48v ac inverters????

I have a 3kw 48v motor fed by a 415/48 transformer.
Is there anyway I can control the speed.
Have tried conventional inverter brands but with no joy.
Any ideas.

Andy

I assume for some reason you dont want to change the motor..ie spec a drive and motor to fit the application.

You may want to look at variable speed reducers...ie gearboxes/transmissions. They make them these days with electrical controls but overall variable speed reducers arent as common these days because of Inverters etc. Even if you had to use a manual style reducer you could use an actuator to change it, might be alot of work involved with doing all this but you do what you have to do.

AndEdTec,

I know with all of the Yaskawa drives (Magnetek, Yaskawa, Omron, Safetronics) you can set the max output voltage for the drive. ie. you can feed a drive with 460 volts and the output can be 208, I am not sure if how low you can go on the output of these drives, but I would look at any of these units to start. You may be able to go as low as 48 volts max output, but I am not sure.

Mike

I do not know of that feature in any brands I deal with. Could you show or state a specific model (like Magnatek/Yaskawa) that has an online manual to show that feature?

All the drives I have dealt with only offer 2 choices ..230 or 460 and that is a fixed setting for some depending on the model.
Example: Magnatek GPD 315 they make a 230 volt model and a 460 volt model with variations dependent on HP/Amps. The voltage is set at Parameter n012 at either 460 or 230

Anyway I would appreciate seeing models that offer that versatility.

The Cutler Hammer SV-9000 VFD, for example, specifies that output voltage can be anywhere from 0 to the VFD input volage. I would suggest that you talk to your VFD suppliers application engineers before going to that low (48 VAC) a voltage, and obviously consideeration will have to be given in supplying the VFD to make sure the amperage capacity of the drive is OK for the required load at that output voltage.

Another possibility is to us a step down transformer between the VFD and the motor. Again, because of the transients and non-uniform sine wave output, application assistance should be sollicited, and you may need output line reactors or isolation transformers to keep the transformer temperature in limits.

I can't change the supply because of the wet conditions it operates in.

Althogh it would be so much easier if I could.

My problem is the motors drives a gearbox but not like anything you would normally associate with a gearbox , I can't make any changes to this gearbox because it is made of two castings with gears inside.

I have a feeling that you can't get a 48vac inverter and bearing in mind the the current on full load is around 40 amps.

My knowlege on AC theory isn't really that good but all I need is two speeds normal and very slow for positioning.

Maybe there is something something I could design to help me achieve this.???????

The specs you stated show 3KW (3000 watts)/48v=62.5 amps so full load at 40 isnt pushing it that hard.

It may be possible to use a plain old Variac in this application, they make them in manual and motorized versions
http://www.elect-spec.com/variacy.htm

Or you can do a search on the net for VARIAC..a variable AC Transformer. These allow full current while varying the AC. Of course his is normally applied in single phase applications.

The other thing you didnt mention was full details about the motor, is this 48volt AC? Single Phase? 3 Phase? Is the transformer being used to step it down to 48v just that or is it possibly converting to DC? If DC then you have a multitude of options.

Last but not least, have you talked to the manufacturer of the Motor/gearbox for assistance?

rsdoran,

I do not know of that feature in any brands I deal with. Could you show or state a specific model (like Magnatek/Yaskawa) that has an online manual to show that feature?

You can use all of the Yaskawa drives are the same, be it Magnetek (which is no more), Omron or safetronics. Go to www.OIDM.com web site, this is for the Omron drives, you can look at the G5+ (this is equal to the Magnetek 515) or the P5+ (this is equal to the 506) units. On the G5+ look at parameters E1-05 Maximum voltage (.01 to 255volts for 230v units or .1 to 510 volts for the 460volt units), and E1-04 through E1-10 these deal with maximum, mid and minimum values for frequency and voltage. If I am not mistaken the smaller units, which are equal to the GPD305 and GPD315, have the same feature.

Mike

Andy,

How about use an ordinary 415V inverter on the 415V side of the transformer. You would have to make sure it was a sinusoidal current type inverter, but most are these days. If you have problems with harmonics through the transformer, you may have to turn the carrier frequency right up, but as long as your transformer isn't too heavyly loaded it should be able to handle it.

Of course I would reccomend a second opinion from one of your local experts. But I can't see any reason I wouldn't work.

Doug

Some earlier generation inverters were capable of more than 300 Hz. freq. output at 240 volts. You would have to size the unit in Hp.that would provide the necessary Ampacity,( 40 amps or more ). My guess is that it would be rated about five times as much HP.Then there was an adjustment for volts per Hertz that could probably be set for 48 volts at 60 hertz output.
I hope this helps since I don't know a specific brand or model using today's currently available units.

Sid

Big Mistake looming!

OOOOOhhh! Be careful guys. It is true that most modern inverters have a maximum output voltage setting (it may be disguised as motor nameplate voltage, field weakening voltage, etc. etc.) but it isn't what is seems to be.

Remember that all inverters convert the incoming AC into DC. This results in a DC bus voltage of about 325 for 230VAC and 650 for 460VAC. 415VAC results in something proportional in between.

The output transistors do nothing but chop this DC voltage into pulses of varying width (PWM) depending on the inductance of the motor to integrate these pulses back to something resembling a current sine wave.

If you connect, for example, a 460V rated drive to a 460VAC line and then set the software to limit the output to 230VAC, you will still get pulses 650VDC high. The voltage limit will simply keep the pulse width from getting as wide as when 460VAC max. output is selected. Your "230V" motor better have insulation good enough for 650VDC pulses or the smoke will leak out in a big hurry.

Now let's take the 48VAC motor (I'm assuming its three phase) and connect it to the output of a 230VAC drive with the software limiting the output to 48VAC. That poor 48VAC motor will be seeing DC pulses in the range of 325V and the original poster's joy is going to go away again, unfortunately.

In summary, the output voltage limits on inverters are suitable for narrow ranges of voltages (say 480, 460, 440, and maybe even 415 on a 460VAC fed drive) but not any wider.

Actually, I have to wonder if the 48V motor in question isn't DC or single phase AC. Worth double-checking before this goes much further.

Thanks for all your views very interesting, it has made me think a little more about my application.

The motor is 48/85 star/delta.

I have taken some readings and the current drawn is actually closer to 60, but that is it is fully loaded.

I'm looking into the idea of a motorized variac.

The motor is an SEW Eurodrive so I could check to see if the motor could stand the effect of an inverter.

AndEdtec,

SEW Eurodrive also makes VFD's, why not check with them to see if they have a drive which will work on their motor?

Mike

AndEdtec,

Forget the Variac! To change the speed of the motor, the frequency and the voltage must be changed. The frequency is what directly controls the speed. The variac does not change the frequency.

Has anyone any experience connecting a transformer to a VFD output and a motor to the secondary of the transformer. If this can be done, the present transformer could probably be used with a 415 volt VFD. Any comments?

Using a transformer between a VFD and motor can be done but its a dicey business. Operation at low frequencies ( low speed) is not possible and speed regulation under changing motor loads will not be very good. On the other hand, operation withing +/-20% of the transformer design frequency is usually possible.

Since the transformer is now bearing the brunt of the PWM pulse integration, you can expect the transformer to operate warmer than normal at all loads--probably too warm at full load unless the transformer is oversized.

Of course, none of this makes any sense if the motor is not three-phase AC.

And, if the motor is single phase AC, there are some motor designs that will change speed with variable voltage such as that from a variac. I don't know much about these types of motors but they clearly would be asynchronous and, I think, would have a commutator like a DC motor. Don't ask me why! I've got my hands full with three phase!