Understanding VFDs

[Jonathan Spool]

I just ordered a VFD for my 3 Ph 2hp RAS. I am trying to gain an understanding of different electrical phases, and the results from different converter methodologies.
When running 120v or 240v from a non 3 ph source you get 120 from each leg as the sine waves are 180 degrees apart from each other. Howevert the phasing of a 3 phase source have the 3 legs only 120 degrees from each other, and two legs only yields 208v.

I do not know how voltage effects hp, but notice that many motors have wiring options for different voltages (208v; 220v; 230v; 240v).

I have heard that when using a static converter you only get two thirds of the motor hp as you are only using two legs. How much efficiency is retained when converting with a VFD, and why?

Electrically befuddled in Port Orchard

[Dan Hintz]

Since a VFD gives you access to all of the motor's designed phases, you get the same efficiency as if you were running the motor with its native power source.

EDIT: I'll also add that the VFD gives you an opportunity to do things that cannot be done with the native power source, such as change the driving frequency or have access to constant torque / constant speed.

[Logan William]

Check out the link below to OWWM.com's wiki section on Electrical information, they have a subsection devoted to 3 phase and VFD questions.

http://wiki.owwm.com/Default.aspx?As...Electrical__50

[Mike Henderson]

I'm not an expert on VFDs but I'm pretty sure that the output of a VFD, whether the input is single phase or three phase, is three phase with the phases 120 degree apart. Note that the shape of the output voltage is not a sine wave but is a series of pulses which approximate a sine wave. Motors run fine on those waveforms because the motor is essentially a low pass filter, so the higher frequencies are attenuated sufficiently that the resulting waveform inside the motor is very close to a sine wave.

The voltage you choose to run the motor at will not affect the motor HP, given that the motor is designed to run on the input voltage. A VFD varies the output voltage as the frequency changes (up to the design frequency - 60 HZ in the US). As the frequency decreases, the voltage is decreased. The reason is to keep the current in the motor within design specs.

The motor can be viewed as a big inductor. When you put 60Hz power into it, a certain current will flow for a given voltage. When the frequency is decreased, the reluctance of the inductor is decreased which means that more current would flow if the voltage were kept the same. In order to prevent excess current from flowing through the motor, the voltage is decreased.

The goal is to keep the current constant as the frequency changes, which leads (generally) to constant torque. This isn't true down at very low frequencies but I won't explain all that.

Since the VFD provides true three phase current, the power of the motor is preserved. You should get full HP from the motor when you set the VFD to 60Hz (in the US).

Mike

[Lewis Cobb]

Mike - are you saying that as you lower the frequency - and therefore the speed of the motor - the torque remains the same ? I've been thinking about a VFD and 1-2hp motor for my drill press for some time and am trying to get my head around the "downsides" if there are any (other than spending money which I see as enjoyable for projects like this )

Perhaps a more accurate question would be - is there any loss of torque as compared to the same speed obtained on the drill press with the change in pulley locations and the single phase motor that's supplied with the machine?

Thanks,
Lewis

I'm not an expert on VFDs but I'm pretty sure that the output of a VFD, whether the input is single phase or three phase, is three phase with the phases 120 degree apart. Note that the shape of the output voltage is not a sine wave but is a series of pulses which approximate a sine wave. Motors run fine on those waveforms because the motor is essentially a low pass filter, so the higher frequencies are attenuated sufficiently that the resulting waveform inside the motor is very close to a sine wave.

The voltage you choose to run the motor at will not affect the motor HP, given that the motor is designed to run on the input voltage. A VFD varies the output voltage as the frequency changes (up to the design frequency - 60 HZ in the US). As the frequency decreases, the voltage is decreased. The reason is to keep the current in the motor within design specs.

The motor can be viewed as a big inductor. When you put 60Hz power into it, a certain current will flow for a given voltage. When the frequency is decreased, the reluctance of the inductor is decreased which means that more current would flow if the voltage were kept the same. In order to prevent excess current from flowing through the motor, the voltage is decreased.

The goal is to keep the current constant as the frequency changes, which leads (generally) to constant torque. This isn't true down at very low frequencies but I won't explain all that.

Since the VFD provides true three phase current, the power of the motor is preserved. You should get full HP from the motor when you set the VFD to 60Hz (in the US).

Mike

[Mike Henderson]

Mike - are you saying that as you lower the frequency - and therefore the speed of the motor - the torque remains the same ? I've been thinking about a VFD and 1-2hp motor for my drill press for some time and am trying to get my head around the "downsides" if there are any (other than spending money which I see as enjoyable for projects like this )

Perhaps a more accurate question would be - is there any loss of torque as compared to the same speed obtained on the drill press with the change in pulley locations and the single phase motor that's supplied with the machine?

Thanks,
Lewis

With a VFD the torque is constant from some low speed to the rated speed.

To answer your second question, let me assume we have two motors, each with one unit of torque, where one motor is powered by a VFD and the other is a constant speed motor and uses pulleys to reduce the speed at the work.

As you slow the VFD powered motor, it will continue to deliver one unit of torque to the work.

If you use pulleys to reduce the speed of the constant speed motor to half it's constant speed, you will have two units of torque at the work (neglecting losses in the pulleys).

With pulleys, the delivered HP is constant (neglecting losses in the pulley system). Since HP is torque times RPM (times a constant), as you slow the speed to the work, the torque must increase to keep the delivered HP constant.

It's the same reason a 1725 RPM motor is larger than a 3550 RPM motor of the same HP. The 1725 RPM motor has to deliver twice the torque of the 3550 RPM motor to produce the same HP. And producing that extra torque requires that the motor be bigger.

On the other end of the scale, universal motors, which may run at 20,000+ RPM, are physically smaller than a 3550 RPM motor of the same HP (real HP, measured in the same way for both motors).

Mike

[Jonathan Spool]

Logan,
Thanks for the excellent link. Thanks for the explanation Mike.
Jonathan

[Lewis Cobb]

Mike -

Thanks for the excellent explanation on this.

As I see it - another way to look at this would be - let's say we start at 1725 rpm and then reduce the speed -
- With a pulley system, the torque goes up according to the pulley ratio
- With a VFD the torque stays the same.

This raises another issue - as we drop the speed on a pulley system and generally raise the diameter of the bit, the larger bits have more torque as a result. A good thing when hogging off large amounts of wood that the larger bits are capable of.

The issue is - for woodworking applications, is the lower relative torque of the VFD system a disadvantage for boring wood with larger bits? I guess all things being equal, you would simply slow the feed of the bit into the wood. Perhaps this is a question for someone that has rigged up a VFD with a drill press.

Thanks again for your insight (and patience).

Cheers,
Lewis

[David Castor]

Generally, the VFD can run the motor to 100% torque at any speed. Above 60 Hz, the torque is rolled off to avoid operation beyond the motor's rated horsepower.

[Lewis Cobb]

This is a sweet deal all around it seems.
Thanks for the clarification.

Generally, the VFD can run the motor to 100% torque at any speed. Above 60 Hz, the torque is rolled off to avoid operation beyond the motor's rated horsepower.