To mitigate harmonic, the existing VFDs were specified as 18-pulse drives and line reactors. The 18-pulse drives were proven to reduce harmonic level below IEEE 519-2014 levels. However, over the past few years, advancements in drive technology and filter designs have been shown to be just as effective at reducing harmonics to IEEE 519-2014 levels for drives lower than 100 horsepower with a smaller footprint and lower cost. For drives greater than 100 horsepower 18-pulse drives are still an option but are slowly being replaced with AFE and the Matrix drive.
Effective harmonic mitigation techniques include:
- Installing 3% or 5% line reactors. The line reactor is an inductor that adds impedance between the incoming power supply and the VFD. The additional impedance helps to protect the drive from input power disruptions and also reduces harmonics levels seen by the source and upstream equipment. Per Transcoil International, line and load reactor basics, if a reactor is specified as either 3% or 5%, this means the reactor will project a 3% or 5% impedance when the current flowing through it is at the rated current of the VFD. Line reactors have been to reduce harmonics level to IEEE 519-2014 levels.
- Installing 12-pulse or higher drives.
- Installing passive/tuned filters (reactors and capacitor); reduces power factor and may cause resonance.
- Installing active filters, which must be oversized due to decreased power factor.
- Installing AFE or Matrix drives.
Reflected waves — Reflected waves, when not mitigated, cause damage to motors and motor leads. Reflected waves are caused by switching on/off the IGBTs in the inverter section of the drive. With newer technology, the IGBTs can be switched on/off in less than a microsecond at upward of 20 kilohertz. The switching causes a pulse to be transmitted down the motor leads. Due to impedance mismatches this pulse can be reflected to the drive.
Because of the higher rates of switching and long cable runs, the reflected waves do fully decay before the next pulse. As a result, the reflected waves encounter incoming waves, their values add up causing higher peak voltages ranging from 1,000 to 2,000 volts at the motor terminals. These reflected waves and peak voltage spikes stress cable insulation, motor insulation and motor windings and bearings effectively reducing the life span of the equipment. Reflected waves are typically not a concern with VFDs rated 208/240 volts AC.
To protect the motor and cabling most of the existing VFDs were equipped with load reactors. Additionally, the motors were specified as inverter duty. The insulation systems for inverter-duty motors should be designed to withstand an upper limit of 3.1 times the motor’s rated line-to-line voltage. This is equivalent to an upper limit of 1,426 peak volts at the motor terminals for a motor rated at 460 volts.
Additional mitigation techniques include:
- Use inverter-duty motors.
- Install motor lead lengths per manufacturer recommendations.
- Install reactors (typically protects to about 500 feet) at the inverter output if motor lead lengths exceed manufacturer recommendations.
- Install derivative of the voltage with respect to time (dv/dt) filters at the inverter output (typically protects to about 2,000 feet).
- Sine filter at the inverter output (not distance limited).
- Snubber circuit at motor (not distance limited).
Electromagnetic interference — When a lot of drives are installed in an area, there is a possibility of EMI or common mode currents. These unwanted electrical signals, mostly known as noise, are introduced into the control system and produce undesirable effects in the system. They can cause communication errors, degraded equipment performance and equipment malfunction or nonoperation. Equipment affected by EMI include instruments that uses 4 to 20 milliamperes current loop such as ultrasonic sensors, weighing and temperature sensors, proximity or photoelectric sensors and computers.
Communication equipment such as programmable logic control communication links including RS-232, RS-484, remote input/output, data highway plus, scan bus and device net also can be affected.
Mitigation techniques include:
- Properly installed low–impedance ground system.
- Install input and output shielded VFD cable.
- Install common mode choke.
- EMI filters.
Reactor and filter designs should be recommended by the drive manufacturer because low loss reactors may actually resonate the voltage.
Temperature — VFDs can generate a considerable amount of heat. This occurs during switching on/off the IGBTs. The switching frequency, ranging from 2 kilohertz to 20 kilohertz or greater, is what the drives uses to control the motor. The higher the switching frequency, the more heat is generated. The switching frequency can be changed to help reduce the temperature in the drive. However, adjusting the VFD’s switching frequency comes at a price.
The following results from adjusting the switching frequency at higher frequencies:
- Lower audible noise is heard from the motor. The typically audible range for a person is 20 to 20,000 hertz. PWM produces harmonic currents in the stator of the motor. The magnetic fields produced by these currents can cause vibrations in the mechanical portions of the motor stator resulting in a high–pitched sound resonating from the motor within humans’ audible levels. The high–pitched noise is directly related to the switching frequency; the higher the switching frequency, the higher the pitch. Increasing the switching frequency will eventually cause the frequency of the high–pitched sound to be above the sensitive threshold of most people, which is between 2,000 to 5,000 hertz.
- Increased internal heating inside the VFD. Increasing the switching frequency increases the rate at which the IGBTs turn on and off. This results heat losses that must be dissipated from the drive.
- Lower harmonics.
- Motor heating is reduced due to lower harmonics. Harmonic currents operate at a higher frequency and result in inductive heating at the motor. By reducing increasing harmonic currents, the inductive heating is reduced. There is an indirect relationship between harmonic currents and the switching frequency. The higher the switching frequency, the lower the harmonic current. By increasing the switching, inductive heating at the motor can be decreased.
- The drive requires a larger heat sink, which increases the size of the drive
At lower frequencies the reverse is true.
When equipment generates heat, this heat energy must be removed from the equipment to prevent overheating, which can cause the internal electronics to fail. Some of the heat is dissipated through heat sinks. Fans installed inside the drive are used to direct heat out of the drive through vents in the enclosure. VFDs reject heat under normal operation conditions and that appropriate ventilation/cooling must be provided to reduce the impact that heat would have on the service life of the VFDs.
Inverter duty type motors (see Figure 6), meet National Electrical Manufacturers Association MG 1: Part 31 and are suitable for use in VFD applications. These motors are manufactured to withstand voltage stress such as reflected waves and ensure reliable operation during the expected 20-year life span.
Inverter-duty motors are subject to effects caused by PWM, including increased motor losses, inadequate ventilation at lower speeds, increased dielectric stresses on motor windings, magnetic noise and shaft currents. The main concern when controlling a motor using a VFD is reflected waves. Reflected waves generated by VFDs produce peak voltages ranging from 1,000 to 2,000 volts that can be seen at motor terminals. As mentioned previously, the reflected wave peak voltage spikes stress cable insulation and motor insulation, windings and bearings effectively reducing the life span of the equipment.
To ensure reliable operation of motors, NEMA MG 1: Motors and Generators provides two parts for specifying manufacturing standards for motors used with VFDs. In Part 184.108.40.206, the insulation of a motor is required to withstand peak voltage of 1,000 volts. The standard noted that as long as the peak voltage is less than 1,000 volts with a rise time greater than 2 microseconds, there will be no significant reduction in service life. However, as newer technology has been introduced into the market, Part 31 was added requiring a 460-volt rated motor to withstand to 1,600 volts at a rise time of 0.1 microseconds.
When VFDs and inverter-duty motors are installed and operated per manufacturer requirements, no significant reduction in service life should occur when the peak voltage at the motor terminals effectively mitigated. Where the installation requires installing a motor at a distance greater than the manufacturer recommended maximum distance, consider installing dV/dT filters or load reactors to reduce peak voltage spikes.
General-purpose versus inverter-duty motor
The inverter-duty motors have the following characters when compared with the standard induction motors.
- Inverter-duty motors are NEMA Premium efficiency type motors.
- Motor winding use vacuum impregnated technique to eliminate the air pockets due to increase in temperature.
- The insulation material is suitable for higher temperature rating. Normally the inverter-duty motors insulation class is greater than F (temperature rating of 155°C).