Basic question about inverters

Hi all,
For a new program I need to set multiple acceleration-times on the a frequency inverter. However the inverter has only one digital input that let's me choose between two different acceleration/deceleration times. So, I decided to program my own acceleration function. The function is working fine but one thing I don't know.
I am gonna set the acceleration-time in the inverter to 50 msec. This is the smallest value that can be set.
If I accelerate (lineair curve) from 1 to 50 Hz in 5 seconds does this mean that the inverter will take 50 msec to accelerate to 1 Hz?
Or will it take 50 msec to accelerate to 1 Hz, then again 50 msec to 2 Hz, then again 50 msec to 3 Hz and so on?

Probably a stupid question but I have no idea.
I think that the acceleration-time in the inverter will only be used as I set the 'run'-input on, correct?

Thanks

I always hated the accel and decel unit of measure in inverters. For some reason it's just not intuitive to me.
With most inverters the accel time is the time it takes to go from zero to max configured frequency of the drive. With some inverters it is the time to go from zero to base frequency. Either way 50 msec is faster than a standard volts/hertz drive will be able to spin up the motor. In you example with a 50 msec accel time and a base speed = max speed = 60 Hz, you will accelerate at 1200 Hz/sec. Pretty freakin' quick for an inverter.
The accel and decel rates are used any time a speed change is requested, regardless of whether or not the speed change is due a run input transition.
I usually do my own speed ramping also, mostly for the same basic reason you stated; IO like the extra flexibility.

Keith

You'd better go to the owner's manual. In addition to the combinations Keith mentioned, I just did a startup on a VFD where acceleration time was the time allowed to go from set minimum frequency (non-zero in my case) to set maximum frequency.

I'd be careful about using that 50 msec for accel time too. If someone wants to run the VFD from manual on the keypad for testing or diagnostics you will get overcurrent trips on acceleration and over voltage trips on deceleration. Set the accel/decel parameters to the minimum time that the motor can accelerate the load under typical conditions without tripping. For a high inertia load that can be 30 or 40 seconds, for a low inertia load just a few seconds might be enough.

You can use your PLC to make the acceleration slower than this time, but no matter what you can't accelerate faster.

you will get overcurrent trips on acceleration and over voltage trips on deceleration.

Click to expand...

A name of this exists behavior? For when the VFD avoids this event, crazy free, increase DC link, and shoot in a level to brake magnetic. Which name of this term tecnique?
Always I have that we read all manual, to know this.
Tranks reply

Good point about running the inverter in manual...

So, say I set the acceleration-time on the inverter to 2 seconds.
(= time to accelerate from 0 Hz to 60 Hz)

In my program I want to accelerate from 0 to 30 Hz in 4 seconds.
How would I do this?

My solution was to set the acceleration-time to the smallest amount possible, and every 20 msec I would change the analog output value with 0,15 Hz.

But with the acceleration-time on the inverter set to 2 seconds I don't see the involved math....

Any help would be very usefull

dandrade: I think you are talking about DC injection braking. This is usually only intended for slowing the motor from almost stopped to full stop. It isn't intended for dymnamic braking under load from high speed on down.

thierry2003: On the VFDs I've worked with the accel time sets the drive's elapsed time to respond to a step change in the analog speed command to the drive. If you ramp your speed command up more slowly, the VFD will accelerate the motor according to that signal change.

You can effectively treat the VFD and the PLC as independent, with the VFD setting a lower limit on acceleration time. In other words, if your VFD acceleration time sets 0-60Hz at 8 seconds, and you slowly ramp up the analog signal from the PLC from 0-30 Hz in 6 seconds, that is how fast the load will accelerate. If you ramp up the analog signal from the PLC from 0-30 Hz in 2 seconds, the VFD will limit the acceleration based on its internal setting, and it will take 4 seconds for the load to accelerate.

Thanks Tom, you helped me out!