CSD Series Variable Frequency AC Drive
MOTORTRONICS
- 2 -
This speed is described by this simple formula:
Where:
• Synchronous speed is in RPMs (Revolutions Per Minute)
• Poles are the number of poles built into the motor.
• Frequencyistheappliedfrequencyof the power fed to the motor.
As you can see, synchronous speed is directly proportional to the
applied frequency. By increasing or decreasing this frequency, you
can increase or decrease the rotational speed of the magnetic field.
This is the underlying theory behind the operation of the adjustable
frequencydrives.
Changingthespeedisonlyhalf theproblem. The motorwasdesigned
torun atafixedoperation pointasshownin thenameplate(frequency
voltage). This point can be described by a “volts per hertz” (V/Hz)
ratio which relates to the strength of the magnetic field. To maintain
constant field strength and constant torque, we must maintain this
ratio.Since we vary thefrequencytochange the synchronous speed,
we must simultaneously change the applied voltage to maintain the
necessary V/Hz ratio. As an example: For a constant torque
application, if the frequency is cut in half, the voltage must also be
cut in half as shown in Figure 1 - 2.
The final concept to be introduced is known as motor “slip”. The
actualtorqueoutputbyaninductionmotorisproportionaltotheproduct
of the V/Hz ratio and the slip. Slip is simply defined as the difference
betweensynchronousspeedandthe actual motor shaft speed. With
constantV/Hz excitation, themotormustslip to produce moretorque.
Thegreaterthetorquerequirement,thegreateramountofmotorslip
and the slower the resultant shaft speed.
1.4.2 Drive Power Section
Refer to Figure 1 - 3. The input (converter) of the power section is a
three-phase,rectifier bridge used to convert theincomingACvoltage
into DC voltage. This DC voltage is then filtered by the DC bus
capacitors to produce a clean, ripple-free DC level. The converter
alsoincludesa current limiting, pre-charge circuit. This circuit is used
tocontrolthecurrentinrush while the capacitors are building up their
charge when power is first applied to the drive unit. Once they are
charged, this circuit serves no further useful purpose, so it is
bypassed. The output (inverter) section consists of six transistors
which are switched by the microprocessor to produce the variable
voltage, variable frequency output waveform necessary to control
theV/HZratioasdiscussedintheprevious section. The result of this
switching is a “chopped up square wave” voltage that produces a
nearly sinusoidal motor current waveform.
Note: The shape of the voltage waveform prohibits accurate
measurement with most types of voltmeters. The most accurate
measurement is obtained by using a “rectifier” type AC voltmeter. If
this type of meter is unavailable, use an analog meter and check to
insure the three-phase output voltage is balanced (this shows all
transistors are switching evenly, even if the actual voltage reading is
meaningless).
Figure 1 - 2
Volts per Hertz Ratio
Figure 1 - 3
Power Section Block Diagram
Synchronous Speed = 120xFrequency
# of Poles
Operating Point
Frequency
Voltage
460 V
230 V
30 Hz 60 Hz
