Variable Frequency Drives (VFDs) and soft starters are essential components in modern industrial and commercial systems. VFDs provide precise control over motor speed, torque, and efficiency, matching speed to load requirements to minimize energy consumption. Soft starters gradually increase motor voltage to limit inrush current and prevent damage to the electrical system. Controlled inrush current reduces wear on mechanical components such as gears, belts, and couplings. By adjusting motor operations to match load demands, VFDs enhance energy efficiency, improve process control, and reduce mechanical wear, making them indispensable for HVAC and manufacturing systems applications such as elevators, escalators, conveyors, crushers, and mixers. Soft starters are more appropriate for constant-speed applications such as pumps and fans. Figure 1 illustrates examples of machines that benefit from the use of VFDs and soft starters.
The advanced electronic components in VFDs and soft starters are susceptible to electrical hazards such as overcurrent events, transient voltage spikes, and harmonic distortions. These hazards can lead to equipment failures, costly downtime, and reduced system lifespan without proper circuit protection. Ensuring the reliability and longevity of VFDs and soft starters requires implementing robust protective measures to safeguard them from electrical hazards.
This article provides designers with best practices for protecting VFD and soft starter designs from overcurrent events, transient voltage spikes, and electrostatic discharge (ESD). It also offers recommendations for maximizing the efficiency of these designs to ensure they consume the least amount of power. Circuit protection and high efficiency will contribute to the development of robust products.
Variable frequency drives
Figure 2 shows an example VFD. The blocks surrounding the VFD indicate the protection and control components that make the VFD robust to electrical hazards and provide efficient control. Figure 3 provides a block diagram of a VFD. The adjacent table on the right lists the recommended components for protection and control and their location in the specific circuit blocks.
VFDs control a range of motors as small as 0.25 HP (0.186 kW) to over 10,000 HP (7457 kW). Most motor applications, such as HVAC systems, pumps, conveyors, and other industrial applications, require motor sizes from 5 HP (3.728 kW) to 10,000 HP (7457 kW). As a result, VFDs are typically powered from 3-phase, 400 V AC, and higher-voltage power lines. These lines can deliver damaging high currents during an overcurrent condition and are susceptible to voltage transients induced by lightning and the switching of large inductive loads.
Protecting Variable Frequency Drives (VFDs)
The following section identifies the challenges to VFDs caused by electrical hazards. Subsections discuss the recommended protection components for each circuit block within the VFD.
The AC Input Protection circuit provides the first line of defense from overcurrent conditions and voltage transients. An overload or a short circuit in the motor or the VFD itself can draw a large current from the AC power line. Voltage transients induced on the AC lines can generate damaging voltages and inject high current spikes into the VFD circuit. Fuses and metal oxide varistors (MOVs) are recommended to disconnect the circuitry from the power line and absorb transients. Placing these components as close as possible to the input terminals of the VFD minimizes the propagation of transients into the downstream circuitry.
Consider a fast-acting fuse in each of the lines of a 3-phase power source. A fast-acting fuse protects surge-sensitive components. Ensure the selected fuse has:
- An interrupting rating that exceeds the short-circuit current capacity of the AC power line
- A voltage rating exceeding the maximum AC line voltage
- UL or CSA component recognition to save time and cost when obtaining a nationally recognized testing lab (NRTL) certification for a VFD system.
For voltage transient protection, select an MOV that:
- Safely absorbs the maximum estimated peak pulse current – MOVs can have peak pulse current ratings of up to 10 kA
- Safely absorbs the maximum estimated pulse energy – MOVs can have peak pulse energy ratings of up to 480 J
- Includes a UL 1449 or an equivalent NRTL rating.
Some MOV models include a thermal element that opens to protect the MOV in the event of a long-duration transient. The thermal element will open to protect the MOV from overheating. These models have a third lead, indicating if the MOV has been disconnected from the circuit and no longer provides transient protection. This type of MOV ensures that the system is always aware of the MOV’s status for maximum system safety.
Inverter circuit
The Inverter circuit supplies 3-phase AC voltage to the motor. Protect the Inverter circuit from a damaging short circuit event using high-speed semiconductor fuses. This fuse type can interrupt an overcurrent, up to five times the fuse rating, within milliseconds. They can operate in circuits with voltages up to 1300 V AC and 1000 V DC. Models can have current ratings up to 2250 A to address a wide range of applications.
Overtemperature protection of the inverter’s power semiconductors is essential for robust, reliable operation. Negative temperature coefficient (NTC) thermistor probes will detect device temperatures, allowing the control system to interrupt power delivery if any power semiconductor reaches an unsafe temperature. Thermistors have long-term stability and tolerances from 2 to 10 %. Designers can integrate thermistor elements into a ring lug assembly to simplify mounting devices or heat sinks.
Auxiliary power supply circuit
The MOSFET and its gate driver in the Auxiliary Power Supply need protection from transients and ESD. A transient voltage suppressor (TVS) diode can provide the necessary protection. TVS diodes can have a range of capabilities, some of which include:
- Absorption of up to 1500 W of peak pulse power or a surge current of up to 120 A
- Protection from ESD strikes as large as 30 kV
- Response times under a nanosecond
- Be either uni-directional or bi-directional
- Surface mount packaging.
Fortunately, protecting the VFD requires a small number of components; however, without these components, the defense of the VFD would be severely compromised.
Efficient control for the VFD
Managing and delivering high power can lead to substantial power loss through the VFD circuitry. The following sub-sections of the article offer component options for optimizing the efficiency of the VFD circuit blocks. Options for the designer range from discrete components to complete assemblies.
Rectifier circuit
The Rectifier circuit converts the power line AC voltage into a DC voltage. Multiple options exist to develop an efficient, high-power rectifier design. Designers can use individual diodes, diode modules, AC bridges, and power stacks. Individual diodes are available with low forward voltage and low microampere and milliampere reverse leakage current. Depending on the power level required, a rectifier can be constructed from single diodes, phase-leg assemblies, or even an all-in-one solution. Power stacks, which consist of a phase-leg or a complete assembly, include diode and thyristor bridges, high-speed fuses, and surge suppressors, and are also available. Power stacks are a drop-in solution for high-power rectification. Options are available that can handle load current as high as 10 kA.
Brake Chopper circuit
The Brake Chopper dissipates the braking energy in a resistor and maintains the DC rail voltage safely. The circuit uses an IGBT as a switch to control the current into the resistor. The IGBTs can sink hundreds of amps and can be paralleled to manage even higher currents. Available modules include an IGBT and a free-wheeling diode. Modes can have international standard or surface mount packaging depending on the current requirement.
Inverter circuit
The Inverter generates the 3-phase AC power output to drive the motor. The designer can utilize options for this application, including half-bridge IGBT pairs, 3-phase IGBT modules, and discrete transistors. See Figure 4. These components can operate in circuits above 1000 V, and some models can deliver up to 200 A. Many of these components have low saturation voltages of VCE(SAT) < 2 V for low-loss on-state operation.
The half-bridge modules are housed in surface mount packages with an isolated back surface. The package has a low coupling capacity between its pins and a heatsink. The 3-phase bridge modules use high-pin-count packages with a copper baseplate and internal direct copper bonding. These modules are rated to survive a 10 µs duration short circuit. One highly integrated module includes the 3-phase rectifier and inverter circuits to enable maximum PCB space saving. This module also has a thermistor for temperature monitoring.
Figure 4 illustrates the various components designers can use for the Rectifier, Brake Chopper, and Inverter stages. The various package styles cater to a wide range of applications.
Figure 4. Component and packaging options for the power delivery circuits
Auxiliary Power Supply circuit
The Auxiliary Power Supply provides power to control, sensing, and communication circuitry. It also powers the pulse width modulator and the cooling fans. Silicon (Si) MOSFETs, silicon carbide (SiC) MOSFETs, and high-voltage IGBTs can control the switch-mode power supply. High-voltage models of MOSFETs can have voltage ratings as high as 1200 V and 1700 V. The IGBTs can have VCES values exceeding 2500 V. MOSFETs offer higher switching frequencies and maximum power supply efficiency with very low RDS(ON), gate charge, and output capacitance values. High-voltage IGBTs eliminate the need for multiple series-connected components with high current output, particularly high peak current output.
Gate driver ICs efficiently drive SiC MOSFETs and IGBTs. These ICs can initiate fast switching of the power transistor with drive signals having matched rise and fall times as short as 7 ns. Propagation delays for the gate driver can be as low as a typical value of 16 ns. Some models include safety features such as thermal shutdown, under-voltage lockout (UVLO), and power transistor overcurrent monitoring with soft shutdown. These components can draw a low operating current of as little as 10 µA. Gate drivers offer a single-component solution for safe and efficient power transistor control. They are recommended for minimizing PCB space and power consumption in a switch-mode power supply design.
Control Switches
The VFD design requires switches for on/off control, drive configuration settings, input/output control, and testing. Selecting durable, long-life switches contributes to system reliability. Switches with an IP67 rating for dust and moisture protection, as well as a lifetime of at least 100,000 cycles, are available. The lifetimes for various models can be as high as 5,000,000 cycles. Numerous switch types and options are available to address a wide range of applications.
The VFD is now protected and optimized for efficiency. The same approach will be applied to soft starters.
Soft Starter
Figure 5 illustrates an example of a soft starter in a NEMA cabinet. The blocks surrounding the soft starter indicate the protection and control components that make the soft starter robust to electrical hazards and provide efficient control.
Figure 6 presents the circuit blocks of a Soft Starter design. The table on the right of Figure 6 lists the recommended protection and control components. Since a Soft Starter does not control motor speed, it has fewer circuit blocks.
Protecting the Soft Starter
The AC Input circuit must protect against electrical hazards. Temperature monitoring keeps the Phase Control circuit within thermal limits.
AC Input circuit
As with the VFD, the Soft Starter requires protection from overcurrent due to the high AC line current capacity and voltage transients induced on the AC line. Select a high-speed fuse like the fuses recommended for the VFD to limit damage from a short circuit. Models of high-speed fuses can interrupt a 10x overload in under 10 ms and can have a voltage rating as high as 700 V AC. For protection from voltage transients on the AC line phases, use the configuration of MOVs shown in Figure 6. The MOVs recommended for the VFD AC Input Protection circuit will protect the soft starter circuitry.
Phase Control circuit
The Phase Control circuit has the critical components that drive the motor. Monitoring the temperature of these components is essential for ensuring the safe operation of the Soft Starter. A thermistor like the one recommended for the VFD Inverter circuit can provide the necessary temperature protection.
Efficient control for the soft starter
The SCRs in the Phase Control circuit control the voltage applied to the motor and limit inrush current. Designers have numerous options for high-power SCR thyristor control. They can use individual thyristors, dual thyristor modules, or high-power packaged assemblies for 3-phase soft starters. When optimizing for efficiency, look for devices and assemblies with low forward voltage, low gate-controlled delay time, low reverse current, low gate power dissipation, and high current and voltage rise time rates.
Individual thyristors are available in through-hole, surface-mount, capsule, or threaded-stud-type packages. The capsule packages can carry as much as 8800 A and have low conduction losses with efficient heat dissipation.
Dual thyristor modules reduce component count and are available with a copper-bonded base. These modules can support up to 2000 V and supply up to 300 A.
High-power packaged assemblies, known as power stacks, integrate 3-phase control with cooling fans and heat sinks in a single package, reducing component count and optimizing thermal regulation. For safety, the power stacks include fusing and transient voltage protection. Power stacks provide a comprehensive solution for high-power phase control, with models capable of supporting currents exceeding 10,000 A at temperatures of up to 45 °C.
Low-profile rotary DIP switches or multi-pole coded rotary switches are good options for controlling motor current and mode selection. These switches are available with contact resistance under 150 mΩ and a life of 10,000 cycles. They offer a surface-mount package option and an IP54 environmental protection rating.
Conclusion
Ensuring reliable performance for VFDs and soft starters requires comprehensive protection against electrical hazards, including overcurrent conditions, overvoltage transients, and ESD. Additionally, the use of high-efficiency components enables significant energy savings. Manufacturers like Littelfuse provide a vast array of protection components, high-efficiency devices, semiconductors, and switches, allowing designers to streamline their supply chain. Designers can also save design and development time and compliance costs by leveraging the services offered by component manufacturers:
- Field application engineers who assist with the selection of cost-effective protection, efficient control, and sensing components
- Field application engineers with knowledge of the applicable safety standards
- Pre-compliance testing services to avoid compliance failures and reduce delays and expenses associated with repeated compliance test cycles.
Utilizing the services and recommended components from manufacturers like Littelfuse will help electronics engineers design robust, reliable, and efficient VFDs and soft starters.
About the Author
Christophe Warin is Systems Engineering Manager of the Semiconductor Business Unit at Littelfuse, Inc.
