Dan P.

Motor question- VFD or single phase

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I am moving to a workshop which inly has single phase power.

I'd be grateful to hear opinions, preferably born of experience, on whether to go with a VFD or whether to mount an appropriate single phase motor.

The power hammer is mechanical, bow spring, 1/2 cwt, the existing motor is probably original (1948) or at least very old, 3 hp.

I imagine the bow springs add considerably greater weight to the business end than coil springs do.

I am electrically illiterate, so am leaning toward getting a new motor, but those old motors are beasts, and I am loathe to separate it from its old friend.

I imagine the cost will likely be about approximate.

Your thoughts?

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I ran a lot of 3 phase equipment by wiring the single phase 220V through a second 3 phase motor that was as large or larger than the one on the lathe, mill, surface grinder, etc. I was given a pallet of surplus 3 phase motors, so I did not have that expense.  If I ever get a shop built, I have a rotary phase converter that will go into it for my equipment. By using the rotary converter it will provide 3 phase for the whole shop, and it will handle a combined 60hp and start up to a 15hp motor. The nice feature of VFDs is that you can vary the speed, unlike a phase converter that cannot do that.

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Old motors often don't play well with VFDs because the older motors tend to generate too much heat for the old lacquers used on the windings when run at less than the rated RPM.  So---it's a shot in the dark whether a VFD would be fine with that old motor or not.

I can say that once you start using VFDs on equipment, it's hard to go back---so danged nice in many regards.  Adjustable soft start and stop parameters, variable speed, on some you can over-freq and actually get more speed from the motor than it's rated for, you can see exactly what speed you were working at in the display and repeat that every time.  Plus they integrate the safety of a "magnetic starter" so that in the event of a power failure, the machine doesn't start back up when the power comes on.

They're not horrendous to set up these days.  Not completely simple but not bad.  Cost has come down enough that they are definitely worth considering.

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On 3/13/2018 at 2:31 PM, BIGGUNDOCTOR said:

I ran a lot of 3 phase equipment by wiring the single phase 220V through a second 3 phase motor that was as large or larger than the one on the lathe, mill, surface grinder, etc. I was given a pallet of surplus 3 phase motors, so I did not have that expense.  If I ever get a shop built, I have a rotary phase converter that will go into it for my equipment. By using the rotary converter it will provide 3 phase for the whole shop, and it will handle a combined 60hp and start up to a 15hp motor. The nice feature of VFDs is that you can vary the speed, unlike a phase converter that cannot do that.

Sounds like this would be rather above my pay grade. Is this what's meant by "slave motors"? 

On 3/13/2018 at 2:44 PM, Kozzy said:

Old motors often don't play well with VFDs because the older motors tend to generate too much heat for the old lacquers used on the windings when run at less than the rated RPM.  So---it's a shot in the dark whether a VFD would be fine with that old motor or not.

You are selling me the VFD option. The old motor in question could do with being sped up a little. I did think of the soft start capacity but I'm guessing that's not so important with a mechanical hammer because it's only turning the flywheel to begin with.

My main problem is that I would be inclined to get an electrician involved, which means £££.

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What rpm is the hammer running at Dan?

I have run mechanical hammers (Goliath) from as little as 1 hp but that’s for a 240 rpm 60lb hammer, 3hp is better 5 better still.Faster would need more hp....is it a fast cutlers hammer?

I would be inclined to get a new motor. Are you limited to 13 amp single phase as you will need more than that for either an inverter or new 3 hp motor.

 

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Thanks Owen. The hammer is 250 rpm (could go for a bit faster, maybe 300 rpm), the motor is 940 rpm.

This motor has full load current of 13 amps, not that I really know what that means. I presume it is suitable?;

https://www.ebay.co.uk/itm/Electric-Motor-Single-Phase-2-2Kw-3HP-4-pole-1400-rpm-3-HP/271904454684?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2055119.m1438.l2649

Do you know  if higher amperage (??) is something that the average sparky can help me with?

On 3/13/2018 at 4:16 PM, basher said:

What rpm is the hammer running at Dan?

The hammer is a Massey spring hammer. 250 bpm by design, though perhaps would benefit from greater speed. The ram weight is 1/2 cwt, but the springs must add another 30+ kg of inertia.

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Is it possible to get 3 phase installed?

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starting current will be a bit more than 13A, if you know a local sparky talk to them.

turning the flywheel will mean a lot of load to get it up to speed, it will be like starting your car engine with it in top gear and getting it to 90mph in 3 seconds, when the motor is going it will take a lot less.

reduce the pulley size on the motor to run the hammer

you will want that same power and try working out the correct pulley size to keep the flywheel at the same speed it was before

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12 minutes ago, Marc1 said:

Is it possible to get 3 phase installed?

No, it is prohibitively expensive in this country, unless you already have it from 30 years ago when they would install it practically free (or so I'm told). According to my electrician mate it's not really necessary either for the size of machines that I'm running. 

13 minutes ago, the iron dwarf said:

starting current will be a bit more than 13A, if you know a local sparky talk to them.

turning the flywheel will mean a lot of load to get it up to speed, it will be like starting your car engine with it in top gear and getting it to 90mph in 3 seconds, when the motor is going it will take a lot less.

reduce the pulley size on the motor to run the hammer

you will want that same power and try working out the correct pulley size to keep the flywheel at the same speed it was before

Thanks. I don't mind the hammer being a little faster, or a lot faster, actually, but if I got a new motor I'd likely have to get a new pulley anyway so no drama there, except of the £££ variety. It's mostly the bare facts of how electricity works that have me bamboozled.

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to run it faster you will need a bigger motor and that means more starting current until it gets the flywheel up to speed

larger motor pulley means needs more power to turn flywheel

there will be a limit as to how much starting current you can get as most houses these days dont have a very heavy supply, my house has a 60A supply but that was built in 1950, my workshop  has a 3 phase supply but that is on a farm

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I would consider a new 3 phase motor with an appropriate VFD.  The  new motors are generally 1800 rpm or 3600 rpm but with the VFD you can run it at the desired speed.  You can get a VFD that will run a 3 hp 3 phase motor using 230v single phase power.  There are several advantages to running a VFD and you could also wire it so that you could run other pieces of equipment.  For an example I run my disk grinder, mill drill and drill press using one VFD.

Let me know if I can help you.

Wayne

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We are playing a bit of a guessing game here.

To replace a motor on a machine, you need to replace it with the same power and the same RPM.

What power is the old 3 phase motor?

What is the capacity of your electricity supply? You can buy single phase motors 240v 50Hz up to 5.5 HP and they draw about 22 amps. That means you need a supply cable that is 4 mm2. Not much of a muchness for any electricity worth his salt. you will need to work out the pulley size to make it run at the RPm you want. 

22 amps is not much at all for any ordinary house supply. 

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If you are limited in the size of motor by starting current as starting under load the current draw can be five or more times running draw, you could fit a backing roller or snub roller to over length drive belts to work as a clutch so you can start the motor under little load and then slowly engage the belt tension to get flywheel up to speed. 940 rpm means its a 6 or probably 8 pole motor which means it will be expensive to get another the same rpm  Cheers Beaver

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So, the hammer is in, and a VFD installed without hitch. 

The motor starts as it should, the flywheel flies, but the movement craps out under load. I saw this hammer under 3 phase power before I purchased it and it was fine.

I lowered the motor speed, and the hammer performs to a functional but not optimal level.

I have not tried to enter any settings on the VFD yet, and the instruction manual was not written for laymen. Is there something that the VFD itself can do for this weak torque?

Any further pointers appreciated. 

Video attached of hammer crapping out before I lowered the motor speed on the VFD

 

904C4942-4068-4872-9568-C8CBE176CD66.MOV

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What model VFD are you using? I bought mine used, and discovered the previous user had programmed a parameter that limits the output current. 

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Dan, can you get a pic of the VFD plate or let us know the make/model?

I'd say it looks like a current limitation from the VFD. The first thing I'd check is that the VFD is not set up for a quadratic torque application (pump/fan) as it's just a pushing buttons job. Next would be to check the motor wiring in the connection box (that it's not star/wye-connected instead of Delta, assuming you are on 230V 3-phase); a screwdriver job. After that, it's a case of checking the settings in the VFD and maybe breaking out a multimeter.

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Hi timgunn1962, it's nice to bump into you here, having benefitted from your many experiments and posts over at British Blades (especially as re/discoverer of "amal" burners, I think? Which has benefitted many over here).

I had a "phone a friend" situation with this motor/vfd set up, and the friend in question did make it clear that a "delta" configuration was necessary, or at least preferred, for 1ph to 3ph VFD connection. However, on opening the conduit/terminal box on the motor, the wires that went into it just carried straight on through deeper into the motor. And of course they were all the same colour. So I crossed my fingers, said a few Hail Marys, and wired her up to the VFD, without dramatic incident.

I will get some more photos when Im back at work tomorrow, but part of my problem is that this motor is, I think, original (1948), and does not have a plate to specify what is going on with it. I presume it is therefore by default in star configuration? Would that be a correct assumption? And thus perhaps an explanation of what is going wrong?

 

On 6/15/2018 at 2:51 PM, tylerdewitt said:

What model VFD are you using? I bought mine used, and discovered the previous user had programmed a parameter that limits the output current. 

The VFD is new, thus;

ebay link removed

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It's about 99% certain that the motor will be designed for 415V (or thereabouts) across each of the windings. If it has not been rewound, it will probably have a laquer winding impregnation, rather than a more modern resin, and will have very little chance of surviving long on the spiky output from a VFD, even if you can sort the Voltage issues.

I think it's probably a mechanical job now, changing out the motor for something modern. 

If you can get hold of an optical tacho, it's worth taking the belts off and checking the speed of the motor at 50 Hz running on the VFD. At no load, it should not matter that the Voltage is low. Then it's a case of measuring up the pulleys, calculating the hammer speed, sourcing a motor and fitting a motor pulley that'll give the same speed. 

We have 50 Hz power: 50 cycles/sec, 3000 cycles/min (North America has 60 Hz, 3600 cycles/min).

A 2-pole motor will turn a little under 3000 RPM

A 4-pole motor will turn a little under 1500 RPM

A 6-pole motor will turn a little under 1000 RPM

An 8-pole motor will turn a little under 750 RPM

Simplest is probably to get a 3 HP, 3-phase motor with the same pole count as the original. I suspect it will be a 6-pole, but I can't be sure.

More poles means a bigger frame for the same power and a more expensive motor. The existing pulley is unlikely to be usable on a new motor because it'll have a different shaft size. It may be that a 4-pole motor with a smaller (and therefore cheaper) pulley will get the job done more economically, but the minimum pulley size will depend on the drive belt section.

Even if you can't readily lay hands on an optical tacho, but have a reasonable idea of the hammer BPM, measure up the pulleys (OD and number/width/depth of grooves) and the belts (length and section width/depth, assuming they are vee/wedge belts). Length can be a pain to measure, so shaft centre-centre distance might be the best you can do. 

We should be able to work back from there.

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Thanks for your extensive reply, Tim!

Photo 1 and 2 are of a box on the side of the machine that may serve as a terminal box, though it does not contain what I would expect to find in a modern motor terminal. (There is an earth wire in the 2nd photo, it is just somewhat obscured).

According to the original drawings, the existing motor should be "C.P. Motor 3 H.P. at 940 R.P.M.". The hammer is intended to run at 250 bpm.

Have twiddled with the speed a bit, the hammer runs pretty well at more or less the rated bpm when I turn the speed controller from 65 (top speed, photo 3), where it would not run at all, down to 45 where it seems to work quite well. If the speed is turned down to 40, the motor craps out again.

Interestingly when the clutch is disengaged, changing the speed has the predicted effect on the speed of the motor. Under load, however, the speed of the hammer remains constant irrespective of what speed the controller is at. 

327DB39E-4FE5-4A4F-949A-F7891AC82A22.jpeg

FC40F843-93C8-4BE2-8572-00214CC08981.jpeg

C1B88C47-FE67-43C1-8FBA-7506DF696541.jpeg

Here is a link to two videos of the hammer working;

As you can see, it seems to work okay, though it is clear that there is a lack of power when the clutch is first engaged, and also when I want to put the hammer "flat out" the motor slows or stalls (not belt slippage)

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Dan is your motor wired in star or delta? 

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7 hours ago, basher said:

Dan is your motor wired in star or delta? 

I don't know! The connection in the photo above is the only thing coming close to anything that could be configured.

 

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If you are running a 3 phaze to 3 phaze inverter it does not make too much diference as the VFD will run both but single phaze to 3 phaze needs to de wired in delta at the motor....sounds to me like this could be the problem... (as I have had similar issues) but just a guess.

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I have a variable speed grinder and a single speed grinder with reducing pulleys- the variable speed is WAY more useful to me. Both are 220 single phase. I bought the single speed before I learned to listen when folks said penny-wise and pound foolish ;)

Dave

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