Download Electronics 3 Class 16 Before the availability of high power

Electronics 3
Class 16
Before the availability of high power electronics, most speed control of electric motors involved DC
motors somewhere.
However AC induction motors have advantages
- No commuter of friction slip connectors
- Easier to manufacture.
- Less Maintenance (Brushes)
- (No sparks)
- Smaller and lighter
- Simple rotors
AC motor speed can be adjusted by adjusting voltage but this is not practical for most motors
Except for some special small motors the slip of the motor would become excessive current would be
too high and control would become unstable.
Variable frequency speed control
- Frequency control can be done in two ways.
o Inverter
 AC signal is rectified to DC
 DC voltage is selected in a sequence to give AC again
 Timing of the sequence sets the frequency of the resulting AC
o Cycloconverter
 3 phase AC signal is trigger in a sequence to give 1 phase AC signal
 Timing of the sequence gives frequency of the output signal.
- Overall View:
Input
- Normally 3 phase power in
- Can be single phase with small 3 phase motor (bread maker)
Rectifier
- Produces DC power
- As covered previously
Filter
- Smoothes out the ripple from the rectifier
Inverter
- Converts DC back into 3 phase AC power
- The frequency can be controlled
- Described later
Load
- The motor can be a common AC induction motor
- As mentioned they are simpler and more reliable than DC motors
Tachometer
- This is optional
- If it is very important to keep motor speed constant with variable load, then feedback is used
Controller, speed pot, triggers
- Electronics control all parts of the process
- The frequency controls the speed of the motor
- Voltage control is need to be linked with the frequency controller
Voltage control
- As the frequency goes down, the motor slows
- However to protect the motor, applied voltage must be kept proportional to frequency
- V(Applied)/Frequency = constant [approximately]
Eg. If the plate says 240V/60Hz
240/60=4
If motor is to be run at 90H:
V/Freq=4
V=4*freq
V=4*90
V=360
@ 45Hz
V=4*45
V=180V
Reason to vary voltage:
- The flux must be kept fairly constant
- If the flux tries to go too high, the steel will saturate
- Energy is wasted, heat is produced
- If the flux is too low, torque in the motor is lower (If you drop the frequency and don’t raise the
voltage to compensate, you will have a weak motor)
How to keep the flux constant
- Flux is proportional to magnetizing current I(mag)
Ignoring resistance
I(mag) = E/X(L)
Where I(mag) = Magnetizing current
E = Applied Voltage
X(L) = Inductive reactance (ohms)
X(L) = 2Pi(f)L
(f) = Frequency (Hz)
L=Inductance in stator coils (Henries)
Substitute
I(mag) = E/2Pi(f)L
I(mag)=[1/2Pi(L)]x[E/(f)]
Flux=Kx[E/(f)]
Ratio of voltage and frequency must remain proportional to keep flux constant
-
The voltage can be reduced right down to zero by delaying to 90 degrees
The filler gives the average voltage as DC to the inverter
The controller circuit sets the voltage relative to frequency to keep the ratio the same