belt grinder build— vfd -rpms -sfpm

A

achillesgr

Joined
Apr 29, 2012
Messages
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Guys,
I want to build a 72x2" belt grinder and i need your experience .
I am thinking a 2hp motor 3phase at 1400 rpms with a vfd inverter.
The motor will have a 4" drive wheel directly attached .

First concern is the rpms. The 1400 and the 4" drive wheel give me about 1450 sfpm and the 6" drive wheel about 2196sfpm.
Witch one do you think is the appropriate?. I heard that the vfd is able to increase the rpms but i do not know how much OR if it is better to use a 2800 rpm motor so the values of the sfpms will double and instead of having the vfd to increase the rpms i will decrease.
For example
A) if iuse 2800rpms with a 4" drive i will get 2850 sfpms that is ok but how much is the lower limit of sfpm?if i need to run at 1000sfpm i will need to slow the motor to 30% ??? Is that ok with the motor?
B)if i use the 1400rpm with the 6" drive wheel and the 2196 sfpms i will be able to get slower speed much easier but i will suffer at the high.how much a vfd can increase the speed?

Please note that i do not know how much will be the rpms drop when the machine is under load.

Second concern i have is about the inverter types.there are cheap ones and expensive. the difference is the sensor less vector technology that i do not know what it is and if i need it.can someone please let me know if i need such a type for a belt grinder and what it does?
 
First question is where are you?

You mention 1400 and 2800 RPM motors, suggesting you are somewhere that has 50 Hz mains power. I'm guessing Europe: North America uses 60 Hz.

50 Hz is 50 cycles/sec, so 3000 cycles/minute. 60 Hz is 3600 cycles/min.

Motors are available with different numbers of poles. These work in pairs, so a 2-pole motor has one pair of poles, a 4-pole motor has 2 pairs, and so on.

Each pair of poles needs a cycle, so more poles means a slower motor.

A 2-pole motor will run at 3000 RPM on 50 Hz, or 3600 RPM on 60 Hz

A 4-pole motor will run at 1500 RPM on 50 Hz, or 1800 RPM on 60 Hz

A 6-pole motor will run at 1000 RPM on 50 Hz, or 1200 RPM on 60 Hz

Under no load, the motor will run at, or very close to, these speeds. As the load on the motor increases, the speed will drop slightly ("slip" in the motor), until it reaches the rated speed at the rated load.

For use with a VFD, you are generally best using a 4-pole motor. This will be rated for 1400-1500 RPM on 50 Hz mains, 1700-1800 RPM on 60 Hz, but because the VFD provides variable frequency, the motor can run at speeds either side of the rated speed.

At speeds below the rated frequency, the drive varies the voltage and frequency together and the motor runs at "constant torque". Above the rated frequency, the voltage is already at maximum and only the frequency can be increased, so above rated frequency, the motor runs at "constant power".

As the frequency moves further away from the design frequency of the motor, various losses become increasingly significant and the motor performance is adversely affected.

Typically, motor and VFD manufacturers advise that a 4-pole motor can be run from 10 Hz to 100 Hz using a VFD, giving 300-3000 RPM on no load, perhaps 280-2800 RPM when loaded.

A Sensorless Vector drive has some fancy electronics built in which can compensate for some of the losses and will usually allow smooth operation below 10 Hz. I don't think this is likely to be particularly significant for a grinder, though it may be worth having if you want to use a very low speed for sharpening.

Most industrial motors are Totally Enclosed Fan Cooled. The fan that blows cooling air over the motor is mounted on the shaft and slows down as the motor slows. At low speed, the motor needs to be derated to avoid overheating. As a rule-of-thumb, derating is needed below about 25 Hz. This is something to be aware of, but tends not to be much of a problem in most grinder applications because they are not normally worked as hard at the lower speeds.

As far as drive wheels are concerned, I would size the wheel for the highest belt speed you might ever want at 3000 motor RPM and use the VFD to get it down to whatever you need for the job you are doing at the time.

The biggest consideration for choosing your VFD is likely to be how to ensure it is dustproof. Grinding dust will kill an unsealed drive instantly. The KBAC series of sealed drives are popular in the USA, but do not seem readily available in Europe. In the UK, Invertek IP55 drives seem popular with the guys that build their own grinders.
 
Timgunn,
Thanks for your time. I appreciate your help.
I am in greece and we have 220v 50hz. Tbe motors are at 1400 rpms. You suggested to choose the drive wheel considering the motor at 3000rpms and max belt speed iwant.do you suggest a 2800 rpm motor and use the vfd to slow it down or a 1400 rpm and use the vfd to increase the frequency?
er
I do not think either that i will run the motor at 10rpms so a sensor less vector is useless for me.

The enclosed types are much expensive. I do not know if i can use an ip20 with a biger fan bolted outside the already mounted fan and a cloth filter?

Please take a look at this link for a cheap chinnese:

(( live ebay link ))

I am sure that these are already long ago an the market.do you have any experience or oppinion about?
yes i know the serius vfds are much expensive but i am curius if someone here used aunit like this.


Do you think it is a good idea to mount the drive wheel directly on the motor?if i use a vfd i do not see why to use pulleys and axles.the problem i see is that the motors have about 2" shaft length and the drivewheels are about 2.5" so the shaft is only incerted about 1.5" inside the wheel?is it a problem? How you guys directly mount the wheels?

Thanks again!!
 
Last edited by a moderator:
Use the 1400 RPM (4-pole) motor and set the VFD to give 100 Hz maximum speed; 2800 RPM.

The HuanYang Chinese drives off ebay seem to be pretty good just looking at the manual, but I've not used one yet myself. A few of the guys on the various diy CNC machining forums seemed to find them OK. They are often used with 24,000 RPM spindles (also Chinese), running at 400 Hz. I'd like to get my hands on one to play with, just to see how easy it is to set up.

I have several drives, most of which I've taken off old machinery, but some have been new. My employer has provided me with a couple of drives for "familiarization" purposes; I persuaded them it's cheaper to get me a small version of the drive, which I take away and learn how to program in my own time, than to drop me on site with a big drive that I don't know and lose production while I try to learn how to program it in the field.

I tend to mount them in IP66 enclosures and have a remote control box. The enclosures need to be quite large to get enough passive cooling from the walls to stop things overheating. It's not a huge problem with the British climate, but you might need even bigger enclosures over there. I'd be inclined to increase the enclosure size or add heat-sinks (salvaged from bigger VFDs that have died) before considering blowing air through the enclosure, even with filtration.

Putting the wheel directly on the motor is pretty much a no-brainer. The motor bearings are pretty meaty, a good distance apart, and are designed to take the radial loads imposed by industrial belt drives. It looks like a foot-mount 90-frame motor has a 50mm long shaft and a 100-frame motor has a 60mm long shaft, according to the IEC standard tables, so fitting the drive wheel shouldn't really be a problem
 
Timgunn,
you helped me very much.i found the invertek you suggested and probably i will use it.finaly the enclosure add a significant amount of cost so i will use an already ip55 unit. I want to mount the inverter upon the grinder and the enclosures i find are quite big.
If i add an enclosure of about 100$ to the huanyang the the cost is significant.also i will need a remote control that ads again another cost in money or time to build it.
I think the ip 55 units are one way to machinery like that.

Timgunn., i have a lathe and i will make my own idlers,tracking and maybe the drive wheel.do you have any experience about?is the rubber or polyurethane a must for idlers that ocasionaly will be used as contact wheels?i am thinking a flaten with 2" and 4" wheels that i can rotate it.
Idlers and tracking with two ballbearings on each side and a spacer between?(stoll thinking how to mount the wheels...i am not sure if that way is good one or if there is another simplest)
Thans again!!
 
The IP55 drive is certainly the easiest way, and often the cheapest.

I've not built my grinder yet, or used anyone else's enough to be able to offer first-hand advice on wheels.
 
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