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VFD Question

3 phase voltages and setup for VFD

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Andrew Johnston13/01/2020 14:39:37
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RUN IT AT 50Hz - IT WILL BE FINE!

I've just had a quick look at the link again. Got as far as the first paragraph where it says motor speed is dependent upon voltage and frequency. No point in reading any further.

Andrew

Steviegtr13/01/2020 16:19:06
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Sorry guys I see what you have done. Not something you could get away with in industry but for home use fine. Run a bigger motor than required to get the lower power you need. Quaint. A few friends have the big buz transverters. They work too , big inducter for Lag & a big capacitor for lead.

Stuart Bridger13/01/2020 16:30:40
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The much discussed document from Inverter Drive Supermarket may not be the best worded. What I believe that he is saying that you can get equivalent peformance supplying a 415V motor with 230V up to 29Hz, Efficiency above this frequency starts to drop off. Setting 29Hz as the "base frequency" of the inverter optimises the inverter operation for this situation. Most importantly the full load current must be set to match the motor as it will draw more current for the same power and this needs to be limited ot prevent overheeating the motor. Nowhere in the article does it say don't exceed 29Hz. I run mine up to 50Hz purely so I don't exceed the design rating of the lathe.

This "trick" is ONLY required if you can't rewire your motor for 230V operation. My 1963 motor falls into this category. As mentioned before it does work, so the article can't be complete BS.

Robert Atkinson 213/01/2020 17:04:05
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I've been travelling so have off-line.

Please READ ALL THE THREAD BEFORE POSTING.

I've already explained the situation on running a 400V motor (does not matter if it's star or delta just that it cannot be wired for lower voltage). Running it at 29Hz (with no voltage reduction, maximum VFD can give) Gves more torque at lower speed. Runnng at 50Hz (same voltage -maximum) gives less torque and higher speed. THE POWER IS THE SAME.
The voltage rating of a motor is only to get enough current to provide the required torque at full speed. Star-Delta starters provide lower inital current and thus torque by putting less than rated voltage (0.577 times) across each winding during starting. This can be done to reduce electrical loading so supply requirements can be less OR to reduce initial torque to prevent mechanical stress.


The VFDs with higher output voltage than input either use a simple two capacitor full wave voltage doubling rectifier. This is only suited to smaller units as it has poor power factor, but it's cheap. larger and better units use a boost switchmode regulator that can also provide power factor correction with a few extra components. While a true Cockroft-Walton voltage multiplier uses the same basic technique of multiple capacitors charged in parallel and discharged in series as the full wave doubler it is defined by many more capacitors and thus higher multiplication.

Robert G8RPI.

Stuart Bridger13/01/2020 17:18:03
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A bit more research on VFD operation (specifically on the Emerson Commander that I use) shows that the the VFD will modulate output voltage progressively up to the "base frequency" set. Above the base frequency full supply voltage is applied and just the freqeuncy is varied This is where the 29Hz comes in. Setting the base frequency to 29Hz ensures that this voltage/frequency curve is optimised for 230V operation. Agreeing with Robert's post above

 

Edited By Stuart Bridger on 13/01/2020 17:19:52

old mart13/01/2020 20:37:42
4655 forum posts
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So anyone who runs a four pole three phase motor at 29 Hz is content to expect a full 783 rpm, you must be a bunch of prats. 

Not only that, but it WILL NOT GIVE ITS RATED POWER.

Edited By old mart on 13/01/2020 20:39:53

Steviegtr13/01/2020 21:00:48
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Posted by old mart on 13/01/2020 20:37:42:

So anyone who runs a four pole three phase motor at 29 Hz is content to expect a full 783 rpm, you must be a bunch of prats.

Not only that, but it WILL NOT GIVE ITS RATED POWER.

Edited By old mart on 13/01/2020 20:39:53

I agree with you but it seems some on here have put a larger motor on with the premise that they will have less than with old motor & run slower too. Cannot see the point really. ????????

Stuart Bridger13/01/2020 21:24:02
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1. No one has suggested that you don't exceed 29Hz. It is a VFD setting to optimise performance at slower speeds. I can still get the rated 3000 RPM spindle speed out of my chipmaster, albeit with reduced torque. The clue is the V in VFD variable.

2. There is no doubt that this solution is a compromise, but it does work for those who have a single voltage motor and no 3 phase 415v supply available. It was much easier for me to fit a VFD than to replace the motor. It certainly is a viable option that can be considered, IF you don't need the full torque of ex industrial machine.

Edited By Stuart Bridger on 13/01/2020 21:25:36

Steviegtr13/01/2020 22:58:39
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Out of interest how many of you guys with the inverters use a ten turn pot. I have fitted one & for the life of me I don't know why. It maybe because all the ones we fitted for Nestle foods had to have fine control for production work & line matching to other inverters in the chain. But for an old Lathe ???

Robert Atkinson 213/01/2020 23:07:59
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Posted by Stuart Bridger on 13/01/2020 17:18:03:

A bit more research on VFD operation (specifically on the Emerson Commander that I use) shows that the the VFD will modulate output voltage progressively up to the "base frequency" set. Above the base frequency full supply voltage is applied and just the freqeuncy is varied This is where the 29Hz comes in. Setting the base frequency to 29Hz ensures that this voltage/frequency curve is optimised for 230V operation. Agreeing with Robert's post above

Edited By Stuart Bridger on 13/01/2020 17:19:52

No it's NOT.

The VOLTAGE is reduced when the FREQUENCY is BELOW the rated frequency of the motor.
The VFD instructions you are reding assumes the motor you are using has the same rated voltage as the VFD output. Some VFDs can increase the voltage as the frequency rises to maintain torque and increase power but this depends on the insulation of the motor being able to withstand the higher voltage. Motors with increased insulaton ae available for this purpose. Note that the motor will not run any hotter with higher voltage because the main heating effect is I squared R and the current and resistance of the windings has not changed (yes there are increased bearing, windage and AC losses like skin efect but these are insignificant for the levels we are talking about).

I'll say it again it is FREQUENCY (speed) and CURRENT (torque) that determine the motor power, the voltage rating just sets the current at one frequency.

Robert G8RPI.

Mark Rand13/01/2020 23:32:24
1505 forum posts
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Basically, a 50Hz 415V motor, without rewiring from star to delta (because the star point is often hidden/internal for older motors) can produce full torque when run at 240V and 29Hz. It will only produce 58% of rated power at that speed, and can produce the same maximum power at all speeds up to full (50Hz) speed when run at 240V. This can be sufficient with a larger machine in a non-production environment.

Edit to add:- Darn it, replying to a post on the last page again...

 

I do have a 1400rpm 2hp delta motor to replace the 1/2hp 3000rpm single phase motor on my dust extractor. Works happily at 100-150Hz (no problem with the fan, that's got some leeway). The batch of motors, from a work location that closed down, were at a suitable price (free) that running one off a VFD was cheaper than buying a replacement for the burned out, crappy original.

 

Edited By Mark Rand on 13/01/2020 23:42:30

Michael Gilligan13/01/2020 23:39:59
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Thank you, Mark

MichaelG.

Robert Atkinson 214/01/2020 01:01:54
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In reality you need to know 3 electical specifications for a 3 phase AC induction motor

1. Voltage to Frequency Ratio (V/Hz)
2. Maximum Frequency
3. Maximum current

For a typical UK 1.5 hp motor these could be:

8 V/Hz, 60Hz and 2.5A

This motor is suitable for direct connection directly to the UK mains (50Hz 230/400V) as 8x50 = 400V.
If used on a VFD with voltage boosting it will run 20% faster at 60Hz and 480V with same max current (torque) so will provide 1.8 hp
Running at reduced speed reduces the power linearly to zero. At zero Hz (DC) you can apply zero volts. Note that you still get torque at 0V but its resisting turning, not providing it. This is how dynaimic braking and DC injection works. Try spinning a disconnected motor by hand and then try with the connections shorte together.

Note that the same physical size motor provides more power at higher frequency. This is why aircraft use 400Hz AC supplies, 8 times the power for the same weight of motor. The V/Hz rule applies to transformers too.
Aircraft voltage is 115/200V so V/Hz for motors is 0.5. This means you can run an aircraft motor on 50Hz but you have to reduce the voltage to 25V so there is little point.

Robert G8RPI

Steviegtr14/01/2020 01:11:59
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For a typical UK 1.5 hp motor these could be:

8 V/Hz, 60Hz and 2.5A.

Where do you live. The frequency in the uk is & has been for a long time 50Hz.

Steviegtr14/01/2020 01:13:52
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2668 forum posts
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I want some of what you are drinking.

Michael Gilligan14/01/2020 07:46:27
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23121 forum posts
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Posted by Michael Gilligan on 13/01/2020 23:39:59:

Thank you, Mark

MichaelG.

.

Just to note: My thanks were for Mark’s succinct explanatory paragraph

... I posted before his edit.

MichaelG.

Mike Poole14/01/2020 08:16:51
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3676 forum posts
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Well dear readers (or not as some of the contributions would indicate) the 29Hz trick is for running a star 415v motor on a 230v inverter, this will never develop max torque and therefore max power given these circumstances. By telling the inverter that the motor base frequency is 29Hz then it will be able to produce the rated torque at that frequency which it would not do at a 50Hz base frequency setting. Any motor will only develop full power when run at its base frequency with enough voltage to drive the full load current through it. A VFD is always going to trade something to have the convenience of variable speed, below base speed you will lose power even if you maintain torque, above base speed you will lose torque.
If you set a motor to 230v in delta and run it on a 415v output inverter which you have set set to 87Hz base frequency then it will be able to produce full torque up to 87Hz which will obviously produce more power from your motor.
Inverters are very clever but if you lie to them they will never know.

Mike

Stuart Bridger14/01/2020 08:48:52
566 forum posts
31 photos
Posted by Robert Atkinson 2 on 13/01/2020 23:07:59:
Posted by Stuart Bridger on 13/01/2020 17:18:03:

A bit more research on VFD operation (specifically on the Emerson Commander that I use) shows that the the VFD will modulate output voltage progressively up to the "base frequency" set. Above the base frequency full supply voltage is applied and just the freqeuncy is varied This is where the 29Hz comes in. Setting the base frequency to 29Hz ensures that this voltage/frequency curve is optimised for 230V operation. Agreeing with Robert's post above

Edited By Stuart Bridger on 13/01/2020 17:19:52

No it's NOT.

The VOLTAGE is reduced when the FREQUENCY is BELOW the rated frequency of the motor.
The VFD instructions you are reding assumes the motor you are using has the same rated voltage as the VFD output. Some VFDs can increase the voltage as the frequency rises to maintain torque and increase power but this depends on the insulation of the motor being able to withstand the higher voltage. Motors with increased insulaton ae available for this purpose. Note that the motor will not run any hotter with higher voltage because the main heating effect is I squared R and the current and resistance of the windings has not changed (yes there are increased bearing, windage and AC losses like skin efect but these are insignificant for the levels we are talking about).

I'll say it again it is FREQUENCY (speed) and CURRENT (torque) that determine the motor power, the voltage rating just sets the current at one frequency.

Robert G8RPI.

Robert, I have to say that I completely agree with your statement, effectively you are just describing what happens when you reduce the frequency from base/rated frequency downwards, whereas I described what happens when you increase frequency upwards to the base frequency. Exactly tthe same.

From the VFD manual
"The rated voltage is used in conjunction with the motor rated frequency (Pr 5.06) to define the voltage to frequency characteristic applied to the motor. The following operating methods selected by Pr 5.14 are used to define the drive frequency to voltage characteristic"

" Fixed boost mode: Fd A linear characteristic is used from 0Hz to rated frequency, and then constant voltage above rated frequency. Low frequency voltage boost as defined by Pr 5.15"

As mentioned previously the "trick" here is to set the base frequency to 29Hz, which changes the voltage /frequency curve to get the best out of the compromise of supplying 230V to a 415V motor

Fror reference see p76 onwards from Commander SK advanced user guide


not done it yet14/01/2020 09:07:56
7517 forum posts
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Here is my view of this waffle of 29Hz.

Electric motors rely on resistance, reactance, back emf’s, core saturation, etc. in their design to perform effectively with the prevailing name-plate supply. Deviation from that voltage will alter the motor characteristics - output power, efficiency, torque, etc while a deviation of frequency will alter the basic speed of the motor - again affecting the original design characteristics.

For the reasons above, I think the 29Hz is unlikely to be set in stone as the frequency at which the writer suggests this phenomenon occurs.

Simple physics shows that at zero rpm (start-up point) the current drawn by the motor will be initially a reactive circuit, but very quickly becoming a resistive current - unless that motor actually starts to turn (stalled motor at start up will either trip the motor overloads, blow the fuse, trip the mcb or burn out the motor windings).

At design voltage and frequency the motor should provide the name-plate output.

Between that frequency and zero Hz, the motor would take more and more current because it would revert towards a resistive circuit as the supply tends towards driving a resistive load. That is OK up to a point.

If that increase is due to mechanical overload slowing the motor, it would surely burn out long before the speed dropped to the equivalent speed of 29Hz whilst being driven by the normal supply frequency!

If the speed is reduced by a controlled frequency reduction, we have a different scenario. The current would tend to rise, but the motor power would be reduced because the VFD sorts out the voltage to maintain the safe current. (This is apart from the extra problem of a slower moving fan which was initially designed to cool the motor at its normal supply frequency). At some point, the motor torque will no longer be attained without eventual damage to the windings - the escaping magic smoke! - and that, presumably, will be around 29Hz.

Most motors will be designed within a fairly close range of working parameters. The better motors will be more expensive to manufacture, of course (more copper, better insulation, higher temperature resistance, higher run cycles, better controlled cooling, etc).

The simple way to avoid trouble, for us hobbyists, is not to overload the motor -particularly at lower frequencies - and to limit the run/rest cycle to below whatever it was rated at for normal frequency running. Forget about this supposedly magical figure of 29Hz, apply some common sense and just get on with life.

SillyOldDuffer14/01/2020 09:56:05
10668 forum posts
2415 photos
Posted by Steviegtr on 13/01/2020 22:58:39:

Out of interest how many of you guys with the inverters use a ten turn pot. I have fitted one & for the life of me I don't know why...

Almost no-one I would guess. I can't think of a good reason for fitting an expensive multi-turn pot to control the speed of an ordinary machine tool. Cutting speeds don't need to be super-accurate and I don't see any benefit in being able to fine-tune to a particular speed. An ordinary cheap pot (often used with a tachometer) allows manual tool speeds to be adjusted quickly and with enough precision to suit the job. And are easily replaced when they fail. The time it takes to go from one end to the other of a 10-turn control is a disadvantage on a home lathe.

Context matters. Two bad mistakes chaps sometimes make on the forum:

  1. Assuming what's done in a small workshop should be applied by industry. OR
  2. Assuming what's done in Industry should be transferred to a small workshop.

Engineering is about problem solving. The answer depends on the question. Copying what's been done elsewhere is often an excellent solution, but it comes unstuck when the circumstances are different. Always pays to recognise when one's expertise has gone off piste and it's time to engage brain or take advice!

Dave

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