Interference solutions?

1.Distance of cable

2.Connecting multiple motors with one VFD

VFD has two control modes V/F and SVC. The parameter is F0.00. V/F mode can support one VFD connect multiple motors but SVC mode only connect one motor.

• Total power matching: the output power of the inverter must be greater than or equal to the sum of the total power of all motors.
• Matching motor parameters: Although the power does not have to be exactly the same, the rated voltage, frequency, number of poles and other parameters of the motor should be the same as far as possible. This ensures that all motors can work stably at the same frequency and have the same rotational speed.
• Load Characteristics: If the characteristics of the load carried by the motors differ greatly (e.g., some light loads, some heavy loads), it may lead to uneven force on the motor, or even overload.
• Starting and stopping characteristics: When the power of multiple motors varies greatly, the starting and stopping process may lead to current impact, especially when high-power motors are started. And if the motors are not started and stopped at the same time, it will also cause the inverter to be impacted by the current and overcurrent or overload.

3.The role of the various reactors of the VFD

Causes and solutions for serial mode interference and common mode interference.

(1)Definition of interference

Voltage and current variations are transmitted through wires in two patterns, which we call "common mode" and "differential mode. Power lines for equipment, communication lines for telephones, and communication lines for exchanges with other equipment or peripheral devices have at least two conductors, which are used as round-trip lines to transmit power or signals. However, there is usually a third conductor in addition to these two conductors, which is the "ground". Interference voltages and currents are divided into two types: one is the two conductors as a round-trip line transmission; the other is the two conductors as the way to go, the ground as the way back to the transmission. The former is called "differential mode", the latter is called "common mode".

(2)Common Interference Generation

Interference sources may exist both inside and outside the instrument. For example, outside the instrument, high-power power equipment, high-power transformers, power grids may become sources of interference. Inside the instrument, power transformers, coils, relays, switches and power lines can be sources of interference.

(3)Pathways for interference sources to enter the instrument

1.   The signal source and the instrument between the connecting wires, wiring within the instrument through the magnetic coupling in the circuit to form interference. In high-power transformers, AC motors, power lines and the surrounding space there is a very strong AC magnetic field, and the closed loop in this changing magnetic field will be induced potential. This induced potential and useful signals in series, when the sensor and the display instrument is far away, this series mode interference is particularly prominent.
2.   The source of interference through capacitive coupling in the loop to form interference, it is the result of the interaction of the two electric fields. By means of electrostatic coupling, can be induced in the two inputs to the ground of the common voltage, in the form of common-mode interference, due to common-mode interference is not superimposed on the signal, it does not have a direct impact on the instrument. But it can be formed through the measurement system to the ground leakage current, the leakage current through the resistance of the coupling can directly act on the instrument and produce interference. Electromagnetic induction, electrostatic induction formed by the interference is mostly industrial frequency interference voltage, but frequency converters, motors with rectifiers, etc. will produce harmonic interference. Due to the role of lightning in the power line will also be induced interference voltage.
3.   In some temperature measurement occasions, when the thermocouple electrodes directly welded to the metal parts heated by electricity, due to the metal parts in parallel to the current direction of the points there is a potential difference, then the introduction of the interference voltage is also very large. In the high temperature state, the insulation resistance of refractory materials drop sharply, thermocouple porcelain protection tube, porcelain beads of insulation will also decline, then the furnace power supply voltage through the refractory bricks, thermocouple casing, porcelain beads, etc. leakage to the thermocouple filament, the thermocouple electrodes and the ground between the generation of interference voltage.
4.   The earth often exists between different points of potential difference, especially in the vicinity of high-power power equipment, when the insulation of these devices is reduced, the potential difference is even greater. The field instrumentation in use, sometimes do not pay attention to make the circuit there are more than two grounding points, it will be different grounding points of the potential difference into the display instrumentation and the formation of common mode interference.
5.   When the instrument's bridge power supply ground, in addition to the bridge output unbalanced signal voltage, the signal line to ground there is a common voltage, the common voltage is not the signal voltage to be measured, but a manifestation of common mode interference.

(4)Serial mode interference (differential mode interference)

• Take the two PCB alignments (wires connecting the various components of the motherboard) on the motherboard as an example, the so-called serial mode interference, refers to the interference between the two alignments;
• Source: High-power transformers, AC motors, frequency converters, etc. have strong alternating magnetic fields, if the instrument measurement and control of the connecting wires through the alternating magnetic field, will be subject to the action of these alternating magnetic fields, in the input circuit of the instrument induced AC voltage, which becomes an interference signal.
• Measurement method: use the electronic tube voltmeter, in the field can be used with AC millivolt gear digital multimeter for measurement. The voltmeter is connected across the positive and negative terminals of the instrument input to measure, usually series mode interference voltage is mostly in the range of a few millivolts to tens of millivolts.
• Reduction measures:

1. such as thermocouples, analyzing the signal line of the instrument to be transported away from the strong electromagnetic field, not too close to the power line;
2. do not put the instrumentation signal line, control signal line and power line parallel to the same bridge tray, or wear in the same pipe when necessary, the signal line should use shielded wires or shielded cables, the line of the shield to take one end of the grounding method;
3. Add a filter circuit to the meter input;
4. for intelligent instrumentation to set the digital filtering constants according to the site conditions and then increase the number of levels of filtering circuits if necessary.

(5)Common mode interference (ground interference)

• It is the interference caused by the potential difference between the two alignments and the PCB ground.
• Sources: high-voltage electric field interference; measuring the temperature of the electric furnace when the introduction of interference, such as at high temperatures, electric heating furnace power supply through the refractory bricks, thermocouple porcelain protection sleeve leakage to the thermocouple on the thermocouple, so that the thermocouple and the ground between the interference voltage; due to the ground potential is different and the introduction of interference; and ammonia synthesis tower with an electric heater to raise the temperature of thermocouples can also be caused by interference. The source of the interference is mostly AC voltage may also be DC voltage.
• Measurement method: Use high resistance voltmeter to measure, or use the AC voltage block of digital multimeter to measure. As shown in the figure above, first connect the voltmeter to the positive end of the meter input and ground measurement, and then connect the voltmeter to the negative end of the meter input and ground measurement, usually common mode interference voltage is mostly in the range of a few volts to tens of volts.
• Reduction measures:

1. Float the measuring thermocouple;
2. Instrumentation amplifiers also take floats;
3. Do not use exposed thermocouples to avoid grounding the hot electrode if the measurement object permits;
4. Thermocouple protection sleeve to be reliably grounded;
5. Adopt equipotential shielding when using shielded wires;
6. Add bypass capacitors to the signal line.