VFD motor controllers

[xray]

I have a lathe that runs faster than what I would like on the slowest
speed. Two answers seem to be available. Replace the motor with 3-phase
motor and a VFD controller, or replace the motor with a DC motor and PWM
controller.

The DC version, I think I understand electronically, although maybe not
effective powerly (I've heard lower DC power motors effectively look
like higher power AC motors -- ?).

The 3-phase VFD (Variable Frequency Drive) seems simple enough in
principle, but I'm wondering what's in the best cost effective versions.

The one I want will take single phase 230V AC and convert it to 3-phase
motor drive output that is variable in freq.

First question: what happens to the input? If it is single phase, I
assume it goes through a full wave rectifier to get it as smooth as
possible. Then what? Caps wouldn't help much at this kind of power I'd
guess, and commercial units are small. Just ignore the bumps?

I think the drive to the three phases is a form of PWM, probably
microprocessor generated, and done by IGBTs.

So, It's obvious I don't know a lot about this except my first-level
assumptions. Can anyone provide a basic description of what is happening
in these VFDs and what might make better or worser implementations?

Thanks. Hope it generates some interesting observations. I know this can
get deep, but at a first level I'd like to hear the the basic theory
about how the input power might be adapted and controlled.

 

[Karl Townsend]

VFDs are generally more cost effective. If you really want the DC route,
some good deals can be found at Surplus Center.

I've converted several machines to three phase/VFD and been really happy. If
you have room in your machine, increase the motor horsepower a bit. First
you never can have too much power, and motors only develop full power at top
RPM. I think they are all constant torque. Horsepower is torque*RPM, so you
have 1/2 the power at half speed.

As far as how they work internally, there's a little box that contains magic
smoke tightly packed in there. You don't have to worry about anything, as
long as you don't let the smoke out. <VBG> You want a 230 volt VFD and a
230 3 phase motor. Connect single phase 230 VAC to L1 and L2. Connect your
motor to T1 T2 T3. You can use the VFD keyboard for for all functions but I
find it far handier to connect switches for forward/stop/reverse and a pot
for speed.

Here's the best place to buy your VFD
http://www.driveswarehouse.com
You probably want the L200 series drive.

 

[Terry Given]

I have a lathe that runs faster than what I would like on the slowest
speed. Two answers seem to be available. Replace the motor with 3-phase
motor and a VFD controller, or replace the motor with a DC motor and PWM
controller.

The DC version, I think I understand electronically, although maybe not
effective powerly (I've heard lower DC power motors effectively look
like higher power AC motors -- ?).

The 3-phase VFD (Variable Frequency Drive) seems simple enough in
principle, but I'm wondering what's in the best cost effective versions.

The one I want will take single phase 230V AC and convert it to 3-phase
motor drive output that is variable in freq.

First question: what happens to the input? If it is single phase, I
assume it goes through a full wave rectifier to get it as smooth as
possible. Then what? Caps wouldn't help much at this kind of power I'd
guess, and commercial units are small. Just ignore the bumps?

bridge rectifier + big cap.

for a single-phase input, the crest factor will be fairly nasty. Better
drives have a small (3-5%) line choke to increase the conduction angle.

A fairly hefty DC bus cap is used, typically sized for lifetime - it has
to deal with the extremely high RMS input current, as well as the
high-frequency output current.

I think the drive to the three phases is a form of PWM, probably
microprocessor generated, and done by IGBTs.

almost exclusively digital PWM nowadays. it used to be done with 3
reference sinewaves, a triangular carrier and 3 comparators. And a whole
bunch of other hardware, to deal with the ratshit waveform quality
caused by very low switching frequencies.

So, It's obvious I don't know a lot about this except my first-level
assumptions. Can anyone provide a basic description of what is happening
in these VFDs and what might make better or worser implementations?

2 types of drive: scalar (V/F) & vector control.

Scalar control maintains constant machine flux by keeping the ratio of
output voltage to frequency constant. At half speed, the drive outputs
half voltage (done by halving the PWM duty cycle).

Vout = Vrated*Fout/Frated

Vrated, Frated are nameplate voltage, frequency of machine.

at very low speeds, the stator IR drop becomes significant, so some form
of "boost" is added - the output voltage is Vboost + Vrated*F/Frated

dynamic response isnt too great, but can be improved with speed
feedback (shaft encoder).

Scalar control is based on a steady-state model of the AC machine, and
as such ignores dynamic effects. For what you want, scalar control is
just fine.


Vector control is based on a full-order dynamic model of the machine,
and requires some form of speed feedback - shaft encoder or speed
observer (so-called sensorless operation). Sensorless drives may or may
not work well at zero speed (observers get tricky here, and all drives
are not created equal), but with a shaft encoder, a vector drive can
easily do shaft position control, and provide full torque at zero speed
- its actually kinda neat to set the shaft speed to zero, and try and
turn it by hand (for a small machine). A well-implemented vector
controller will *not* allow the shaft to move.

Vector-controlled induction machines routinely replace DC drives
nowadays, even in demanding applications like steel rolling mills.

Thanks. Hope it generates some interesting observations. I know this can
get deep, but at a first level I'd like to hear the the basic theory
about how the input power might be adapted and controlled.

HTH

Cheers
Terry

 

[The Real Andy]

I have a lathe that runs faster than what I would like on the slowest
speed. Two answers seem to be available. Replace the motor with 3-phase
motor and a VFD controller, or replace the motor with a DC motor and PWM
controller.

The DC version, I think I understand electronically, although maybe not
effective powerly (I've heard lower DC power motors effectively look
like higher power AC motors -- ?).

The 3-phase VFD (Variable Frequency Drive) seems simple enough in
principle, but I'm wondering what's in the best cost effective versions.

The one I want will take single phase 230V AC and convert it to 3-phase
motor drive output that is variable in freq.

First question: what happens to the input? If it is single phase, I
assume it goes through a full wave rectifier to get it as smooth as
possible. Then what? Caps wouldn't help much at this kind of power I'd
guess, and commercial units are small. Just ignore the bumps?

I think the drive to the three phases is a form of PWM, probably
microprocessor generated, and done by IGBTs.

So, It's obvious I don't know a lot about this except my first-level
assumptions. Can anyone provide a basic description of what is happening
in these VFDs and what might make better or worser implementations?

Thanks. Hope it generates some interesting observations. I know this can
get deep, but at a first level I'd like to hear the the basic theory
about how the input power might be adapted and controlled.

My father has been at me for years to make him a single phase to 3
phase VFD for his lathe. I have finally got around to it, just
tweaking power supplies ATM (never use a voltage mode smps for
industrial control) and writing the code.

The unit I have designed is based on a microchip design. Single phase
rectified and filtered. Microcontroller using sine look-up tables to
generate PWM. An IR IGBT module that takes care of all the high side
switching stuff. My estimate is that it will cost under AU$100 for a
hobbiest to make (no certifictions) providing that an aussie company
will stock the IR modules. So far it is working quite well, cept the
damn smps to drive the uP and other stuff.

I wish someone would design the switcher for me (hint hint), i just
cant seem to get a smps to work reliably for me. It would be nice to
have it working with a UC3842 with a non isoloated output of +5 and
+15dc )))) (hint hint again) )) I will get there...

 

[Rod Richeson]

Real Andy do you post to cnczone.com? They have a bunch of forums, and
the electronics section is pretty active.

This is very interesting to me, but my skill level is about the magic
smoke level.

Rod

 

[Richard J Kinch]

I know this can
get deep, but at a first level I'd like to hear the the basic theory
about how the input power might be adapted and controlled.

A typical VFD is just a switching mode power supply feeding a programmable
signal generator. The hard part is doing it reliably at high voltage and
current.

 

[xray]

HTH

Cheers
Terry

Thanks, Terry. Exactly the sort of explanation I was looking for.

 

[Terry Given]

xray writes:




A typical VFD is just a switching mode power supply feeding a programmable
signal generator. The hard part is doing it reliably at high voltage and
current.

Oh yes. the word "just" involves a whole lot of grief

We once ran some tests on a tiny little 1/2hp toshiba inverter. After
slamming it from regen voltage limit into motoring current limit, it
locked up at about twice rated current. for a short time

we then ripped it apart, and found the DC bus cap was about the same
size as my little finger. what the ?! buried deep in the manual was a
requirement to replace it every *year*

Cheers
Terry

 

[xray]

we then ripped it apart, and found the DC bus cap was about the same
size as my little finger. what the ?! buried deep in the manual was a
requirement to replace it every *year*

Makes one wonder WTF they were thinking (or not thinking.)

 

[Spehro Pefhany]

Makes one wonder WTF they were thinking (or not thinking.)

VFDs, like industrial temperature controls, are an insanely cutthroat
business.


Best regards,
Spehro Pefhany

 

[Richard J Kinch]

Makes one wonder WTF they were thinking (or not thinking.)

They were thinking, "must get edge of probability distribution just past
the warranty period".

 

[xray]

VFDs, like industrial temperature controls, are an insanely cutthroat
business.


Best regards,
Spehro Pefhany

Scary -- being a cost-consious buyer who is about to shop for one.

Always planned to buy one, but started this thread because I wanted to
know what is in them.

If I get one, should I immediately crack it open and see if it should be
repopulated with better parts?

 

[Terry Given]

VFDs, like industrial temperature controls, are an insanely cutthroat
business.


Best regards,
Spehro Pefhany

yep. Yaskawa made more drives per month than we had done in 30 years....

Cheers
Terry

 

[Terry Given]

xray writes:




They were thinking, "must get edge of probability distribution just past
the warranty period".

LOL. but yes.

Cheers
Terry

 

[John Woodgate]

I read in sci.electronics.design that xray <[email protected]>

Makes one wonder WTF they were thinking (or not thinking.)

The PHB is not extinct in Japan. Someone presumably specified the
outside dimensions of the product before it was designed and remained
deaf to objections.

 

[Spehro Pefhany]

Scary -- being a cost-consious buyer who is about to shop for one.

Always planned to buy one, but started this thread because I wanted to
know what is in them.

If I get one, should I immediately crack it open and see if it should be
repopulated with better parts?

;-) I guess that's one approach. Or get a heavier duty one to begin
with. Watch the derating factors for single-phase power input and
temperature.


Best regards,
Spehro Pefhany