Thread: Calling all electrical experts - DC motor conversion

Originally Posted by BobL

More explicitly, that motor looks like its a 420V "Y" , or 400V "∆" 3HP
If you don't know what "y" and "∆" mean you will need to do a little bit of homework.

Running in Y Mode will give 420/240 = ~1.7HP
Running in ∆ mode = 400/240 = ~1.8HP

To get the full 3HP at 240V the Y connection inside the motor will need to be converted to ∆ connection.

Hi Bob,
That doesnt sound right. If the motor is ∆400/420.... then there is no star point to dig out, you already have it. But if its a Y start ∆ run motor why doesn't the plate say that?

Stuart

Originally Posted by Stustoys

Hi Bob,
That doesnt sound right. If the motor is ∆400/420.... then there is no star point to dig out, you already have it. But if its a Y start ∆ run motor why doesn't the plate say that?

Stuart

Good point.
I didn't look at the pic closely enough.
It looks like the 400/420 refers to the ∆ connection side of the motor so running this connection at 240V will generate ~half power.
Running it on Y will generate 1/4 power.

Still plenty of go at 1.5hp, especially vs what I have on there at the moment which is 0.5hp and probably only 0.15 of those left and working properly

What are the pitfalls in looking for motors? Will any 3ph motor work? 230, 400, 420, with the only impact being final HP when run on 230v or are there some that will never work?

I assume a 1-2hp 230v 3ph motor is the ideal purchase for my little 9" Hercus?

Originally Posted by neevo

Still plenty of go at 1.5hp, especially vs what I have on there at the moment which is 0.5hp and probably only 0.15 of those left and working properly

Even really old motors still tend to generate their rated HP. The oldest motor (rated at 0.5HP) I have tested was 105 years old and it still generated 0.5HP!

What are the pitfalls in looking for motors?

Pitfalls include
Dud Bearings, its not a proper test by any means, but the rotor should spin freely and noiselessly by hand.
Uneven coil impedances (resistances). Also not a complete test but the resistance across each coil should be the same. If these resistances are not the same the motor will overheat
Low insulation resistances. These should be tested with a Megger type instrument.
Running Noise/squeal. This may be indicative of some other problems and can be VERY irritating. This can be tested just by running the motor

[QUOTE)Will any 3ph motor work? 230, 400, 420, with the only impact being final HP when run on 230v or are there some that will never work?[QUOTE]
220 - 250V rated connection, connected to 240V should produce full rated power
380 - 440 rated connection, connected to 240V should produce half rated power
The motor also should be a 1400 rpm motor

I assume a 1-2hp 230v 3ph motor is the ideal purchase for my little 9" Hercus?

On my Hercus I started with a 1/2HP 415V Y connected motor (so only 1/4 HP) and as I do mainly small stuff I thought I would be OK but found it quite restrictive especially at lower frequencies (<40Hz) and so I was constantly changing gears, especially.
Then I upgraded to a 1 HP 240V ∆ motor (so full 1HP) and I now can get down to about 20Hz without changing gears for most things.
2 HP should theoretically allow me to operated down around 10Hz but this is also problematic as operating the motor lower than about 20Hz may running into cooling problems.
The other advantage of more power is being able to take deep cuts but I really question whether extra loads on the bearings on these small lathes are up to this for extended periods.

Great info, thanks Bob. Just heard back on that motor on gumtree. It's 400mm x 280mm diameter, that sounds huge for a 3HP motor doesn't it?

No way can I fit that on the mount unfortunately

Originally Posted by neevo

Still plenty of go at 1.5hp, especially vs what I have on there at the moment which is 0.5hp and probably only 0.15 of those left and working properly

What are the pitfalls in looking for motors? Will any 3ph motor work? 230, 400, 420, with the only impact being final HP when run on 230v or are there some that will never work?

I assume a 1-2hp 230v 3ph motor is the ideal purchase for my little 9" Hercus?

For a 9" Hercus I would recommend a 3 phase 240V 4-pole TEFC motor as follows (based on personal experience with my 10" Hercus 260 fitted with 3/4HP VFD and motor)
- a 1/2HP motor would be the smallest choice. If 90 % of what you do fits over the cross slide, you will be happy.
- 3/4HP would be what I regard the ideal size for either a 9" or 10" light duty lathe such as Hercus.
- 1HP if you do a lot of large diameter work or like roughing with deep cuts at the limits of the lathe's rigidity (or slightly beyond).
Make sure you buy a motor that is a drop-in replacement to your existing motor (meaning the pulley fits the motor shaft, and the mounting holes line up). So first thing you need to find out what frame the existing motor has, is it a metric IEC frame or an imperial/American NEMA frame. If its a very old lathe you may not find a simple drop-in replacement - or it may be cheaper to modify the mounting/pulley to suit whatever motor you can get for cheap.

Pitfalls when looking for a used motor? I would personally NEVER buy a used 3phase motor under 2HP! There is no simpler and cheaper motor than a 3 phase motor. In the power class of 1/2 to 1 HP, these are really dirt cheap: you are looking at between $80 and 150 brand new off the shelf. Sometimes it pays off at visiting a motor dealer, telling him its for a hobby lathe, and asking if he has some one-off "new old stock" motor that would suit. If you must buy second hand, choose at least something that looks near new or well looked after. There are some motors on eBay auctions..... old and rusty and grotty on the outside.... that actually belong in the tip. I have found bargain NOS motors on eBay, like never used at 1/4 the former list price, but you have to be patient and look up regularly and just buy it when one pops up. If you need a motor NOW, all I can recommend is buy a new one off the shelf. Whatever the salesman tries to tell you, you do NOT need an expensive VFD rated motor, you want a stock standard low cost off the shelf motor.

Thanks all. Search continues. Will see if I can find a local place for motors near/around Penrith NSW.

Originally Posted by neevo

Great info, thanks Bob. Just heard back on that motor on gumtree. It's 400mm x 280mm diameter, that sounds huge for a 3HP motor doesn't it?

No way can I fit that on the mount unfortunately

Another reason to get a smaller motor.

I'm very happy with all the small used 3P motors I have acquired over the last 5 or so years.

These include

The 2 x 1/4 HP, 1 x 1/2 HP and 1 x 4HP I picked up for nothing.
All except the 1/2 HP have been converted to 240V/∆ operation
The 1/4HP motors are being used on Blowers to vent the shed - these motors are more than 40 years old and were left out in the open for some time (so quite rusty) but they still meggered and ran OK, although one had a noisy (corroded) bearing. I converted and restored these more as a challenge than any sort of practical consideration.

I bought a never used 240V/∆ 1/5 HP motor for $20 because it looked cute and for a project that never eventuated, so it now sits in the back of a cupboard.

Two 1HP motors I bought are on my wood and MW lathes each cost $30 and both are running fine after 3 and 4 years of operation respectively.
Another 240V/∆ 1HP (Danish ASEA) motor cost me $70 as felt sorry for the old guys selling it. This has become a sort of a test motor.

The 240V/∆ 1.5HP Leeson on my DP also cost ~$30 and probably gets more use than any of them - running well after 3 years.

A never used 2HP 240V/∆ Fasco cost me $50. I was going to put that on a home made linisher until I picked up a 3HP linisher for nothing.

A never used 3HP 380V/Y, dual speed ABB that I bought for $30 has been converted to 240V/∆ single speed - awaiting a future project.

A 4HP GE 240V/∆ -415V/Y motor cost me $35 awaits a future project.

The near new 5HP 240V/∆ - 415V/Y cost me $35 and is on my compressor.

As others have said there are motors out there, you just have to be patient and wait for then to come up.
At our mens shed we have more than a dozen, mostly quality never used 3P motors. About half of these have come as a donation from an air con company that closed down. Most of these will not be used so we will eventually look to move these on as they do take up considerable space.

I actually have updates!

I ended up going down the PWM drive path, mainly because it was $200 delivered and I could use the existing motor.

Step 1 was to get a fan that would blow air into the motor when I reversed it. I bought a PC fan with a housing and turned up a custom mount for it on the lathe plus made a sheet metal frame for the fan housing:





I've hooked it up to the motor and spun it and on mid speed the motor actually gets colder to the touch!

I freshened up the motor with fresh brushes which caused some issues as I though the system was broken, until I worked out the brush guide was bent and one of the brushes wasn't touching the motor.

The next part was to work up a proper control box. 1) to house the DC drive and 2) all the controls.

The box also has a 12v feed which turns on the RPM sensor, plus also a fan in the top of the box to draw out air around the drives cooling fins. Works a treat.

Also all input wires are detachable so I can separate the box from the motor and power socket.

Here she is in all her glory:









It works a treat. Just need a slightly shorter a section v belt.