Choosing a Soft Start or VFD for large 480V motor

[11bgrunt]

Will a soft start draw more starting current than a VFD?
I have seen VFDs installed when motor speed control was needed but if ramping to 100% to reduce starting current is all you want, why choose one over the other?
The POCO has said that a soft start can exceed their motor starting limits, a VFD will not.
Is this all to do with programming the device?

 

[mike_kilroy]

1) Will a soft start draw more starting current than a VFD?
2) I have seen VFDs installed when motor speed control was needed but if ramping to 100% to reduce starting current is all you want, why choose one over the other?
3) The POCO has said that a soft start can exceed their motor starting limits, a VFD will not.
Is this all to do with programming the device?

1) Yes and no. A VFD can be programmed to take more or less than the softstart.
2) BIG question. Too big to give pros and cons in a minute. Both have their place.
3) Yes, you can program so "starting" current is very low by programming a long acceleration ramp to speed. That said, most VFDs output only to 150% of nameplate current rating of the motor. The actual starting current, if for instance you set a 100hp motor to 100 second accel to base speed, might be only 10% of nameplate current.

 

[Jraef]

Well, I think 10% is a little unrealistic... it would mean your motor is 10x the size it needed to be.

11bgrunt
What the Utility is referring to is that with a Soft Starter, the motor is highly unlikely to be able to accelerate without the current getting to AT LEAST 200% of motor FLA, and then only if the motor can start uncoupled (i.e. a clutch system of some sort). For pumps and fans it usually 300-350% minimum (open flow), for anything else it could be a lot more than that. this is because with a soft starter, you cannot accelerate over an unlimited amount of time, because the motor is heating up since the frequency is fixed and 60Hz, only the voltage is limited, so the motor operates at high slip at first. A VFD on the other hand can be programmed to take 1 minute, 10 minutes, 10 hours or, 1 year to accelerate if that's what it takes to keep the current low. Only a VFD can give you 100% torque at 100% FLA.

The down side of a VFD is that it is a very expensive soft starter if you have no need to change the speed of the machine. SOMETIMES it is what you need to do, it's just a hard pill to swallow. I once had to use a VFD to start some 2500HP gas compressors, even though they had no need to vary the speed. But even with soft starters, they were going to dim the lights in the nearby town, VFDs were the only solution that would work. Set it for Current Limit at 100% FLA, it took 12 minutes to accelerate to full speed, but it worked.

 

[mike_kilroy]

Well, I think 10% is a little unrealistic... it would mean your motor is 10x the size it needed to be

Thanks Jraef; I should have been clearer in my reply stating 10%.

It is fairly easy to accel most motors with only 10% current draw from the Poco to the VFD. One has to remember that there are two currents in a motor, 90 degrees out of phase.

The 90 degree out of phase is imaginary current and cycles back and forth from the motor to its power source; there is no real power in it other than I^2R heating in the wires. Once started, it is self-sustaining. Without a VFD drive, this out of phase current flows from the motor all the way back to the Poco generator source and back to the motor over and over, once per cycle. With a VFD in between, this imaginary current flows from the motor to the capacitors in the VFD bus and back: it does not get thru to the AC power input side of the VFD so the Poco never even sees the I^2R wire heating or power factor phase issues a motor can produce.

The other one is real current, in phase with the Poco supplied voltage, this is the current that actually produces torque. This current is linear with respect to the torque produced. This is the only current the VFD pulls from the Poco and it is in phase so nearly 1.0 power factor. There are often non linear power factor wave distortion on the Poco input but that is a different discussion and not significant here.

So you can see that if you accel an AC motor slow enough, the torque required to accel = 1/2Jw/t and can be very small. Of course any load on the motor during the accel will also require more current in addition to the current to accel, but typically as Jraef said, the load at slower speeds than normal run speed are usually small.

So a 10% of rated nameplate current to accel any AC motor on a VFD is perfectly valid.

 

[kwired]

Thanks Jraef; I should have been clearer in my reply stating 10%.

It is fairly easy to accel most motors with only 10% current draw from the Poco to the VFD. One has to remember that there are two currents in a motor, 90 degrees out of phase.

The 90 degree out of phase is imaginary current and cycles back and forth from the motor to its power source; there is no real power in it other than I^2R heating in the wires. Once started, it is self-sustaining. Without a VFD drive, this out of phase current flows from the motor all the way back to the Poco generator source and back to the motor over and over, once per cycle. With a VFD in between, this imaginary current flows from the motor to the capacitors in the VFD bus and back: it does not get thru to the AC power input side of the VFD so the Poco never even sees the I^2R wire heating or power factor phase issues a motor can produce.

The other one is real current, in phase with the Poco supplied voltage, this is the current that actually produces torque. This current is linear with respect to the torque produced. This is the only current the VFD pulls from the Poco and it is in phase so nearly 1.0 power factor. There are often non linear power factor wave distortion on the Poco input but that is a different discussion and not significant here.

So you can see that if you accel an AC motor slow enough, the torque required to accel = 1/2Jw/t and can be very small. Of course any load on the motor during the accel will also require more current in addition to the current to accel, but typically as Jraef said, the load at slower speeds than normal run speed are usually small.

So a 10% of rated nameplate current to accel any AC motor on a VFD is perfectly valid.

To accelerate to a certain point yes. You can keep current down by controlling voltage and frequency, but at some point in a majority of applications there is a good chance you eventually have the motor loaded to more than 10% of it's rating or you have much more motor than the application needed to begin with.

 

[winnie]

If the motor is unloaded, then I bet a VFD could accelerate it with less than 5% of full rated current drawn off the line. But if the machine is connected to its mechanical load then eventually the VFD has to supply both the acceleration power and the load power, and that should approach full rated current as the motor comes up to speed.

-Jon

 

[mike_kilroy]

Remembering what the original question/concern was, "The POCO has said that a soft start can exceed their motor starting limits, a VFD will not," I replied regarding the STARTING input current. The POCO never said the motor was too large to run its load, that was never in question, so I did not address it. Just starting current: to me that means as it begins rotating; regardless of WHEN the actual load is seen. The load itself was never an issue. If the load itself WAS an issue then I would have also recommended a VFD over softstart as it would at full motor load have reduced the input current draw from POCO by the PF ratio too.

 

[kwired]

Remembering what the original question/concern was, "The POCO has said that a soft start can exceed their motor starting limits, a VFD will not," I replied regarding the STARTING input current. The POCO never said the motor was too large to run its load, that was never in question, so I did not address it. Just starting current: to me that means as it begins rotating; regardless of WHEN the actual load is seen. The load itself was never an issue. If the load itself WAS an issue then I would have also recommended a VFD over softstart as it would at full motor load have reduced the input current draw from POCO by the PF ratio too.

You still eventually transition from "accelerating" to "running" at some point. If "running" amps is 100 amps (using that figure for simpler math) then how do you only draw 10% (10 amps) throughout the acceleration process? You are not going from 10 to 100 instantly, some acceleration is still happening over that range. Even getting an unloaded motor to speed by only drawing 10% and then throwing the load on it is still going to have similar issues if you throw the load on it all at once, you just raised the threshold from where you started load acceleration is all.