Thread: Motor and VFD Questions

Some reason the photos have all rotated, I hope you don't get a sore neck looking at them

If it was a low HP motor the individual coil resistances can provide a clue to the power rating.
For example a 1/2HP motor has a typical coil resistance of ~20Ω, a 1 HP ~10Ω, and a 2HP has a typical single coil resistance of ~ 5Ω.

To measure these resistances, the coils have to be disconnected from each other including the common point if they are connected in Y mode.
OR
If you know they are connected internally in Y mode, then the resistance between any two terminals in the motor connection box is divided by two.

Unfortunately the resistances are not clear cut once you go >2 HP
EG some 3 and 4HP motors I have measured have had coil resistances similar to some 2HP motors.

The reason for this is that it is not just "electrical resistance" that affects "power draw" but also something called "electrical inductance". It al gets pretty tricky very quickly.

Another matter that is worth considering is that some old motors like yours I have experience with have not played nicely with VFDs. They ones I have tried have sometimes emitted unusual noises that signify something is not right.
Perhaps before you purchase a VFD you should try the motor out on a borrowed VFD and see what it does?

If the motor is not under any load it should not matter and will not damage the motor or VFD if say the motor is 4HP and you try it on a 2HP VFD and use a soft start (ie start up time of say 10s). The motor will not draw much current in starting and even less for running.

Before doing anything I would make sure that a motor insulator test is performed - it may well be the motors are under spec and not worth mucking about with.

Hi Bob,
Thanks for your reply. I think this motor would be more that 2hp as is was running on the big cast iron bandsaw? So I think I may have to just over compensate with a vfd. I was thinking I could buy one that was say suitable for a 5hp motor but then change the parameters so it is suitable for a 3hp (or a bit less) motor and see how it goes?(That way I have some flexability) I happy to buy one as I'm fairly confident I would like to use them in the future anyway (and I don't know anyone I can borrow one from) The motor rewinders said it was a really well made motor and they could supply a VFD but that would cost a minimum of $500. Perhaps I'll ask them to do an insulation test. Thanks for the tips!

Just gave them a call, and the lady said that they definitely would have checked the insulation. She said "We are happy to sell you another one but that's a very good old motor." She also said that they can't test to see the horse power but based on there best guess and experience it would be roughly a 3hp motor. With that is mind, do you think my idea of buying a VFD suitable for a slightly more powerful motor and adjusting the parameters would work? Looking on the powtran sight at the PI150 range the largest is 2.2 kw, 10amp (PI150 2R2G1Z) which I think is roughly suitable for 3hp? So maybe I'll need a different model?

The size of old motors is not a very good indicator of their power rating.
eg We had an old DP at the mens shed powered by a motor that looked like a 3HP but turned out to be 1/2HP.

The 600kg 16" jointer I have in storage has a honking big motor on it and that is only 3HP.

Old large 3P bandsaws were not used that often to full depth of cut anyway - they were more often than not that size because of better throat capacity and because old bands were more likely to crack/break when rotating around smaller wheels.

I had a 2HP single phase motor on my 19" BS and after conversion to 3P 3HP +VFD I have been able to observe that with a sharp band it never utilises more than about 2.5HP to cut even the thickest hardest wood. Significant improvements in BS cutting speed are obtained not by greater motor grunt and pushing harder but by increasing the band/motor speed speed - this requires a motor that plays nicely with a VFD. older motors are less likely to be able to do this but if all you want to do is to run it on 50Hz it sounds like you will be OK with a 3HP VFD

They look like lovely old motors. I hope you're able to use them.

As an indication (and who's to say whether these have been sized for the machine, or just any old motors the PO had laying around), my 30" 1928 bandsaw has a 3 hp motor (although up to 5hp is not uncommon). I don't have a 16" buzzer (mine is just 9") but a reputable firm like Wadkin would have a 4hp motor on a buzzer of that size.

Again, as an indication (and reading from the "Motor Current Table" in an early MEM catalogue, a typical 4hp motor on a 400/415v supply would draw 6.5amps per phase (but 20amps on single phase). A 3 hp motor on a 400/415v supply would draw 5.0amps per phase (but 17.5amps on single phase).

Hopefully that might help with VFD size selection.

Cheers, Vann.

I see the last photo (1st post) shows a DOL starter. You might be able to read what amps it's set at. That will give you an indication of the max load.

It's possible that the DOL is set too high - but if it was set too low it would keep cutting out, and would therefore be adjusted up until it no longer cut out. So it's either set right, or too high. Whatever it's set at could be used to size a VFD. You'll either get a VFD that's the right size, or bigger than needed - but at least you won't get one that's too small.

Another really slim possibility would be to contact the manufacturer to see if they still have records. AEG (Allgemeine Elektrizitäts-Gesellschaft) is still in existence today. It's a slim chance - but you have a serial number stamped on the motor.

Cheers, Vann.

Originally Posted by Vann

Again, as an indication (and reading from the "Motor Current Table" in an early MEM catalogue, a typical 4hp motor on a 400/415v supply would draw 6.5amps per phase (but 20amps on single phase). A 3 hp motor on a 400/415v supply would draw 5.0amps per phase (but 17.5amps on single phase)..

I think these are over estimates.

A a general rule, 3P to SP current requirement conversions are done multiplying by 1.73, so a 3P 6.5A/4HP machine would draw 6.5 x 1.73 = 11.3A and 5 x 1.73 = 8.7A.
This seems on the low side as the sorts of single phase currents I see driving a fully loaded 3HP 3P motors via a SP VFD on my dyno is ~ 10A, above that current, the motors start to suffer significant slippage in RPMs and heat up fast and would not be able to run continuously.
For 4HP 3P via a SP VFD motors the max continuous working load currents are around 14A.
These motors will run at higher currents but will produce less HP due to the significant reduction in RPMs.

Measuring fully loaded currents on the motors of working machinery is very tricky as the machines have to be driven to just before stall point and RPMS measured. It's much safer and easier to do with a dyno.

Originally Posted by Vann

AEG (Allgemeine Elektrizitäts-Gesellschaft) is still in existence today. It's a slim chance - but you have a serial number stamped on the motor.

Cheers, Vann.

It's not an AEG. It's an Australian General Electric. Made by the American GE company in Australia. Likewise GE Australia still exists but i think that motor is pre WWII....or the bottom number is a manufacturing date and it was made in 1941.

Thank-you for everyone's input, lots of useful info! This is all new to me so its good to hear from people with more experience. Although, its frustrating trying to get my head around 3-phase motors in use with VFDs it is also extremely interesting. I never thought not having the name plate would become such an issue. (I've learnt my lesson). The bandsaw is roughly a 1930 model (It's also missing its name plate!) So the motor being made in 1941 would make sense to me.

Originally Posted by NCArcher

It's not an AEG. It's an Australian General Electric...

D'oh .

Oh well, I tried. I wonder which records have a better chance of survival: those in a German factory in WW2; or those in private ownership in an English speaking nation. The German ones I suspect .

Cheers, Vann.

CBaulds

I thought I recognised the brand as a Motor I have on a large homemade wood lathe, which I have never used! This motor is only 1HP, but four pole (1440rpm). The dimensions are 235mm long excluding the shaft and, coincidentally, the diameter is also 235mm excluding the terminal box. I thought it may help you at least work out if your motor is bigger again.

This motor is on a lathe with a bed around four meters between centres, but as I said it is fabricated and I have never actually used it so I have no idea whether the motor is appropriate.

P1050660 (Medium).JPGP1050661 (Medium).JPGP1050662 (Medium).JPG

Regards
Paul

It may depend on what you are trying to achieve. If you primarily want to get your machines operational, I would be inclined to buy suitable 240V motors, secondhand if you can find suitable ones. The cost of a quality VFD, plus the risk of the motor having faults or being incompatible with a VFD may make it a costly and risky venture to try and keep the current motors.

If you are keen to keep the machines intact with the original motors for heritage reasons, then that is a different story and there is plenty of advice above.

Good luck.

Bauldy

Cheers, for those measurements Bushmiller. Yes that one looks just like mine, even looks like who ever you got it off had the same taste in paint colour! Have you got any plans to get that lathe up and running?

Bauldy, you make a great point. I agree it would be safest to get a knew motor. But I would really like to run this motor with the machine and I can't seen myself getting 3-phase hooked up any time soon. So I've decided to take the gamble and treat it as a learning exercise. Fingers crossed.