PWPM Power Control
(With Registered Name: TurboDigital®)
PWPM algorithm for AC motors and UPS systems is based on the new approach of modulation of DC line into AC sinusoidal line by variation of pulses width and position too.
This outstanding algorithm has following amazing characteristics:
1. It eliminates third and fifth harmonics directly by shape of pulses,
2. It does not requires filters, because motor's or transformer's coil works as filter,
3. It is very cheap and it does not require DSP and thus it can be based on cheap micro controller only.
4. It minimizes number of pulses and their frequency and thus it reduces loses and heating of switches,
5. It minimizes higher harmonics and thus it reduces heating of motor or transformer core.
The algorithm enables production of cheap sinusoidal UPS with continual accommodation of output to the UPS battery's voltage and current because battery's output varies during discharging and power consumption. The low frequency transformer then is used for voltage rectifier and also sinusoidal filter.
There are various approaches for generation of AC motors driving power from a DC line. Generally for successful control of AC motor the variable frequency and amplitude should be available on the motor’s input. The frequency is determined by actual motors velocity and variation of amplitude is used for power control.
Magnitudes of harmonics and width of pulses in respect to amplitude of sinusoidal output for PWPM algorithm is shown on the following picture:
Magnitudes of harmonics of classic PWM algorithm is shown on the following picture:
The picture shows the pulses' positions and widths in classic PWM 3 pulse vector driver for asynchronous electromotors. Pulses are not moveable.
During switching cycle switch changes its resistance from zero to infinite and vice versa and on the transition it acts as resistor. During transient time there is thermal losses on finite resistance of the switcher. Higher number of pulses of PWM modulation suppresses higher harmonics but it increases losses on switchers. PWPM algorithm minimizes both parameters: switching frequency and harmonics.
BREF INTRODUCTION INTO AC MOTORS OPEARTION AND CONTROLL
Operation of asynchronous motor is based on the effect discovered by Nikola Tesla. The effect is described on the picture below:
On above picture is shown one conductive ring and a permanent magnet below it. Rotating magnet transfers its rotation, i.e. torque to
a ring. It is interesting to be noticed that in the case magnetic lines are tangential to a ring and thus Lorentz law cannot be directly applied in this
Rotating magnet can be replaced with three solenoids mutually angled for 120°, as it is shown on the following picture:
If the solenoids are driven by sinusoidal potential, the disk will rotate, as there is a permanent magnet instead of solenoids.
In real asynchronous motor rotor is consisted of several parallel disks, not rings, mutually isolated.
Three-phase driving voltages for all solenoids are shown on the following picture:
The force acting to a ring is continual and thus three-phase asynchronous motor is extremely gently to gears' teeth.
Function of force momentum acting to the ring depending the difference between the rotation of magnet and ring is shown on the following picture:
Maximal mechanical coupling is obtained when the angular speed of magnetic field is about 20% greater than the angular speed of disk.
Block schematics of general controller of asynchronous motors is shown on the following picture:
Power is controlling by variation of amplitude of sinusoidal function while regulator itself should keeps angular velocity of rotating
field (i.e. frequency of sinusoids) optimally higher than angular velocity of rotor. So we can affect the amplitudes, but not the frequency – frequency
is automatically determining regarding above graphic.
For proper regulation it is important to obtain as better sinusoidal input to motor as possible. Higher harmonics could be represented as smaller magnets on same shaft on fig. 1, where each one rotating faster then nether one as harmonics do.
Phase itself is not important for driving of asynchronous motors. Asynchronous motors can be using without controls directly plugged to public electric network. Modern controllers use minimization of reactive power instead of measuring rotor's velocity because reactance is also minimal in optimal control.
Asynchronous motors are invented by Nikola Tesla.
Synchronous motors are three phase AC motors containing permanent magnet in rotor, as it is shown on the following picture:
In the case Lorentz law can be fully applied and these motors have much higher density of power than
asynchronous ones, i.e. for the same
weight synchronous motors offer more power than asynchronous ones. Nearly all-modern electric and hybrid vehicles use synchronous AC motors. Synchronous motor
offers precision control of both angle and velocity because rotor is not magnetically symmetrical.
Controller of an asynchronous motor must keep phase (i.e. angle) of rotating field identically to phase of a magnet, i.e. rotor. The angular velocity of the rotating magnetic field and magnet must be the same. Phase is crucially important parameter in control of a synchronous motor. Any discrepancy between phases of magnet and rotating field can seriously damage either motor or controller.
Block schematics of general controller of synchronous motors is shown on the following picture:
Power is controlling by variation of amplitude of sinusoidal function while controller itself should keeps angular velocity and phase of
rotating field equal to angular velocity and phase of rotor. So we can affect the amplitudes, but not the frequency and phase – frequency and phase are
automatically determining to be equal to instant velocity of magnet, i.e. rotor. There are realizations of the controllers without measurements of rotor's
angle and velocity based on minimization of reactance.
Synchronous motors cannot not work without controllers.
BASIC THREE PHASE EQUATIONS
Current trough null wire whereas all three phase's currents I1, I2 and I3 are known is:
Potential U1,2 between two phases whereas potentials U0,1 and U0,2 between phases and null are known is:
As depicted on the following figure:
Continual sinusoidal generators have limited applications due their small efficiency. They can be realized in class A or class B. Class A is not producing any more because significant DC component heats primary coil of transformer that must exist and its purpose is to eliminate the DC component. Capacitor in serial connection can be used only for smaller powers and its usage significantly improves efficiency.
B class linear converter.
Class A is totally inefficient and cannot be used without condenser in serial connection, suitable only for very small AC motors:
A class linear converter.
Dipl. Ing. Andrija S. Radović
Tel: +381 64 1404075