Variable speed motor suggestions for pin router project

[joe milana]

I have been in the process of restoring this pin router for some time now. What a project. Rebuilding a spindle that uses (3) 6205 bearings & spins nearly 20k is no easy task! Not to mention all the heavy cast iron components, mostly seized from rust. I'm really looking forward to getting this thing up and running. I expect it to replace my router table & be much more versatile.
The machine uses a 3hp 3450rpm motor with a large pulley & belt to turn the spindle at high speed. I would like to replace the motor with a 3phase motor & vfd to make the machine variable speed. I have some experience with vfd's but on this machine, I am concerned that when the motor is slowed down, it will loose power & under-perform. Any guidance or suggestions are greatly appreciated.

machine before disassembly...

[John McClanahan]

One way to deal with the power loss you describe would be to use a 5 HP motor and VFD.


John

[David Malicky]

Also try to use a 1725 rpm motor -- if it's reasonably modern and from a good manufacturer, it should be able to go 200% speed (or faster). This approach gives rated HP at 1725, and roughly that HP on up to 3450 rpm. The fan will get noisy at 200%; if a problem, it can be replaced with a constant speed fan. Of course a 1725 rpm motor will be heavier and more $ than a 3450.

[joe milana]

One way to deal with the power loss you describe would be to use a 5 HP motor and VFD.

I read somewhere that hp loss is exponential as the motor is slowed down, so at 50% Hz, i would only have 1.25hp. Also, an inverter duty motor would be required to run at speeds this slow. 'Dont know if any of this is true. ..


John

Also try to use a 1725 rpm motor -- if it's reasonably modern and from a good manufacturer, it should be able to go 200% speed (or faster). This approach gives rated HP at 1725, and roughly that HP on up to 3450 rpm. The fan will get noisy at 200%; if a problem, it can be replaced with a constant speed fan. Of course a 1725 rpm motor will be heavier and more $ than a 3450.

Interesting idea! So in theory, if i can source a 3hp 1725 motor, i might be good to go. I'll look into it. Thanks!

[David L Morse]

Interesting idea! So in theory, if i can source a 3hp 1725 motor, i might be good to go. I'll look into it. Thanks!

Or put a smaller pulley on a 3450 motor.

[joe milana]

Also try to use a 1725 rpm motor -- if it's reasonably modern and from a good manufacturer, it should be able to go 200% speed (or faster). This approach gives rated HP at 1725, and roughly that HP on up to 3450 rpm. The fan will get noisy at 200%; if a problem, it can be replaced with a constant speed fan. Of course a 1725 rpm motor will be heavier and more $ than a 3450.

This seems to be the best solution! The constant speed fan is a must. 3hp weighs about #100 so I'll have to make sure the mount can handle the weight. I'm looking at about $900 for new, but there are lots of used on ebay. I't a considerable investment when you throw in the inverter, but I have almost nothing in the machine so far, and I think it will be a great machine when finished.

[David Malicky]

David, that works fine at a constant speed. For VFD use, the 3450 motor running at 1725 only gives 50% its rated HP.

This seems to be the best solution! The constant speed fan is a must. 3hp weighs about #100 so I'll have to make sure the mount can handle the weight. I'm looking at about $900 for new, but there are lots of used on ebay. I't a considerable investment when you throw in the inverter, but I have almost nothing in the machine so far, and I think it will be a great machine when finished.

The weight will depend on the casing (cast iron vs rolled steel) and frame size. 3HP 1725rpm frame sizes are 56, 143, and 182. The smaller frames dissipate heat less easily at lower % speed, but a constant speed fan fixes that. A 3HP rolled steel 56 frame is typically about 50 lb. For VFD use, look for insulation class F (standard in ~all motors now). A SF of 1.15 (or higher) is helpful if it will run at full amps continuously. TEFC is best for woodworking, but ODP can work fine depending on duty and dust exposure. Price should not be more than $400 new for a Leeson or similar (Baldor will be higher). Probably $100-$150 on ebay or craigslist. New examples:
http://www.electricmotorwholesale.com/LEESON-116594/
http://www.walkerindustrial.com/1166...631-00.htm?gsn
http://electricmotorwarehouse.com/3-hp-1725-rpm-56hz-145t-frame-208-230-460-volts-tefc-leeson-electric-motor-116594#sthash.EDTaMyq1.dpbs

[joe milana]

Thanks David, this really helps. Heres what i came up with by following the categories for motor choices:
http://www.electricmotorwholesale.com/LEESON-811320/

Maybe its more than i need. The steel motors didnt say anything about variable speed, and they didnt have the constant speed fan. Maybe that is a simple add on. I dont know.

[David Malicky]

Thanks David, this really helps. Heres what i came up with by following the categories for motor choices:
http://www.electricmotorwholesale.com/LEESON-811320/
Maybe its more than i need. The steel motors didnt say anything about variable speed, and they didnt have the constant speed fan. Maybe that is a simple add on. I dont know.

That is a true "inverter duty" motor, which means it can go lower than 1% rated speed and not overheat, 24/7, for years. It has other features to protect it from hazards with older VFDs or long transmission lines (voltage spikes). It's a very nice motor, but this type is typically only needed for very-low RPM apps in a factory, where reliability is paramount.

Any 3 phase motor can go variable speed, and most VFD apps just use one of those with class F insulation. Say the pulleys are setup for these speeds at the spindle and motor:
20,000 rpm spindle = 3450 rpm motor = 200% rated motor speed = 120 Hz
7,000 rpm spindle = 1200 rpm motor = 70% rated motor speed = 42 Hz
That is easy for a standard 3ph motor to do. Milling machine motors often go as low as 25% rated speed in intermittent duty.

The fan is the only hiccup, as it will be noisy at 200% -- but not nearly as loud as the cutter. For constant airflow and less noise, the standard fan can be removed and a big muffin fan put on top of the fan shroud (sizing that fan may take some measurements).

If you have an idler pulley (like many drill presses), you could setup the pulley ratios to achieve 20,000 rpm this way:
20,000 rpm spindle = 2800 rpm motor = 160% rated motor speed = 96 Hz
7,000 rpm spindle = 1000 rpm motor = 56% rated motor speed = 34 Hz
160% would give a reasonably quiet fan, and 56% is plenty safe for intermittent duty. The only loss is a bit less power at 7000 rpm (1.7 HP vs 2.1 HP).

[joe milana]

Got it! I keep forgetting that with a 1750 motor, i would not need to go below 60hz, only above, therefore cooling would not be an issue. Not sure where to source a " muffin fan" though.
All great news, and makes used an option for sure.
No idlers on this machine, gearing changes just aren't an option.

[Rick Lizek]

http://www.designworldonline.com/how...r-duty-motors/

Here's something I'll bet you didn't know...

[David Malicky]

I would try the stock motor at 200% and hear the noise first -- most people find it acceptable. Easiest next step is some sound insulation around the motor / fan housing -- this kind of noise (high freq) is easy to muffle. A muffin fan is like a computer fan (but bigger). Some sources:
http://www.newark.com/wcsstore/Exten...html?page=2388
http://www.mouser.com/catalog/catalogusd/648/2142.pdf
Choosing the cfm is the hard part. The stock fan could be measured and spec'd/overkilled, or felt by hand and then try something overkill. As a pure guess, I'd start with at least 100 cfm.

Yes, bearing arcing can occur, but it only seems to be a problem for very large 480V+ motors running 24/7. I've had a few VFD/motors running 2000+ hours with no issue. I've seen VFDs retrofitted to many pumps and blowers (standard 3 phase motors), for years with no issue. Practicalmachinst has some good discussion on it:
http://www.practicalmachinist.com/vb...ailure-156525/
http://www.practicalmachinist.com/vb...arings-189035/

[joe milana]

http://www.designworldonline.com/how...r-duty-motors/

Here's something I'll bet you didn't know...

Yeah, i was warned about that before i purchased my first inverter. The benefits have far outweighed the risks.

I would try the stock motor at 200% and hear the noise first -- most people find it acceptable. Easiest next step is some sound insulation around the motor / fan housing -- this kind of noise (high freq) is easy to muffle. A muffin fan is like a computer fan (but bigger). Some sources:
http://www.newark.com/wcsstore/Exten...html?page=2388
http://www.mouser.com/catalog/catalogusd/648/2142.pdf
Choosing the cfm is the hard part. The stock fan could be measured and spec'd/overkilled, or felt by hand and then try something overkill. As a pure guess, I'd start with at least 100 cfm.

Yes, bearing arcing can occur, but it only seems to be a problem for very large 480V+ motors running 24/7. I've had a few VFD/motors running 2000+ hours with no issue. I've seen VFDs retrofitted to many pumps and blowers (standard 3 phase motors), for years with no issue. Practicalmachinst has some good discussion on it:
http://www.practicalmachinist.com/vb...ailure-156525/
http://www.practicalmachinist.com/vb...arings-189035/

Thanks again david. I think i am ready to move forward. I will post back when i have made progress.