

A2 - A3
B2 - B3
C2 - C3

-------------------------------

16.5 Ohms each:

A2 - B2
A2 - C2
B2 - C2

-------------------------------

10.5 Ohms each:

A2 - B3
A2 - C3
B2 - A3
B2 - C3
C2 - A3
C2 - B3

 

[6ply]

Looking at the readings, I believe the internal connections are thus:

 

[6ply]

I think I've figured it out.

 

[6ply]

So to connect the motor "high speed", line (L1, L2, L3) could be connected to

A2, B2 and C2

or

A3, B3 and C3

 

[Erie Fred]

Old motors don't like the noisy power from Inverters, modern "Inverter Duty" motors
have extra insulation for spikes.

YMMV

 

[6ply]

Old motors don't like the noisy power from Inverters, modern "Inverter Duty" motors
have extra insulation for spikes.

YMMV

Yes, I've seen that mentioned elsewhere, hence my original statement that I'm aware it's not inverter rated. But I've run many old motors using vfds and not had a problem yet so I'm willing to chance it again.

 

[6ply]

So to connect the motor "high speed", line (L1, L2, L3) could be connected to

A2, B2 and C2

or

A3, B3 and C3

Correction, I don't think these combinations would perform equally. I suspect only one of them is correct, but I'm not sure I know or can explain why. I need to give it some further thought.

 

[Guineafowl]

Speed switching 2:1 and different amp ratings suggest Dahlander to me. The hp rating is quite a loose ‘power class’.

21.38 same terminal numbering system:

So, looking at your delta in post#5, I’d roll it round until the 3s are at the vertices. You strap these together and feed power in at the 2s for high speed double-star.

The key thing you’re doing when switching delta-double star is reversing the current through half of each winding.

 

[6ply]

Speed switching 2:1 and different amp ratings suggest Dahlander to me. The hp rating is quite a loose ‘power class’.

21.38 same terminal numbering system:

So, looking at your delta in post#5, I’d roll it round until the 3s are at the vertices. You strap these together and feed power in at the 2s for high speed double-star.

The key thing you’re doing when switching delta-double star is reversing the current through half of each winding.

Thanks for the reply and video. I'll have to watch it later due to all the expletives.

 

[Guineafowl]

Thanks for the reply and video. I'll have to watch it later due to all the expletives.

You don’t really need to watch it - just that timestamp showing the 2s and 3s numbering. I remember watching it a long time ago and wondering why not 1s and 2s.