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AC drive speed control
 Posted by paru on 26 May, 2012 - 5:09 am
Hi

Can someone explain me few things related to AC drive.

1) AC drive (V/F) without speed sensor connected to the rotor, How does the drive calculate the actual speed?

2) What happens when the drive is running at a speed say 100o rpm and you start increasing the load on the motor? Or if there is sudden change in load?

3) When the speed is changed, the voltage also changes to keep the V/F same. What happens to the current?

4) At low speed does the current really remains same as the current at full speed?

 Posted by driveguy on 29 May, 2012 - 2:12 pm
> 1) AC drive (V/F) without speed sensor connected to the rotor, How does the drive calculate the actual speed?

It doesn't. It just "sends" out a V/Hz signal… i.e. it assumes. Hence the term "open-loop".

> 2) What happens when the drive is running at a speed say 100o rpm and you start increasing the load on the motor?
> Or if there is sudden change in load?

There is more slip(motor speed will vary/drop) and the motor current will increase.

> 3) When the speed is changed, the voltage also changes to keep the V/F same. What happens to the current?

If the speed is changed alone and the load does not change the current will not change noticeably. See my note below about friction and windage.

> 4) At low speed does the current really remains same as the current at full speed?

The current is load dependent (i.e. doesn't matter what the speed is). So it should relatively stay the same. Things that may not make it stay the same with speed change is friction and windage.

Keep in mind with a VFD in V/Hz mode you can't run real low speeds. It is more likely to stall below 5%. Also some VFD's have a special algorithm that compensates for slip (it usually requires the rotor magnetization current to be entered as one of the additional parameters… and it would need to be enabled.) Some also have a low speed boost that allows you to run slower i.e. <5% without stalling. Some also have a sensor-less vector mode, but that gets into a whole different conversation... still open-loop but you control speed better than V/Hz.

 Posted by paru on 4 June, 2012 - 6:22 am
thanks driveguy

good explanation....Can you explain little about the vector drive also. If i assume all above questions with vector drive.

 Posted by driveguy on 4 June, 2012 - 12:22 pm
Sensor-less vector (open-loop_

>How does the drive calculate the actual speed?

you enter motor nameplate data into the motor, the do an autotune which is used to calc things like induction, mag current, rotor resistance and time const etc., then the sensorless-vector algorithm calculates torque and magnetizing current vectors for more precise motor control.

> How does the drive calculate the actual speed?

Vector (closed-loop) - motor has a sensor on it e.g. encoder, resolver, sin/cos etc.

> 2) What happens when the drive is running at a speed say 100o rpm and you start increasing the load on the motor?
> Or if there is sudden change in load?

Sensorless vector - algos work to more precisely control speed... to be more specific I would have had to written the algos myself...which I did not do.

Vector - uses PI, PD, or PID type control depending on the drive brand/model

There is more slip(motor speed will vary/drop) and the motor current will increase.

> 3) When the speed is changed, the voltage also changes to keep the V/F same. What happens to the current?

this should be the same for all three control modes.

If the speed is changed alone and the load does not change the current will not change noticeably. See my note below about friction and windage.

> 4) At low speed does the current really remains same as the current at full speed?

again should be the same answer for all three control modes.

Note with true vector control (closed loop) and if you have the right motor selected you can pull full torque at zero speed (i.e. standstill).

a good estimation is you can get 1-5% speed control with V/Hz and 10 times better with sensorless vector (i.e. 0.1-0.5%) and with closed vector you would get 100 times better (i.e. 0.01-0.05%)

 Posted by CSA on 4 June, 2012 - 3:14 pm
AC machines run at a speed that is pretty much directly proportional to the frequency of the applied voltage/current source. The relationship is stated by the formula:

F = (P * N)/120

where, F = Frequency (in Hertz)
P = number of Poles of the rotor (always an even number)
N= Speed of the rotor (in RPM)

Synchronous machines run at a speed that is exactly described by the above formula.

Induction machines have necessary slip, but, still the speed is proportional to frequency. Increase the frequency, the speed will increase. Decrease the speed and the frequency will decrease.

I think that's what's being referred to as "open loop" control here; the drive controller 'understands' the above relationship and is able to control speed without any speed feedback because it knows the speed of the motor is proportional to the applied frequency, and if the motor nameplate data is entered into the controller it knows how many poles the rotor has, etc.

Also, lots of applications requiring variable speed drives are not primarily concerned with the speed of the prime mover. They are pressure- or flow-control applications, and so the speed is varied as required to make the pressure feedback equal to the pressure reference, or the flow-rate feedback equal to the flow-rate reference. The speed gets adjusted to whatever value is required to make the pressure or flow-rate equal to the reference. Some applications do require very tight speed control, and in those applications there probably is some kind of speed sensor providing speed feedback.

But, again: The speed of an AC machine (motor or generator) is directly proportional to the applied frequency. Vary the frequency, and the speed will vary in direct proportion.

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