How to choose a suitable control method for the inverter

       How to choose a suitable control method for the inverter
　　Electric motors account for at least half of America's energy consumption. Choosing the right control method for the application allows the motor to run efficiently while simultaneously optimizing torque and overall performance. Efficiently running motors can also reduce energy consumption and reduce downtime, thereby saving more costs.
　　For a motor controlled by a frequency converter (VFD), most of the control methods used determine the efficiency and performance of the motor in the application. Once engineers and designers understand the advantages, disadvantages and specific specifications of each control method, choosing the right control method for any application becomes simple.
　　The frequency conversion technology was born in response to the need of stepless speed regulation of AC motors. After the 1960s, power electronic devices experienced SCR (thyristor), GTO (gate turn-off thyristor), BJT (bipolar power transistor), MOSFET (metal oxide field effect transistor), SIT (static induction transistor) , SITH (static induction thyristor), MGT (MOS control transistor), MCT (MOS control thyristor), IGBT (insulated gate bipolar transistor), HVIGBT (high voltage insulated gate bipolar thyristor) development process, device update promotion The continuous development of power electronic conversion technology. Since the 1970s, the research on pulse width modulation variable voltage frequency conversion (PWM-VVVF) speed regulation has attracted people's attention. In the 1980s, the PWM mode optimization problem as a frequency conversion technology attracted people's great interest, and many optimization modes were obtained, among which the saddle wave PWM mode was used. Beginning in the second half of the 1980s, VVVF inverters in developed countries such as the United States, Japan, Germany, and Britain have been put on the market and have been widely used.
　　How to choose the inverter control mode reasonably
　　The control mode is the key to determining the performance of the inverter. At present, there are many brands of low-voltage general-purpose inverters on the market, including more than 50 kinds of European, American, Japanese and domestic ones. When selecting the inverter, don't think that the higher the grade, the better, but according to the characteristics of the load to meet the requirements of use, so that it can be used only in quantity and economical.
　　How to choose a suitable control method for the inverter
　　Control Method
　　Many people in the industry believe that the control method is the sequencing method of controlling the VFD, such as setting to determine whether the input control terminal of the VFD is interfaced with a maintained contact or a momentary button to start and stop the drive. The control method that this article focuses on may be more accurately called the motion control method. They determine how the VFD controls the motor.
　　There are four main types of motor control methods for induction motors connected to VFDs: V/f (Volt/Hertz), V/f with encoder, open loop vector and closed loop vector. These methods all use pulse width modulation (PWM), which modulates the pulse duration to change the width of a fixed signal to generate a variable analog signal.
　　By using the fixed DC voltage of the VFD DC bus capacitor, PWM is applied to the VFD. A set of insulated gate bipolar transistors (IGBTs) on the output side quickly turn on and off to generate pulses. Changing the output pulse width in the output voltage waveform can construct an analog AC sine wave. Even if the driver's output voltage waveform is composed of a square wave caused by a DC pulse, the current waveform is also a sine wave because the motor is inductive. All motor control methods rely on the PWM voltage waveform to control the motor. The difference between the control methods is how they calculate the voltage demand of the motor at any given moment.
　　Pulse width modulation is usually used to control AC motors. During this process, the carrier frequency (shown in red) is the rate at which the VFD output transistor is gated or tuned. The carrier frequency can usually be 2 to 15 kHz. The frequency reference (blue) is the speed signal sent to the motor, usually 0 to 60 Hz. When covering two waveforms, engineers can use the intersection between the two curves to modulate the output DC pulse (black) to provide the required speed control.
　　V/f control
　　Different V/f modes enable VFD to control multiple different applications while maintaining the performance of each application. The constant torque mode is straight, which results in a constant V/f ratio, providing constant motor torque throughout the speed range. Variable torque mode has lower voltage at lower speed to prevent motor saturation.
　　Volt/Hz, usually called V/f, is a simple motor control method. It is often used because of its "plug and play" simplicity and the amount of motor data required by the drive. It does not require an encoder, nor does it need to tune the VFD to the motor (but recommended). This means lower costs and less wiring. V/f control is usually used when it needs to run, and may exceed 1,000 Hz, so it is usually used in machine tool and spindle applications.
　　V/f is a control method that allows multiple motors to run from a single VFD. In this case, all motors start and stop at the same time and follow the same speed reference.
　　V/f has some limitations. For example, when using V/f, there is no guarantee that the motor shaft is rotating. In addition, the starting torque of the motor is limited to 150% of the 3 Hz output. For most variable torque applications, the limited starting torque is more than sufficient. In fact, almost all variable torque fan and pump applications in the field use V/f control.
　　The relative simplicity of the V/f method is partly due to its "more relaxed" specification. Speed adjustment is usually 2% to 3% of frequency. The speed response rating is 3 Hz. Speed response is defined as the degree to which the VFD responds to changes in the reference frequency. When the reference frequency changes, the increase in speed response results in a faster motor response.
　　The control method also has a speed control range (expressed as a ratio). The speed control range of V/f is 1:40. Multiplying this ratio by the frequency can determine the operating speed of the VFD controlled motor. For example, using a frequency of 60 Hz and a speed control range of 1:40, a drive using V/f control can control the motor at 1.5 Hz.
　　AV/f mode defines the ratio of voltage to frequency of the motor, VFD can only program one V/f mode. The V/f mode or curve determines the voltage output to the motor based on the given speed reference (frequency).
　　Operators or technicians can use a single parameter to select the preset V/f mode in VFD programming. The preset modes are optimized for specific applications. Users can go a step further and program a custom V/f mode or configuration file to tune the VFD to a specific application and the motor being used.
　　Applications such as fans and pumps are variable torque loads. The variable torque V/f mode prevents malfunctions and improves performance and efficiency. This mode reduces the magnetizing current at low frequencies by reducing the motor voltage at lower frequencies.
　　Similarly, constant torque applications, such as conveyors, extruders and hoists, should use the constant torque V/f mode. Constant torque applications require full magnetizing current at all speeds. Therefore, a straight ramp is constructed and followed in the entire speed range.
　　Normally, the VFD will output voltage according to any speed set by the motor while referring to the V/f mode.
　　How to choose a suitable control method for the inverter
　　V/f with encoder
　　If the application requires more speed adjustment and can run at a higher reference frequency, you can add an encoder to the V/f control. Encoder feedback reduces the speed adjustment to 0.03% of the frequency. The output voltage is still determined by the selected V/f mode programmed in the VFD by the technician. This allows high-speed control without high dynamic response because the voltage and frequency are predetermined.
　　This kind of control method is not common, because it needs the cost of encoder and feedback card, and its advantages over standard V/f control are. Starting torque, speed response and speed control range are the same as V/f control. In addition, the higher operating frequency is limited by the number of pulses per revolution produced by the encoder.
　　Open loop vector control
　　Open loop vector (OLV) control is used for larger and more dynamic motor control. It independently controls the motor speed and torque, just like controlling a DC motor.
　　How to choose a suitable control method for the inverter
　　According to the direction of the motor (forward or reverse) and whether the motor is electric or regenerative, the torque limit can be divided into four quadrants. For example, the bottle cap will need to set a torque limit for quadrant 1. Alternatively, an unfolding application will require positive motor rotation to feed the wire, but a negative torque limit due to regeneration due to the wire being pulled to generate tension. Therefore the torque limit will be set in quadrant 4.
　　When running OLV, the motor can generate 200% of the rated torque at a frequency of 0.3 Hz. Higher starting torque at lower speeds opens the door for various applications. This control method also allows four-quadrant torque limitation.
　　Torque limit mainly limits the motor torque to prevent damage to equipment, machinery or products. They are divided into four different quadrants, depending on the direction of the motor (forward or reverse) and whether the motor is electric or regenerative. The limit can be set individually for each quadrant, or the user can program the overall torque limit into the VFD.
　　Motor state means that the speed and torque of the motor are in the same direction. For example, the forward speed and forward torque will move the conveyor forward. Regeneration refers to the overhaul of the motor by the load. On an AC motor, when the rotor rotates faster than the magnetic field in the stator, it acts as a generator. This causes the regenerative energy to flow back to the VFD.
　　How to choose a suitable control method for the inverter
　　For example, the bottle cap can use the torque limit in quadrant 1 (forward rotation and forward torque) to prevent excessive torque on the bottle cap. It moves forward and uses positive torque to place the cap on the bottle. Applications involving elevators with counterweights heavier than empty cars have limitations in quadrant 2 (reverse motor rotation and positive torque). If the empty car is transferred to a higher floor, the torque is in the opposite direction of the speed to maintain control of the counterweight and the speed and position of the elevator under the action of gravity.
　　The machine drill (reverse motor direction and negative torque) that takes out the screw from the wooden block can use the limit in quadrant 3. Unwinding applications can use quadrant 4 limits (positive motor rotation and negative torque). The electric motor rotates the rotating electric motor to feed the line, but because the line is pulled to create tension and cause regeneration, a negative torque limit is also required.
　　Vector control reduces each torque by keeping the torque generating current (Iq) and magnetizing current (Id) at 90°. I1 represents the total motor current (Iq + Id). If ¨ "90°, then sin¨ "1; if ¨ "90°, then sin ¨" 1; but if ¨ = 90°, then sin¨ = 1 and the torque reaches the value. The VFD tries to keep ¨ at 90° to simulate a DC motor. In a DC motor, the brushes and commutator are mechanically positioned at 90° to continuously generate torque.
　　The current feedback loop in these VFDs allows the user to set torque limits and operate in all four quadrants. As the motor current increases, the motor torque also increases. If the application requires more torque, the output voltage input to the motor can be increased; when the torque limit is reached, the output voltage can be reduced. Unlike V/f control, this makes the open loop control dynamic.
　　In addition to torque limitation, open-loop control also has a faster speed response of 10 Hz, enabling it to handle more dynamic responses to shock loads. For example, the load of a rock crusher will constantly change, depending on the size and amount of rock being processed.
　　OLV control uses a vector algorithm to find the output voltage of the running motor, rather than a fixed V/f mode. Vector control achieves this by using current feedback from the motor. The current feedback is measured by the current transformer inside the VFD. Constant current readings and fast calculations performed in the VFD determine the current torque demand and flux. Basic vector mathematics decomposes the magnetizing current of the motor and the current that produces torque into vectors. OLV control largely depends on motor dynamics, so some type of motor self-tuning must be performed to ensure that the VFD has as much motor data as possible.
　　With the help of reliable motor data/parameters, VFD can calculate magnetizing current (I d) and torque generating current (I q) as vectors. In order to obtain efficiency and torque, the VFD must separate these two vectors by 90°. 90° is important because sin(90) = 1, and the value 1 represents the motor torque.
　　Overall OLV control leads to tighter control. The speed adjustment is +/- 0.2% of the frequency, and the speed control range jumps to 1:200, allowing low-speed operation without sacrificing torque.
　　Closed loop vector control
　　Closed-loop vector control uses a vector algorithm to determine the output voltage, which is very similar to open-loop control. The key difference is that the closed-loop vector uses an encoder. The use of encoder feedback in conjunction with vector control means that 200% of the rated motor torque can be obtained at 0 rpm. This is a selling point for applications that need to keep the load without moving, such as elevators, cranes, and lifts.
　　Encoder feedback allows the speed response to exceed 50 Hz, and the speed control range is 1:1500, which is among all control methods. Closed loop control can also run the motor in torque control mode. Torque control allows the VFD to control the motor torque instead of the motor speed. This is required in any application where torque is more important than speed. Winders, rewinders, cappers and web applications are good examples of using torque control.