Speed control with VFD ( from newbee)

If the 3 phase motor is rated ca. 1750 rpm @ 60 cycle, what happens when the VFD puts out a higher frequency such as 350 Hz? Does the motor rev up to an unsafe speed?

Presuming same stator lag 350hz would make this 1750 rpm motor turn 10,208 rpm. The spindle it is driving would probably fail first. How much RPM the motor would take would be determined by the stator diameter, don't ask me what that would be for sure, 6,000 sfpm sticks in my head but don't quote me on that one.

Someone help me out with this. But it is a definite cause for caution.

This is the reason for parameters in the controller, an electronic govenor, as well as making the VFD work best in given situation.

1/2Nut,have you seen or are you aware of any 'built from scratch'VFD's? It should be possible to make a bare bones VFD it seems,without a lot of bells and whistles. I'll wager Mr.Potter probably came up with one! George

Just noticed the time on my last note is 1 hour later than it should be?

George, click on the post titled simply "VFD" under this forum and read Forrest's answer to this very question already asked.

Me again.
I guess I'm confused 'cuz I bought a 1 hp motor and inverter combo from Dealers Electric. The motor is rated for inverter use and rated 60 Hz, but the inverter can be programmed to go to 400 Hz. All of the motors in the "Dealers" catalog, from .5 HP to 300 HP, are rated 60 Hz. So where is the motor that can use the 400Hz output of the inverter???

One example of a 400hz capable motor would be a direct drive router motor, such as used on CNC routers in furniture manuf. plants, which have special bearing arrangments and are typically designed for up to 23,000 RPM spindle speed.

Don uses the example of high speed spindles for routers. I work in a furniture plant, we use a lot of frequency inverters to increase spindle speeds. Before VFD's the only way to do this was with a rotary rig, a motor driving an alternator. 300hz was the output of all I have seen. Gets a 2 pole 3ph motor up towards the 18,000 rpm mark, minus stator lag of course.

The stator of these motors for high speed usage are often small in diameter and long. They have to be small in diameter to handle the rpm, longer to get the power out of the them.

The VFD duty motors have more cooling capacity ala larger fan, for when motor is operated at lower rpms.

Back to the wood working equipment, I wondered how they got the spindle RPM up for the moulders, I was under one of them one day to fit an adjuster. Flat belts, plain old flat belt drive, big driver, small driven pulley, no alligator lacing, an endless belt. This was not an old machine, it was a Unimat 23, top of the line from Europe.

Yep, manual overhead routers are often that arrangement...with flat belt drives...a thin, endless special purpose flat belt. I once owned a nice modern Italian router with that arrangement and a 2 speed motor, for 10,000 and 20,000 rpm possiblities.

Hi, fellas. I sure am enjoying this conversation about vfd’s. I’m not an expert on vfd’s, but I think that’s part of the point I’m fixin’ to make about them. We have a moulding plant and a finger jointing operation . Any body that knows what a finger jointing operation is knows that it gobbles up a large number of short blocks of wood. In other words it takes short blocks of wood that you aquire from ripped lumber and glues them
Together to make longer lumber without the defects. It’s used on mouldings that will get painted anyway so the fingerjoints don’t show anyway. Our fingerjointer can put out up to 20,000 ft in 8 hours so that’s a lot of short blocks that need to be prepaired at a low labor cost.

One of the things I’m working on in the shop is an automatic defect saw that will cut out the defects based on where the people mark the lumber. In other words it will read the marks and feed the board in such a way that it stops where the board needs to be cut. A popup saw does the cutting. The point about all this is that the mecanism that feeds the board has to have variable speed. I used what they call a “smart inverter”. The one I used was made by Mitsubishi. These are amazing things. You can program the parameters so that you can have as many as 17 speeds. You can have acceleration/deceleration ramps and braking motions. You can stop, you can reverse you can vary the speed. You do all this by simply sending several electrical signals to the vfd. Seems like it was 5 signals on the one I used on this saw. Depending on the combination of signals (the signals are 0V or 24 VDC) the vfd “tells the motor what to do”. I use a plc to send the signals. But a plc is not necessary. You can use toggle switches or selector switches or whatever. My point is that I’m not a trained electrical technician. Matter of fact I’m not a trained machinist or any kind of technician. I’m self taught. If a person understands a little about 3 ph motors, contactors, he can use these things. The manual was a little intimidating. Over 200 pages.
Mostly about how to set the parameters. After reading thru it a couple of times I realized that I was making it way too complicated. During the third reading what was important was fairly strait forward and fell into place. Most of the parameters you don’t even need to mess with. Mitsubishi would give you help over the phone, I’m sure, but seems like I always do things the hard way. A little perseverance and any reasonably technically oriented person can use these things. I have a whole bunch of dc motors laying around. But when you need to vary a motor a lot the vfd is hard to beat. You can get new ones on the internet at good prices. The finger jointer uses three vfd’s to get the motors to do what it’s plc wants them to do.

Talking about Weinig Unimat 23’s we’ve got two new ones we also have two Weinig Hydromats. Weinigs spindle arrangements work great. You never have trouble with the motors, pulleys, or belts. Unless some idiot asks the moulder to do something it don’t want to do and then you get a melted belt real quick like. My shop is 200 ft from the moulders and I usually can tell by sound when somebody tries to get one to do something it don’t want to do. We had a IMC overarm router with the pulley arrangement that you described for years and years and years and never did have to change the belt once. Incidentally, if your interested you can see the fingerjointer and moulders on our web site: www.mouldingsunlimited.com

Anybody who loves to tinker and has not used these smart vfd’s will love them and all kind of possibilities will start to dance thru your head.

How about the bottom end of the range? HOW SLOW is it possible to run a standard squirrel cage motor from an invertor & get decent torque?

Don, how slow you can run an induction motor depends on the drive.

A plain vanilla VFD will deliver full motor torque to about 6 Hz givng you a 10:1 rangeability below rated motor RPM.

A more expensive sensorless flux vector drive will give you full motor torque to 1 Hz and below ot a 60:1 range.

A much more expensive closed loop flux vector drive can give you full motor torque of a 1000:1 range and in their most expensive form can be used as axis drives servo motors in CNC machines.

How slow you can go for extended periods of time also depends on motor cooling. You may have to install a 5" biscuit fan to blow air over (or through) the motor if you run it at speeds below which it can self-cool.

How fast you can go depends on several factors:

Practically speaking, you can spin an induction motor until the armature brusts and them back off a notch ot two. You may be too late but you'd know the RPM limit. Induction motor armatures are very strong. I'd go out on a limb to sau you can spin 10 HP or less armatures to 6 X their 60 rated RPM without fuss or bother. I've experimentally run a 50 year old 10 HP motor to 400 Hz (12,000 RPM) with no trouble except its ancient bearings let me know they didn't like it.

The point is you can run a 60 Hz motor to any freq you want but the Volts to frequency has to be kept to some degree. 230 Volt to 60 Hz works out to 3.833 V/hx and 460 to 60 = 7.667. Hold those ratios and you can run a motor to full torque (theoretically) to 400 Hz.

But look at voltage. NEMA rated motor have to be insutlated to withstand double the rated nameplate rated voltage plus 500 Volts. 460 x 2 + 500 = 1420 Volts. Working backwards; Let's say you kept the 500 Volts cushion for electrical safety and ignored the other NEMA 2 x voltage limit. Let's say you had a magic VFD with no voltage limit. You would need a way to vary the frequency and voltage in proportion within the 920 Volt limit. If the motor was connected for 230 Volts you could run the motor to 240 Hz or, if 460 volts, 120 Hz at full torque.

All that said and practically speaking, you can run a 230 volt connected three phase induction motor to 90 without fuss or bother but you do lose a little torque. The HP stays about constant. If you push it, and try to run the motor to 120 Hz you'll find the motor has about 1/5 the 60 hz torque - that is dang little and not enough to do practical work.

Here's a way you can run a 60 Hz induction motor to way above rated motor freq but it's a little expensive. Obtain a 230 to 460 step up transformer if you're working from 230 volts use it to supply the VFD with 460 volts. Obtain a 460 volt VFD. Wire it to a motor connected for 230 Volts. Adjust the VFD parameters for 3.833 V per Hz (Look this up in the manual. How-to will vary with VFD manufacturer.)

Adjust the VFD's max freq to 180 Hz. Once you get the wrinkles worked out you can run the motor to 3 times rated RPM. Sync RPM for a 2 pole motor will be 10800, less slip. One thing: use a VFD that's DOUBLE the rated motor HP. in this case double voltage and Hz equaly double RPM at the same torque. The 5 HP motor (what ever) now develops 10 HP.

Think about this: If you're made out of money you can afford to have a motor shop rewind (or internally reconnect) a 5 HP motor to suit 400 Hz at a proportionately lower turns per volt. A 230 volt 60 Hz motor rewound for 34.5 volts at 60 Hz becomes a 230 Volt 400 Hz motor with a full load Amp rating about 6.67 times the original rated Ampa.

The rewound 5 HP motor will develop full original torque and deliver about 30+ HP at400 Hz. You'll have greater losses because the stator iron is less efficient at 400 Hz.

Do not attempt to use a 230 Volt VFD on 460 Volts. It will fry.


[This message has been edited by Forrest Addy (edited 03-18-2003).]

Thanks Forrest. I have an altivar 16 rated at 460 volts and my shop supply is 420v 50hz.
I did not have any documentation, but I have today managed to find the info. from the Square D site,so I can set it up as you say.