powering a 480v ac drive from 240v

[vinmann]

I have a 440v only 3ph 1hp motor as part of a variable speed gear reduction system. The motor is connected to an ac drive rated at 3 hp.

The problem is I do not have 440v or 3ph available. The question is would a voltage doubler circuit be able to power this motor? How much capacity is needed for 1 hp? I have a pair of 3900uf 450v caps.

http://files.engineering.com/getfile.aspx?folder=a4522588-d362-4e44-84d7-baef623dd3d9&file=paint.bmp

 

[itsmoked]

You are opposed to getting a transformer?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

You can easily get a transformer to convert your 240 V to 480 V. And small VFDs that can run on a single-phase input and provide three-phase outputs are available. But I'm not sure an existing VFD expecting three-phase input is going to work.

Standby for a better answer...

 

[itsmoked]

vinmann; You never actually say what power you have available...

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

He said it in the title of the thread

Don't use a voltage doubler, too prone to damage. Just use a simple transformer. If you have 3 phase available, no problem, just get a minimum 6kVA (standard sizes are 3kVA, then 6 and the 3 will be too small).

If you don't have 3 phase available, you need to check your VFD to see if it can take 1 phase input; most 480V drives cannot do so without de-rating by at least 50%, meaning you will need at least a 7-1/2HP drive. A few also have built-in phase loss protection that cannot be defeated, in which case they will not work at all.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[vinmann]

Sorry about the incomplete data. I have 240v single phase. The ac drive is an Emerson Prism 2950-8002 which I don't have a manual for. I read where some ac drives will allow single phase hook up but at 2/3 of output rating. In my application would the 3hp drive be de-rated enough to allow full output of the 1 hp motor with large enough capacitors in the doubler?

I'm not against using a transformer. I just don't have a big enough one. I have several .5 and a 1KVA. I may use one to test the drive to see if it will allow single phase.

I saw a 1hp 3ph 240v drive drive with a 120v input and the thing wasn't very large. I figured this is how they did it.

Thanks for the response.
Vin

 

[itsmoked]

Ahem.. See Jeff!

Single or 3ph.

Now we know.

Vin; Your correct there are a lot of small 120V 1ph units around. It's your 460V that's a problem.

Hey can your motor be rewired to 240V? A lot can be.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

You should not automatically assume that the 3 hp VFD is oversized for the 1 hp motor. It may be, or there could be high torque requirements for the motor that require a larger drive.

Remember a VFD is a current output device. The horsepower ratings of VFD are for convenience and are secondary to the current rating.

There may be a good reason a 3 hp VFD was applied, especially if the VFD and motor were provided by a single vendor as a package system.

 

[vinmann]

The motor has just three wires coming out of it and the plate has just one number for voltage and that is 440. The motor looks to be a proprietary part of reduction unit. I do not believe a replacement motor can easily be found.

This unit drives the unload auger on a 30+ year old continuous flow grain dryer also known as a batch dryer. The output is varied to bring the grain temperature up. The dryer was bought (without the fan)and installed not knowing the motor could not be wired for 240v. The ac drive is a planned add-on by me to convert single phase to three phase and to give the system some automation. The selection is what was found quick and cheap.

Vin

 

[jraef]

If the motor is 30+ years old and not easily replaceable, I for one would not apply a 440V VFD to it, it will most likely accelerate the motor's demise. Motor windings more than 10 years old were never designed for the rigors of VFD use. You can add filtering to the output of the VFD to help prolong the inevitable, but the added cost would likely outweigh the other options. A better choice for that task would be a rotary phase converter, or get a good motor dealer involved in finding you a 1 phase motor that will fit.

I would also be concerned for the viability of that VFD. Emerson divested themselves of the group that made that drive over 15 years ago, so even if it is still working, it is likely on it's last legs. If it has been un-powered for more than a year, you will need to go through a special power up routine to ensure the capacitors have not dried out or you will fry the drive.

FYI:
The "official" formula for 1 phase to 3 phase current conversion is to multiply the 3 phase current by the square root of 3, 1.732 to get the 1 phase input current. So assuming your 1HP 440V motor is going to draw 1.4A FLC, the VFD current rating will need to be a minimum of 2.42A. However, the secondary problem with 1 to 3 phase conversion in a VFD is ripple on the DC after the rectifier. You need considerably more capacitance on the DC bus to smooth it out or you will toast the transistors in short order. So that is why the "rule of thumb" is actually 2X the HP size. In your case, a 3HP VFD should be fine for a 1HP motor, even 1.5HP, but no larger. That is, IF your VFD will allow you to apply 1 phase power.

As to the voltage doubler issue;
Yes, you have likely seen many VFDs for 230V motors that can accept 120V input, and you are correct that they use a Voltage Doubler on the front end. But that does not easily translate to 480V applications. For a 240V motor and drive, the voltage doubler has to boost the 170VDC from the 120VAC rectifier to 340VDC; not too much of a problem for components. But for a 440V motor, the voltage into the VFD needs to be at least 622VDC and if you start with 240VAC input, it is almost 680VDC; problematic for the components and wiring you will need to build the doubler and very prone to catastrophic failure.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

So now we have the information.

I'd get a transformer/s. 240 to 460V

A 2x sized VFD.
To deal with the 1->3ph

And an output reactor.
To give the motor the best chance of life.

Mount the VFD physically close to the motor.
To reduce voltage peaks to the motor.


Alternatively use a fixed speed setup and modulate the heat input.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[vinmann]

Thank jraef,
Thats what I was looking for. I don't like the sound of 680 at least the catastrophic sound of it.

Should I look for a transformer that has multiple taps that allow a lower output like 440 or less to improve longevity of the motor?
If I stay under full load amps should I decrease the ratio and decrease the motor speed for the same output speed to improve longevity?
Should I add capacity to the bus if I have the ability to do so?

Vin

 

[itsmoked]

decrease the motor speed for the same output speed

?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[vinmann]

This unit is a mechanically adjustable variable speed gear reduction. It has a belt and adjustable pullies. I was thinking I could slow the motor with the drive and adjust the speed back up with the mechanical adjustment if that is easier on the motor. Perhaps speeding up the motor and slowing the mechanical adjustment would be better for the motor.

 

[waross]

It will probably be a lot cheaper and more dependable to throw the motor and reduction gear away and buy a new gear motor to connect to the auger shaft. A grain auger drive is not rocket science! Get a 240 volt motor and a VFD sized to run it from single phase power.
Or, you could use the old motor with the old drive and see which fails first. After the first failure is repaired, wait for the other component to fail. Remember that one of the constrictions you gave us was the difficulty of replacing the motor. Bite the bullet and do it right in the first place. I understand the lure of making something work by using the parts available, but you need some fairly expensive parts that are not on hand. Rather than buying expensive transformers for old equipment and then trying to cope with a burned out motor for which a replacement cannot be found, do the mechanical work now save the worries of the future.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

If you want to fully consider the drive sizing from 3-phase to 1-phase then knowing that the 3-phase drive has a 6-pulse rectifier and the 1-phase drive has a 2-pulse rectifier I would assume that the drive needs to be de-rated to 1/3 of it's 3-phase rating for 1-phase input.

 

[ScottyUK]

Lionel,

On diode rating it would be 2/3 of full rating, each diode conducting for 180[°] rather than 120[°]. The reservoir caps for a 3-ph rectifier are usually fairly small, whereas for a 1-ph drive they are relatively large because the rectfier output voltage drops to zero twice a cycle and the caps have to hold up the DC bus. Caps which are sized adequately for a 3-ph input are desperately undersized for a 1-ph input. If the DC bus is accessible then adding external capacitance can help when using a 3-ph drive on a single phase supply. My lathe uses a 208/3ph A-B 1336 Plus II drive with 230V 1-ph input and 4700uF of external capacitance. Without the cap the drive tripped on bus undervoltage and the DC link ripple was huge; with the cap the ripple is about down to about 15% and the drive behaves normally.


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