Thread: Another VFD & 3 phase question

Simon,
Re the delta/star winding, current passes through all 3 windings in both configurations. If the motor is wound for 415V in star it will have the same output if connected to 240V in delta. You can run a 415V star wound 3 phase motor on 240V, but with reduced power - the VFD parameters can be fiddled to improve the situation a bit and sometimes that will be enough for the home workshop situation.
Bill

Originally Posted by WCD

Simon,
Re the delta/star winding, current passes through all 3 windings in both configurations.
Bill

Thanks Bill,

I was under the impression (with my basic electrical knowledge) that in Delta, the current from each phase only passes through one winding before exiting the motor into the adjacent phase as return current?

Thanks for your reply!

Simon

Bill is correct in saying that the current goes through all 3 windings in both cases, but I think you are asking why you need to wire the VFD to the delta connected motor, and not a star connected motor, and if that is what you mean, the answer is that the VFD generates a synthetic 3 phase, but( assuming 240V input to the VFD), it is 240V between phases not 415V , as is the norm with mains reticulated 3 phase. So a dual voltage motor which is 415V in Star will be 240V in delta, and both configurations will give a full power output. If you had a motor wound for 240V in star configuration, you could connect your VFD to it and have the motor develop full power, providing the ratings of motor and VFD were a match.
Further to your question, as to which way would be best, the cheaper VFDs on eBay, will not give you anywhere near the speed range you require, while maintaining adequate torque, unless you use mechanical speed reduction as well. You are talking a 50:1 speed range here, and the cheaper VFDs will have a range of more like 10:1, and even Vector drives would struggle I think for the range you want, especially when you factor in the need for maximum torque at the slowest speeds rather than the highest speeds. The D.C. option looks much better, with maximum torque developed at 0 r.p.m., but as you say with more mechanical complexity and motor maintainence, although with the motor you have, that might not be much of an issue. If you dont mind some speed changing by changing belts and pulleys, you could reduce the number of times you need to change your belts and pulleys over, by using electronic speed control with a VFD, but not eliminate it altogether.
Rob.
On reflection, and after re-reading your post Simon, I thought that you meant that only one winding was used in delta, and 2 windings in star. I see now, or at least I think I see, that you are referring to an instant in time, and not overall. I'm having trouble in expressing myself here, but if I understand your question correctly, you are correct in that in star configuration, in any one instant the current passes through 2 windings from phase to phase via the neutral point, while in delta, the current passes through one winding from one phase to another. Stuart did a much better job explaining it below,
Rob

Originally Posted by WCD

Re the delta/star winding, current passes through all 3 windings in both configurations.

While current passes through all 3 windings, wouldnt it be true to say that in star is must travel though two windings and in delta only one?

Hi Simon,
Remember torque isnt power, 2hp is 2hp no matter what you do with the gearing. What speed was you motor designed for? assuming its 1440 and assuming torque was the same regardless of rpm range(which it isnt) running at 144rpm to get your spindle to 72rpm you would have .2hp.

Stuart

Hi Bill, Ropetangler and Stuart,

Thanks for the responses, they answer most of my questions. I was hoping to bypass all the gears in the head and have a single fixed ratio with a belt pulley set up to reduce noise and simplicity. I have seen these mods done on youtube and other forums. But I guess they must not have the low down RPM that I wish to retain. Like you said Stuart, 100 -2500 RPM is a large range with a fixed drive ratio. Perhaps I could get away with just using the H/L gear changer....

Stuart, the motor I plan on using does not have one single specified speed or frequency as it has been designed from the get go as a variable speed 3 phase motor. It has spent the good part of 10 years of its life in a treadmill in a commercial gym. This is another reason for wanting to use this motor as it is obviously designed and happy run all day everyday at a reduced speed without any cooling issues. The motor itself nor the treadmill had any special cooling system.

The plate states to use it only with the dedicated emerson controller but I have a copy of the circuit diagram and it clearly shows the motor as a 240V 3 phase so I would be very disappointed if I could not get it to work on a VFD. In any case for $150 I'm willing to have a crack!

Cheers,

Simon



2012-09-19_12-33-06_216.jpg

Originally Posted by Stustoys

While current passes through all 3 windings, wouldnt it be true to say that in star is must travel though two windings and in delta only one?

Hi Stuart,
In a basic three phase induction motor - star or delta configuration - there are three windings and current does indeed pass through all three, with each winding contributing to the output power.
Bill

Originally Posted by WCD

Hi Stuart,
In a basic three phase induction motor - star or delta configuration - there are three windings and current does indeed pass through all three, with each winding contributing to the output power.
Bill

Hi Bill,
Yes, in operation. I thought Simon was asking more about how it was wired??

How about. In star you have two windings in series across each pair of phases, in delta you have one winding across each pair of phases?

Hi Simon,
For all I know that motor maybe 240V star? either way as it is already wired for 240V it wont need to be changed from star to delta if thats what you were thinking?

As always I could be wrong

Stuart

Hi Bill,

We are all correct but I think there maybe some confusion about what is being said. If we were to label an individual electron and watch it's path of travel, we would see in star configuration that it enters the motor, passes through one winding then it is at the middle of the "star" it then passes through another winding and emerges at one of the other connections. In delta mode, the same electron would pass from one connection, through a single winding before emerging to another connection, hence only passing through one winding as opposed to two in star mode. I'm assuming that in Delta mode, the equivalent impedance would be half that of star and this is why it requires roughly half the voltage ie 220 instead of 415 for the same 3 phases.

Cheers,

Simon

Your question DC drive vs. VFD drive:

- Well made DC drives in the 2-3HP range can easily cover a constant torque speed range of 20:1. A modern VFD in the 2-3HP range can barely cover a constant torque speed range of between 4:1 and maybe 6:1. Therefore, in theory a well made DC drive would be much better suited to drive a machine tool.
- The problem is cost; an industrial quality 2HP permanent magnet brushed DC motor (like a Baldor) costs new about $1,500 (that is only the motor, the controller is extra). A good quality TEFC 3-pase motor in comparison can be bought for a fraction of that cost. And this is one reason, why most machine tools in the past 15 years have been using AC and not DC drives.
- A VFD drive can be easily made to cover a larger constant torque speed range, by simply choosing it one or several sizes larger. Note that VFD controllers have become a cheap commodity with dozens of makers, whereas DC controllers with comparable features are nowdays rather rare and expensive.
- In the context of upgrading an existing lathe or mill to variable speed, the VFD wins. Particularly if you keep it low cost by reusing existing step pulleys or gearboxes to preselect speed/torque. Upgrading to variable speed does NOT mean to do away with speed changes!

Your mention of Treadmill motors:
- a treadmill motor is usually not rated for continuous duty. A 2HP treadmill DC motor with typical 60% duty cycle is only comparable to a 3/4HP industrial quality DC motor. Very much like the overstated motor ratings for things like vacuum cleaners or small air compressors.
- a treadmill motor has its power rated at 4,000 to 6,000rpm, whereas an industrial quality DC motor has its power rated at 1,700 or 3,000rpm. High speed treadmill motors are more useful for vacuum cleaners than for machine tools. And the lower speed treadmill motors often used to drive machine tools are very much sought after and therefore hard to get and more expensive.
- many treadmill motors have offset timing, and/or brushes set at an angle to the commutator, making them unidirectional, not suitable to run in reverse.
- most treadmill motors have an open frame, meaning the fan is blowing metal dust, small chips and coolant onto the open collector. The sparks of an open collector can also be a fire risk if a volatile/flammable coolant is used and the motor is installed very close to the worpiece, like typically in a lathe.
- treadmill motors do not use standardised frame and shaft sizes, making mounting difficult (compared to 3-phase motor that are usually bought as a drop-in replacement for an existing motor).
- treadmill motors have only a short design life, about what you would expect from a household vacuum cleaner. Compared to an industrial DC motor which has almost the same life expectance as a 3-phase motor, or about 100.000 hours = 8 years of nonstop running.
- treadmill motor armatures are often not very well balanced, and being used at higher rpm can cause vibrations
- treadmill motors can be as noisy as a vacuum cleaner
- treadmill controllers are usually only very primitive single quadrant thyristor types. A single quadrant DC controller can only drive the motor in one direction, a two quadrant DC controller can drive and brake a motor in one direction, only an expensive four quadrant DC controller can drive and brake the motor in both directions like a cheap VFD controller can. Do not be fooled by using a reversing switch between a cheap single quadrant controller and the motor. Flipping this switch before the motor comes to a complete halt will instantly fry the controller. A good 4-quadrant DC controller with pwm modulation and back-emf detection will cost considerably more than the most sophisticated VFD of same power.
- treadmill motors used for hobby machine tools are very popular in the USA, because there is a thriving treadmill industry. Brand new treadmill motors and 110V controllers can be bought for real cheap from industry surplus outlets. In Australia treadmill motors are sourced from the tip, it is pure chance if one scavenges a low rpm model or a useless high rpm model.
- playing around with open treadmill speed controllers can be a hazard to people without some basic electrical background.

My recommendation:

* go for a VFD drive, you will not be disappointed. If you already own a VFD drive, I can almost guarantee that you will not be perfectly happy with a treadmill motor.

** But if for any reason you WANT to go with a DC drive, I highly recommend you search for a used industrial quality motor, like a Baldor brand, these sometimes pop up on Australian ebay, even new old stock ones that did once cost a fortune. Some are even fitted with a tachogenerator that with the right controller can be the ultimate machine tool drive - such a DC drive with tacho feedback can cover a 100:1 speed range.
And as a side note: many small hobby class bench lathes and mills made in China come with treadmill-like DC drives, think minilathe and the Sieg X3 mill. I own an X3 mill. I spend some time and effort to convert it to VFD. Needless to say I was not happy with what it came with.


Your question regarding star or delta wiring:

- in 220/240V countries it is normal practice to wire the motor to a single phase VFD in delta. This is so, because the 240VAC are rectified and filtered into a DC voltage of 240 * 1.41 = 338V. This is the DC bus voltage, which is then chopped in an H-bridge mosfet transistor module into an AC voltage of +/- 338V (= 676V peak to peak, or divided by 1.41 = 478V rms). 478VAC (less svitching losses) is very close to the 415V that a normal 3-pase motor is rated for when wired in delta. In Australia, a normal low voltage 3 phase motor is wired to either 3x240V in star or 3x415V in delta.

- In other countries like the the USA with different voltages supplied to homes, it is quite common to hook up a VFD either star or delta, depending on the particular situation.

Chris

Hi Chris,

Couple of questions

Originally Posted by cba_melbourne

Well made DC drives in the 2-3HP range can easily cover a constant torque speed range of 20:1.

Constant torque, wouldnt this mean if you had 2hp at full speed you had .1hp at low speed? Even if gear so 200% rpm is your max speed this still only gives you .2hp at low speed?

Originally Posted by cba_melbourne

is very close to the 415V that a normal 3-pase motor is rated for when wired in delta.

delta?

Originally Posted by cba_melbourne

a normal low voltage 3 phase motor is wired to either 3x240V in star or 3x415V in delta.

Dont you have that back to front?

Stuart

> Constant torque, wouldnt this mean if you had 2hp at full speed you had .1hp at low speed? Even if gear so 200% rpm is your max speed this still only gives you .2hp at low speed?

power = torque * rpm

constant torque means the power rises as the rpm rise. Like half the HP at half the rpm, and twice the HP at twice the rpm. For example, 1/2HP at 500rpm, 1HP at 1000rpm, 2HP at 2000rpm, 3HP at 4000rpm.

note VFD drives have a different characteristic. They are near constant torque below base frequency, but near constant power above base frequency. Constant power meaning the power does not rise, as the rpm rise. For example a 4-pole motor, 1/2HP at 500rpm, 1HP at 1000rpm, 2HP at 2000rpm, 2HP at 4000rpm.

>delta?
yes partially back to front. But you still wire a 240V single phase VFD to a stock 240/415 motor in delta.

Hi Chris,

Thanks for you're replies. I have had to read and re-read your first post in order to absorb it all. I am 90% sold on 3 phase and VFD but I can't shake the idea of DC at the moment as I have a 220V 3HP DC motor with continuous duty rating and like the idea of using that too. My main problem is finding a 220V DC speed controller rated to 12A. I can't find one, or I find find one for a price I would be happy to pay!

Here's the pic of the DC motor attached.

Also, If I was to purchase another 3 phase motor, what would be the prefered setup, 2 ,4 ,6 pole motor. I realise that they run at different RPM's but is it better to have a 2 pole running at 3000 rpm or perhaps a 4 pole at 1500 rpm? Is it safe to say that the 4 or 6 pole motor would give more torque if run at lower rpm with vfd but at the same time a 2 pole motor would be better at the other end of the rpm scale?

Cheers,

Simon

Ooops. Here's that DC motor I'm looking to use. If I can.

Simon
2012-09-16_15-34-36_188.jpg

Hello all,

OK. I'm about to pull the trigger on one of those popular VFD's from ebay. Looking at a 2.2Kw. I will most likely only need a 1.5Kw but I figure for an extra $20 I may as well buy a machine that will do the job easier than flat chat and besides, you never know!

Question is, there seem to be about 5 different players on the market, all about the same total price (including freight) and seem to be selling the exact same product. Am I missing something or it just a matter of looking at the seller with the best feedback ratings?
One seller bangs on about having Mitsubishi Japan chipset, do they all have that? Also, one thing I have noticed is that one product seems to have a pot on the machine front panel but the others don't. Am I splitting hairs here?

I probably won't buy it now until Monday. I could buy now over the weekend but it won't make any difference. So I will wait to see if anyone has anything to enlighten me on a decision one way or the other.

3HP 2.2KW PROFESSIONAL VARIABLE FREQUENCY DRIVE INVERTER 10A 220-250V VFD r3 | eBay
VARIABLE FREQUENCY DRIVE INVERTER VFD NEW 3HP 2.2KW 10A | eBay
PROFESSIONAL 2.2KW 10A 220-250V 3HP VARIABLE FREQUENCY DRIVE INVERTER VFD p3 | eBay
2.2KW 220V Variable Frequency Drive Inverter VFD for CNC Engraving Spindle motor | eBay
NEW 2.2KW 220V VARIABLE FREQUENCY DRIVE INVERTER VFD 3HP 10A CE | eBay

Thanks,

Simon