Self control of synchronous motor drives
Phase-Controlled Cycloconverters
Synchronous Motor Drive
Self controlled sync. Motor drive using a cycloconverter
2. Introduction
Cycloconverters directly convert ac signals of one
frequency (usually line frequency) to ac signals of
variable frequency.
These variable frequency ac signals can then be
used to directly control the speed of ac motors.
Thyristor -based cycloconverters are typically
used in low speed, high power (multi-MW)
applications for driving induction and wound field
synchronous motors.
9. Synchronous Motor Drive
Due to power supply system limitations, low starting currents
(locked rotor) are usually required.
A combination of high torque with low starting current can be
better achieved with the application of synchronous motors
without affecting operating characteristics.
The various type of synchronous motor are :
Cylindrical rotor motor
Reluctance motor
Salient pole motor
Permanent magnet motor
10. Self controlled sync. Motor drive using a
cycloconverter
• Cycloconverter obtain a variable frequency
output from it. Infact the cycloconverter will
divide the line frequency by 2,3,4 etc.
• The drawback of a cycloconverter is that it is
possible to change its output frequency in
steps. Smooth variation in the output
frequency is not possible to obtain.
11. CONT…
• The output frequency will always be less than
the line frequency and therefore, the operation
of the motor above the rated speed will not be
possible.
• In very low speed and large horsepower
applications, it is possible to use
cycloconverters to control the speed of
synchronous and induction motor.
12. CONT...
• The basic cycloconverter circuit utilize line
commutated converters. Each phase consists of
two back to back connected line commutated
thyristorised converter.
• The maximum output frequency is limited to
about one third of the input ac frequency. This is
done to have a waveform close to a sinewave
with a low percentage of distortion and low
harmonic contents.
13. Self controlled sync. Motor drive using
a cycloconverter
Digital signal
Cycloconverter
Control circuit for
cycloconverter
Rotor position
sensor
sync. Motor
3 phase
AC supply
control
voltage
14. CONT...
• Cycloconverter provides a 3 phase variable frequency
stator voltage to a 3 phase sync. Motor. A rotor
position sensor produce digital pulses on the basis of
the actual position of the rotor.
• The pulse are applied to the control circuit which
produce triggering pulses to turn on the thyristors
connected in the cycloconverter.This is the way the
cycloconverter is actually controlled by the rotor
position sensor.
15. Advantages…
• The cycloconverter are inherently capable of power
transfer in both the directions. Therefore the direction
reversal of the synchronous motor are possible. In
other words four quadrant operation is possible.
• They have a smooth low speed operation.
• Their dynamic response is good.
16. CONT...
• The cycloconverter in the above speed range gives a
high quality sinusoidal output voltage. The resulting
current are also nearly sinusoidal . The harmonic
content of the motor current is small. Therefore the
effect of harmonic current such as losses, heating and
torque pulsations are minimal.
17. • The cycloconverter drives require no. of thyristors so
these drives are economically acceptable only when
the drives rating is high.
• The control circuitry is complex.
• Their cost is high.
Disadvantage…
18. Applications
• The drives are attractive for low speed operation and
frequently used in large, low speed reversing mills
requiring rapid acceleration and deceleration. Typical
application are large gearless drives for reversing
mills, mines hoists etc.
