which motor for this variable speed inverter (VFD)

Here's my take on motor RPM

It doesn't matter , LOL Unless your going to run a direct drive set-up (no belt and pulleys with the drive wheel right on the motor shaft) In which case you WANT the fast motor.

I figured it like this- the less RPM's the motors turning the longer the bearings and windings will last. This is just my theory not based on any scientific study or facts so TIFWIW just my opinion.

The RPM of the driveshaft is all that matters so a lower rpm motor with pulleys increasing the driveshaft RPM = longer motor life (MAYBE) LOL.

To be honest the quality of the motor is most likely the biggest factor that will determine it's life.

Some things require great consideration and others are easily overthought and worried about. I think motors are easily overthought in the context we're talking about here.

Get the highest HP you can run and get the driveshaft turning 3450 RPM (or even a touch more :) ) and you'll have a great grinder that'll last a LONG time.

-Josh
 
makes sense! lol

hopefully i'll get the grinder soon, will check everything first and then buy the motor, wont hurt to ask about prices and specs now tho :)

thanks Josh
 
The slower motors have more contacts inside the motor and are better motors. That is why they are a little more expensive. The fan set up is more effecient because of the slower speeds also. So if you are running a fast motor at slow speeds there is more of an over heating problem. The torque is also better at slower speeds with the slower motor.
 
wow a year and half passed

i now got almost everything ready, need to connect the invertercto the motor.

i really like what Josh Dabney did. the idea to be able to connect different motorscand the sort of remote control.
Josh, Wayne or anybody would know how or where to get the electrical plans for such connection?

thanks
 
I'm only familiar with my Teco inverter so things may be different with your brand.

I'd call the phone number for the inverter mfg. and ask if you can wire an external Potentiometer and forward/stop/reverse switch. They should be able to tell you exactly how to wire this set-up and what size wire to use.

All the components for my set-up are available locally here in the States.

This is my Potentiometer-
http://www.radioshack.com/product/index.jsp?productId=2062357

This is the switch-
http://www.radioshack.com/product/index.jsp?productId=2062531

You could use about anything as a substitute but I have my Pot and Switch mounted in this box-
http://www.radioshack.com/product/index.jsp?productId=2062279

I use standard 4 prong cords made for clothes dryer on my motors like this one-
http://www.lowes.com/pd_172517-83-W...29&Ntt=dryer+cord&pl=1&currentURL=&facetInfo=

Instead of wiring your inverter directly to the motor wire it to an outlet like this one-
http://www.lowes.com/pd_246881-1571-3884-CC6_0__?productId=3235978&Ntt=dryer+power+outlet


This set-up has served me very well as I now have my KMG, Disk Grinder, horizontal Grinder, Surface Grinder, and Milling Machine which are all powered from a single VFD.


Great looking grinder ! Should be a fantastic set-up

-Josh
 
I'm way too late to be of any use to the OP, but my understanding of the motor speed issue is as follows:

Pretty much every motor manufacturer standardises as many components as they can across the range of motors they offer. It's normal manufacturing practice.

They will have a casing, rotor assembly and bearings for each frame size they offer. Within a frame size, there will be different numbers of poles to give the different speeds. The number of poles is related to the number of windings in the stator, but as the name suggests, the stator is static and therefore does not form part of the rotating assembly.

Because roughly half the world runs on 60 Hz mains and the other half runs on 50 Hz, and it's not economic to make different motors for each market, you can be reasonably sure that all the main parts of pretty much any motor you buy are designed to run at 3600 RPM, which is the 2-pole, 60 Hz speed.

When driven by a VFD, the speed is not limited to the mains frequency and the motor can be run above the 50 Hz or 60 Hz local mains frequency. The further the frequency gets from the design frequency, the less efficiently the motor will run, so there are limits, but most motor manufacturers seem to recommend a speed range of 10 Hz to 100 Hz.

For a 4-pole motor, this gives a speed range of 300-3000 RPM. The top end of this speed range is still within the normal operating speed range for which the rotating parts of the motor were designed and the mechanical life of the motor should therefore not be compromized.

Running a 2-pole motor over the same frequency range would give a speed range of 600-6000 RPM. If the rotating parts of the motor were designed for a maximum normal running speed of 3600 RPM, then 40% of the speed range with a VFD is above the design speed. I'd expect the mechanical motor life to be shortened somewhat by this.

There again, I could be completely wrong and missing something blindingly obvious.
 
As for having more than one motor to a VFD I think that the simplest way is to wire from the motor to a 4 prong outlet box. From the VFD to a 4 prong plug. Then you can plug the VFD into whichever motor you want to run. You should have all the same hp and rpm motors. This can be done with a 4 pole on/off/on switch also but it is not as simple and the switches are expensive and you would be limited to only two motors.
 
timgunn, thanks, that actually made me understand a bit the whole thing

Wayne, thanks, i actually didnt even know there were 4 prong plugs, we use two and on occasion 3 to add earth, whats the 4th for?
i think thats a stupid question and i probably already know the answer, but does it matter if the VFD has the plug and motors have boxes or vise versa?

thanks again for all the help, i'll try to find plans, either online or with manufacturer and will post pics when im done.
 
The most common 4 prong plugs and outlets are for dryers and ovens. Most are 3 prong but there are 4 prongs too. You might also check www.grainger.com MSC and McMaster-Carr. The outlets should be either in a box if open but you might also find a female plug like to go on an extention chord.
 
Plugs have exposed conductors and are used on the device. If it's not plugged in, there's no power on the conductors, so no risk of electrocution if someone touches them. The sockets are used on the power supply side and have fingerproof receptacles, so the live conductors can't be touched when nothing is plugged in. It's a safety feature and applies universally as far as I am aware. There are lots of different regional designs of plugs and sockets though.

The reason for the 3Poles + Earth/Ground is that the VFD outputs 3-phase power to the motor, so a separate conductor is needed for each phase. The Earth is for a protective Earth/Ground connection.

I tend to use blue 16A 3P + E plugs and sockets conforming to IEC 60309 (also known as BS4343 in the UK) between my drives and the machines they drive.

http://en.wikipedia.org/wiki/IEC_60309

I don't know what is available in Egypt, but IEC 60309 is a widely-used international standard, so you've probably got a reasonable chance of finding stuff.
 
thanks timgunn, yeah i believe we have these here.

i think this is bit much for me tho, will probably need the help of an electrician, or maybe pay a hungry college student :p
 
iittsssaaaalliiivveee!!!!

20120710_201942.jpg

its a temp setup(my living room), but just wanted to share and thank all that helped!! thanks A LOT!!
 
so i now plan to upgrade motor, because apparently i've been grinding at grandma-speed (if i remember the calculations, 2094f/m)

a very experienced knife maker advised me to increase HP of motor, but i'm limited by my inverter which is 2HP

the one i am using now is 1400rpm, i planned on buying 2hp 3000rpm

i tried before changing pulleys but that removed all the torque i kept getting motor overload error on the inverter...

so just buy the new motor or is there anything else to consider? because i really really really don't want to make mistakes this time and don't want to waste away money

thanks

extra notes: motor pulley is 12.5cm, grinder pulley is 8.5cm. driver wheel is 10cm(which i guess i can upgrade but that would be secondary option)
 
Pretty much every VFD manufacturer and motor manufacturer I've come across recommends using a 4-pole motor and running over a range of 10 Hz to 100 Hz for a usable speed turndown of 10:1.

The motor will run at constant torque between minimum speed and rated speed, then at constant power between rated speed and maximum speed.

I have found that it is often possible to run up to 120 Hz on a 4-pole motor, though I suspect that the torque is starting to drop quite fast once the speed gets up that high.

I have also run down to 3 Hz smoothly with a "sensorless vector" VFD, where basic "V/Hz" VFDs seem to feel like there is "cogging" once they get much below 10Hz.

Making the assumption that the rated speed is the maximum speed the motor should be run at is probably not quite right.

Motors are designed for mass production. The only thing that really decides the speed of a motor is the stator winding arrangement. It is much more cost-effective to make all the motors of a given frame size exactly the same in all other respects.

All the mechanical components of the motor will therefore have been designed/specified for 2-pole operation on a 60 Hz supply to give 3600 RPM, regardless of which windings get fitted.

Comparing a 2-pole and a 4-pole motor, both rated at 2 HP full power, at various speeds:

At 300 RPM, the 4-pole produces 0.4 HP at 10 Hz

At 600 RPM, the 4-pole produces 0.8 HP at 20 Hz
At 600 RPM, the 2-pole produces 0.4 HP at 10 Hz

At 900 RPM, the 4-pole produces 1.2 HP at 30 Hz
At 900 RPM, the 2-pole produces 0.6 HP at 15 Hz

At 1200 RPM, the 4-pole produces 1.6 HP at 40 Hz
At 1200 RPM, the 2-pole produces 0.8 HP at 20 Hz

At 1500 RPM, the 4-pole produces 2.0 HP at 50 Hz
At 1500 RPM, the 2-pole produces 1.0 HP at 25 Hz

At 1800 RPM, the 4-pole produces 2.0 HP at 60 Hz
At 1800 RPM, the 2-pole produces 1.2 HP at 30 Hz

At 2400 RPM, the 4-pole produces 2.0 HP at 80 Hz
At 2400 RPM, the 2-pole produces 1.6 HP at 40 Hz

At 3000 RPM, the 4-pole produces 2.0 HP at 100 Hz
At 3000 RPM, the 2-pole produces 0.6 HP at 50 Hz

At 3600 RPM, the 2-pole produces 2.0 HP at 60 Hz

Draw the power:speed curves and choose your motor from the curves. I don't think there will be any doubt at all which is the better choice.
 
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