Increasing motor speed with a VFD?

[Bill Edwards]

As some know I have got another Harrison lathe (a big one) which is of course three phase.

The motor plate says 380-440v 3 phase, BUT shows the delta triangle so presumably will happily work with a typical Chinese inverter?

However, just getting it to run isn’t enough as the top speed of the machine is a mere 4 or 600 rpm which is pants. I could mess around with pulleys to a degree but don’t want to lose torque, I desire every bit of power available.

Changing the motor from a 1440 to 2800/3000 rpm unit would be perfect, but is it possible and practical to over speed the existing unit without causing problems or suffering power loss?
An inverter to run this will set me back about £200, so it’s not to be purchased on a ‘it might work’ basis.

 

[Bill Edwards]

I didn't get to pull the motor cover (was late, cold and dark). Normal single phase is all I have, and not a very good supply at that.

I am running a smaller machine with a VFD that's actually configured for 400v in star (with help from daleyd), I just accepted some power losses and wasn't interested in speeding things up any more that it's designed for. But this machine is so slow I know I'll have to do something.

 

[ronan]

We were warned in tech about vfd's that its easy to drive the motor too fast for its bearings. Also with a lathe make sure the chuck is ok for high speed use. A lot of the far eastern cheapies are not.

 

[Pete.]

Ball bearing motors will do just fine. They don't put better bearings in a 2800 rpm motor than in a 1440.

 

[Bill Edwards]

We were warned in tech about vfd's that its easy to drive the motor too fast for its bearings. Also with a lathe make sure the chuck is ok for high speed use. A lot of the far eastern cheapies are not.

It's a Harrison, from the 50's, the chuck certainly won't be a cheapie! Also, they did offer a faster motor so I know that the lathe is designed for the higher speeds. But certainly a good point.

The motor bearings were something which I was wondering about, as well as whether any of the spinning internals would have issues from the increased centrifugal forces.
Due to being an old Brook I'm sure the bearings will have grease nipples, the one on my smaller Harrison certainly has. But what type of bearings are used I don't know, never having taken one apart. And having only ever wired up one motor with VFD I know that I'm no expert on the electrical aspects.

The machine will see very little use, I can't see it replacing the smaller L5 for the majority of my work (and that doesn't actually do much in terms of hours), it'll just be for jobs that either won't fit in a small machine or which are harder in a small machine. So although I don't want to do anything daft which could destroy the motor a little extra wear and tear on the bearings isn't likely to ever affect me.

 

[Luke]

Taking a motor in to field weakening is constant power, rather than constant torque. Increasing the speed above synchronous will require more current for less torque.

Also, pwm puts increased stresses on the winding insulation and the bearings, so older motors in poor health can suffer. Often, you'll get away with it, but it's worth being aware of the risk.

 

[Bill Edwards]

The smaller one that's 400 star is almost certainly able to be rewired for delta.

Same cover off exercise to find out.

By covers, I don't mean the cap of the wiring box, I mean the motor end covers where the bearings live. And by dig around, I mean actually in the windings. No handy jumpers, talking internal connections...

I have an old motor that was originally 440 star, and I've got that reconfigured to delta in that way and it runs fine with no apparent loss of power.

I was told that it'd be possible to dig out the windings to configure it correctly, with time and patience. But I didn't really want to mess around with it unless necessary, and I had already bought the inverter for another job so as it was sat there it got wired up as I figured the power loss probably wouldn't be an issue.

TBH, it turns out that the power loss is an issue, though how much is down to it being wired up wrong and how much is the motor being too small I don't know. I can't take anywhere near as much metal off as I'd like (I get bored on the lathe at times) but the answer is probably to replace the 1.5hp motor with a bigger one.
I think though that when the big lathe is set up (which will have a powerful motor, whether the original or a replacement) I can use that for the heavy cuts if there's much material to be removed. Two powerful lathes isn't so necessary.

 

[KemppiFrog]

In your shoes, I would get a 2800 motor. I find jet-wash ones do well. My cut off saw is powered by a 4hp one, saved when the pumps gave out on the washer and free.

 

[Anguz]

does the original motors name plate mention insulation class? looks quite old a vfd could break the insulation down, needs to be class f

 

[Bill Edwards]

In your shoes, I would get a 2800 motor. I find jet-wash ones do well. My cut off saw is powered by a 4hp one, saved when the pumps gave out on the washer and free.

I'd get a 2800 motor, but a 3 phase one and run it on a vfd.

Properly variable speed is so nice

Variable speed is great and one reason for considering three phase, I'm just starting to get used to it on the smaller machine.
Trouble is, the inverter will cost £200, and unless I can find a suitable second hand one a new motor is about the £200 mark, so there's 4-500 gone just like that - it can soon cost more than the lathe!

does the original motors name plate mention insulation class? looks quite old a vfd could break the insulation down, needs to be class f

You know as much as I do from the information plate, which isn't great to read.

 

[Bill Edwards]

TBH, I was thinking about the torque reductions last night that I'd see from fitting different pulleys. Lets say I doubled the speed with pulleys, in top gear I'd lose the torque.
But when running at slower speeds provided the gearing is dropped (not the motor speed) I shouldn't actually l shouldn't actually lose anything. For simple figures lets say the top speed is currently 400, and with pulleys I doubled it to 800.
Yes above 400 rpm I'll lose a lot of torque, but if I select the gear which would previously give 200 (giving me 400 with bigger pulleys) it'll be exactly the same as having it in top gear with smaller pulleys, no power loss?

That might not actually present any problems, because the high speeds are only really going to be used for finishing cuts, small drills and small diameter workpieces (which might be done on the smaller lathe anyway). You don't use 5hp to push a 10mm drill or work on a 6mm bar.
Maybe using the existing motor at the correct speed and upping the pulley ratio won't actually lose any power where it's needed?

 

[Bill Edwards]

View attachment 129635

aliexpress to the rescue!

(I've not used them, I can't vouch for them, but they get reasonable reviews...)

I was looking at the 7.5kw inverters which are a little more. Possibly overkill, but I've found a couple of times in life that it's often necessary to get a little bigger than should be necessary as pushing things like that to the limit doesn't always pay?

 

[Bill Edwards]

I might have found a 7.5kw one for 190 though which is slightly better!

 

[daleyd]

does the original motors name plate mention insulation class? looks quite old a vfd could break the insulation down, needs to be class f

For home use I wouldn’t bother(esp if it will be run on 230v) - ideally it “should” be class f (but really that’s referring to the temperature range it can operate in) but I’ve seen plenty of old motors run fine when converted to vfd use.

 

[Bill Edwards]

Yeah, overkill doesn't hurt.

The motor will only draw what it needs - a 10kw vfd won't blow up a 1kw motor...

As for gearing, if you can add an "overdrive" it'll work as you thought - the existing gears will remain unchanged and higher speeds will lose torque.

It's almost a direct relationship, double the speed through gearing, get half the torque.

I wasn't thinking anything as fancy as an overdrive, just fitting a bigger pulley to the motor which will increase the speeds all the time. But the bottom speed is currently 25rpm, say that was doubled would it really matter to me? Probably not. I reckon most of the work it does will be within the speeds the machine is already geared for, up to 400 which should still be available by changing gears, it's just every now and then that faster speeds are going to be called for.

 

[hareng]

I
I can't take anywhere near as much metal off as I'd like (I get bored on the lathe at times) but the answer is probably to replace the 1.5hp motor with a bigger one.

1.5HP seems very small for a larger lathe the 11" had a 2HP 3ph motor whilst the 140 i had a 1ph 1 1/2HP and could double the depth of cut on a similar 3ph 3HP M300.
Might be best to get another motor as a starting point, my 7 years of troubles turned out to be the motor and not worth a rewind when managed to source a Brooke Crompton delivered for £187 from up Scotland last October.
Other problem will face changing motors is the pulley sizings and motor foot print, caused me a right headache so bare that in mind mine was ultra ultra tight and had to make a new adjustable extended motor mount.

What lathe is it or the nearest to it?
Cant make out the Amps or power.

 

[Bill Edwards]

1.5HP seems very small for a larger lathe the 11" had a 2HP 3ph motor whilst the 140 i had a 1ph 1 1/2HP and could double the depth of cut on a similar 3ph 3HP M300.
Might be best to get another motor as a starting point, my 7 years of troubles turned out to be the motor and not worth a rewind when managed to source a Brooke Crompton delivered for £187 from up Scotland last October.
Other problem will face changing motors is the pulley sizings and motor foot print, caused me a right headache so bare that in mind mine was ultra ultra tight and had to make a new adjustable extended motor mount.

What lathe is it or the nearest to it?
Cant make out the Amps or power.

The lathe I currently use is an L5 (9”) which has a 1.5hp motor.
The big machine which is the subject of this thread is a 16” gapbed L21 with a 5hp motor.

 

[hareng]

Bit of checking Bill motor 5hp 1000 rpm gives 38 to 600 rpm, suspect this is the one.
Or dual speed motor 3/5hp gives 32 to 1000rpm.

Since its one of the larger examples though has Timken taper bearings i wouldnt want to push it past 1500rpm - dependant upon chuck fitted. A standard 3ph 5hp 4 pole modern motor 1425-1550 rpm would take yours to around 900 rpm. More if eventually go vfd route.

Almost certain will have to change motor pulley or bore it out and slot again.

Looks a nice oldish machine, cant beat British iron.

 

[Bill Edwards]

Bit of checking Bill motor 5hp 1000 rpm gives 38 to 600 rpm, suspect this is the one.
Or dual speed motor 3/5hp gives 32 to 1000rpm.

Since its one of the larger examples though has Timken taper bearings i wouldnt want to push it past 1500rpm - dependant upon chuck fitted. A standard 3ph 5hp 4 pole modern motor 1425-1550 rpm would take yours to around 900 rpm. More if eventually go vfd route.

Almost certain will have to change motor pulley or bore it out and slot again.

Looks a nice oldish machine, cant beat British iron.

Yes according to the lathes website it should be a 600 rpm machine - but I’m pretty sure that the speeds shown on the lathe are only up to 400. Something could be amiss.

However, even if it is 600 that’s still darn slow, I wish for more speed from the other sometimes at 750. So I really ought to be trying to double the speed even if it’s actually a 600 machine, or even triple if it’s the extra slow type.

Yes it’s a proper chunk of British engineering, I’ve yet to do anything with it but the smaller one is a gem, hard to find fault. Brilliant to use.

The more I read about Harrison lathes the more pleased I am to have ended up with them, if I were to make a shortlist of ideal makes they’d be at or very near the top.