VFD Help Sought

[wildcat]

I am piecing together information for a VFD purchase. Single phase 220V power is available and the motor will be a Leeson 2HP 3phase. The VFDs that I am considering include the Hitachi L200 and SJ200. My main question concerns the derating that many have mentioned in this forum. At least one sales rep says that Hitachi says that derating is no long necessary when running their drives from single phase power. Does that match the experience of those here or should I go with a more expensive 3HP VFD?

My main reason for replacing the motor on my mill is to double the upper speed. I am curious though what the reasonable range of VFD increased and decreased motor speed. Can a person expect to alter the speed range +/-5%, 10%, more? Is there potential damage to the motor? Are there any other negatives to be aware of?

Thanks

[HuFlungDung]

If you buy a new motor, get one that is rated to be suitable for VFD service. I think this implies that the motor can handle 120Hz, which would make a 1725 rpm motor run at 3450. Running at a higher rpm than that may require more accurate rotor balancing and a modified cooling fan that won't explode. Quite often a 1725 rpm motor has a quite beefy fan, with large fins, and it gets quite noisy at high rpm. Centrifugal force on those large fins may cause them to break off after fatigue occurs.

At the lower end of rpm, if the motor is working hard, it will not be properly cooled at very low rpm if run there for extended periods. This may not seem to be an immediate problem in your search for more speed. However, the motor can be used down to quite low frequencies, until it starts to power out. This might be down around 5 to 10Hz or so, if using a vectorless VFD.

If you are trying to get more spindle rpm than 3450, you might be as far ahead to use a mechanical speed increase, via belts and pulleys, to get the spindle rpm up in the range where you want it. This will allow you to cycle your VFD in a reasonable range to preempt the purchase of an ultra spec motor.

[SuperChuck]

Wildcat,

Motors have a typical 1.15 service factor, this means they are generally rated to allow for up to 15% over the stamps on the motor. Its not carte blanche to overrun the motor, but it can be used inside that SF range frequently without any expected problems. If you redesign the pulleys to allow the motor to run at 66-72Hz(110-120%) while giving you the final speed you wanted, then all could work out well.

The only issue with increasing the speed, mechanically or with the VFD is the loss of torque at your tool(at the final drive point). To compensate for this, you may need more than a 2 HP motor with 1.15 SF.

If you detect a problem with the tool stalling thoughout the speed ranges, or just the upper limits, you may need to invest in a more powerful motor. To allow for that contingency, I'd recommend you go ahead and purchase the 3 HP VFD, that way if you do experience problems, its ready to run a replacement 2.5 or 3 HP inverter duty motor.

SC

[Al_The_Man]

I have fitted a few VFD's to Vector and non-vector rated motors, and this has been my personal experiences and practice.
I would not run a standard non-vector motor above 3600rpm, this means if you have a 2 pole motor you cannot run much over 60hz, if you have a 4 pole motor , I run these at 120hz ( I double the rpm).
For Vector rated motors, I follow the manufactures rated top RPM, for example some Baldor vector motors are rated at a base speed at 60hz but can be run from 0-133hz, almost 4000rpm.
I have not yet lost a non-vector rated motor using these prcatices.
Usually the Vector rated motor may have better bearings, be better balanced and are wound with high insulation wire to prevent damage due to the High switching pulses they are subject to.
Al.

[unterhaus]

I thought that the recommended derating was very conservative, the manufacturer could easily get rid of the need for it. That being said, I'm always worried about listening to salesmen.

[wildcat]

The motor that I plan to get is the Leeson 192074 which you can see the details of at http://www.leeson.com/findaproduct/ after entering the this number in the first field. This motor has a native speed of 3470, SF of 1.15, draws 5.2A at 230V under full load, and has marked in the inverter type field PWM. Oh, in the speed range field they have 3-60Hz. So, should one stay only within this range? Would the addition of a cooling fan help with lower speeds? Do the Hitachi VFDs do breaking without the need for a breaking resister? Can one switch from forward to reverse quickly across the speed range (for tapping)? Again, thanks for the help - VFDs are a completely foreign item to me.

[JRaef]

So many questions... Responses in order of when I think of them;

Your motor selections is TEFC, Totally Enclosed Fan Cooled. There is no method of adding an external fan anyway. That is why they give you the low speed rating of 3Hz. Any lower than that and your rotor will overheat, and fans etc. have no effect on that.

Since you have chosen a 3470RPM motor, was your old design using 1725RPM? If so, then you have already doubled your speed and will not need to go over 60Hz with the VFD. If on the other hand, your machine has already been designed around 3450RPM and you want to take it to ~7000RPM, you must consider more than just the motor consequences anyway. Please clarify this issue if you want more advice.

Of those 2 Hitachi drives, the L200 is intended for LIGHT DUTY loads, i.e. pumps and fans. I would not use that on a machine tool unless I bought a larger one and derated it. Just use the SJ200, it's a better drive for the money.

If you want to do fast reverses for tapping, you need Dynamic Braking which takes the kinetic energy of the motor and load to a resistor bank to be burned off as heat. The Dynamic Braking issue is that in the SJ200, the braking control (transistor) is built-in, but you still need the resistor externally. On the L200 the control is not included, so you need an additional Braking Unit, which is the resistor plus the transistor in a package, which of course costs more than just the resistor alone.

Most 230V VFDs 3HP and under do not need derating for 1 phase input, that capability is inherent. Starting at 5HP 230V, and ALL 460V drives however must be derated by 50% for 1 phase input (with one exceptions at 5HP 230V). So even if you bump your HP size to 3HP in order to compensate for the torque loss at higher frequencies, you should still be OK.

[wildcat]

JRaef - Yes, my current motor is 1725RPM. I thought perhaps it may be possible to go slightly faster but I guess you are alluding to possible consequence mechanically of driving the gear head faster than 3600 RPM.

Within the 3-60Hz range motor spec. will there be torque lost?

Sounds I should get a 3HP SJ200 plus the resister bank for this 2HP motor and go outside the 3-60Hz range. Is that correct?

Thanks

[JRaef]

... I guess you are alluding to possible consequence mechanically of driving the gear head faster than 3600 RPM.

Yes I am.

Within the 3-60Hz range motor spec. will there be torque lost?

No. You lose torque using a VFD only when you go over the motor's base design speed. In order to maintain torque, you must keep the Voltage and Frequency ratio (V/Hz ratio) constant with respect to the motor's design. Once you get to 60Hz, you are at full voltage. When you go above 60Hz, you have no more voltage to give, so the torque begins to drop rapidly. Within the design frequency range however, that VFD will allow the motor to provide full torque. Just keep in mind that this is radically different than changing speed with pulleys or gears. When you lower speed mechanically, you are INCREASING torque. When you lower speed electronically, you can at best, MAINTAIN torque.

Sounds I should get a 3HP SJ200 plus the resister bank for this 2HP motor and go outside the 3-60Hz range. Is that correct?

I would, as long as what I just said above is not a problem for you. What I usually recommend is that if you have a pulley system now, leave it in place with the sheaves that give you the speed you will do most of your work at when the motor is at full speed. Use the VFD to trim speed for accuracy and convenience, but if you need high torque at low speeds, change the sheaves.

[wildcat]

Thanks for all the help everyone. I ordered a 2HP motor and 3HP SJ200 today (the sales people were not sure about the resister and the Hitachi tech said to give the VFD a try without the resister first). Should be fun to hook it up.

[SuperChuck]

I may be on the wrong track here, but I'll give it a go. They were probably thinking that the inertial load of the drill is not enough to worry about. The more rotating mass/inertial load of the system(loaded belt conveyors, revolving mills) , the more significant the regeneration. If you can deal with a longer decelleration time, then the surge is manageable, but if you actually use the drive to control the slow down, then you'll see more current and need somewhere to send it, so it gets directed to the resistors to be converted into heat.

The really high end models are true regenerative models that actually convert the braking energy back into A/C and return it to your process.

Anyhow, good luck with the purchase and installation, we'll be glad to hear how the combination of parts work out for your purpose.

SC

[wildcat]

Got the VFD and motor hooked up this evening. Wow! VFDs are really cool! Thanks everyone for the advice. The SJ200 was a really good choice. The smallest AWG wire I could get from the hardware store was 12/4 SOOW which has to be complete overkill for a 2HP motor but oh well. I am still not clear if the motor I used, a Leeson 192074, is a 4 or 8 pole motor though. Is there an easy way to know? There are nine wires coming out of the motor. Three are connected together, then each pair of remaining wires connects to each of the three lines from the VFD. Does that make sense?