I have been developing a software for scalar control of three phase induction motor with squirrel cage rotor. This control method is based on voltage-frequency characteristics of the motor. The used motor nameplate contains 400V, 50 Hz and the stator is in wye connection. Based on these values I expect that the slope of the voltage-frequency characteristics will be \$K_1 = \frac{(\sqrt(2)*400)}{(\sqrt(3)*50)} = 6.5\,V/Hz \$. The problem is that I would like to rotate the motor at 60 Hz. Based on the above given voltage-frequency characteristics the operating point would be in the field weakening region. I had an idea that I can prevent the field weakening of the motor by changing the slope of the voltage-frequency characteristics \$K_2 = \frac{(\sqrt(2)*400)}{\sqrt(3)*60)} = 5.4\,V/Hz \$ i.e. the nominal stator volatge would
be achieved at 60 Hz and not at 50 Hz (please see the attached graph).
Is it possible or is this idea wrong? (I see that below the 60 Hz the magnetic flux of the motor will be lower than the nominal value. But the motor starts with no load and the load torque is applied as soon as the motor achieve the no load speed.) Thanks for any ideas.
Electronic – Induction motor scalar control
controlinduction motormotor controllervfd
Related Solutions
A variable frequency drive (VFD) will generally try to maintain a constant magnetizing current in the motor with any frequency and load. The results will vary depending on the VFD design and set-up. That will result in an operating power factor that is generally similar to the pf vs load plot provided by the motor manufacturer for rated voltage and frequency sine wave operation.
To calculate output power, motor efficiency must also be considered. That will vary with speed.
You can not accurately determine the output mechanical power from the motor using the method that you propose. If the VFD provides a torque display, that is likely to be more accurate than any determination made by measuring motor current. A power determination using DC bus current of the VFD might also be more accurate. There are clamp-on wattmeters on the market that can accurately measure VFD output power. Of course, that still leaves the problem of estimating motor efficiency. I have some empirical data that might help, but I don't have time to post it right now.
Here is the data that I mentioned above. It is calculated from motor design data, not empirical test data.
I believe this data was calculated in the 1970's by Alberto Abbondanti of Westinghouse, but I can't find where it was originally published. I received it in class notes for a tutorial.
I was expecting some kind of closed loop control of the stator field with respect to the rotor field
Indeed, there is. The ESC senses the back-electromotive force (back-EMF) produced by the motor, and keeps track of the rotor position using that information.
The back-EMF is a term for the voltage induced into the stator windings due to them being exposed to a time-varying magnetic field, in turn created by the spinning permanent magnet rotor. In essence, the back-EMF is the voltage that would be present at the terminals of an extenally spun motor without anything being connected to it.
If the motor is at rest, the rotor isn't spinning, so there is no back-EMF to sense. The only thing that the ESC can do is to blindly spin the rotor by driving the motor phases open loop, until back-EMF builds up enough for the ESC to figure out the true position of the rotor. This works acceptably with just a propeller or the inertia of a lightweight transmission and chassis of a RC car as a load, not so much with a high geared motor driving a human.
Related Topic
- Electrical – Three phase induction motor control
- Electronic – Asynchronous induction motor, current in stator
- Electrical – How to calculate the number of rotor and stator poles in AC motor
- Electrical – Frequency of stator current in induction motors
- Electronic – How to calculate rotor flux of the three phase squirrel cage induction motor
Best Answer
If the motor nameplate says 400 V, 50 Hz, wye connection, that is the line to line voltage that must be supplied for the wye connection 8 V/Hz is required. If you re-connect to the delta connection, you can supply 4.62 V/Hz. That would be 231 V for 50 Hz or 277 V for 60 Hz. If you do not provide the rated V/Hz the motor can not produce the rated torque without overheating. Rated V/Hz is required to produce the magnetizing flux for which the motor is designed.