AC Drive Characteristics
The use of adjustable speed industrial equipment is increasing due to the need for better equipment control and for energy savings where only partial power is required.
The principle of speed control for adjustable frequency drives is based on the fundamental formula for a standard AC motor:
In the formula:
Ns is the synchronous speed in RPMs
The number of poles in a motor is determined when it is manufactured and cannot be changed.
An adjustable frequency control regulates the frequency (f) supplied to the motor. The motor speed (Ns) increases and decreases in proportion to the supplied frequency. The output frequency of the control is adjustable by means of a potentiometer or external signal.
The control can automatically maintain the required voltage/frequency ratio for any motor speed. This provides for maximum performance over the motor's speed range.
Because the frequency output of the control is infinitely adjustable over the speed range, the speed of the motor is also infinitely adjustable.
AC Drives, AC Motor Drives
Common terminology for AC Motor Drives during early days was ‘Inverters’ due to a notable step involving inverting DC power to AC power.
Most common current terminology today is VFD which stands for Variable Frequency Drive. Some call it Adjustable Frequency Drive or simply AC Drive.
The speed of a standard induction motor can be varied by adjusting the frequency and voltage of the power source to the motor. The incoming AC power source is rectified to effect a DC power source which is subsequently inverted to provide AC power for the motor of the proper frequency and voltage.
Speed varies linearly with frequency where full motor speed occurs at line frequency. Adjustable, Variable Frequency VFD Electric Motor Drives can generate frequencies up to multiple times the line frequency. Voltage is generally maintained proportional to frequency to maintain the desired flux density within the motor.
Adjustable, Variable Frequency VFD Electric AC Motor Drives are differentiated by one basic characteristics of torque it produces.
Variable Torque Drives
Variable Torque Adjustable, Variable Frequency VFD AC Motor Drives have variable volts/hertz ratio. Variable torque machine loads are those which exhibit a variable torque over their entire speed range, such as centrifugal pumps and fans. The use of Adjustable, Variable Frequency VFD AC Motor Drives for the speed control of pumps and fans are the simplest applications, because the starting torque is very low which increase with speed.
The required starting current is low, overload protection is seldom required during the acceleration phase. Running for long period of time at low speed is seldom a problem. However operating at speeds above motor base speed could be a problem because power requirement of the drive increases as the cube of the speed. High viscous and slurry pumps need high starting torque, therefore the variable torque Adjustable, Variable Frequency VFD AC Motor Drives may fall short of the capability to complete the job.
Constant Torque Drives
Constant Torque Adjustable, Variable Frequency VFD AC Motor Drives can maintain constant volts/hertz ratio over their entire speed range, so that motor can have constant torque characteristics such as conveyors and positive displacement pumps.
Starting torque is theoretically equal to the full speed load torque, however in practice the starting torque is much higher due to starting conditions.
Running for long period of time at low speeds can result is motor thermal overload. Separate forced cooling might be necessary is some cases.
The selection of the correct AC Drive is impacted by the speed range within which it is expected to operate continuously. Most important requirement is that the motor should be able to drive the load continuously at any speed within the speed range without stalling or overheating the motor. The torque and thermal capacity of the motor must be adequate for all speeds within the speed range, within the load limits.
Flux Vector Adjustable, Variable Frequency Drives (VFD), AC Motor Drives
• High starting Torque 200% at 0.3Hz
Improved Sensorless Vector Control and Auto Tuning produce high starting toque of 200% or more at 0.3Hz. Easy Setup of motor constants, ideal for applications which need high torque such as cranes, extruders and lifts.
• Hitachi Exclusive 0Hz Domain Sensorless Vector Control Develops 150% torque at 0 Hz speed reference ideal for cranes and other applications that require high starting torque. • Position Control Function
The SJ700 with optional feedback board installed together with an encode equipped motor can perform position control. For many applications, suitable performance can be achieved at lower cost than servo systems. Based on your four motion parameters (position command) speed command, acceleration time and deceleration time), the SJ700 will move an object from original position A target position B. After the movement, the inverter keeps lock status.
• Over Current and Voltage suppress function
Higher internal calculation speed improves current control performance. Over -Current suppress and Over-Voltage suppress functions avoid inverter trip during acceleration and deceleration.
• DC Bus AVR Function during Deceleration
The SJ700 controls deceleration time so that the DC bus voltage does not exceed the over-voltage trip level, providing trip-less operation during deceleration.
• Inverter control by built-in Programming function
Sequence operation is realized by downloading to an inverter a program created with Hitachi’s EzSQ software. Tailor inverter operation to meet changing process requirements and replace separate PLCs in some cases. By simplifying or eliminating external hardware, significant cost savings can be achieved. Password functions is incorporated to provide security for proprietary program data against loss or unauthorized modification.
• Built-in EMC Filter up to 150kW (200HP)
Cost and space reduction compared with external EMC filter. Reduces electromagnetic noise. Meets EN61800-3 2nd Environment.
• Brake Circuit up to 22kW
Cost and Space reduction compared with external braking controller.
• Easy –removable construction for maintenance
Field replacement of cooling fan(s) and DC bus capacitors can be accomplished in a fraction of the time. Using Logic terminal move to SJ700 without wiring change. Read SJ300 Parameter by SRW remote operator and write them in to SJ700.
• Long life time components and Life time warning Function
Design lifetime 10 years or more for DC bus capacitors and cooling fan.
Cooling fan ON/OFF control function for longer fan life.
Ambient temperature average 40 deg C.
Perform preventive maintenance before a failure occurs using the Lifetime Warning Function.
DC bus capacitor, cooling fan. Heat sink temperature and motor temperature can be monitored in order to replace components prior to failure.
• User selection of Displayed parameters
Data Comparison Function – allows display of only parameters changed from default.
User Selected Function – Display of upto 12 user defined parameters.
Basic Mode – Basic display mode of commonly used parameters.
Other Functions – The direct input of function code selection is possible rather than scrolling through the list.
Holding down the functions key for 3 seconds, causes the display to jump output frequency monitor mode from any menu locations.
• Network Compatibility
A serial RS-485 Modbus-RTU port is standard.
The SJ700 can communicate with DeviceNet, PROFIBUS-DP and other networks with communication options.
-DeviceNet is a trade mark of Open DeviceNet Vender Association, inc.
-PROFIBUS-DP is a registered trade mark of PROFIBUS Nutzer organization.
• Conformity to Global Standards.
CE, UL, c-UL, C-Tick appovals.
Logic Input and Output Terminal apply sink & source logic.
Wide Input power voltage range.
• Instantaneous Power Failure Disregard Function
The SJ700 ignores instantaneous power failure when power fluctuation happens frequently, as long as DC bus voltage remains higher than under-voltage trip level.
• Emergency Stop
Shuts down the inverter by hardware, bypassing the CPU to achieve a reliable, emergency stop function.
• Intelligent input terminal and output terminal ON/OFF delay function.
Helps simplify external circuits.
• Active Frequency matching function.
• Controlled deceleration and stop on power loss.
• Analog Input disconnection Detection Function.
The SJ700 outputs a disconnection signal when frequency command through analog input is lost.
• Analog Command Holding Function (AHD)
Output frequency can be changed with UP/DOWN Function or with an analog as reference value. The set frequency at power shutdown can be saved too.
• Pulse train input function
Frequency reference or PID feedback signal with SJ-FB.
• Integrated Input Electric Power Monitor
Input electric power (kW) and integrated input electric power for monitoring energy saving.
• Automatic Carrier Frequency Adjustment Function
The SJ700 detects motor current and automatically reduces carrier frequency according to the current.
The resolution of analog outputs (voltage & current) improved by 10 bits.