VFD with analog signal and motor speed

[tahir4awan]

Hi,
I was doing my project to drive an induction motor with VFD. The problem occured, when I tried to calculate the speed of the motor. I mean, it is a calculation problem and I am not sure what is correct? We have learnt in our institute that VFD always work with synchronous speed and not with the rated speed. Becasue rated speed is different different due to slip. I had an induction motor with rated speed 1390 r/min. Which means 1500 r/min synchronous speed. If VFD shows 50Hz on display at 1500 r/min then it should show 33.66Hz. at 900 r/min. Because 900 r/min+110 r/min=1010 r/min. And (1010 r/min * 50hz)/1500 r/min = 33.66Hz.

But my supervisor told me that I should use this calculation, i.e at 1390 r/min VFD shows 50Hz then at 900 r/min it shows, (900 r/min * 50Hz) / 1490 r/min = 32.37 Hz. Which is a little bit different. Has anyone any idea, what is the correct way of calculating speed?

Thanks

 

[BradtheRad]

There are different types of AC motors (4 shaded pole, 2 shaded pole, PSC, EC). Speed may be fixed depending on frequency (eg, my reel-to-reel tape recorder), or speed may be variable via triac control (eg, my wood stove fan). Articles seem to focus on operating principles and how we pick a motor to drive certain loads.

As for what determines speed, your supervisor seems aware of details in your motor's construction that cause the slight difference in formulae.

 

[KlausST]

Hi,

3 phase AC asynchronous motors have a slip.
But mind: the slip depends on torque.

I assume the slip in RPM is about proportional to torque.

So if you apply 50Hz you may get close to 1500 RPM at no load. The rated 1390 RPM are at full torque.

So it will not show you the actual RPM, just the estimated RPM.
So at 50Hz you may display "1500 RPM" or "1390 RPM" or something inbetween as you like.
It´s not accurate anyways. It´s on you.

Klaus

 

[dick_freebird]

Does your supervisor deny you the use of a shaft mounted
tachometer?

There are probably voltage or current artifacts which could be
"sniffed" to build a speed feedback term for your controller if not.

Is the speed accuracy requirement such that you need feedback,
or loose enough that something like old school (Varmature -
Iarmature*Rarmature) (whatever its synchronous-machine
equivalent is - I've only worked with DC motor control) can
be an adequate feedforward, no loop, approach?

 

[D.A.(Tony)Stewart]

It depends on the load torque curve and desired accuracy of RPM.
The load may be inverse, linear , square or cubic with RPM such as pumpers, conveyors, or fans and you did not specify the condition for 900 RPM
Assuming max torque slip is desired target use constant V/f for rated RPM ratio = 900/1390 = 64.7% times 50 Hz = 32.4 Hz and same % Vin.

At reduced speeds VFD and motor assembly, the efficiency is lowest for square and cubic torques loads compared to linear and constant torque loads.
Thus we would avoid using V/F mode at low speeds for pumps and fans with induction types and only use for synchronous types.
Your motor is rated at (1500-1390)/1500 = 7.3% slip (low).

Generally % slip at peak torque is not voltage dependent, but % slip is not the same for maximum power point.

Torque is inverse to applied frequency Constant V/f is used to avoid flux saturation but if too low, there is insufficient coupling to rotor.

Before the peak the torque zone is fairly flat but cooling is reduced so temp. rise is greatly affected.