Motor size
 

Last weekend I bought an old lathe with 20" swing. It came with
a massive 3kW (4hp) 3-phase motor. This draws too much current and
thus I will replace it with a smaller 3-phase motor and variable
frequency drive. I was thinking about to get a 1.1kw (1.5hp) motor.
Would that be powerfull enough for turning larger bowls at lower
speeds?

Frank

william_b_noble wrote:
leave the 4 HP motor in place. Program your VFD to treat it as a 2 HP.
That way, if you want a larger motor, you just reprogram the VFD. If it
came with a 4 HP motor, I'd assume the belts, etc, can carry the HP and not
slip.

It came with a strange pulley system. One small pulley on the spindle
and three larger ones on the motor side and no belt. Minimum RPM
is probably somwhere around 2000. So I have to change the pulley
system in any case.

I've thought about getting a Yaskawa VS Mini J7 inverter. That is a
scalar drive (vector drives a probably too expensive). Would I have
enough of torque at all RPMs if I had a 2-pulley system with ratios
of 1:3 (or 1:2?) and 1:1?

It seems that the J7 cannot handle the 4 hp motor.

Frank

no opinion about the belts - sounds like the motor and lathe are not
related. But, look around at VFDs - I bought a refurbished Delta (not the
tool company, rahter delta electronics) for $200 from a distributor - and I
have a used Minarik vector drive that I could sell for $150 - and of course
I'm making money on it - similarly, 3 phase motors are pretty cheap - a 2
hp motor shouldn't cost you over $35 or so in great condition.


"Frank" wrote in message
...
william_b_noble wrote:
leave the 4 HP motor in place. Program your VFD to treat it as a 2 HP.
That way, if you want a larger motor, you just reprogram the VFD. If it
came with a 4 HP motor, I'd assume the belts, etc, can carry the HP and
not
slip.

It came with a strange pulley system. One small pulley on the spindle
and three larger ones on the motor side and no belt. Minimum RPM
is probably somwhere around 2000. So I have to change the pulley
system in any case.

I've thought about getting a Yaskawa VS Mini J7 inverter. That is a
scalar drive (vector drives a probably too expensive). Would I have
enough of torque at all RPMs if I had a 2-pulley system with ratios
of 1:3 (or 1:2?) and 1:1?

It seems that the J7 cannot handle the 4 hp motor.

Frank

IMHO, motorsize is almost irrelevant, as long as it is 1/2 hp or more.

Bjarte

There is a certain logic to what Bjarte says. It's not like this is a metal
lathe where the tools, tool feed and chips get larger as the lathe gets
larger, resulting in a larger requirement for Hp. As wood lathes get larger
the gouges, feed and chips don't increase all that much. The power required
to remove chips using a 1.5" gouge on a 8" bowl is about the same as the
same gouge removing chips on a 24" bowl. What needs to change is the pulley
ratio so the tangential speed stays about the same. Since the pounds of
wood removed per hour stay about the same, then the Hp stays about the same,
too. Dan

I sort of agree. The problem, though, is that as we get better at turning we want to remove
more material faster. And with bigger diameter pieces there is a lot more material to be
removed. Underpowered lathes become more frustrating as time goes on. So do we want to buy
one motor or several motors? If you only do spindle turning -- even big spindles -- you
don't need all that much power. Bowls are a different matter, though, I think.

Bill

In article ch3xd.217079$V41.10198@attbi_s52, says...
IMHO, motorsize is almost irrelevant, as long as it is 1/2 hp or more.

Bjarte

There is a certain logic to what Bjarte says. It's not like this is a metal
lathe where the tools, tool feed and chips get larger as the lathe gets
larger, resulting in a larger requirement for Hp. As wood lathes get larger
the gouges, feed and chips don't increase all that much. The power required
to remove chips using a 1.5" gouge on a 8" bowl is about the same as the
same gouge removing chips on a 24" bowl. What needs to change is the pulley
ratio so the tangential speed stays about the same. Since the pounds of
wood removed per hour stay about the same, then the Hp stays about the same,
too. Dan

no, not correct - with a larger lathe you turn larger diameter, and take
larger cuts - If I put a 35 inch blank on my lathe and lean into a 5/8
glaser gouge, I can pull 1/4 inch thick shavings - that takes a fair amount
of power, and near the rim I can easily stall a 2 hp motor. Torque X RPM
gives you power, and the shaving thickness gives you pounds, so shaving
thickness/width times radius gives you torque.


"Dan Bollinger" wrote in message
news:ch3xd.217079$V41.10198@attbi_s52...
IMHO, motorsize is almost irrelevant, as long as it is 1/2 hp or more.

Bjarte

There is a certain logic to what Bjarte says. It's not like this is a
metal
lathe where the tools, tool feed and chips get larger as the lathe gets
larger, resulting in a larger requirement for Hp. As wood lathes get
larger
the gouges, feed and chips don't increase all that much. The power
required
to remove chips using a 1.5" gouge on a 8" bowl is about the same as the
same gouge removing chips on a 24" bowl. What needs to change is the
pulley
ratio so the tangential speed stays about the same. Since the pounds of
wood removed per hour stay about the same, then the Hp stays about the
same,
too. Dan

william_b_noble wrote:
I have a used Minarik vector drive that I could sell for $150

That would have been exactly what I was looking for, but unforunately
wrong continent wrong voltage.

Frank

Yesterday I went to see a guy nearby here who stocks used and new
motor and VFDs. I bought an 1.1 kW (1.5 hp)(1420 rpm, 3-phase) motor
and the Yaskawa J7 VFD. Both new. Based on the discussion we had and
some posts on this newsgroup it seems that that kind of a scalar drive
can be used to reduce the rpms roughly by the ratio of 1:5 without
loosing much torque. On the other hand the torque will start to
decrease after about 100Hz. That would mean that the workable range
of motor rpms would be 300-2800. With pulley ratios of 1:2.5 and 1:1
I would have two rpm ranges: 120-1100 and 300-2800.

The guy also had a used 1.5 kW (2hp, 700 rpm) motor. I thought that I
would sacrifice high rpm torque with that motor. Am I correct? The guy
said that I can still change the motors. Should I do that or keep the
one I have?

I will go to a shop that sells pulleys tomorrow. If I cannot find the
wright ones from there I thought about turning temporary pulleys out
of birch plywood (and hardening the surfaces by epoxy). I will have
access to metal lathe later on so I could replace them by aluminium
ones.

Frank

well, these particular drives' input voltage is 220 or 110, so it's probably
OK for that, and shipping across the atlantic is not terribly expensive.
But, you may be able to find one locally if you hunt around.
bill
"Frank" wrote in message
...
william_b_noble wrote:
I have a used Minarik vector drive that I could sell for $150

That would have been exactly what I was looking for, but unforunately
wrong continent wrong voltage.

Frank

personally, I'd go with the higher horsepower. And, I'd see what extra you
have to pay for a vector drive (used, or refurb) - you should be able to get
the drive for under $200, and the motor for closer to $50 (used)


"Frank" wrote in message
...

Yesterday I went to see a guy nearby here who stocks used and new
motor and VFDs. I bought an 1.1 kW (1.5 hp)(1420 rpm, 3-phase) motor
and the Yaskawa J7 VFD. Both new. Based on the discussion we had and
some posts on this newsgroup it seems that that kind of a scalar drive
can be used to reduce the rpms roughly by the ratio of 1:5 without
loosing much torque. On the other hand the torque will start to
decrease after about 100Hz. That would mean that the workable range
of motor rpms would be 300-2800. With pulley ratios of 1:2.5 and 1:1
I would have two rpm ranges: 120-1100 and 300-2800.

The guy also had a used 1.5 kW (2hp, 700 rpm) motor. I thought that I
would sacrifice high rpm torque with that motor. Am I correct? The guy
said that I can still change the motors. Should I do that or keep the
one I have?

I will go to a shop that sells pulleys tomorrow. If I cannot find the
wright ones from there I thought about turning temporary pulleys out
of birch plywood (and hardening the surfaces by epoxy). I will have
access to metal lathe later on so I could replace them by aluminium
ones.

Frank