VFD Question

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

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

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

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.

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.

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.

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

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

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.

a motor driven by a 240v inverter would have to be connected in Delta as you already know. (my underlining)

Tell us again, ‘cos I’m not hearing you!🙂. Fact - My mill is currently running a 240 volt output VFD into a star connected motor. If there is something wrong with it, I’ve not noticed it yet. Works far better than the one horse single phase motor I replaced (that motor would not start the machine in top gear from cold).

I believe another motor on the mill is also connected star and is supplied from another 240V VFD. Not checked that one, but it, too, works fine for the variable speed duty required.

Steviegtr

so just to be clear

a 750watt motor running delta connected to a single phase VFD outputting 240v

and a 750 watt motor running star to a VFD outputting 415v but fed from 415 3 phase

which one has more starting torque? And why?

And Like NDIY i have run many 415v star connected motors of a single phase VFD outputting 240v with no apparent detriment

This is a good explanation and the technique I use on my Chipmaster with a 415V star connected motor with a 240V VFD
**LINK**

Thanks Andrew for all the time you are devoting to my question

I got the information from this page As this is from the supplier I felt I should follow their directions

Thanks

Gerard

Ah, that's the page MichaelG linked to earlier in this thread. I think it's also cropped up on the forum in the past. I didn't understand it then, and I don't understand it now. Suppliers are no better than anyone else; there are those who know what they're doing and those that don't.

Andrew

Thanks Andrew,

It was my reason for posting the question here

regards

Gerard

As mentioned above I use the technique highlighted in the article by Colin@Inverter Drive Supermarket.
I don't pretend to fully understand it either but it works! They are a very helpful supplier.
The 29Hz is not the maximum you can set, but will be the value up to where you get full torque with 240V. Above this frequency the motor still runs, but with reduced power. I run mine up to 50Hz and the reduction in power is not significant for hobby use. More significant I beleive is getting the current setting correct.

Edited By Stuart Bridger on 10/01/2020 16:36:01

This is the motor I bought for the Tom Senior light vertical R8 conversion, it is run in delta and through a Schneider VFD from the domestic mains. The spec gives the expected speeds, torques at different frequencies. The machine runs between 25 and 75 Hz.

**LINK**

Edited By old mart on 10/01/2020 17:02:37

Andrew - you posed a question earlier about when a star-delta starter was appropriate.

I don't remember where I got it from, but my experience is to choose star-delta starting at 7.5 Kw (yes, it's a capital K) or above.

On 400 volts a typical 7.5 Kw motor pulls 14amps full load, so starting current DOL is about 7 times this ie knocking 100 amps. This might be why this is the decider point, as most of the factories I worked in had a 100 amp supply.

If we use a star delta starter the inrush is about three times full load current, so that makes the initial inrush about 42 amps. Second stage inrush is about 2 times full load current so is less arduous.

HTH Simon

I remember occasionally starting up the compressor at the plating works back in the 60's. It was manual start, and it had a lever which you pulled up to get the thing rotating at about 200 rpm and then down quickly before it stalled. It would run normally after that. I was told that it started in star, then went to delta.

Picking up on the point about the maximum frequency that a nominal 50 Hz motor can be driven I've outline my experience with my lathe.

I have a Chipmaster lathe which I've modified by removing the variator but have retained the original 3kW 4 pole 1428 RPM motor which is configured in Delta for 240V working. I drive the motor from a 3 phase 7kW VFD which I operate from a 240v single phase supply. The VFD is programmed to have a max output frequency of 100Hz that spins the motor at 2800rpm and I have fitted a motor pulley to gives a spindle speed of 2200rpm. This works fine and produces sufficient torque at low speed for most of the turning I do whilst providing adequate higher speeds.

However the Chipmaster is capable of running its spindle at 3000 rpm so to try and get this I re-programmed the VFD max frequency to 150Hz but still couldn't achieve much more than 2500rpm which I was due to the motor's back emf and the supply voltage.

I next tried a 6 pole 980 rpm 3kW motor, fitted a bigger pulley but still couldn't get this motor to run at much more than twice its design speed confirming that the back emf appears to be the issue.

Therefore to get the higher speed I either have to drive a delta connected 240V motor from a 440V VFD or find a 110v motor and drive it from a 240V motor.

Clive