SOFT STARTING AC MOTORS
Most motors in most applications do not need special starter controls. The motors are built to take the brief start-up periods where high rotor circuits are experienced until the motor picks up speed and approaches the desired slip point. However, there are many other applications where, if unchecked and uncontrolled, high start-up currents may cause premature motor failure if not immediate breakdown.
Before we begin considering different methods used to limit the start-up current, let us recall the equation for the start-up current:
This is not a very complicated equation and does not render itself to many different methods of controlling the current, I'R. The obvious ways are
Reducing the stator voltage ("star-delta starter")
The simplest technique is by using a so-called star-delta starter to start the motor. In this method, two configurations exist for the stator circuit: Y (or star) and Delta. The motor is started with a Y-connected stator. It automatically switches to a delta-configuration when the rotor speed exceeds a preset value (typically 70-80% of the full-load speed). We have already seen that, for the same line voltage, the per phase voltage for a Y-connected stator is 1/31/2 or 58% of a delta-connected stator.
The catch is, and this applies to all other methods where starting currents are controlled by controlling the stator voltage, that while the start-up current is proportional to V1, the start-up torque is proportional to the square of the same voltage. By reducing the voltage we pay a severe penalty in reduced start-up torque. Therefore, the technique is not suitable for applications requiring high start-up torques.
Increasing the Rotor Resistance
Additional resistors are added to the rotor circuit to increase the rotor impedance and thereby reduce the current during start-up. This can only be done with wound motors. Since squirrel-cage designs operate solely on induction principle with no slip rings or commutators, no extra component can be added to a squirrel-cage rotor circuit.
We have already seen how the rotor resistance can be changed actively (wound rotor) or passively (shaped bars).
It should be emphasised that increasing the rotor resistance works at many levels:
This last point requires some elaboration. We developed the following torque-speed relation a few lectures ago:
According to this equation, the start-up torque (s=1) will increase with increasing resitance only if R'R<<XS+X'R
This is seen in the following chart, which shows the variation of the start-up torque with rotor resistance.
The other motor parameters are as in the example we have seen before in the AC Motor Dynamics section.
AC MOTOR BRAKING
Induction motors can quickly be stopped by by either of the following two methods:
Plugging
Stator phase sequence is suddently switched completely reversing the stator magnetic field. The reverse field quickly decelerates the rotor to a standstill. Unless the stator is disconnected at zero speed, the rotor then starts accelerating in the reverse direction.
Dynamic Braking
The stator is suddenly removed from the AC supply and connected to a DC source. The result is a stationary magnetic field and the rotor brings itself to a standstill by trying to orient itself with this new field.
AC MOTOR SPEED CONTROL
There are basically two mechanisms available to us to control the speed of an AC motor:
Changing the number of poles
By changing the number of poles, we can change the synchronous speed. A given squirrel cage rotor will work with any number of stator poles. Therefore, if all we want is to have two discrete speeds then the stator can be wired so that one can switch from two poles to four poles when demanded. The same result can be obtained by a variable speed gearbox. One has to look at both options and choose the most cost-effective one.
Line Voltage Control
We have already seen that the motor torque is proportional to the square of the line voltage. The above figure was for a line voltage of 450 V. The following shows the effect of changing that voltage. The new operating point in each case is the intersection of the new motor torque curve with the load curve.
The line voltage can be controlled by
The voltage delivered by a solid-state controller is non-sinusoidal and can only be tolerated in small induction motors. For large motors, it may cause large harmonic currents fed back to the supply lines, which is usually not acceptable by the electricity supplying authorities. Appropriate input filters need to be used in such instances.
Frequency Control
By controlling the line frequency, we would be directly controlling the synchronous speed. It is important to change the line voltage at the same time because the air gap flux (the magnetic flux from the stator onto the rotor) is proportional to V/f:
If the voltage is not increased when the frequency increases, the torque will drop at higher frequencies because of the drop in the flux. Compared to the voltage, it is relatively easier to change the frequency. Especially at high frequencies, it is not practical to achieve proportional increases in the line voltage. Therefore, the torque for a variable-frequency drive would be constant up to a certain point but would drop afterwards. This is seen in the following chart. Each motor curve in the following chart corresponds to a different line frequency:
There are several options of changing the frequency. The recommended textbooks will give you a good list. I basically group them under two headings:
The PWM has only been economic in the last several years (and even then at a stiff cost but improving) and since then it has revolutionised the AC motor industry. We do not have time in our scope to cover Pulse Width Modulation but be aware of it when you are designing electro-mechanical systems in tie future. Using PWM you can drive AC motors almost like servomotors if you want to. Obviously, if your application requires a servomotor, you are better off with one. But in many applications where DC motors have been the only option for a long time, AC motors are catching up real fast.