VFD questions

I got a VFD and after reading through the manual and the internet I am kinda confused by a few things.

Torque

So some VFDs lose torque when you reduce speed, others keep constant torque below the rated Hz and other can get more torque when running the motor at a lower speed.

If I have an ATV11 inverter which is a "Sensorless flux vector control" type does anyone know what I should expect in torque?

Also do I need a braking resistor / module if I want the VFD to decelerate instead of coast to a stop? The manual says it needs a resistor of more than 51 ohms but I wasn't sure if that was only required for fast breaking.

What size of motor (power) are we looking at?

I hope someone answers , i was considering buying one of those to power a mill brand new old stock but considering their age i didnt as the capacitors may need reforming which i know nothing about.

I have a siemens  micromaster 440 on my lathe and after reading the manual ,i ended up more confused than when i started . Far too complex for me to understand.(also bought as brand new old stock but fortunately worked out of the box after a bit of fiddling about with the settings)

Such as i think they can operate in sensorless vecter and the normal mode (vxd???) where the torque does fall at lower speeds.Do you have to set these up to use sensorless vector or do they operate automatically in sensorless vector mode.I just got the lathe motor working and left it at that.Probably isnt set up properly as it seems to lack power to get it started . The manuals are mindblowingly confusing if you have no experience and know what your doing.

Edited By mark smith 20 on 24/07/2016 18:45:46

Sensorless flux vector can maintain good torque down to a few hz, it uses some very complex maths to approximate the position of the rotor and hence improve low speed torque. Some inverters can use an encoder to to give rotor position and give 100% torque at 0rpm. These are not in the price bracket of the hobbiest but the sensor less type are, and worth having, remember that cooling will be compromised at low speeds. You will only have full torque but not full power, Using mechanical speed reduction maintains the power and multiplies the torque, using the extreme range of an inverter will not defy the laws of mechanics. I use a flux vector inverter on my Myford with a 1hp motor which is over motored for the lathe but gives a wide useful range. I still change belt range for the extremes of speed, but being able to adjust the speed to get the tool cutting sweetly is most useful. Much work will fall into the useful range in the mid speeds and a bit fast or a bit slow is no problem and swapping belts is not required too often.

Mike

Edited By Michael Poole on 24/07/2016 19:22:38

BPlease loopraking using the VFD is a good attribute but do make sure it is appropriate for your application.

If it is required for 'fast breaking', the resistor power may be important, too. Beware something may 'brake' if you 'break' too quickly. Or is that the other way round?

It is a 1.5kw / 2 hp motor in this case.

I also was wondering what power resistor to get. 200, 500, 1000 w ones seem to be whats on offer. I had a feeling getting a bread board resistor wouldn't have worked.

In simple and approximate terms the torque aspect is simple - it doesn't change what ever speed is set. Various control strategies offer closer approximations to that ideal. This means that the horse power of the motor changes as the speed is changed. Horse power or kw as it's usually called these days is a product of rotational speed and torque. So horse power drops as the speed is reduced. A belt drive increases torque when it is used to decrease speed.

The manufacturers default settings are usually a good place to start and the menu's usually give quick access to the major parameters that are needed. These are the ones on the plate on the motor.

Some also have 2 modes, constant torque and constant speed. The constant torque mode as I understand it is for things like pumps. The constant speed for things like machine tools where load may vary suddenly. The manuals I have seen do explain this some where.

Just hope I haven't got them the wrong way round as it was some time ago since I set mine up. I don't think I have.

John

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Edited By Ajohnw on 24/07/2016 21:01:46

Most of the VFDs available for our application use flux vector control. It basically means digital control. And most offer sensorless control ie no speed sensor required. Apart from the need to cool the motor (the motor needs to be spinning in order to cool itself), many of them can actually provide as much as 50% extra torque at low or zero speed.

You only need an external braking resistor if you have a large inertial load and need to stop it quickly. Decelerating a load results in the inertial energy being dumped as electrical energy in the internal DC bus, so doing this too quickly could result in the voltage becoming too high for the power devices. You can set the acceleration and (more importantly) deceleration rates in the setup to avoid causing an internal overvoltage.

Contrary to popular opinion, VFDs do not pop simply because you have tried to decelerate the motor too quickly. Any half decent VFD will either limit the deceleration or inhibit the drive once the internal voltage reaches a max threshold. If your drive has popped during a mundane event like this, perhaps you should have spent a few more pounds on a better quality one!

As an alternative to a braking resistor, you often have the choice of applying "DC injection" which slows the motor down more rapidly, by generating motor torque without dumping energy back into the VFD - it ends up in the rotor instead. The rotor gets a little hotter but unless you have a really large inertial load it isn't likely to be a problem.

Murray

And that requires no extra equipment? Huh I guess I might for for that instead then.

I wonder if its possible to add a small fan to a motor to aid cooling? Half the warnings I see on the internet are about overheating the motor because the native motor cooling works best at the rated RPM and similar.

Inverterdrive.com ended up showing the various torque / rpm / power ratings at different Hz. Think I understand it better now. Also seems to be better to gear something down too low then increase motor RPM instead of reducing the speed at any point.

Ok wow according to the manual the factory preset makes the VFD constant-torque. No mention of how to change to constant speed. 

Edited By Rainbows on 24/07/2016 23:18:21

Constant torque sounds very unlikely - are you sure you have read correctly? The default settings tend to be constant speed and the simplest settings will use a pot to set the speed and 2 or 3 switches to control direction and run/stop, although you could simply run it from the front panel without remote controls. If it's been pre-loved, you may be safest to do a factory reset and go from there.

All the internet warnings? How many motors have you heard of that actually died through overheating in this way? Unless you are using them flat out, continually at 50% speed or less, I doubt you'd have a problem - but who does that? But yes, you could fit a fan.

Even though you have a VFD you should still operate the motor around base speed (50Hz) otherwise you won't be able to develop rated shaft power for one thing. So select the right gear / pulley ratio to get it in the right zone and just use the VFD to vary the speed perhaps 30% either way.

Edited By Muzzer on 24/07/2016 23:30:21

An inverter properly set up with your motor parameters will calculate how hot your motor gets and will reduce the power if its in danger of overheat (as long as you don't block the cooling fan).

Braking resistor doesn't have to be rated as high as the motor, as you only brake for short periods with cooldown in between. No point having a 2kW braking resistor for a 1.5kW motor. A google suggests less than 10% of motor power so your 200W example should be more than adequate.

Neil

The VFD will maintain whatever speed you tell it to and apply enough power to give up to the rated torque.

The non-constant torque setting will allow over-voltage/current to give greater torque at low speeds at the expense of more rapid heating so should only be exploited for short bursts/starting under load.

Neil

My 3kW Yaskawa VFD has a 150W external braking resistor, as specified in the Yaskawa manual. However, when specifying or designing one of these, the most important parameter is the peak power rating, not the average. Internally, the mass of the resistance wire must be enough so that when the baking energy is dumped into the resistor, it does not approach its melting point.It must also be able to withstand the repeated thermal stresses arising from this application. So you need to obtain a proper braking resistor, not just any old resistor.

I don't think that is correct at all Neil. It just makes it behave like a normal motor but will interfere if the motor is overloaded. Then comes modelling aimed at predicting the motor temperature more aimed at when it's run at reduced speed 'cause if it is at it's rated current it will overheat once any leeway the motor manufacturer has built into the cooling is used up. There will generally be some leeway due to allowing for higher than normal ambient temperatures. Motors will take significant overloads for shortish periods providing they have time to cool down in between them.

Early inverters were slated for not maintaining constant speed then came flux vector control which improved things a lot.

Some one mentioned adding a fan. I don't see why not. It is possible to buy inverter rated motors that use a separately powered fan for cooling. There are other changes as well eg aimed at getting rid of the circulating currents caused by high frequency switching.

Personally I would be very wary about what some motor inverter retailers reckon can be done with them. Some people have bought twice for instance. 2nd time fitting a more powerful motor than the original. I extended that a bit by also fitting a 6 pole motor. That helps get round reduced speed running even more because at for normal speed it has to be driven at a frequency that will give 1400 rpm. While the motor manufacturers wont state figures there doesn't seem to be any problems running motors over speed - within reason. When I asked directly I was told THAT THEY DIDN'T THINK THAT RUNNING ANY MOTOR AT 2,800 RPM would harm it but they wont state this in the catalogues so I run mine well below that.

Wasn't there a post some where or the other recently indicating that some retailer had software available that allowed them to re rate inverters ? If so they could do all sorts of other things. Personally I would buy from some one else. The overload factors the inverter makers build in are there for a reason.

John

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I have run a 3/4 hp motor on a Super 7 via an inverter for many years. I have used it prolonged for low speed screw cutting and have never found the motor warm enough to warrant supplementary cooling. Yes it gets a little warmer than is usual when running at normal speed (50Hz) but nothing to worry about and a simple hand test suffices to confirm. DC injection used to control stopping.

I have a Bridgeport inverter fed and that has a brake resistor. Proper brake resistors are obtainable from REO(UK)Ltd [usual disclaimer]

None of my other inverted fed machines have brake resistors

I thought the whole point of an inverter is so you can modify the motors behaviour otherwise you might as well stick to single phase and belt drives, theres equally nothing stopping you from experimenting with different belt setups with a 3 phase motor.

Michael W

Not sure what you mean? This is from the manual for my inverter, the engilsh is a bit stilted but later it specifically says that if the motor won't start (under load) increase torque boost F09:

Manual torque boost
In manual torque boost mode, the inverter maintains the output at a constant level
regardless of the load. When you use this mode, select the appropriate V/f pattern
(square reduction torque or constant torque characteristics) with Select Load (F37).
To keep the motor starting torque, manually select optimal inverter output voltage
for the motor and load by setting an optimal torque boost rate to F09 in accordance
with the motor and its load.
Setting an excessive torque boost rate may result in over-excitation and overheat of
the motor during light or no load operation.
Manual torque boost keeps the output voltage constant even if the load varies,
assuring stable motor operation.
Variable torque characteristics (F37 = 0) Constant torque characteristics (F37 = 1)
• Set an appropriate torque boost rate that will keep the starting torque
of the motor within the voltage level in the low frequency zone. Setting
an excessive torque boost rate may result in over-excitation or overheat
of the motor during no load operation.
• The F09 data setting is effective when F37 (Load Selection/Auto
Torque Boost/Auto Energy Saving Operation) is set to 0, 1, 3, or 4.
Automatic torque boost
This feature automatically optimizes the output voltage to fit the motor and its load.
Under a light load, it decreases the output voltage to prevent the motor from
over-excitation; under a heavy load, it increases the output voltage to increase
torque.
Since this feature is related to the motor properties, it is necessary to set the rated
voltage at base frequency (F05) and motor parameters (P codes) properly.
For the automatic torque boost feature, which is related to the motor
characteristics, you need to consistently set the voltage at the base
frequency (F05) and motor parameters P02, P03 and P99 appropriately
for the motor rating and characteristics.

Micheal W.

One thing you will discover with the ability to marginally alter a speed is that should you have a tool resonance which usually exhibits itself as chatter then a change may well illiminate it. It is so much easier to change speed with VFD and you still have the option to belt change if you wish.

Another instance, I can run my mill on 60Hz to raise the cutter speed for small carbide cutters!

This is what I have on torque boost Neil.

To me this implies it still wont allow over current and it's main purpose is to get thing rotating when there is a significant mass to get rotating - in other words if a lathe for instance spins up don't use it.

I've not found the constant speed section that explains why yet but I have found that this plays with motor slip so that the speed achieved is closer to the one demanded. It needs a self tuning facility to obtain the motor parameters. As I understand it this is what flux vector control is for.

However it does mention 2 types of motor - high efficiency standard motor and constant torque motors. I read high efficiency motors as any standard motor. There are also setting to cause the inverter to change the drive frequency to avoid stalling and or over current.

John

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Found part of it !

And a comment elsewhere

**LINK**

It also points out that load pattern settings which is what these appear to be don't function when flux vector is selected. Also add that mine defaults to 150% overload but there are tight time limits one it.

John

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Edited By Ajohnw on 28/07/2016 23:44:20