Usage of Variable Frequency Drives & Impact on Machinery Performance

What is Variable Frequency Drive (VFD) ?

A Variable Frequency Drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. Other names for a VFD are variable speed drive, adjustable speed drive, adjustable frequency drive, AC drive, micro drive, and inverter.

Frequency (or hertz) is directly related to the motor’s speed (RPMs). In other words, the faster the frequency, the faster the RPMs go. If an application does not require an electric motor to run at full speed, the VFD can be used to ramp down the frequency and voltage to meet the requirements of the electric motor’s load

What is working principle of VFD?

When operated from a constant frequency power source (typically 50Hz),AC induction motors are fixed speed devices. A variable frequency drive controls the speed of an AC motor by varying the frequency supplied to the motor.

The drive also regulates the output voltage in proportion to the output frequency to provide a relatively constant ratio of voltage to frequency (V/Hz),as required by the characteristics of the AC motor to produce adequate torque.

The first step in this process is to convert the AC supply voltage into DC by the use of a rectifier. DC power contains voltage ripples which are smoothed using filter capacitors. This section of the VFD is often referred to as the DC link.

This DC voltage is then converted back into AC. This conversion is typically achieved through the use of power electronic transistors using a technique called Pulse Width Modulation (PWM). The output voltage is turned on and off at a high frequency, with the duration of on-time, or width of the pulse, controlled to approximate a sinusoidal waveform.

The entire process is controlled by a microprocessor which monitors the:

• incoming voltage supply,
• speed set-point,
• DC link voltage,
• output voltage and current to ensure operation of the motor within established parameters. 

What are advantages of using a VFD?

Keeps starting current in control: A VFD has the capability of starting the motor at zero voltage and frequency, which keeps a check on motor winding flexing and heat generation. This helps in extending the motor life.

Reduces power line disturbances: Any voltage sag caused in the power line can adversely affect voltage sensitive devices such as proximity switches, sensors, and computers. Using VFDs eliminates voltage sag.

Demands lower power on start: Power required to start an ac motor across the line is substantially greater than with a VFD. When industrial customers start these motors during peak hours of electrical consumption, they are likely to be charged with surge prices. However, with VFD demanding lower starting power, the issue can be addressed.

Helps in controlling operating speed and acceleration:  Allows speed to be remotely adjusted by a controller. Control is speed and acceleration is a big bonus to industries in a production process.

Limits and adjusts torque: The drive is capable of limiting and adjusting the amount of torque so the ac motor never surpasses this limit. This protects machinery from damage and protects the process or product.

Saves energy and cost: A VFD regulating a pump motor that usually runs less than full speed can cut down energy consumption over a motor running at constant speed for the same period. In addition, it eliminates the need for mechanical drive components, which also helps reduce overall costs.

What is the impact of VFD on Machinery performance?

Following factors elucidate the impact of VFD on pumps and compressor performance

Potential for Energy Reduction:

VFDs can be used to save energy at a pump / compressor station.

Potential for Extended Pump / Compressor Life

VFDs can enhance extend pump / compressor life. 

Adaptable Pump / Compressor Curve

Pump performance changes as speed changes. How the pump reacts is governed by a set of equations called the affinity laws. Pump flow changes linearly with speed and pump head (or pressure) changes with the square of speed.

Simplify Selection

VFDs can simplify the pump / compressor selection. Instead of potentially having a pump / compressor for peak day and another one for low flow periods, we can get serve both purposes with the increased flexibility of operation.

Closed Loop Control

VFD on a pump / compressor is a valuable tool in closed loop control. Many utilities run their rotating equipment’s in simple on/off control loops. However, if a pump station needs to hit a target flow or pressure, a closed loop control scheme takes advantage of a VFDs speed to hit a setpoint more precisely. Often, these are implemented with PID loops. 

Mitigate Pressure Transients

When a pump / compressor starts or stops, it changes the velocity of the fluid in the piping system which sets up pressure waves that echo until friction dissipates them. VFD can adjust the speed in discrete increments at a programmable rate to drastically slow the change of velocity in the pipe system and resulting in a lower amplitude of the pressure wave instead of using a pressure control valve as is the case with fixed speed motors

Controlling Inrush Currents

When a fixed speed motor starts, there’s a large electrical surge from the motor overcoming the inertia of its stationary shaft. The magnitude of this draw might be several times the normal full load current of the motor. These high loads can damage the motor and any upstream electrical equipment. It also makes the sizing of current protection equipment (like circuit breakers) difficult. By ramping up speeds slowly, VFDs reduce the start-up motor load and drastically reduce resultant inrush currents thereby keeping the motor amps below the configured level to protect the motor.

Phase Loss Protection

VFDs can keep a motor running through a phase loss. Again, the DC signal sent to the motor is removed from the AC supply, so as long as the motor isn’t near full speed (and full power),the drive can continue feeding it power. This feature is also configurable. 

Apart from above positive impact of VFD on rotating machinery there is flip side of it as well.

Inverter Duty Motors are Required

With traditional AC motors, pulse width modulation produces increased stress on motor winding and greater magnetic noise. These can break down a motor’s insulation and lead to motor failure. Inverter duty motors have improved insulation and other technologies that prevent the modulated signal and its noise from prematurely wearing the motor.

Ground Rings, Insulated Bearings

Pulse width modulation can induce currents on the motor shaft. Without sufficient grounding and insulation, these currents will go to ground through the bearings of the pump and/or motor. This phenomenon is called Electrical Discharge Machining and will pit or flute the bearings, quickly leading to motor failure. Adding a grounding ring and insulated bearings to the motor will prevent these discharges and prevent motor damage.

Supply Harmonics

A VFD has a rectifier circuit that converts the AC current to DC current. This circuit has a non-linear (non-sinusoidal) current draw that creates distortion on the AC supply line. VFDs, particularly larger horsepower drives, need harmonic filters on their line side so this distortion isn’t transmitted to other (potentially sensitive) electrical equipment.

Environmental Concerns

VFDs are more sensitive to cold and heat than across the line starters. Drives need climate control and may need to be oversized to cope with intense heat. Also, VFDs need to be kept in a relatively dust free environment.

Increased Operational Complexity

While many VFD user interfaces are getting simpler and better, they’re still more complicated than across the line starters. 

Cost

While VFDs are inexpensive for small pumps, VFDs are more expensive than across the line starters.