Drive Tech FAQ
VVVF AC drive is the power electronic controller used to control the speed of 3ph AC motors (synchronous or inducution) by varying the frequency and the voltage applied to the motor terminals. Voltage and frequency relationship is decided based on the motor name plate data and the load characteristics.
Typical power circuit configuration involves 3Ph. diode rectifier at the input, which converts the AC input to DC voltage. LC or C filter reduces the ripple in the DC voltage. 3Ph IGBT AC drive stage converts this DC voltage into variable voltage variable frequency output as per the desired pattern.
VVVF AC drive are generally classified into three types based on the type of control philosophy adopted for motor control:
Scalar control/PWM control.
Sensorless vector control.
Vector control (with sensor) or Flux vector control.
In scalar control, relationship between voltage and frequency of the AC voltage applied to the motor terminals is predetermined by the user. This relationship is marginally altered in scalar drives sometimes, to improve the performance of the drive. Scalar controlled inverters can have only speed control and these are ideal for group/multi motor drives.
What are the typical applications?
In Vector Control motor, current is controlled with two independent components i.e., torque component and flux component. These components are computed based on the rotor position, rotor speed and motor parameters. Motor speed is controlled rather than output frequency. Relationship between voltage and frequency is decided by operating conditions. Vector controlled inverters invariably use encoders for rotor speed and position feedback. As flux and torque components of current are decoupled, fast dynamic response is obtained. It is possible to get more than rated torque at zero speed also. Vector control can be achieved for single motor only. Vector control inverters are used for applications demanding zero speed regulation, wide speed control range and excellent dynamic response. Ex. Paper machine drives, film line drives.
In sensorless vector control, motor speed is estimated based on the measured motor terminal parameters and hence speed sensor is avoided. Based on motor parameters and computed rotor speed, flux and torque component of motor current are computed. As flux and torque component of current are independently controlled, fast dynamic response is achieved. Speed regulation is better than scalar drives and typical value is +/0.5%. This speed regulation is typically achieved in the range 1:50. High starting torque (>150%) is also achieved by this control.
Pulse Width Modulation (PWM) is the method of control where variable voltage (AC/DC) is achieved from a fixed DC voltage using switching devices. DC voltage is applied for sometime in the cycle and in the remaining period, no voltage is applied to the load. By adjusting the duty ratio, (ratio of on period to cycle time) output voltage is adjusted between zero and rated voltage.
Applied motor rating specifies the maximum rating of the 4-pole motor that can be connected to the inverter to obtain its rated output power at the rated speed. It is necessary that the rated input voltage of the inverter and motor are matched or else specified out power may not be achieved.
It is the apparent power that can be delivered continuously by the inverter at the rated frequency. This is calculated as [SQRT(3) x rated output voltage x rated current]/1000.
Rated output voltage is the fundamental rms value of the output terminal voltage at rated input and output conditions.
Output rated current is the rms current the inverter can continuously supply irrespective of the output frequency.
It is the rated supply voltage and frequency for which invertor delivers its rated output.
Input voltage and frequency variation range specifies the range wherein the inverter can deliver the rated current without affecting the life of the equipment. Other specifications as output power, voltage etc., may not be met during the variation.
Input power factor can be specified in two ways i.e., displacement power factor and harmonic power factor. If diode rectifier is used displacement between the fundamental voltage and current is nearly zero and hence displacement power factor is approximately 1.0. Harmonic power factor in the ratio of input effective power and input apparent power. This depends on output speed and load conditions. It is normally specified at rated input and output conditions. Power factor depends on the power circuit configuration.
It is the input apparent power drawn by the inverter at the rated output conditions.
Frequency/speed range is the ratio of minimum and maximum frequency/speed in the defined operating condition. Ex.1:10 speed range with constant torque.
Frequency stability specifies the variation in output frequency with the defined temperature variation keeping frequency reference constant. Ex. +/-0.5% of max frequency for 25° C. +/-10° C.
Inverter efficiency is the ratio of the output power to the input power of inverter at rated output conditions i.e., with rated voltage, rated current & rated power factor at the output of inverter.
AC Reactor is used when supply line has to be isolated from commutation notches caused by the inverter and to reduce the rectifier peak current.
Noise filter is the element involving inductor and noise capacitor to suppress high frequency voltages, which can cause interference to sensitive electronic equipment.