Some general simple tips about 3 ph motors please

[DPittman]

While I'm not ready to do it yet, I suspect some day i will want to do the vfd 3ph motor switch on my little milling machine.
What does one need to look for in the used market to find a suitable motor for said application?
Same hp as original?
Same rpm as original?
Same mounting holes/style
Are there other differences in motors once you get those basic parameters met?

 

[DavidR8]

The advice I received when I upgraded my mill was to increase the hp to help offset the loss of torque at lower rpm. I had a 1hp single phase and I went to 1.5 hp 3 phase. works great so far.
Look for an inverter duty motor as they as built to deal with the current coming from a VFD or phase converter.
Same rpm or you'll have to change pulleys.
I lucked out and found a motor with the same mounting holes. I could have fabbed up an adapter if I had to but it was easier to have a direct mount.
There is basically no used market where I live so I bought my motor and VFD from emotorsdirect.ca.

 

[DPittman]

The advice I received when I upgraded my mill was to increase the hp to help offset the loss of torque at lower rpm. I had a 1hp single phase and I went to 1.5 hp 3 phase. works great so far.
Look for an inverter duty motor as they as built to deal with the current coming from a VFD or phase converter.
Same rpm or you'll have to change pulleys.
I lucked out and found a motor with the same mounting holes. I could have fabbed up an adapter if I had to but it was easier to have a direct mount.
There is basically no used market where I live so I bought my motor and VFD from emotorsdirect.ca.

Excellent, thanks for the info. I'm not in a hurry to do the change over but having some basic info will be helpful in my search and application.

 

[Dabbler]

Advice is dependent on your application.

I VFD'd my big mill with a 2HP Teco - matched to a normal 2HP 3 phase motor. it coincidentally can also serve my other 3ph mill, and my surface grinder. (all are 2hp)...

The prime component is that all the motors are very nearly the same amperage rating, within 10% is best.

A normal (not inverter rated) 3ph motor will lose torque at both extreme ends of the speed range. At 10% of the rated speed, expect at least 30% loss of torque, even with sensorless vector option. At 150% of rated speed, expect a similar loss of torque. An inverter rated motor will lose a little less torque.

Heat build up is also a factor. both inverter and non-inverter motors will heat up appreciably at slow speeds. You may need to rob a computer and mount external cooling fan to help with this if you do a lot of cutting at slow speeds needing higher torque. I haven't needed to (yet). I tend to take a lot of short cuts or drill smaller holes with my mill. Your experience will vary.

With a typical 2 HP mill, this loss of torque isn't very relevant for typical hobby work.

Now a lathe, that is a seriously different matter.

 

[DPittman]

So if original motor on a milling machine is 1725 rpm can I use a 3450rpm motor successful with just pulley changes?

 

[DavidR8]

So if original motor on a milling machine is 1725 rpm can I use a 3450rpm motor successful with just pulley changes?

I think I’m theory yes but my experience was that I would have struggled to find the correct size pulleys to fit.
No idea what mill you have but the 3-step motor pulley on my mill has a shaft about 2” longer than the bottom groove.
I couldn’t find one like that anywhere


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[DPittman]

I think I’m theory yes but my experience was that I would have struggled to find the correct size pulleys to fit.
No idea what mill you have but the 3-step motor pulley on my mill has a shaft about 2” longer than the bottom groove.
I couldn’t find one like that anywhere


Sent from my iPhone using Tapatalk

Ah good to know....im just sort of keeping an eye out for a used motor and see a couple of 3450 rpm ones for sale.

 

[Dabbler]

@DPittman A 3450 motor isn't -quite- like a double speed 1725 motor. The starting torque and how it develops torque through slippage works is weaker... It has a lot less starting torque, and if you are solely depending on using a tiny pulley, the tight bend can cause the motor to stall or overheat on startup - if there is a lot of starts and stops.

A way to help with this is to get one you can wire for 220V instead of 110V. Wiring it that way gets you better starting torque (but the same torque at max speed), because of better slip characteristics with the stronger fields. It isn't quite as much as a 110V 1725 motor, but almost as much.

or - you can just get a slightly larger motor, a 1.5HP motor instead of a 1 HP motor...

 

[Tom Kitta]

I have like 30 motors, let me know what do you need.

It is best not to change speeds - through you can do a 1:2 pulley from 3600 rpm motor to act as 1800 motor with exact same torque you loose some power - as much as 10 - 20% on the pulley system. That pulley system also needs to follow rules - i.e. your smaller pulley cannot be to small etc. in order to transfer full motor torque - best to not do it.

Also you cannot over-speed 3600 rpm motor too much - you can over-speed 1800 rpm motor.

"Inverter duty" motors, which I have, simply have stronger insulation to deal with possible inverter related harmonics - no change in torque is claimed (at least anything meaningful). Also stronger insulation help when overheating your motor.

All motors are sized by "frame size" - best to get same size frame in 3ph. You can get a bit different frame if your machine is pulley run.

Remember, HP = torque * RPM. Also a bit different motors will have a bit different start & growth torque curves - but not by much - this is only important for starting under heavy load & is not applicable to a milling machine (which by definition starts under low load).

From HP = torque * RPM you can see how running your motor at only 50% of speed will at least halve available HP. This is why some people pick larger motors - say original was 1hp they go for 2hp.

I even have 6 pole motors - you want to try 1200 rpm? Weight of motor increases with more poles. Also weight of motor increases with amount of copper which determines efficiency. Farm duty == low efficiency motor.

 

[Dabbler]

Inverter duty motors use a combination of factors that change the inductance in order to generate less heat at lower frequencies. A bonus is that they can generate much more torque - as much as 20% more at very low frequencies. There is still a considerable drop off of torque, but they still perform better at the low end.

- if you can afford it, using a VFD with an inverter duty motor is a great combination. None of my VFD'd motors are inverter duty - I wish they were!

100% agree about frame size. On many of my machines, they can take bigger motors. For the low end torque, it can be cheaper to buy a used non-inverter duty 1.5 HP motor to VFD than buy a brand new 1HP inverter duty one, etc.

 

[DPittman]

Great info. It sounds like I should be looking for about a 2hp 1750rpm INVERTER duty 3ph motor to replace a 1.5hp single phase.
What's does "Class B" mean? Is that the frame size?

 

[Tom Kitta]

Class B means design - most motors take into account ambient temperature of 40C as max & 80 degree rise for max temp of whopping 130C accounting for hot spots of 10C. So your motor can get very hot yet still be within specs. see http://www.drivesandautomation.co.uk/useful-information/nema-insulation-classes/

As for inverter duty motors you have more info here: https://www.automationdirect.com/products/motors/index

 

[DavidR8]

Farm duty == low efficiency motor.

I did not know this. I always thought farm duty meant it would stand up to more abuse.

 

[DPittman]

I did not know this. I always thought farm duty meant it would stand up to more abuse.

That's what I kind of assumed also.

 

[Brent H]

The “Farm Duty” should indeed be able to take more abuse - typically they have a higher starting torque so will inherently draw more power off the bat and may be less efficient due to this.
There are many designs of motors for Various applications. For milling and lathe work I would be happy with a TEFC (totally enclosed fan cooled) or at the very least drip proof motor. Duty cycle of 1 or 1.2 and rated for continuous operation. Sealed bearings preferred. Three phase for smoother operation, speed control, ease of reversing and simple design. Single phase motors offer ease of plug in but also have capacitors in play for starting and possibly running and will be generally larger - they will also have centrifugal switches and a more complicated reverse and speed control.
As folks have mentioned before, a VFD for speed control will typically require a larger Hp motor to achieve the same torque results as a geared speed application or use of a DC motor for lower RPM operation.
If you are going to run a 3 phase motor and use it only for the speed of your mill or lathe and not use the gearing or belt combinations then you will require a higher HP motor to achieve the lower RPM power you want. You can also purchase a VFD rated motor etc and carry on that way .....life is like a bunch of electric motors.... LOL

 

[Tom Kitta]

When you look at when farm duty motors appeared add to it fact that they are cheaper then general purpose motor and add to it that they have low efficiency you notice that they are the old general purpose motors. After all if Farm Duty was able to take more abuse it would be *better* then general purpose motor and thus more expensive.

When efficiency rules came in there was a loophole placed in for "special duty" motors. Thus manufacturers used that loop hole to create whole range of "special" motors. You notice there are also "compressor duty motors" - I have some modern ones (two in fact) that are very light weight and rather poor efficiency - they are very similar to farm duty motors.

There could be some "farm duty" motors that are indeed designed for rough life, but these that I own & have seen all follow the pattern of cheap build and low efficiency.

 

[Brent H]

So here is a nameplate off a Farm Duty motor - 1 Hp. It is a well built motor and good for continuous service and capable of running at 100% load all the time. - efficient - no - may use almost double the amps but should deliver consistent torque. A typical 1 HP will draw about 6 to 7 amps at 120 V
That being said - the “efficiency” of the motor may not be an issue provided it gets the job done without problems.
We recently replaced motors on our Circa 1985 crane - old type 3 phase in place before all the “green” rules. The “new high efficiency” motors draw more power to operate than the old motors -(up to 25%) big disappointment and are larger. Our fans are being replaced and the original frame size was discontinued under the “efficiency” act that Tom mentioned above and guess what - still takes power to drive the fans so all the fans now have higher HP ratings and draw the same or more power for the same operation.

I guess it is all relative to what your application is. As well there will be Farm Duty motors built on the cheap as there will be other electric motors. I can say for a fact, all the “new “ motors we have installed on the ship are of cheaper construction, draw more power than before for the same operation and nearly all the installations have some cheapened electrical feature that is supposed to be the “best thing today”.

 

[Tom Kitta]

In order to figure out (mostly) what you are getting you need full spec sheet from manufacturer of the motor - even here it is "mostly".

New motors do not imply better - "better" is relative to "what" - i.e. motor A is better then motor B for certain application under certain conditions. Frequently "better" is cheaper to make.

The high efficiency motors generally need more power to start then old low efficiency motors and show off their high efficiency at close to full load. For lower loads they may draw a lot of apparent power - as seen by amp draw & may need power factor correction to bring it down. However, they should draw less real power then similar low efficiency motor.

There are so many generalizations in motors that it would be best to either just take everything as "roughly" or take a "spec sheet" out + there are always exceptions to everything (hence spec sheet).

Take for example a 10hp Baldor inverter duty motor I have - all great but its outer shell is cheap stamped metal. Or compare a 15hp motor I use for rotary converter to another 15hp motor right next to it (both 4 poles) I have as a spare. Weight difference is 3x. Technical plates are similar but clearly different design choices were made (I admit one of these is aluminium smooth skin vs. ribbed cast iron).

It would be great to have someone in the group that designs motors - I am sure they could just go on and on and on about all sorts of technical stuff.

 

[trlvn]

A typical 1 HP will draw about 6 to 7 amps at 120 V

Normally I would take your word as gospel but I think you slipped this time. 6 or 7 amps at 240 volts would be right. For example, following is the motor from my Kraemer dust collector:



Another 1 horsepower motor I had at one time was a a CGE:



I'm with the others that think that farm duty means that the motor will tolerate rain, dust and maybe other adverse operating conditions. I'm thinking of a grain auger or bale elevator application.

I've never been clear on electric motor efficiency. I have a "Handyman In-Your-Pocket" reference book (by Young & Glover) that has a section on decoding motor tags. It says efficiency is usually given as a percent (eg. 87%). I don't think I've ever seen that on a small electric motor. The book also says that there is a NEMA "index letter for efficiency rating of a motor". Maybe that is the "KVA Code K" on the Franklin tag. If so, that "K" corresponds to 84% efficiency.

Craig

 

[Brent H]

Hey @trlvn - the 6.5 to 7 amps would be perfect world by only the math:

1 Hp = 746 w / 120 volts = 6.21 amps but that would be 100% efficient. Divide by your efficiency and you are up to 7.4 amps - but that is in some miracle world. Most single phase 1 Hp motors will draw between 12 and 16 amps - so many factors come into play. Efficiency on small motors is not usually a major concern to manufacturers as you plugging in and running a 1/2 hp electric motor is not the same as me closing the breaker on a 750 Hp motor.

I agree, a proper Farm Duty motor should be better built for conditions. In my fast review - most say higher and consistent torque - like you would want driving a grain belt or bailer. There are also - Agriculture duty motors etc.

In my line of work ( maybe it’s a “marine” thing) I have been seeing more “crap” than cool being introduced. We have some motors on board 35 years old that run 24/7 and have only had a bearing change or two. And we have had 3 new Baldor “marine rated” 30 HP motors crap the bed only months into operation. Couple bearings and one melted down.