VFD Motor Speed Question

[David DeCristoforo]

Is there any reason one could not use a 3400 RPM 3 ph motor with a VFD? I know this is "too fast" for a lathe but with pulleys, the spindle speed could easily be brought down and with the VFD, speed is controllable anyway. Most of the used motors I am finding are 3400 RPM motors. I'm looking for a 1700 RPM motor but they seem to be not as common. So for all the motor pros out there…?

[Dave Mueller]

David,
A 3400 RPM motor will work just fine as long as you use the pulley to reduce the spindle RPM. For example, a 50 RPM spindle speed will only be about 0.8 Hz from the controller for a 1:1 pulley ratio on a 3400 RPM motor. You might check with your local motor repair shop, either to have your existing motor checked or to buy a new or used motor from them. I bought a new 2 HP 3 phase motor from a local shop for about $130.

[Dale Miner]

Horsepower is torque X speed. An 1800 rpm 3 Hp motor has twice the torque of a 3600 rpm 3Hp motor. So, all things being equal, a 3600 rpm motor geared/belted down to produce the same spindle speed as a 1800 rpm motor with the 3600 rpm motor running twice as fast would work. In practice, when belting down the speed of the 3600 rpm motor, getting the first reduction on the motor pulley is the issue. V Belts don't like to run over small pulleys, so if using V belts, the pulley on the motor has to remain at a reasonalbe size, and the pulley that the belt runs on in the first reduction then needs to be about twice as large as it would be if an 1800 rpm motor was used. Typically, on a wood lathe with VFD, only a single reduction is used. If using a 3600 rpm motor, it might be necessary to use a double reduction AKA jackshaft. If using a jackshaft, getting the needed reduction ratio can be accomplished without the need for a large pulley on the lathe spindle, but slippage of the smaller pulley on the second reduction may occur.

In my opinion, trying to use a 3600 rpm motor without a jackshaft by dialing down the speed with the VFD will be less than satisfying. Heavy cuts are usually taken at the lower end of the speed range, and the 3600 rpm motor dialed down with the VFD will be lacking the needed torque.

The micro V or Poly V belts do much better than standard V belts on smaller pulleys. The thinner body is more flexible, has more surface area in contact with the pulley, and accepts wrapping on the smaller pulleys without slipping. Kinda pricy though, compared to standard V belts.

Seems like in your situation, a larger VFD would be more econimical than re-pullying your setup, and who wouldn't want the bragging rights of a 5 HP lathe. Bet even Mike Mahoney couldn't stall that much HP.

[David DeCristoforo]

The thing is I can find used motors for fifty to a hundred bucks. It's amazing how worthless a 3PH motor seems to a guy with only single phase power! A 5hp VFD is more money than I can spend right now. It was a stretch to spring for the 3HP one. I have to wait now while the VFD issue gets resolved so I have a bit of time to look for a 2-3HP 1700RPM motor.

[Dale Miner]

The thing is I can find used motors for fifty to a hundred bucks. It's amazing how worthless a 3PH motor seems to a guy with only single phase power! A 5hp VFD is more money than I can spend right now. It was a stretch to spring for the 3HP one. I have to wait now while the VFD issue gets resolved so I have a bit of time to look for a 2-3HP 1700RPM motor.

Good choice to stick with the lower rpm motor.

[Harry Robinette]

DD
You might try HGR Surplus in Cleveland Ohio.

[curtis rosche]

most farm motors are 1700rpm. try a ag supply place

[roger oldre]

Rpm of the motor realy makes no diferance. If you analyze the torque transmission they are a horse apiece when considering the primary drive and driven section. a two to one from 3450 or one to one from 1725 with the same diameter driven pulley and the same center distance if the center distance is properly designed to begine with. if you incorporate a secondary drive it can make quite a differance because of the low speeds. I have run both ways presently because they were available I am running 1725 with a speed up arrangement and also overdrive the VFD to get in the 3000 rpm range. Its overkill and i will likely reset the max output hertz back to 60 from 100 when I get around tuit.lol. Be careful on your motor purchase its best to have an inverter duty motor especialy if you want to run the low speeds. the high frequency feilds can destroy a standard motor in short order.

[Colin Prince]

A 5hp VFD is more money than I can spend right now. It was a stretch to spring for the 3HP one. I have to wait now while the VFD issue gets resolved so I have a bit of time to look for a 2-3HP 1700RPM motor.

You don't need a 5hp VFD to run your 5hp motor. A 3hp VFD will run it just fine, but obviously it will only deliver 3hp. The VFD will supply up to and including 3hp, and the motor will still work fine, as long as the load on it is 3hp or less.

[Thom Sturgill]

You don't need a 5hp VFD to run your 5hp motor. A 3hp VFD will run it just fine, but obviously it will only deliver 3hp. The VFD will supply up to and including 3hp, and the motor will still work fine, as long as the load on it is 3hp or less.

Along these lines, horsepower in terms of electrical power drawn is:

Php = (motor efficiency * voltage * amperage) / 746

While your motor is capable of drawiing 5hp in amperage at its rated voltage, the VFD will only provide 3/5 of that amperage (3hp). The additional cost of a 5hp VFD is because the need to handle a higher amperage = more expensive components.

[Bill Boehme]

Is there any reason one could not use a 3400 RPM 3 ph motor with a VFD? I know this is "too fast" for a lathe but with pulleys, the spindle speed could easily be brought down and with the VFD, speed is controllable anyway. Most of the used motors I am finding are 3400 RPM motors. I'm looking for a 1700 RPM motor but they seem to be not as common. So for all the motor pros out there…?

Yes, there is a reason. If you want to use a two-pole motor and run your lathe at the same speeds as you would get using a four-pole motor, you will have to do one of the following, both of which have significant consequences:

Rig up some sort of mechanical 2:1 speed reduction such as using a jack-shaft and pulley arrangement or use a smaller half size motor pulley. The jack-shaft and two pulley arrangement may create a Rube Goldberg situation not to mention some power loss because standard V-belts have an efficiency in the neighborhood of 90%. The second option of a smaller diameter motor pulley is even worse because there is a speed vs. diameter limitation where the efficiency and belt life may be much less than the first option.
Use the same drive train, but just run the VFD to get the slower speeds. This is a far worse option than using the mechanical speed reduction from the perspective of mechanical power output. I think that many woodturners do not know that the maximum mechanical power output is approximately proportional to the fraction of base speed at which the motor is being operated. In other words, if the motor is running at 25% of base speed then the maximum mechanical horsepower output would be a bit less than 25% of the nameplate rated horsepower at base speed. (Base speed is the rated speed of the motor -- for a two-pole motor it is approximately 3500 RPM and for a four-pole motor, it is approximately 1750 RPM).

How is mechanical power determined? It is the product of speed and torque. At base speed the ratio of mechanical power out to electrical power in is the efficiency of the motor. With a VFD running at lower frequencies, the efficiency no longer is valid. The efficiency will be lower for speeds that are both below and above base speed.

[David DeCristoforo]

"...The VFD will supply up to and including 3hp, and the motor will still work fine, as long as the load on it is 3hp or less...."

This is news to me! I would love to keep the 5HP motor against the day I can afford to "upgrade" the VFD to a 5HP model. But if I can safely run the 5HP motor off a VFD with a 3HP rating, that would be excellent. As long as nothing would be damaged, this would be ideal. In truth, a 3HP output would be more than sufficient for my present needs so if it is indeed true that the 5HP motor can be safely run with the 3HP VFD, then I'm keeping this motor!!!

[Scott Conners]

It will work, but if you cut too hard, you'll trip the VFD's protection circuits. Depending on the exact motor torque/power use, it may work great, or may not be quite ideal (giving less torque than a 3hp motor at the same current). I'd confirm it with the VFD manufacturer if you want to be certain.