VFD Question

Wonder if this helps.

The diagram is typical of a UK sub-station transformer wired to supply a mix of single and three-phase consumers.

Electricity from the power station arrives at high voltage on 3 wires and is connected to the input side of a 3-phase transformer wired in delta. Using 3 wires to transfer power at high voltage reduces resistive losses and hence the size of the wires needed. Unfortunately, 3-phase at high voltage is dangerous and inconvenient for most domestic purposes: imagine the circuit needed to switch a 3-phase light bulb on and off! On the other hand, industry needs 3-phase for big motors, electric furnaces and other heavy equipment.

Fortunately, both single and three phase can be produced by wiring the output side of the transformer in star (aka 'Y', and making the centre point made available as 'neutral'. Note the consumer side of the transformer has 4-wires, plus - not shown - some kind of earth safety as well.

The voltage between Lines is 440v 3-phase. The voltage between any Line and Neutral is 230V single phase. In a factory, the system might be used to supply 3-phase to equipment and also single phase to domestic services like lighting. Care is taken to ensure 'Live' from different phases is kept well apart in the same building.

Houses are connected in groups to different phases of the same supply. Be careful borrowing power from a neighbour - there's a slight risk his house might be on a different phase! Trying to buy a 3-phase supply for a home workshop meets with mixed results because the convenient availability of all three phases depends entirely on local details. My village has 3-phases and neutral on overhead wiring in the street outside: easy! Not everyone is so lucky - in suburbia, they might to dig a long trench back to the substation.

Has to be said 3-phase electricity is the work of geniuses! Not obvious how it works, or that it neatly solves several different awkward supply problems. For example the system is self-balancing, ie able to cope with one phase being lightly loaded whilst the other two are working flat-out. Clever stuff squared!

Dave

I am using a VFD with a 1hp motor taking advantage of the speed control. I have set the frequency to vary between 25 and 75 Hz. I feel that going lower than 25 Hz is not suitable for my requirements, because at that frequency, the motor speed is halved from the standard 50 Hz, but the power is also halved. Below 25 Hz the drop off of power is rapid.

I regularly use the VFD on my mill at 10 Hz and below for power tapping. Though the power may be low there's plenty of torque. In fact you can drive an induction motor at very low speed or even zero speed and get full torque using a vector drive. Power not necessarily a good indication of usefulness.

I regularly use the VFD on my mill at 10 Hz and below for power tapping. Though the power may be low there's plenty of torque. In fact you can drive an induction motor at very low speed or even zero speed and get full torque using a vector drive. Power not necessarily a good indication of usefulness.

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

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.....

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.

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.

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

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.

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.

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

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🙂.

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...

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

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.

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

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

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.