VFD Question

[SillyOldDufferModerator @sillyoldduffer]

Posted by Steviegtr on 13/01/2020 00:11:46:

Posted by Mark Rand on 13/01/2020 00:02:31:

Simples! There are VFDs which are either originally manufactured or later modified to use a voltage doubler so they can produce 415V output from a 240V input. It's a circuit that was around even before Messrs Cockcroft and Walton used it.

Could you point me to one of those.

In pedant mode, I'm not convinced a VFD would use a Cockroft Walton multiplier, but Mark's point is correct. It's not difficult to make a VFD to output almost any voltage the designer wants.

Transforming mains down in the home to power electronics is common as muck. A typical wall-wart contains a switch-mode power supply. These oscillate at several tens of kHz, at which frequency power transformers are remarkably small compared with 50Hz units. In a switch-mode power supply the high-frequency transformer can be wound to step up or down to any voltage. Also possible as Mark suggests to multiply mains voltage with diodes and capacitors.

High-voltage VFD's come with a few disadvantages compared with the usual types. First, for a consumer, wiring up 400V 3-phase is more exciting, especially if you don't know much about electrics and struggle with manuals! (Has to be admitted VFD manuals can be difficult.) Second, the need for the electronics to handle high-voltages tends to make them more expensive and perhaps a tad more likely to fail. Third, they're harder to find for sale. Fourth, because star motors tend to be big, multi-horsepower VFDs soon get into professional territory, price on application rather than ebay. But apart from that, a high voltage VFD is good solution for chaps stuck with industrial motors that can't easily be converted from star to delta.

Dave

[not done it yetParticipant @notdoneityet]

Posted by Steviegtr on 12/01/2020 23:22:46:

I still do maintain that fact. I was not pointing at you. But a star connected motor on 240v will have the windings basically connected in series for each pair so 2 240 windings in series with 240v applied will give a pretty crap power curve

Your ‘generalisation’ appeared to include me, as well as others. Power at design speed will be 415V power/square root three? That is basically good enough for me. I am a hobbyist. It works more than adequately.

My replacement was a 2HP 3 phase motor, so provides about 1.2HP at 240V.

Three phase – so far better than the single phase 1HP motor it eventually replaced. Soft start, choice of speed, not limited to number of starts per unit time, no motor capacitors to worry about, smoother output (less vibration), does not object to starting in highest gear when cold. What more could I want? I don’t even consider if the ‘power curve’ might be crap or not – it just works.🙂

Edited By not done it yet on 13/01/2020 10:07:23

[Anonymous]

Posted by SillyOldDuffer on 13/01/2020 09:57:05:

A typical wall-wart contains a switch-mode power supply. These oscillate at several tens of kHz

I would think that they're up in the 100s of kHz these days. Even 100kHz is so passé.

Low power DC-DC buck and boost converters are up to the low MHz, and use inductors rather than transformers. An offline switcher may need a transformer depending upon isolation requirements. Inductors are preferred as they're small and surface mount and don't need fancy formers for the windings. Higher power converters that need bridges tend to use transformer based designs, although the transformer windings can be part of the PCB.

I don't know what a 240V in, 415V out VFD uses to boost voltage. But given that more expensive VFDs probably incorporate a power factor corrector at the front end rather than a simple rectifier it would be simple to use the PFC to generate the higher voltage needed for the DC link. A PFC is basically a boost converter. There are two big cost impacts caused by the higher DC link voltage. The bridge transistors and DC link capacitors need to have much higher voltage ratings.

Andrew

[Stuart BridgerParticipant @stuartbridger82290]

Posted by Steviegtr on 12/01/2020 23:22:46:

I still do maintain that fact. I was not pointing at you. But a star connected motor on 240v will have the windings basically connected in series for each pair so 2 240 windings in series with 240v applied will give a pretty crap power curve

Your statement may well be true for industrial use. For hobby use, I have been running a Colchester Chipmaster on 240V with the original 1963 415V motor star connected motor using the 29Hz configuration suggested by Inverter Drive Supermarket for the last 9 years. I have never been lacking in torque/power for the use I put it to. I did try turning 1.25 inch mild steel at 2000RPM witha hefty DOC and feedrate with carbide tooling once. It did slow down the spindle speed a bit, but I was getting seriously hot chips and a stunning surface finish. But that is way beyond my typical usage.

Edited By Stuart Bridger on 13/01/2020 12:19:51

[Gerard O’TooleParticipant @gerardotoole60348]

…

.., I have been running a Colchester Chipmaster on 240V with the original 1963 415V motor star connected motor using the 29Hz configuration suggested by Inverter Drive Supermarket for the last 9 years. ..

Edited By Stuart Bridger on 13/01/2020 12:19:51

CAn I ask, is the 29Hz configuration only needed for Star connected motors.

My motor is connected Delta but the maximum HZ is also set at 29Hz ,as per Drive Supermarket page. Should I ignore this for delta wired motor.

Also, if I change the configuration to 50Hz, what should I look out for in terms of problems. Will the motor be noisier, or more erratic or hotter.

Thanks

[old martParticipant @oldmart]

Motors in this country are designed to run at 50 Hz and in the USA, 60 Hz, not 29 Hz.

[Anonymous]

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

[SteviegtrParticipant @steviegtr]

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 BridgerParticipant @stuartbridger82290]

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 2Participant @robertatkinson2]

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 BridgerParticipant @stuartbridger82290]

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 martParticipant @oldmart]

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

[SteviegtrParticipant @steviegtr]

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 BridgerParticipant @stuartbridger82290]

 

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

[SteviegtrParticipant @steviegtr]

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 2Participant @robertatkinson2]

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 RandParticipant @markrand96270]

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 GilliganParticipant @michaelgilligan61133]

Thank you, Mark

MichaelG.

[Robert Atkinson 2Participant @robertatkinson2]

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 8×50 = 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

[SteviegtrParticipant @steviegtr]

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.

[SteviegtrParticipant @steviegtr]

I want some of what you are drinking.

[Michael GilliganParticipant @michaelgilligan61133]

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 PooleParticipant @mikepoole82104]

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 BridgerParticipant @stuartbridger82290]

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 yetParticipant @notdoneityet]

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.

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