VFD Question

[Anonymous]

Posted by John Haine on 08/01/2020 15:58:09:

To save Andrew doing it.

Thanks John. Worth re-iterating that for the motor to produce full power from the reduced voltage it needs to be configured for delta.

Not sure I agree with SoD on a couple of points. The use of high voltage for power transmission is not connected with the 3 wires needed for 3-phase distribution. It's equally valid for single phase. It's simply the application of Ohms law. For a given power transmitted the current goes down as the voltage rises. And since power dissipation is proportional to current squared as the voltage rises the power lost falls faster. Or thinner, and lighter, wire can be used. The big advantage of 3-phase over single phase is that one can transmit three times as much power for an increase of only 1.5 times the amount of cable.

The same is true for industrial power users. Although of course 3-phase has other advantages such as smoother running and easily reversible motors, and constant power if the phase currents are equal.

Not sure what SoD means when he says 3-phase is self-balancing. If you have unbalanced phase currents and a neutral then there should be a neutral current flowing. But, for instance, most electric motors don't have a neutral connection! So voltages start to go awry.The utilities go to great lengths to ensure that phase currents are balanced over a large number of individual loads. A 3-phase generator, and 3-phase transformers, are not going to be happy if the phase currents are unbalanced.

Andrew

[SteviegtrParticipant @steviegtr]

Hello all. at last a subject I know plenty about. I was an electrical engineer & contractor of sorts for 35 years, in that I designed , manufactured & installed control panels to machinery. Mainly in blue chip food factories. We installed many inverters in different machines. If 3 phase was available then of course we fitted 3 phase inverters, usually imo Jaguars. If however only single phase was available & speed control was required then we would use a single phase inverter. Usually Imo Jaguar Cubs. 2 things you need to know is that a motor suitable to this application has to be what is called dual voltage. That is the windings are each actually 240volt wound. If they are 415v then it is no good. So always make sure the 3 ph motor has 240v windings. Most new motors are. The 2nd thing is that a motor used with a 240 single phase to 3ph inverter, must be connected in delta. The problem with Delta connected motors is that the starting torque is reduced by route 3. An example once was an engineer came to me with a box of bits he had purchased for the speed control of a Glucose pump where the only supply available was single phase. I advised the engineer that it would not work because pumping thick liquid would require a high starting torque & that the motor would have to be connected in star. He was adamant that it would work so we built a control panel with the parts supplied. Outcome was that as you guessed the pump once primed with glucose & stopped, would not restart because of the lack of torque connected in DELTA. The reason I say this & I myself are building up a Myford super 7 with a 3 phase motor is when you fit the system & set it up you may find that the best set up is to say use a low gear on your pulleys as you may find that if you run the motor slow & plunge a cutting tool into the work that is may stall. This is normal. So use a lower gear so the motor is running faster & you will not have a problem. I myself have probably gone for a overkill as the single phase motor was 3/4 hp & I have gone for a 1.1 kw 1 1/2 hp 3ph motor & Omron 1.1kw inverter. Also running the inverter at around 50Hz will give you 1425rpm. the same as your single phase motor. The inverters you will be fitting can be turned up to pretty high Hz settings. The one I have will run up to 400 HZ. Just be careful, you will be able to get some silly speeds from your inverter but if run too high 2 things may happen. The 1st is the motor may come apart, explode if you like. The other is the noise will be bad. If you buy a good quality inverter then you can tune within the parameters to stop the high pitched noise associated with these drives. We would regularly overdrive a motor to say 75Hz with no problems so happy speed control. God I can prattle on. I am usually the one asking questions But anything electrical is steviegtr's speciality…..

[noel shelleyParticipant @noelshelley55608]

SODs drawing shows why we get electrocuted, we're standing on one of the wires ! Earth/neutral !!

Why in a 3phase generator wired star should you earth star point ? Does this not create the hazard ?

Is not the reason why there are only 3 wires from the power station more about cost than safety ?

In the village I live in they only brought 2 phases in ! even the farms have to make do. Or instal there own generators.

[SteviegtrParticipant @steviegtr]

The star point of the generated power from the power station is grounded to earth. It takes some understanding but earth is earth. Neutral is earth. The Pilons only carry 3 phases, usually up to 133,000 volts then reduced to what is needed. That is another subject.

[Clive SteerParticipant @clivesteer55943]

Taking Andrews point about how a street is wired with 3 phase and each house is fed with a one of these phases it is likely that your neighbour either side of you may not on the same phase as you. So it may be possible to get a true 3 phase supply to your property if you can persuade you neighbours to throw an extension lead connected to their mains over the fence.

Although not a serious suggestion is this a possible safety risk as there would be 440v between the live of your house wiring and the live of the feed from your neighbours.

Clive

[SteviegtrParticipant @steviegtr]

It was a project when I was an apprentice to design a building with 3 phase power supply . You got marks for balancing the load. Don't know if they still have but 3 phase polymeters had a out of balance dial on them. The lower you could keep the dial the cheaper your electric was.

[John HaineParticipant @johnhaine32865]

Posted by noel shelley on 09/01/2020 00:03:20:

….In the village I live in they only brought 2 phases in ! even the farms have to make do. Or instal there own generators.

I think there is a transformer arrangement that will generate a 3 phase supply from 2 phases at 120 degrees.

"The Pilons only carry 3 phases, usually up to 133,000 volts then reduced to what is needed. " – Actually I think you will see another cable right at the top of the pylons, which connects them all together and to the neutral/earth at each end. Partly so that stray cows and walkers don't electrocute themselves when they touch the legs if there's an earth fault, but also to carry the unbalance current which otherwise could take an unpredictable route home and cause various problems.

[EmgeeParticipant @emgee]

I think what Noel is speaking of is a 480v AC supply as taken to many farms in the area in previous years, 2 x 240v lines in with 480v between lines, not the usual 415V AC between phases.
It was used so large motors could be started and run more efficiently than when using a single phase 240v supply.

Emgee

[not done it yetParticipant @notdoneityet]

The repair crews just dug up the road – well, both pavements – down from us (generator running 24h for several days over the Christmas/new year period&#128578.

Apparently each pair of semis are wired on the same phase. Don’t know how they wired the detached dwellings. Bungalows at one end and larger detached houses at the other…

[Martin ConnellyParticipant @martinconnelly55370]

The star point at the alternator supplying the grid is connected to earth through an NER (neutral earth resiztor). If the load is un-balanced then the voltage across the NER rises and if it exceeds a given value or rises above a set rate then the system assumes a fault and trips the alternator off the grid to avoid damage.

Martin C

[Anonymous]

Posted by Steviegtr on 08/01/2020 23:58:28:

The problem with Delta connected motors is that the starting torque is reduced by route 3.

I don't understand why that would be? If I've got a dual voltage motor then in both star and delta the power available (from voltage/current values on the plate) is the same. In both cases the speed is also the same. Since power is torque time angular velocity, and power and angular velocity are same for delta and star, surely the torque is also the same in star and delta?

The issue of stalling a motor running at low speed is simply explained. Below base speed an induction motor runs at constant torque (*). Whereas using pulleys or gears to reduce speed has a constant power characteristic. So using the formula above running at slow speeds, through a mechanical reduction, results in an increase in torque in proportion to the decrease in speed.

Andrew

(*) Before any smartypants points it out I know VFDs can be programmed to output more than normal current at slow speeds. But it's not a good thing in the long term as it results in significantly more losses in the windings.

[Gerard O’TooleParticipant @gerardotoole60348]

Thanks Andrew, John, Dave , MArtin and Steves for the clear explanations.

Just one other question.(Sorry!)

Frequency. I have been told that the maximum frequency must be reduced in proportion to the voltage. Currently my VFD is set at maximum 31 Hz.. Is this correct or can I increase it to 50Hz.?I note that 'Old mart' runs his VFD up to 75Hz.

regards

gerard

[Anonymous]

In the UK the main distribution pylons now run at 400kV, not 132kV. What is completely mind boggling is that people work on these lines while they're live.

Andrew

[SteviegtrParticipant @steviegtr]

Posted by Andrew Johnston on 09/01/2020 11:24:54:

Posted by Steviegtr on 08/01/2020 23:58:28:

The problem with Delta connected motors is that the starting torque is reduced by route 3.

I don't understand why that would be? If I've got a dual voltage motor then in both star and delta the power available (from voltage/current values on the plate) is the same. In both cases the speed is also the same. Since power is torque time angular velocity, and power and angular velocity are same for delta and star, surely the torque is also the same in star and delta?

The issue of stalling a motor running at low speed is simply explained. Below base speed an induction motor runs at constant torque (*). Whereas using pulleys or gears to reduce speed has a constant power characteristic. So using the formula above running at slow speeds, through a mechanical reduction, results in an increase in torque in proportion to the decrease in speed.

Andrew

(*) Before any smartypants points it out I know VFDs can be programmed to output more than normal current at slow speeds. But it's not a good thing in the long term as it results in significantly more losses in the windings.

Rather than me ranting on. Look up the function of a star delta starter.

[SteviegtrParticipant @steviegtr]

Posted by Gerard O'Toole on 09/01/2020 11:27:15:

Thanks Andrew, John, Dave , MArtin and Steves for the clear explanations.

Just one other question.(Sorry!)

Frequency. I have been told that the maximum frequency must be reduced in proportion to the voltage. Currently my VFD is set at maximum 31 Hz.. Is this correct or can I increase it to 50Hz.?I note that 'Old mart' runs his VFD up to 75Hz.

regards

gerard

You can run it as high as you want within reason. 75Hz is not that high. If fitted to a lathe or a milling machine just make sure you do not go over the manufacturers top speed. Steviegtr

[Alan Waddington 2Participant @alanwaddington2]

Many years ago i helped a mate who lived a few doors away build a wood turning lathe.

We knocked it up in my garage and fitted a used single phase motor, it ran perfectly.

When he plugged it in at home it ran in reverse…….much head scratching.

Eventually brought the motor back to mine and it ran the correct way every time, but at his place it ran the other way.

Never really got to the bottom of it.

[Mike PooleParticipant @mikepoole82104]

A star delta starter reduces the current because the motor is rated at 415v in delta and about 718v in star so as we only have a 415v supply then the current will be reduced in star which is handy for starting but the torque will also be less. The motors we typically use for small inverters are 415v in star and 220v ish in delta and cannot be used for star delta starting on a 415v supply. The torque is not a function of being configured in star or delta but having the correct voltage for the configuration.

Mike

[SillyOldDufferModerator @sillyoldduffer]

Posted by Andrew Johnston on 08/01/2020 22:45:37:

Posted by John Haine on 08/01/2020 15:58:09:

…

…

Not sure I agree with SoD on a couple of points. The use of high voltage for power transmission is not connected with the 3 wires needed for 3-phase distribution. It's equally valid for single phase. … he big advantage of 3-phase over single phase is that one can transmit three times as much power for an increase of only 1.5 times the amount of cable.

…

Not sure what SoD means when he says 3-phase is self-balancing. If you have unbalanced phase currents and a neutral then there should be a neutral current flowing. But, for instance, most electric motors don't have a neutral connection! So voltages start to go awry.The utilities go to great lengths to ensure that phase currents are balanced over a large number of individual loads. A 3-phase generator, and 3-phase transformers, are not going to be happy if the phase currents are unbalanced.

Andrew

The first point, I agree! My poor wording, I didn't mean high-voltage was exclusively a 3-phase thing. Works just as well for single-phase and DC because W=VI and W = I²R. As I = V/R Big Volts = small losses.

I may be talking rubbish again about 3-phase having balance advantages due to misremembering or misunderstanding what my dad told me. Unlike me he was a qualified Electrical Engineer and could do the maths, though it has to be said his job didn't keep his skills well-honed! Anyway, I don't mean that 3-phase systems don't have balance problems, only that they are less vulnerable. In this diagram the supply side of the transformer is connected to a 3-phase generator, but only one phase on the consumer side has a load attached. (Not by design, but imagine someone crashed into a pole and broke the other two live wires.)

By 'self-balancing, I mean the generator doesn't bump because only one winding is generating power while the other two idle due to an odd load. The generator is isolated from consumer-end phase imbalances by the transformer. The magnetic flux in the transformer's core is generated by all three input windings and the same flux is shared by all three output windings. It's not fatal if only one phase is drawing power. Obviously not ideal or efficient but having a phase or two disconnected certainly isn't disastrous. Normally the transformer would be wired to balance the load, balanced phases are a good thing, but unlike riding a motorbike at 120mph, perfect balance isn't mission critical!

More. My diagram shows only one generator and one transformer: in the real world there are many generators and transformers in the distribution system, and as they all tend to minimise phase imbalances, the system as a whole is resilient, even in the face of faults and atypical customers.

This is my understanding: I hope it's right! Given my diagram, anybody out there able to calculate the current flowing in each generator side phase when only one single phase consumer is taking power from the system?

Dave

[Anonymous]

Posted by Steviegtr on 09/01/2020 12:30:40:

Rather than me ranting on. Look up the function of a star delta starter.

A star-delta starter is used to start large motors without causing uneccesarily large inrush currents while the motor comes up to speed. The crucial point is that they operate at a fixed input voltage. So I agree with Mike. If the motor is designed to run at full power, at full voltage, in delta then in star the phase currents will be lower, by the square root of 3, and hence inrush currents will also be lower. Since the phase currents are lower the torque during startup, in star, will also be lower. But that seems to be the opposite of what Steviegtr is saying?

I don't know at what motor power it is generally advisable to use a star-delta starter. The biggest motor in my workshop is 5hp (horizontal mill) and that is started DOL (direct off line). The mill has a clutch so starting torque is fairly small.

Andrew

[John HaineParticipant @johnhaine32865]

The transformer doesn't do anything to correct the balance. The load current can be resolved as "symmetrical components" which are fwd and bwd rotating balanced sets of 3-phase currents plus equal "common mode" currents which are equal and in-phase with each other on the 3 lines. The same symmetrical components will exist on both sides of the transformer but be differently manifested because there's no neutral. What I think it means is that two of the windings on the transformer supply all the current while the third carries no current. Whether this matters depends on the transformer capacity relative to the load.

[SteviegtrParticipant @steviegtr]

Another little ditty when using an inverter is, it is advisable to use screened cable to the motor. The screen ideally should then be connected to earth only at one end only. This is to shield the inverter output from emitting noise. Interference is caused by inverters so any delicate electronic equipment nearby would be effected. Does. not matter if you only use plain cable. Just advisable. Not sure if it would interfere with a CNC setup. I cannot get any deeper into the if's & but's that are being bounced around. I spent most of my career working in industry as an electrician. But will not argue with any comments said.

[Robert Atkinson 2Participant @robertatkinson2]

I've come to the thread late but OMG what a confusion. Lot's of good info, some "simplified electrical" and some just wrong.

One key point that seems to be missing is that (assuming same speed) the output power of a motor is determined by the torque. The torque is directly proportional to the CURRENT in the windings. If the current required to provide the load torque is less than the available current the motor provides the required power at the output (note this could be an overload and damage the motor). If the available current is less tthan the requirement the motor will slow and stall. A much better way to consider motor rating is the maximum torque and speed not power.
The above applies to any motor, 3 phase, single phase, universal including DC. For permanent magnet motors the magnet field is an additional limting factor.
Note there is NO mention of voltage. Voltage does not affect the available power IF enough current can be drawn. Voltage applied to a motor sets two parameters – A/ the off-load current (and thus speed for a DC motor) and B/ the maximum available current and thus (indirectly) available power.

So what controls the current? Well according to mr Ohm's law, Voltage divided by resistance for DC. We are mostly talking about AC but if we simplify things its Voltage divided by impedance (AC & DC resistance).

For a given frequency (speed) the impedance of the motor windings is fixed. This sets the stalled current. So what set the running current? That is the apparent voltage across the windings divided by the impedance. This apparent voltage is the applied voltage less the "Back EMF". This is the voltage developed by the the current flowing in the motor and opposes the applied voltage. It decreases with the load on the motor. At no load it is equal to the supply voltage and no current is drawn (This is a gross simplification, there is always some load, bearings windage etc so some current. and ignores phase relationships beteen voltage and current in coils). As load increases the Back EMF decreases and the current increases. The voltage rating of the motor is that which limits the input current and power (and thus output power) to the rating of the motor (set by the amount and quality of the iron and copper in the motor). For AC motors this includes the frequency. Too much current will overheat the windings due to rsistive losses.
The impedance of the motor is directly proportional to the frequency applied (all else being the same) so if you halve the frequency the impedance will halve and the current will double. This is why when running at reduced frequency (speed) the VFD reduces the voltge proportionally to keep the currrent the same. The available power reduces not becase of the reduced electrical input but because of the reduced speed at the same torque. You cannot exceed the torque rating of a motor without overloadding it and causing damage. If you increase the frequency (speed) the VFD increases the voltage to compensate for the increased impedance. This means you can get MORE power from the motor assuming you don't exceed the mechanical ratings (mostly bearings and centrifugal forces) or the voltage rating of the electricl insulation. Some "inverter" rated motors allow for this in their design. So ideally yous hould run a motor as fast as possible. 60Hz is OK for most normal motors.

So;

1 you do not need to limit the upper frequency when running a motor at less than its rated voltage. Running a higher voltage rated motor (e.g. delta) with at lower speed and maximum available voltage allows you to get more torque out of it so keep the voltage up.

2 The ratio of line to line and line to neutral is always root 3 (1.732) for a 3 phase system (five phase is root 5) so 230V/440V is not possible.

Robert G8RPI.

[Anonymous]

Posted by Gerard O'Toole on 09/01/2020 11:27:15:

I have been told that the maximum frequency must be reduced in proportion to the voltage. Currently my VFD is set at maximum 31 Hz.

Not sure where that came from? Internet 'expert' maybe.

Let's consider a standard motor, in delta, running from 240V phase to phase and designed for 50Hz. At 240V and 50Hz the difference between the applied 240V and the backemf will be sufficient for the motor to drive rated current through the windings. In simple terms winding current controls torque so the motor can provide rated power. This is known as the base speed, ie, the speed resulting from a frequency of 50Hz. So if you have a standard motor there is no reason not to run it at 50Hz.

Now if we increase the frequency the speed of the motor increases in proportion, as does the backemf. The rise of backemf with a fixed applied voltage means that the rated current cannot be reached, and the torque drops as well. But since the speed has increased the power stays pretty much constant. The limit to the upper frequency will most likely be the motor itself – too fast and bits come off! The 2-pole high speed spindle on my CNC mill will run at 400Hz (24000rpm), but it's designed to do so. For a normal motor I'd go to 75Hz or so, unless I was sure it wouldn't come apart – like manufacturers data.

Conversely if we decrease the frequency from 50Hz the backemf drops and the larger difference between the backemf and 240V applied voltage will drive more than the rated current. That's not a good idea long term due to overheating of the windings. So the applied voltage needs to be reduced in proportion to the frequency to maintain rated current. Since the current stays constant so does the torque. But the motor speed has decreased so the power has decreased.

To summarise, above base speed the motor characteristic is constant power. Below base speed it is constant torque. And there should be no problem running a standard motor at 50Hz and a bit above.

Andrew

[not done it yetParticipant @notdoneityet]

2 pole induction motors gleefully turn at 2800rpm with a clear safety margin on the mechanical side, so I would expect a 4 pole motor to likely emulate the higher speed motor on the mechanical side, if of similar construction. Most 4 pole motors have the same form for 50 or 60Hz operation, so I would consider 80Hz as only a 25% increase.

As I’ve said before (somewhere), motors with plain bearings may well be over-speeded but those with rolling bearings would likely cope. I don’t exceed 80Hz – as long as the rest of the machine is still within maximum design spec.

That could up my lathe spindle to 2800rpm. The lathe was made with a 2500rpm design spindle speed for some versions, but did not run gears for screw cutting, so I don’t go there! Those who over-speed their motors need to consider the machine; those who run more slowly need to consider the motor

[SteviegtrParticipant @steviegtr]

Too many people reading too many books. Not many that have actually worked on these. All theory & no experience. I Spent many weeks just setting up a control circuits to control the speed of motors on machines like. Dosing pumps for Brazilian mint. (What you may say). This mint costs £3500 a barrel & one touch on your tongue would burn a hole in it. You may have guessed but it is one of the ingredients in a after 8 mint. I designed & built the panels that did the dosing. A while ago Nestle decided to make a Baileys after 8 mint & asked me to design a dosing system for that too. It was very delicate as the mint was not meant to contain much alcohol. Also a orange version. We also designed the control of dosing for the various Kit Kat brands. All I said at the beginning of my posts was that a motor driven by a 240v inverter would have to be connected in Delta as you already know. Also that a motor connected to a single phase inverter that is 3 phase, would not have a high starting torque. We proved this many times by R&D work. Some of you think this is bull. It is not it is fact. A 3 phase motor with 240v windings which it must have to work on single phase will not have the same torque as a 3 phase motor connected to a 3phase supplied inverter. I give up. Going to move to a desert island with no roads or electricity & catch fish with a hook.

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