The primary function of a variable frequency drive (VFD) is to vary the speed of a three-phase ac induction motor. VFDs also provide nonemergency start and stop control, acceleration and deceleration, and overload protection. In addition, VFDs can reduce the amount of motor start-up inrush current by accelerating the motor gradually. For these reasons, VFDs are suitable for conveyors, fans, and pumps that benefit from reduced and controlled motor operating speed.
A VFD converts incoming ac power to dc, which is inverted back into three-phase output power. Based on speed setpoints, the VFD directly varies the voltage and frequency of the inverted output power to control motor speed.
There is one caveat: Converting ac power to a dc bus—and then back to a simulated ac sine wave—can use up to 4% of the power that would be directly supplied to a motor if a VFD were not used. For this reason, VFDs may not be cost-effective for motors run at full speed in normal operation. If a motor must output variable speed part of the time, and full speed only sometimes, a bypass contactor used with a VFD can maximize efficiency.
Consider your reasons for choosing a VFD
Typical reasons for considering VFDs include energy savings, controlled starting current, adjustable operating speed and torque, controlled stopping, and reverse operation. VFDs cut energy consumption, especially with centrifugal fan and pump loads. Halving fan speed with a VFD lowers the required horsepower by a factor of eight, as fan power is proportional to the cube of fan speed. Depending on motor size, the energy savings could pay for the cost of the VFD in less than two years.
Starting an ac motor across the line requires starting current that can be more than eight times the full load amps (FLA) of the motor. Depending on motor size, this could place a significant drain on the power distribution system, and the resulting voltage dip could affect sensitive equipment. Using a VFD can eliminate the voltage sag associated with motor starting, and cut motor starting current to reduce utility demand charges.
Controlling starting current can also extend motor life because across-the-line inrush current shortens life expectancy of ac motors. Shortened lifecycles are particularly prominent in applications that require frequent starting and stopping. VFDs substantially reduce starting current, which extends motor life, and minimizes the necessity of motor rewinds.
The ability to vary operating speed allows optimization of controlled processes. Many VFDs allow remote speed adjustment using a potentiometer, keypad, programmable logic controller (PLC), or a process loop controller. VFDs can also limit applied torque to protect machinery and the final product from damage.
Controlled stopping minimizes product breakage or loss, as well as equipment wear and tear. Because the output phases can be switched electronically, VFDs also eliminate the need for a reversing starter.
Select the proper size for the load
When specifying VFD size and power ratings, consider the operating profile of the load it will drive. Will the loading be constant or variable? Will there be frequent starts and stops, or will operation be continuous?
Consider both torque and peak current. Obtain the highest peak current under the worst operating conditions. Check the motor FLA, which is located on the motor’s nameplate. Note that if a motor has been rewound, its FLA may be higher than what’s indicated on the nameplate.
Don’t size the VFD according to horsepower ratings. Instead, size the VFD to the motor at its maximum current requirements at peak torque demand. The VFD must satisfy the maximum demands placed on the motor.
Consider the possibility that VFD oversizing may be necessary. Some applications experience temporary overload conditions because of impact loading or starting requirements. Motor performance is based on the amount of current the VFD can produce. For example, a fully loaded conveyor may require extra breakaway torque, and consequently increased power from the VFD.
Many VFDs are designed to operate at 150% overload for 60 seconds. An application that requires an overload greater than 150%, or for longer than 60 seconds, requires an oversized VFD.
Altitude also influences VFD sizing, because VFDs are air-cooled. Air thins at high altitudes, which decreases its cooling properties. Most VFDs are designed to operate at 100% capacity up to an altitude of 1,000 meters; beyond that, the drive must be derated or oversized.
Be aware of braking requirements
With moderate inertia loads, overvoltage during deceleration typically won’t occur. For applications with high-inertia loads, the VFD automatically extends deceleration time. However, if a heavy load must be quickly decelerated, a dynamic braking resistor should be used.
When motors decelerate, they act as generators, and dynamic braking allows the VFD to produce additional braking or stopping torque. VFDs can typically produce between 15% and 20% braking torque without external components. When necessary, adding an external braking resistor increases the VFD’s braking control torque—to quicken the deceleration of large inertia loads and frequent start-stop cycles.
Determine I/O requirements
Most VFDs can integrate into control systems and processes. Motor speed can be manually set by adjusting a potentiometer or via the keypad incorporated in some VFDs. In addition, virtually every VFD has some I/O, and higher-end VFDs have multiple I/Os and full-feature communications ports; these can be connected to controls to automate motor speed commands.
Most VFDs include several discrete inputs and outputs, and at least one analog input and one analog output. Discrete inputs interface the VFD with control devices such as pushbuttons, selector switches, and PLC discrete output modules. These signals are typically used for functions such as start/stop, forward/reverse, external fault, preset speed selection, fault reset, and PID enable/disable.
Discrete outputs can be transistor, relay, or frequency pulse types. Typically, transistor outputs drive interfaces to PLCs, motion controllers, pilot lights, and auxiliary relays.
Relay outputs usually drive ac devices and other equipment with its own ground point, as the relay contacts isolate the external equipment ground. The frequency output is typically used to send a speed reference signal to a PLC’s analog input, or to another VFD running in follower mode. Typically, general-purpose outputs of most VFDs are transistors. Sometimes one or more relay outputs are included for isolation of higher-current devices. Frequency pulse outputs are usually reserved for higher-end VFDs.
Analog inputs are used to interface the VFD with external 0 to 10 VDC or 4 to 20 mA signals. These signals can represent a speed setpoint and/or closed loop control feedback. An analog output can be used as a feedforward to provide setpoints for other VFDs so other equipment will follow the master VFD’s speed; otherwise, it can transmit speed, torque, or current measurement signals back to a PLC or controller.
Select the proper control mode
VFD control mode choice greatly depends on the application. The three VFD control modes are volts-per-Hertz (V/Hz), sensorless vector (sometimes called open-loop vector), and closed-loop.
V/Hz-type VFDs use the ratio between voltage and frequency to develop the operating flux to supply operating torque to the motor. Sensorless-vector VFDs have accurate torque control over a wide speed range without having to use encoder feedback. Closed-loop VFDs use encoder feedback to obtain motor speed and slip information.
V/Hz control is adequate for many applications such as fans and pumps. However, for applications that require greater degrees of speed regulation, sensorless vector or closed-loop control types may be necessary. Applications such as paper mills, web printing presses, or material converting require the added speed regulation that closed-loop control provides.
Typically, general-purpose outputs of most VFDs are transistors. Sometimes one or more relay outputs are included for isolation of higher-current devices. Frequency pulse outputs are usually reserved for higher-end VFDs.
Analog inputs are used to interface the VFD with external 0 to 10 VDC or 4 to 20 mA signals. These signals can represent a speed setpoint and/or closed loop control feedback. An analog output can be used as a feedforward to provide setpoints for other VFDs so other equipment will follow the master VFD’s speed; otherwise, it can transmit speed, torque, or current measurement signals back to a PLC or controller.
Joe Kimbrell is product manager for drives, motors, and motion control for AutomationDirect.
