Thread: VSD for3 Phase Motor

Hi SurfinNev,
When you look at the Terminal Block of the Motor there should be 6 Terminals, and three pieces of metal the connect two each of the terminals. if this is so then wiring the Motor in a Delta Configeration will be easy.
Using a VFD to take your single phase in and give you three phase out for motor is a good way to control the motor, there are terminal blocks in the VSD where you can take a Forward and Reverse connections out to Push Buttons on Mill. You can also use a 10k Pot to adjust speed.
Threre are a lot of VSD Manufacturers so shop arround for the best value for you.
If you are not confident in wiring up the Motor to the VFD get an Industrial Electrician to do it for you as you dont want to blowup the Drive.

Regards,
Keith.

Nev
Do you want the vfd to control motor speed as well as convert from single to 3 phase? If so you might have to fit a bigger motor. Apparently you have to down rate the motor if you want to use it as a variable speed. So in order to get the same sort of power from the machine you have to install a bigger motor. Hope that makes sense.
If you only want to run the mill using the belts for speed change then you may as well just fit a single phase motor. The motor you have is only small and you would probably find one from something else that will do the job. ie and old washing machine motor or maybe from a treadmill. Some of these have variable speed DC motors and power supply. You would probably get a new single phase motor for less then the price of the the VFD you are looking at.
I discovered last week there is an Australian company called CMG who make electric motors. ( I bought one of theirs from a seller on ebay last week)
Anyway when I did a search for them I found out that they make a motor with a self contained VFD built in. it has a speed control on the side of the motor and you just plug it in to the 240V and away you go.
have a look the link below. Just follow the links to the motors page and then look at "speed master" I have no idea what the cost of one of these would be.

http://www.cmggroup.com.au/


Re VFD's, phase changers etc.
I'm not fully up on it myself as I don't have an electrical background. There is lots of info here
http://www.practicalmachinist.com/vb...sprune=-1&f=11


hope this helps

regards bollie7

Ha re and forbes sell them , they have a 1 hp, and 2hp,
https://www.machineryhouse.com.au/Pr...stockCode=E120

bollie7

Do you want the vfd to control motor speed as well as convert from single to 3 phase? If so you might have to fit a bigger motor. Apparently you have to down rate the motor if you want to use it as a variable speed.

I'm not sure you are quite correct, most of the smaller VSD's vary the frequency to achieve control, hence VFD, and as I understand it (from the limited reading I have been doing since I bought a 3 phase machine) you will only lose torque at fairly low speeds of 30% and less. When running at 50hz there would be no difference and with most small VFD's you can program the to exceed 50hz so you can increase the speed above that of the simple 3 phase connected motor.

Craig.

If you go for a VFD that has Vector Control you can keep the Torque up down do relatively low speeds.
You dont need to go for a different motor as there would be pulleys to change speed ranges, when you get to a situation where the speed would be too low for the VFD to operate efficiantly, change the pully belt speed then you can range the speed with the VFD.

Regards,
Keith.

Originally Posted by CraigMBeckett

bollie7


I'm not sure you are quite correct, most of the smaller VSD's vary the frequency to achieve control, hence VFD, and as I understand it (from the limited reading I have been doing since I bought a 3 phase machine) you will only lose torque at fairly low speeds of 30% and less. When running at 50hz there would be no difference and with most small VFD's you can program the to exceed 50hz so you can increase the speed above that of the simple 3 phase connected motor.

Craig.

Craig
I don't know enough about electrics to argue .(Its a bit of a black art for me. I do know letting the smoke out doesn't do them any good.) I just remember reading somewhere about having to increase the size of the motor to get the same power when running at lower speeds using a VFD. The other thing was that if you run a motor at a lower frequency thus a lower speed the shaft driven fan doesn't work too well in cooling the motor. easily fixed by mounting a seperate electric fan to the motor.
I was just looking through my favourites and found this link that I had saved.
http://ecmweb.com/mag/electric_matin...ble/index.html
Lots of info there, but gets into the theory a bit.

regards
bollie7

Hi bollie7,

As I said my info came from limited reading, but as the majority of reports I have read only talk of problems at low rpm and no noticable loss in power/torque until at approx 30% speed and less I don't believe ther would be much of a problem with using a VFD. I have read the article you gave a short cut to and maybe the author is stating the worst case scenarios. As I understand it a 3 phase motor provides better torque than its single pahse equivalent, and as you can generally buy 3 phase motor for considerably less than an equivalent single phase I don't see that you have a lot to lose in going for a VFD. Indeed you have a lot to gain in terms of dynamic brreaking etc.

Craig.

Thanks for the replies everyone. I will sort this out eventually with your help and a lot of other research. I guess there is no simple yes or no when it comes to a question like this. I'm not in a hurry as I plan to rebuild the machine first. BTW Some of the bearings for this are not cheap.

Nev

Hi Nev, the reason you derate the drive is because with a single to three phase drive you are using line voltage of 240Volts, usually a three phase motor uses 415Volts as the line voltage, since you are supplying a single phase input it will never go above that for the line voltage.

If you had a three phase input VFD it would not be derated.

Having said that I put a small VFD on a little lathe that I CNC'd for fun and it was very nice, you get good power over a large PRM range. The torque does drop off at low speeds but these speeds are usually too slow to do anything with.

The motor I used was a 0.55 KW WEG and it cost about $350.00 from memory brand new so if you need a larger motor it isn't the end of the world.

As mentioned you need to run it in Delta as in star it will simply not have a high enough voltage and the motor will not perform very well and probably burn out.

Good luck.

Russell.

SurfinNev,
There has been a lot comment here on VFD, most is correct, I work for a large Electrical Company where Motor Control is a major part of the business. We sell VSD's as well as other methods of motor control for all types of industry.
I suggested Vector Type Control, its the best for the type of power speed you want, I also mentioned that you would have pully arrangements that would put you into certain speed ranges where the Vector Control would work best.
What was said about Single to Three phase is true against Three Phase to Three Phase, if you are to run the Motor at low speeds for long times and had high torque it will get hot and there would be deteriation of the wiring laminations. Using an external fan will help in keeping the motor cool, use a large enclosed type fan like the ones used for cooling enclosures, also remember if you are putting VFD in enclosure that it needs a lot of air arround it as they generate a lot of heat. You could place a contactor line side of VSD with a thermal overload so if Motor is getting too hot it will trip overload turning the contactor off. If you are going to enclose the VFD I can give you recomended enclosure sizes.
It is not hard what you want to do as many others have done this, just needs some thought of how you combine the Motor Control parts and what will be the end speed/torque limitations.

Regards,
Keith.

Your choices then are

1. replace existing motor with single phase. IF you can get a suitable size motor in the correct frame size you get a simple solution. Fixed motor speed, but you have the pulley system for speed selection.

2. Use existing motor with the VFD. What hasn't been said so far:

a. You use single phase input at 240v to get three phase 240v output. Thus, your motor must be capable of being connected in delta (It has to be rated 415v Star in order to be connected 240v delta. A motor wired 415 delta can't be used with a 240v VFD. The connections in the motor terminals has already been described.

b. The motor does not have to be replaced. You simply use it the way you would normally, but with a potentiometer you can get +/- (say) 50% motor speed so you can get lazy with belt changes or fine tune spindle speeds between the belt steps. If you plan to run it at less than 50 Hz for long you will need auxiliary motor cooling...a fan ducted to the motor bell housing will do. This is necessary because the motor's internal fan's speed is reduced proportional to the motor speed just when the motor is making lots of heat.

c. If you had a machine with no mechanical speed reduction (belts or gears) you WOULD need a large motor to deliver enough power at low speed*

d. Early VFD's caused a torque loss at low speed. Newer designs don't. Torque is not the same as power. Horsepower is torque X rpm. Thus low speed by definition gives lower power, even with constant torque. Power is what removes metal.

e. early VFD's allowed for single phase input as long as the drive was derated by about a third. Most modern drives in the 5hp and under size are rated for single phase input so no derating is needed.

f. Not asked, but the only way to get 415v three phase from a 240v single phase input is to run the 240v three phase output into a 240v-415v transformer. Not a factor in your installation but this question comes up from time to time.

g. a 1/2 hp motor like yours takes a very inexpensive VFD (compared to the other options). The VFD allows you the variable speed, allows you to mount emergency stop buttons and incorporate the machine's original fwd/rev and on/off switching.

Have fun
Greg

This is most interesting topic, thanks everyone for the input.

To clarify something. I have a 'modern' 1hp 415v star connected motor ( 6 way terminal block)
I also have a 3 phase supply.( workshop wired for 3 phase)
If I want to connect it to a VFD, ( only need speed control, so torque etc not an issue)

1.do I wire it in delta?
2. buy a single phase to 3phase VFD
3. buy a 3 phase to 3 phase VFD

Which is the cheapest option?

Is there any problems with running it of a VFD that is made for a larger motor ( eg 2.2kw etc) then I could use the one drive for different motor in the future?

Any help would be much appreciated.

In that case, assuming you want to use the existing three phase to power the vfd, get a locally sourced 415v vfd. No phase changing required, you just gain the speed variability. Leave the motor connections as present (415v Y)

OR

Get a 240v vfd, use single phase input, connect the motor in delta.

In either case torque is still a factor, so a constant torque drive would be better than the earlier V/Hz type.

Good luck

Gregoryq thanks for the quick response and info.
Do you know if a larger VFD unit would be OK?? ( future proof for other machine etc)
the particular motor is driving through a reduction box to get down to 1rpm (purely to drive slow) so its not like the motor will ever get overloaded past its 1hp rating. ( I just happen to have the motor and its size fits etc)

Do you happen to know what unit would be cheaper 1-3phase or 3-3 phase ?