Electrical and Electronic Engineering Department
Open Loop 𝑽/𝒇 Variable Frequency Drive
The open loop V/F control of an induction motor is the most
common method of speed control because of its simplicity
and these types of motors are widely used in industry.
Traditionally, induction motors have been used with open
loop 50Hz power supplies for constant speed applications.
For adjustable speed drive applications, frequency control is
natural. However, voltage is required to be proportional to
𝒗
frequency so that the stator flux [𝜳𝒔 = 𝝎𝒔 ] remains constant
𝒆
if the stator resistance is neglected. The power circuit
consists of a diode rectifier with a single or three-phase ac
supply, filter and PWM voltage-fed inverter. Ideally no
feedback signals are required for this control scheme.
Scalar control of Induction machine based on the
steady state model per phase equivalent circuit.
Rectifier/Converter is one of the three primary sections of a VFDs main power circuit.
The AC line voltage is rectified into DC voltage in this converter which consists of diode,
Silicon Controlled Rectifier(SCR) or Insulated Gate Bipolar junction Transistor (IGBT)
connected in a full bridge configuration
DC bus is the second part of the main VFDs power circuit which comprised a capacitors
that store the power rectified by the rectified.
Inverter is the final part of the VFDs main power circuit and it consist of IGBTs which
creates sinusoidal current by means of pulsed DC bus voltage of Pulse Width Modulation
(PWM).
Carrier Frequency in PWM-based VFD the rate at which output transistors are gated
or turned-on usually (15 KHz), Higher values results in better current waveform but more
VFD losses.
PWM is a VFD control scheme in which a constant DC voltage is used to reconstruct a
pseudo AC voltage waveform using a set of six power switches usually IGBTs. Varying the
width of the fixed-amplitude pulses controls effective voltage. This PWM scheme works
because the motor is a large inductor that dose not allows current to pulse like the
voltages. Sequenced correctly, PWM outputs motor current in a nearly perfect sinusoidal
waveform.
Constant V/F control
Given a load 𝑻 − 𝝎 curve, the steady state speed can be
changed by altering the 𝑻 − 𝝎 curve of the motor.
Variable Frequency
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Speed control above the rated speed
Require PWM Inverter to control the frequency of the motor
Frequency increased ( (𝝎𝒔 ) increased)
Stator voltage kept constant at rated value
Airgap flux and rotor current decreased
𝟏
Developed torque decreased 𝑻𝒆 ∝ (𝝎 )
𝒔
For the control below the rated speed, use constant Volts/Hz
method.
Open loop Scalar Control of Induction Motor
Air gap flux in the motor is related to the induced stator voltage 𝑬𝟏 :
𝑬 𝟏 𝑽𝒔
∅𝒂𝒊𝒓𝒈𝒂𝒑 =
≈
𝒇
𝒇
Assuming small voltage drop in 𝑹𝒔 and 𝑳𝒍𝒔 .
For below base speed operation:
• Frequency reduced at rated 𝑽𝒔 , airgap flux saturates (𝒇 ↓, ∅𝒂𝒊𝒓𝒈𝒂𝒑 ↑ and enters
saturation region on B-H curve) → excessive stator currents flow , distortion flux
wave and increase in core and stator copper losses.
• Thus, need to keep airgap at rated value.
• Stator voltage 𝑽𝒔 must be reduced in proportional to the reduction in the frequency
( maintaining V/f ration constant)
Constant 𝑽/𝒇 curve
• Boost- to compensate for voltage
drop in 𝑹𝒔 and 𝑳𝒍𝒔 , at low speed
operation.
• Linear offset curve- for high starting
torque load and employed for most
application.
• Non-linear offset curve- for low
starting torque load.
• Open loop speed control is easy to implement.
• Used in low performance applications, where precise speed control is unnecessary, but
this cab be solved by 𝑽/𝒇 drive with slip compensation or by closed loop speed control.
• Speed reference 𝝎𝒔 ∗ is primary control variable. The actual motor speed is not measured.
• Voltage 𝑽𝒔 ∗ is generated from the 𝑽/𝒇 ratio.
- Boost voltage 𝑽𝟎 is added at low speeds.
- Constant voltage applied above the base speed.
• Sinusoidal phase voltages 𝑽𝒂𝒃𝒄∗ is generated from 𝑽𝒔 ∗ and 𝜽𝒔 ∗ , where 𝜽𝒔 ∗ is obtained
from the integral of 𝝎𝒔 ∗ .
• 𝑽𝒂𝒃𝒄∗ is used for the PWM inverter connected to the motor.