Variable Frequency Drive

[Guest]

Does anyone have experience this this VFD? It is supposed to convert single phase to three phase and operate any motor size up to the HP it is rated for. I was hoping to wire my grant pump and kettle pump to the same VFD as they aren't used simultaneously.

I couldn't get a direct link, but it's at www.grainger.com, part # 3XA45

thanks,
Josh

[grs]

I don't have any experience with that particular one but it looks fairly simple to use. VFD's in general will always take a single phase input and convert it to a 3 phase output. Just make sure the load is within the specs.

[Larry Horwitz]

No, No!!!! Freq drives do NOT convert single phase to 3 phase power. 3 phase has to come into the building as 3 phase...you can't "build" it. A VFD takes the power and digitally recreats the 3 seperate sine waves of power allowing you to regulate the flow up or down...to, for instance, control the speed of a 3 phase pump motor. If you tried to do this with 3 plain old pots you'd burn up your motor 'cause the 3 legs would be ever so slightly out of phase causing your motor to pull incorrectly.

VFDs start around $300 and go up. We use allen and bradley units, but they are a commoditiy so buy what's cheap.

[grs]

Vfd 101

I was trying to speak in laymans terms. Indeed, a VFD does not convert single phase to 3 phase power. In actuality, the output waveform from a VFD is a pulse width modulated DC voltage which looks nothing like true sine wave AC when viewed with an ocilliscope.

If you break down a VFD there are 3 basic components:

1. The input rectifier - this converts the incoming AC to DC (google wheatstone bridge if interested how).
2. The DC bus.
3. The output switching transistors.

Let's look at the output transistors. We have a DC bus voltage that we have to drive an AC motor with. An AC motor basically represents a large electrical reactance, ie, it will resist changes in voltage. If we expose the motor to a DC voltage of, say, 10 volts, the motor will take a certain amount of time before the voltage at the motor equals 10 volts. If, before the voltage at the motor equals 10 volts, we remove it, the voltage will drop slowly. If we turn on the DC voltage again before the motor voltage reaches zero the voltage at the motor will begin to rise again from where it left off. By chopping up the DC bus voltage into pieces we can effectively create a sine wave of voltage at the motor, if we were really quick at turning the switch on and off. This is what the output switching transistors of the VFD do. Typically the operating frequency of these switches is in the range of 2-10 khz.

So lets look at the input to the drive and the DC bus. With a 3 phase, 600 volt VFD the typical bus voltage will be in the range of 810 volts DC. If we were to single phase the input to the VFD it will reduce the voltage of the DC bus (again, google wheatstone bridge for specifics). Ultimately what this means is that the amount of current that the the VFD can produce on its output by chopping up this reduced DC voltage is less, reducing the size (hp) of the motor that the VFD can drive. Most manufacturers provide a derating table for their VFDs to dictate the motor size that the VFD can spin based on a single phase input, as is the case of this VFD.

So, in laymans terms, the VFD DOES take single phase power and convert it to 3 phase power. Incidentally, I've worked mostly with Allen-Bradley VFDs since the early nineties from 1 hp up to 800 hp. It's been a few years since my college days so the above information may not necessarily be 100% correct but the basics are there.

There is one thing, biz82. If you are operating two different sized pumps (say, 1/2 hp and 1 hp) from the same VFD only the larger of the two will be protected from a motor overload by the VFD. You could put in two separate overload relays but the cost of doing that probably isn't realistic as the pump motors are probably pretty cheap. All this means is that you may have premature failure of the smaller pump, depending on how heavily loaded it is.

[premierstainless]

Using VFD's

We use VFD's on most of our pumps and motors with great results. No reason not to, especially for a wort grant or wort transfer applications.

Yes, if you have 208-230V single phase available, you can operate a three phase motor. I have even seen one that will take 110V input and output 230V, up to 1/2 HP.

Always size the unit for your largest HP motor.

Not all three phase motors like VFD's power output. If you don't have the motors yet, be sure to get an "Inverter Duty" motor. Most washdown motors are inverter duty. I have used regular motors with VFD's but I am told the service life may be reduced.

Cheers,

Rob Soltys
Premier Stainless Systems
760-752-7032
rob@premierstainless.com

[Guest]

Thanks for all the responses. It seems clear that this unit does what is claims to but not quite the way the salesman said it would. Either way, It'll work for me.
Josh