I have a question about VFD's

I have a question about VFD's · 10-17-2009, 09:09 AM

I have a question about how these work and their electrical efficiency. I am debating between a homemade RPC or a VFD, but need to ask someone a few questions about how the VFD works.

A) Can you control the amp output if say I buy a 2HP VFD and am running a 1.5HP and the VFD is putting out +2 more amps than the motor is rated for, can you manually decrease this?

B) I know that some of them have speed control as an option also, is that controlling the amp, Volt, or hertz output to vary the speed.

C) Also how many amps input at 240V (two single phase legs) is required to output say 7 amps on the 3-phase. My intuition tells me that 7 X 3 = 21amps + small % of loss = 11 amps 240V. Is this on the right track?

Reason I am asking is that I have a Powermatic 81 bandsaw that I would like to keep. It has a 3-phase motor, and I am also planning my electical layout to hook up new subpanel in the basement to get my shop set up.

Thanks,
ALG

merrill77 · 10-17-2009, 03:36 PM

Originally Posted by algustaf

A) Can you control the amp output if say I buy a 2HP VFD and am running a 1.5HP and the VFD is putting out +2 more amps than the motor is rated for, can you manually decrease this?

ALG,

In general, electricity doesn't work that way. Assuming the electrical input to the motor (output from the VFD) is the correct phase, frequency and voltage, the motor will draw exactly the current (amps) it needs, no more. This assumes the motor is functioning properly, of course. It is not possible to force more amps into it without changing one of the above-mentioned parameters. The rating is there to tell you how much power the VFD can supply. A motor requiring more current may either malfunction in use or damage the VFD by drawing more current than the VFD can safely handle.

Chris

disclaimer: I'm no expert on VFDs.

algustaf · 10-17-2009, 04:03 PM

I had kinda thought that about motors as well as other electical devices, but however due to the way that VFD's work (at least my limited knowledge of them) I wan't sure if it (motor) would act the same as normal devices drawing only the power they required. I have a decent grasp of electrical concepts regarding circuits, and residential wiring- everything but motors and electronic controls.

Thanks for the input.

scsmith42 · 10-17-2009, 08:15 PM

Originally Posted by algustaf

B) I know that some of them have speed control as an option also, is that controlling the amp, Volt, or hertz output to vary the speed.

C) Also how many amps input at 240V (two single phase legs) is required to output say 7 amps on the 3-phase. My intuition tells me that 7 X 3 = 21amps + small % of loss = 11 amps 240V. Is this on the right track?

Thanks,
ALG

Al - regrding question B - VFD's work by varying the frequency (also know as the hertz output).

Re C - you select your VFD based upon the HP motor that you need it to operate. Based upon your selection the VFD manufacturer will let you know what the amp draw is.

cpowell · 10-18-2009, 01:12 PM

You need to answer a few questions to get the best response. Are you using a 3-phase or single phase motor? Do you have 3-phase or single phase voltage supply? Those questions will determine the size VFD (inverter) you'll want.

The inverter will be the most efficient means of delivering power since the only losses are electrical where the RPC will have mechanical and electrical losses. Now, if the RPC is free or very low cost compared to the VFD it may still be the best choice from a financial standpoint.

You didn't mention the machine type. For many WW apps the inverter would be set up for constant torque (constant volts/Hz) application. Say your motor nameplate is 480 volts, 60 Hz. If you select constant torque control then the following volts and Hz would apply:

480 V 60 Hz
360 V 45 Hz
240 V 30 Hz

and so on. Of course this will be transparent to you since the drive will manage these ratios.

The inverter will give you the option to set acceleration up/down speeds (within a range of limits unless you use resistive braking), will allow you to select appropriate torque and current limits, and will generally allow programming beyond most WWers basic needs. If you are installing on a lathe then you can change directions from fwd to reverse by pushing a button. The lathe will slow down, reverse, then accelerate in the opposite direction.

The inverter will provide overload protection for the motor based on nameplate data. You will not need to worry about separate over-current protection for the motor.

Chuck

Alan in Little Washington · 10-18-2009, 02:30 PM

It would help if you provided your intended application, etc.

All good answers above. I assume from your question, all you want to do is power a 3 phase motor, but all you have available is single phase power. If that is all you want to do, shop based on price. There is a point where the cost of a VFD vs an RPC cross. Typically for motors 3 hp and below, a VFD is cheaper, simpler and more compact. Above 3 hp and depending if you can find some cheap motors for your homemade RPC, then an RPC may be cheaper.

While some VFD's will work with multiple motors, a typical installation is to run only one motor. A properly sized RPC can be used to power a couple of 3 phase motors. Some folks have wired three phase outlets/circuits to their shops using a RPC as the source of 3 phase power since it is usually much cheaper than to get the power company to run 3 phase to the shop.

Most/many(?) 3 Hp and below VFDs can run on single or three phase input, most/all(?) VFD's designed to power higher horsepower motors require 3 phase input. Something to remember is the primary purpose of a VFD is not to turn single phase into 3 phase, it is to allow for variable speed, etc., control of a 3 phase motor.

If you want to control the speed (RPM) of the motor and add things like electronic braking, ramp up/down time, etc., then a VFD is the only way to go. For a non-variable speed application, you can have a pretty basic "set it and forget it" VFD arrangement.

FYI, a VFD is rated by the horespower of the motor it is designed to power and the rating is often listed in KW, e.g. 1.5 KW = 2.0 hp, 2.5 KW - 3.3 hp. You will need to check the VFD ratings also. Depending how the ratings are written you may need to de-rate the VFD if it is powered by single phase source (I don't remember, but believe it is aoubt 20%). Some VFD's ratings have already taken this into account.

Lastly, for variable speed applications, due to their typical constant torque and unlike pulley reductions, motors operated by a VFD lose horspower at low RPM and it can be considerable.