Most of the applications preferred VSI fed to
synchronous motor due to its self controlled nature and rotor position sensor.
A normal VSI with 180° conducting mode required only forces commutation.
The circuit for VSI fed synchronous
motor is given below. It consists of three thyristors and diodes combination to
provide variable voltage and frequency to the synchronous motor. The voltage
control is obtained by inverter using phase controlled rectifier which is
connected external to the circuit. The output voltage is square wave hence the
inverter is named as variable voltage or square wave inverter.
There are
two techniques to get desirable outputs
1.Controlling
the inverter voltage by PWM technique
2.Connecting
the DC chopper in between rectifier and inverter.
A VSI is a
self or separate control feeding synchronous motor. Open loop and closed loop
operation is possible but VSI provides instability and hunting problems.
The stator current drawn by the motors when
fed to VSI has sharp peaks and is rich in harmonic content which causes
additional losses and heating. This technique is used for high and medium speed
applications.
PWM
technique:
This technique is used to reduce harmonic,
the voltage is
controlled with in the inverter itself. The stator currents are
less peaking and harmonics reduced, all the effects become minimal and
commutation is also possible at low speeds. Dynamic braking can be employed.
When PWM inverter
is used two cases may arise
*When
inverter is fed to a constant DC source regeneration is straight forward.
*The DC
supply to inverter may also be obtained from a diode rectifier which requires
additional controlled converter on line side.
The phase control decides the line power
factor. The line power factor improves to unity if a diode rectifier is
sufficient in PWM inverter. The field control improves the machine power factor
by minimizing the losses. So it is fetching to operate motor under Unity power
factor (UPF).
Advantages:
1.VSI drive
provides good efficiency
2.Multi-motor
operation is possible
3.Commutater
less motor (CLM) mode is very stable
4.PWM drive
has better dynamic response
5.VSI used
for high and medium power applications, PWM used for low and medium power
applications.
Disadvantages:
1.The converter cost is high
2.By using
VSI there is instability and harmonic content in output
3.VSI is not
applicable for low speed operations.
CSI (Current Source
Inverter)
CSI is used
in many applications due to its flexibility, reliability and commutation. CSI
operates on closed loop and capable of generation, by using CSI variable
frequency is obtained. The CSI operates in two mode i.e. Load commutation and
forced commutation.
1.
Load commutation:
A load commutated
self control synchronous motor is also known as Converter motor. In load
commutation mode motor works in CLM (Commutator Less Motor) mode. No additional
equipment is required for the commutation, the commutation is done by over
excitation of synchronous motor under Leading power factor. Four quadrant
operation is possible in simple way with good efficiency.
The damper winding is required to minimize the voltage sprikes. It provides lower speed limit, the speed range is about 10% of base speed, low speed operation is impossible.
2.
Forced commutation:
By using forced
commutation the speed ranges from zero to maximum speed. The power factor
operates at unity. The inverter cost is high due to thyristors and capacitors
utility. Open loop condition is not stable but efficiency is good, the
harmonics and torque pulsations are same as load commutation. Forced commutation
is used for low to medium power range applications.
Advantages:
1. CSI is
robust and simple
2. Four
quadrant operation is possible
3.
Controlling the speed in simple way
4. Better
controlled performance.
Disadvantages:
1. Cost is
expensive with PWM technique
2.
Unsupported for multi-motor operation
3. CSI not
used in open loop control drives
4.
Undesirable dynamic performance
5. At
no-load condition it is difficult to operate.
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