Doubts

Why should we maintain the v/f constant during the Induction motor control?

Reason:

Induction motor control revolves around the formula of synchronous speed i.e., Ns = 120*f/p.

As frequency is the only variable to be controlled other than the poles (p), I used to think how it can be sufficient to just vary the frequency of the supply and increase the speed. I reached to a satisfying level of understanding that “it is sufficient”. This is how I understand.

The formula quoted above is the synchronous speed i.e., the speed of the stator flux revolving in the air-gap and not the actual speed of the motor (rotor). If the frequency of the stator current is increased, obviously the revolving speed of the air-gap flux will increase as it has no other constraints but to satisfy the relation depending on the number of poles. But, our induction motor control doesn’t mean just to increase the speed of the air-gap flux but to increase the motor speed i.e., the rotor speed which we do it by increasing the asking speed i.e., the synchronous speed. (Air-gap flux asks the rotor to rotate at its speed via the induction principle)

To do that, let us recollect the principle of induction motor. The torque required to drive the rotor depends on the rotor current which in turn depends on the emf developed in the rotor (because of the induction principle). This torque producing component of current magnitude depends on the cause i.e., the air gap flux strength. The current should be sufficient to drive the rotor (motor) to attain the speed at which the air-gap flux is asking it to rotate. Just asking the rotor to rotate at its speed is not adequate. It has to be given enough energy (current) to achieve that. This poses a requirement that the flux should be strong enough to develop a strong emf. In turn the magnitude of the current given to the stator is to be increased which can be done by increasing stator voltage. This is how the voltage is varied along with the frequency to maintain the ratio v/f constant. This in short, the text books tell us in a sentence which is well-known i.e., “in order to maintain the magnetizing current.”

Conversely, if only the voltage is increased then it means that energy is fed to the stator and rotor which is more than required for that speed. Increasing voltage doesn’t result in increase in speed as voltage holds no direct relation with the synchronous speed. It results in excess current which results in heating up the windings.


Why are system models and control theories not sufficient for analysis & design?

Reasons:

1. No mathematical system model is perfect
2. Dynamic systems are driven not only by our own control inputs, but also by disturbances which we can neither control nor model deterministically.
3. Sensors do not provide perfect and complete data about a system.

How is Speed, N converted from RPM to rad/s i.e, ω?

N is rotational speed and ω is angular speed.

Rotational speed tells as how many rotations(revolutions) have been completed per minute or per second. The standard unit is Revolutions per minute. RPM.

Angular speed tells the change(shift or displacement) in degrees per seconds.

one complete revolution is 360 degrees i.e. 2π radians.

one revolution per second is 2π radians per second.

one revolution per minute is 2π/60 rad/s.

N rpm is 2π x N/60 rad/s.

Similarly angular frequency (cycles per second) is calculated.

1 cycle per second = 2π radians per second.
f cycles per sec (Hz) = 2πf radians per second.

What is the difference between MV7000 (Voltage Source Inverter (VSI)) & SD7000 Current Source Inverter (CSI)?

The DC link after the converter (network) bridge in the VSI is made up of capacitors with voltage available for the inverter (machine) bridge. So, the voltage source is presented for the inverter. Hence, the voltage source inverter, VSI. In the CSI, the DC link is made up of a reactor which is to smoothen the current and give good DC current to the inverter bridge. It acts as a current source for the inverter present after the DC link. Hence, the current source inverter.