[SOLVED] Help me understand the concept of motor acceleration by vfd

[abc_de]

Help me to understand concept of motor acceleration by vfd

hello
in commerical VFDs up to 1HP minimum acceleration time is .01(10msec) what it does mean?
can motor accelerate 0HZ to 50HZ within 10msec. or frequency increment time is 10msec.

please check image

 

[KlausST]

Re: Help me to understand concept of motor acceleration by vfd

Hi,

in the datasheet there are a lot of "timing values". Which one do you refer to? Highlight it in the datasheet.

***

Generally:
It should be obvious that the motor can´t accelerate from 0 to 100% within 10ms.
But for sure the VFD frequency can.

Klaus

 

[abc_de]

Re: Help me to understand concept of motor acceleration by vfd



for 50hz time 1/50hz = 20msec how motor achieve full speed

i want know how this is possible i am referring " delta vfd m series"

 

[KlausST]

Re: Help me to understand concept of motor acceleration by vfd

Hi.

You say "motor ... speed"...
but the document refers to VFD (frequency) speed.

--> read post #2

If you choose the lower limit for Pr.10 ... it will be similar to "switch ON" 50Hz.

Klaus

 

[abc_de]

Re: Help me to understand concept of motor acceleration by vfd

My doubt is that what this parameter says..
You say this means direct apply 50hz in that case motor starting current is to high around 4amps. But when driven with vfd its around 2amps under no load means no high starting current.
I want to know if i choose 10msec acceleration time then how vfd apply frequency. Vfd start from 0hz and target frequency is 50hz.
Is vfd increment each frequency 0> 1> 2> 3 ....49> 50hz in 10msec.
Please help me to under that

 

[KlausST]

Re: Help me to understand concept of motor acceleration by vfd

Hi,

Is vfd increment each frequency 0> 1> 2> 3 ....49> 50hz in 10msec.

Theoretically possible.
But I doubt this.

Just use EXCEL to show the resulting waveform. --> For the motor current you won´t see any significant change in current compared to directly applied 50Hz.

--> Use a meaningful Pr.10 value.

Klaus

 

[tsan500]

Re: Help me to understand concept of motor acceleration by vfd

My doubt is that what this parameter says..

I think it says, that the resolution of acceleration time setting is 10 ms. You could set 10,00 seconds or you can set 10,01 seconds acceleration time. 10 ms resolution is good. Even 1 second resolution would be good enough for most applications. I assume you noticed that factory setting is 10 seconds->they are not saying that 10 ms acceleration time setting makes sense.

But when driven with vfd its around 2amps under no load means no high starting current.

No load current requires that there is no load on the motor and the motor speed is constant. During acceleration, speed is not constant and we store energy to motor+load inertia. That's why it is necessary to use longer acceleration time with higher inertia load.

I want to know if i choose 10msec acceleration time then how vfd apply frequency. Vfd start from 0hz and target frequency is 50hz.
Is vfd increment each frequency 0> 1> 2> 3 ....49> 50hz in 10msec.

We don't know how the ramp is implemented. At least the idea of running each integer frequency is not possible. One cycle of 1 Hz takes 1 second and we should get to 50 Hz in 10 ms. Less than 1 second can't make 1 Hz. The ramp has to be some sort of frequency sweep. This is the case also for 10 second ramp time. How many discrete frequency steps there are during 10 ms? Maximum amount of discrete steps is determined by carrier frequency. If the carrier is 10 kHz, there is 10 kHz x 0,01 = 100 semiconductor switching during 10 ms. This means the motor sees maximum 100 different steps.

 

[FvM]

Re: Help me to understand concept of motor acceleration by vfd

Usually the acceleration/deceleration ramp time refers to the full scale speed. If you ramp to a smaller speed, e.g. 10 Hz with 50 Hz full scale, the actual ramp time is only 1/5 of the specified time.

It has been clarified that the parameter range has nothing to do with feasible motor acceleration. The VFD drives I'm used to have mostly 0.1 sec resolution for the ramp time, but may even allow 0.0 input. To know the smallest feasible setting, you need to put maximum VFD and motor current and motor moment of inertia in the calculation. Modern VFD sense motor current and rotor speed during spin up and delay the ramp if the motor can't follow it.

The more critical situation is however during ramp down, a too fast deceleration pushes the DC linke voltage and causes an overvoltage fault, unless a brake chopper is installed with the VFD.

 

[D.A.(Tony)Stewart]

Re: Help me to understand concept of motor acceleration by vfd

There are 2 acceleration zone times and 2 setpoint frequencies (Pr3,Pr6) so the times in seconds are from 0 to Pr.6 ( eg 10s to 10Hz) and Pr.6 to Pr.3 (e.g. 10s to 60hz)

The resolution options may be determined elsewhere for xxx.1 and xxx.01 you can ignore for now that depend on some counter frequency.

This is my understanding from reading the manual briefly.

The start stop acceleration times depend on your friction, start load and dynamic load. e.g a compressor has a big start load but fan does not

 

[c_mitra]

Re: Help me to understand concept of motor acceleration by vfd

minimum acceleration time is .01(10msec) what it does mean...

If I understand correctly, it means the resolution of the accl time setting. The underlying software increases the frequency every 10ms. Of course your motor must be able to follow this accl else the motor will stall. You need to see how the VFD senses the motor speed.

can motor accelerate 0HZ to 50HZ within 10msec...

The driver can but (most) the motor cannot.

 

[abc_de]

Re: Help me to understand concept of motor acceleration by vfd

May be here is some hidden concept.
But i am preety sure motor can not reach at full RPM within 10msec.
Still i do not know how they implement it.
For less starting current may be they 1st apply 10hz and after 10msec 50hz under controlled current ...

 

[c_mitra]

Re: Help me to understand concept of motor acceleration by vfd

May be here is some hidden concept....

The controller does not know much about the motor. It just supplies pulses. High end controllers will also read the motor rotation speed. Motor rotation speed can be read either via Hall sensors or by the back emf produced - but I think Hall sensors are actually simpler and can provide some independent measure of actual speed of rotation.

When you want to change the speed of a motor, you need to follow the following steps:

Increase the VFD frequency a little - wait for a while and see that the motor has reached the speed a little.

Repeat the above step till you reach the final speed you want. But continuous monitoring of the speed is important else the motor may stall.

If your motor is fully loaded then you need to accelerate slowly- else your motor can even be damaged.

If the motor is rather small and lightly loaded (say the motor of a gas centrifuge), then you can accelerate faster.

The controller can figure out whether the motor is actually speeded up only in some cases. You need to dig more in the documentation.