Thread: VFD, have I made a blue?

Originally Posted by electrosteam

On the subject of changing speed while energised - don't do it.
Inverters have delicate current and voltage components that are tuned to the connected load.
An internal series inductor, for example, carrying current into the load has no where to send the current if you disconnect the load.
The current has to go somewhere, as that is what inductors do, maintain a current flow against changing load and voltage conditions.
The stored energy in the inductor looks to be dumped anywhere it can - possibly into sensitive devices that cannot accept the energy.

John.

John, out of curiousity more than anything, do you have any thoughts on what differs between the inverters used in a VFD, and the inverters used to power houses in a rural application?

My parents house is not on the power grid, and runs predominantly on solar (having done so for the last 25 years). Everything is always turned on and off at a powerpoint like any normal house, with apparently no ill effects (one (primitive!) inverter failure in 25 years I think can be attributed to age, can't it?), which I find interesting given the apparent sensitivity of VFDs to this behaviour. They have in the last few years upgraded their system to a fancy new one, in which the inverter looks very much like an oversized VFD, and given the amount of lights and things on it, I imagine has similarly sensitive components inside. Yet you may still switch things on and off at will. Obviously there must be SOME basic design difference here, but surely the inverter part itself must work essentially the same?

Jekyll and Hyde You can buy VSD that operate in the same way as your parents inverter. Of course they cost more.
Stuart

Originally Posted by gregoryq

Now. With regard to the switching: The motor gets wired directly to the VFD. NO switches between them.

To go a little OT here. You can have switches, you just cant use them while the VSD is "on". In fact on my VSD I was given the ok to switch the coolant pump on and off as I please while the main motor is running(although I havent tried this).

Originally Posted by gregoryq

I have a bunch of emergency stop switches here somewhere. I'll give you one for mounting in a handy spot. The VFD supports them, so why not have a bit of modern safety for free?

Not sure EMG stop switch will work with these VSDs. They require a NO switch. I wouldnt use a NO circuit for an EMG stop.(you could of course add it to the normal control circuit)

Originally Posted by gregoryq

***these mills were sold in both the 50Hz market ("the world") and the 60 Hz market (the world according to North America). The 60 Hz machines had a 20% higher top speed, so why not take advantage of that knowledge

Thanks, I'd never thought about the 60Hz rating. I'll be checking my motor tomorrow. I was going to run it up to 60Hz, there may be some more room.


Stuart

Hi Greg and others,

As you pointed out a little while ago, that Arboga on eBay by Tissuescars, was not a mill, but a drilling machine with an x-y table. You say it sold for what? $3400, that's pretty steep for a drilling machine, albiet a good one.

The other Arboga EM825, I couldn't find. Do you have an item No.?

Ken

Originally Posted by gregoryq

I am planning to use 20 Hz to 130 Hz on my Arboga clone for example)

Greg...is 130hz going to be ok for your standard 50-60hz motor...ie is the motor able to sustain much above 70hz without possible damage?

My understanding is that motors have to be designed and manufactured to operate at those types of frequencies?...where as your off the shelf standard 50-60hz motor is not.

Originally Posted by Stustoys

Not sure EMG stop switch will work with these VSDs. They require a NO switch. I wouldnt use a NO circuit for an EMG stop.(you could of course add it to the normal control circuit)

Stuart

Really? Shows what I get for assuming. I have a couple of VFD's here that use Normally Closed switches for e-stop. You can run a chain including door microswitches etc. I thought that was a common feature. I agree about using Normally Open switches for e-stop isn't the brightest idea.

Greg

Originally Posted by eskimo

Greg...is 130hz going to be ok for your standard 50-60hz motor...ie is the motor able to sustain much above 70hz without possible damage?

My understanding is that motors have to be designed and manufactured to operate at those types of frequencies?...where as your off the shelf standard 50-60hz motor is not.

Good point for a single speed motor, but don't forget that both Ken and I are talking about using only the lower rpm windings of a two-speed motor. The motor is designed for the equivalent of 120 Hz on the low speed windings. Going above that shouldn't tax the bearings too much, and the rotor and fan were balanced nicely at the factory. (I did check when I took mine apart, and the bearings are new SKF rated for 20,000+ rpm).

Greg

Originally Posted by neksmerj

Hi Greg and others,

As you pointed out a little while ago, that Arboga on eBay by Tissuescars, was not a mill, but a drilling machine with an x-y table. You say it sold for what? $3400, that's pretty steep for a drilling machine, albiet a good one.

The other Arboga EM825, I couldn't find. Do you have an item No.?

Ken

Ken, item number: 270693275699

On second look I guess its a U-2508, the later version of your EM825. Squared-off head, slightly beefier main tube.

$2,990. It does have power down feed on the quill, and a powered X-axis too. No tooling. Looks like it was "rode hard and put away wet".

Originally Posted by Jekyll and Hyde

John, out of curiousity more than anything, do you have any thoughts on what differs between the inverters used in a VFD, and the inverters used to power houses in a rural application?

My parents house is not on the power grid, and runs predominantly on solar (having done so for the last 25 years). Everything is always turned on and off at a powerpoint like any normal house, with apparently no ill effects (one (primitive!) inverter failure in 25 years I think can be attributed to age, can't it?), which I find interesting given the apparent sensitivity of VFDs to this behaviour. They have in the last few years upgraded their system to a fancy new one, in which the inverter looks very much like an oversized VFD, and given the amount of lights and things on it, I imagine has similarly sensitive components inside. Yet you may still switch things on and off at will. Obviously there must be SOME basic design difference here, but surely the inverter part itself must work essentially the same?

Not really, motor control VFD's are vastly different animals to the mains inverters you are describing. The output circuitry is similar in some respects, (apart from 3phase versus usually single phase) but the mains inverters usually put a lot of effort into producing a sine wave output at 50hz. Which is required to minimize RFI and some applicances just don't like high levels of harmonic distortion.

Motor control drives produce ugly looking square waves by comparison, and require LC filtering to reduce harmonics. Not the sort of power supply you want to run your plasma TV on.

The other KEY difference with motor control VFD's is the control logic that drives the power output stages. (not counting the el-cheapo V/F types) they use internal software models of the motor and actually monitor motor feedback voltages control torque and try to make sure the motor is actually running at the set reference speed. In it's simplest form, if the drive senses the motor is slowing under load, it will compensate by increasing the drive to the output stages to compensate and maintain the setpoint frequency. Sometimes called vector control or torque compensation, varies a bit between drive manufacturers.

Mains inverters have no such feedback, they just crank out 50hz sine waves regardless of load, (up to the rated wattage anyway).

Therein lies the problem with switching, you need to start at 0hz and ramp up (at the set acceleration rate) to the reference speed. Likewise stopping should ramp down (at the set decelleration rate) to stop. Just disconnecting the load, would probably produce a fault. However some drives will allow you to "catch" an already spinning motor, like an auto-rotating fan or pump. But they do this by reversing the search to start from 50hz and search downwards till it "picks up" the motor) Sometimes there is a configuration option called "fan or pump motor" option.

If you have the drive running at 50hz with no motor connected, then suddenly connect the motor, the drive will probably trip with a fault. DON'T DO IT

You should only switch the output when the drive is stopped at 0 hz, or better yet do what GregoryQ suggests, and, that is wire the drive directly to the motor.

Hope that helps

Regards
Ray

Originally Posted by gregoryq

Good point for a single speed motor, but don't forget that both Ken and I are talking about using only the lower rpm windings of a two-speed motor. The motor is designed for the equivalent of 120 Hz on the low speed windings. Going above that shouldn't tax the bearings too much, and the rotor and fan were balanced nicely at the factory. (I did check when I took mine apart, and the bearings are new SKF rated for 20,000+ rpm).

Greg

I am not at all worried about the bearings nor balancing concerns...but rather the effects that 130hz will have on the windings/rotor?...i'm no expert here and hence my questions, but it just doesnt sound right that one can get a "standard off the shelf motor" stick it on 130hz and expect it to work without some form of possible problems?

I think the ABB site have specific motors for frequency drives....does this mean that their standard motors are'nt really suitable for vsd?? at above 70-80hz.

I have also got a vsd and 415v motor that i am wanting to stick on the mill so all this info will be of assistance when I actually get around to doing it....
Got my switchboard and new motor relays, overloads, switches etc etc the other day...now just need to get some consumables and start. The electrician ran the new 3ph to the mill a couple weeks back...it was easier to get him to do as I had him wire up some aircon units on a job and he did this for zilch

Originally Posted by RayG

Not really, motor control VFD's are vastly different animals to the mains inverters you are describing. The output circuitry is similar in some respects, (apart from 3phase versus usually single phase) but the mains inverters usually put a lot of effort into producing a sine wave output at 50hz. Which is required to minimize RFI and some applicances just don't like high levels of harmonic distortion.

Motor control drives produce ugly looking square waves by comparison, and require LC filtering to reduce harmonics. Not the sort of power supply you want to run your plasma TV on.

The other KEY difference with motor control VFD's is the control logic that drives the power output stages. (not counting the el-cheapo V/F types) they use internal software models of the motor and actually monitor motor feedback voltages control torque and try to make sure the motor is actually running at the set reference speed. In it's simplest form, if the drive senses the motor is slowing under load, it will compensate by increasing the drive to the output stages to compensate and maintain the setpoint frequency. Sometimes called vector control or torque compensation, varies a bit between drive manufacturers.

Mains inverters have no such feedback, they just crank out 50hz sine waves regardless of load, (up to the rated wattage anyway).

Therein lies the problem with switching, you need to start at 0hz and ramp up (at the set acceleration rate) to the reference speed. Likewise stopping should ramp down (at the set decelleration rate) to stop. Just disconnecting the load, would probably produce a fault. However some drives will allow you to "catch" an already spinning motor, like an auto-rotating fan or pump. But they do this by reversing the search to start from 50hz and search downwards till it "picks up" the motor) Sometimes there is a configuration option called "fan or pump motor" option.

If you have the drive running at 50hz with no motor connected, then suddenly connect the motor, the drive will probably trip with a fault. DON'T DO IT

You should only switch the output when the drive is stopped at 0 hz, or better yet do what GregoryQ suggests, and, that is wire the drive directly to the motor.

Hope that helps

Regards
Ray

Thanks for the time you've taken there Ray, its much appreciated! I always stop the VFD, and then change the switches as needed and fire it back up, as per the warnings given nearly everywhere on the subject. It may only be a cheap inverter, but I'd rather not replace it unnecessarily! I just found it interesting that 'house' inverters could survive such behaviour, and wondered how they got around it.

Thanks again, Ben.

Since I'm not a sparkie's bootlace, would a kind member give me the line up of electrical gear from the 240V wall switch, to VFD.

These are my thoughts. A lead running from the wall socket will enter a metal enclosure. The first obstacle would be a two pole isolating switch. From there into an emergency off switch, then into the inverter. The output side I reckon is pretty straight forward. Just 3 cables, one for each phase plus earth.

I will set the Arboga speed switch to low, then disable it by removing the knob, for example.

I have also ordered a remote Pod for the VFD that contains controls for REV/OFF/FWD and SPEED.

How am I doing so far?

PS, just had a thought, maybe a gutted computer case would do for the enclosure.

Ken

Jekyll and Hyde,
I agree with Ray's description of the way inverters work with the comment that true vector control requires the inverter supplier to have detailed modelling of the motor, rarely available, hence Rays's comment that the definition varies between suppliers.

Although it is true heavy-duty industrial inverters will survive disconnection under load, some suppliers warn that this is a fatigue issue - each time you do it you reduce the fatigue life, just like metal being stressed.

For consumer inverters selling on price alone, you can bet the inverter has been highly optimised by the back-room guys to eliminate 'unnecessary' components and 'excessive' component ratings.
You play with these inverters outside the recommendations at great risk.

Just be careful of assuming a two-speed motor effectively operates internally at 100 Hz in high speed.
The winding re-arrangement simply puts the internal flux into a different pattern.
The flux is still operating at 50 Hz and any part of the magnetic structure is only seeing 50 Hz.
Going to 100 Hz increases losses but most experiences reported say that operation is satisfactory.
Lifetime and associated costs are rarely reported.
Operating any modern motor at 60 Hz should not be a concern.

I am in the same boat with the switching - I have an Off-Low-High switch.
I want to retain the Off position for safety without having to power-down the inverter.
I do not trust any input signal to an inverter to make it safe - either turn off the supply to the inverter or insert an isolating switch (or contactor) in the motor leads.
Keeping power on to the inverter reduces the fatigue factor mentioned above.

John.