Thread: Most affordable electric motor supplier?

Originally Posted by pixelfixed

My question to you is,why in the hell do you need a three phase motor on a grinder?

Because variable speed using a frequency convertor is a lot easier using 3 phases. 3 phase is industry standard.
My grinder has a 3 phase drive. The frequency convertor is 1 phase, but outputs 3 phases specifically for this purpose. Having 3 phase motor also allows for controling the direction via the frequency convertor.

Originally Posted by 32t

Assuming we are talking about 3 phase motors I don't understand that most can't handle a VFD for long. Other than shaft voltage possibly effecting bearings and that for your use and size of motor needed should be no problem. If anything it should extend their life. At work we don't run anything under 20 Htz.

There is probably a miscommunication, because a 3p motor can run indefinitely on a VFD.
In fact, most motors in our plant are continuously controlled via a VFD, and run for years and years on end.

But it requires a VFD and not a cheaper solution with a dimmer, for example.

From what I understood from the salesman, motors have different ratings. WHen I was explaining what I was using it for and about the vfd, he stated that the motor wouldn't make it very long. Now that I think about it, I wish I would asked "how long is not very long". The one he found for me (only $25 more) was rated to go down to 1/10th the original shaft speed. There was something he kept on about "fractional ratings" & most of the others had a larger fractional numbers. Maybe he was talking about speeds, I'm not sure. I know on my vfd, the speed rating is shown on the lcd as "%". If I were to apply logic as I understand electricity & motors, I could see how a lower rpm could build up more heat (remember I know squat about 3ph)...but I am not certain by no means by making that statement. What I did take out of that conversation is that unless doing a finish grind I plan on keeping the rpm's up for a longer motor life.

It is true that if you go very low in rpm, than is kinda bad for the motor. Especially if it is a 3450 rpm drive, because in order to make it go slow, you'd have to get the frequency to half what you would use for a 1750 rpm motor. One easy rule of thumb I've heard is not to drop below 20% of max rpm unless the motor is designed for it. And 3p motors can handle it better than 1p motors.

It's also important how much load you would put on the motor. Asking the motor to deliver a lot of torque at very low RPM is a sure way to burn it. But that is not what we do when grinding razors or knives.

And yes, it is correct that this is expressed in % of shaft speed. Also yes: if you want a long motor life, keep up the rpms while hogging out blades. Not only will it go faster, but you will also go easy on the motor.

I chose a 1800 rpm motor just for this reason. I would rather the belt running a little slow than a little fast. I have been looking forward to having a real grinder for some time, but the original motor took the wind out of my sails, the play fund had been exhausted as well with purchasing the drive wheels & the "NWG in a box". I'll get some pics up once it's complete. Next on the "bucket list for 2013" is to build a gas forge, but the expense will not be near as great as this one. As it stands right now, I would say I have about $800 in this one...but it wasn't all at one time. I may have been ahead getting a commercial one, but what fun would that be...I just LOVE banging my head against a cinder block wall!!!

Just a reminder, the easy way to reduce your speed by 1/2 is to change from a 4" drive wheel to a 2" drive wheel. So if you want to run you VFD at no lower than 20% but "need" a slower speed then you can use a smaller diameter drive wheel.

Motor is here!!! Bad part is that the shaft size is different & I now have to order a new drive wheel as my old motor had a 7/8" shaft & this one looks like a 1/2" shaft.

Who has the quickest turn around on drive wheels?

I've heard many great things about sunray.

Drive Wheels | Industrial Wheels | Heavy Duty Wheels

Wheel - Contact Wheel 5" 1/2" bore

I think Sunray is the mfg supplier. Check out what they have to offer. Do you need a "keyed" wheel?

Hope this helps.

I'm away from the house for a day or two & having to access emails & have to access emails by my phone. I do think USA Knifemaker will have what I need. The contact wheels I have are from Sunray.

Thanks guys! I'll get the drive wheel on order once I am certain of the shaft size, but at least the motor is in hand and I can wire & bolt everything back up.