Variable Frequency Drives

If you are using single speed motors to drive fans or pumps, you may be able to save energy through use of a variable frequency drive.

The variable frequency drive (VFD), one type of SCR-based device, can convert a single speed motor to variable speed with no modification to the motor itself. This can be an efficient way to convert, for example, constant volume air systems into variable volume. VFDs are readily available for motors from 1 hp to 300 hp and are easily installed directly into the power line leading to the motor, replacing the existing motor starter.


The VFD operates on a simple principle. The rotational speed of an AC induction motor depends on the number of poles in that stator and the frequency of the applied AC power.

Although the number of poles in an induction motor cannot be altered easily, variable speed can be achieved through a variation in frequency. The VFD rectifies standard 60 cycle AC line power to DC, then synthesizes a variable frequency AC output. Motors connected to VFD provide variable speed mechanical output with high efficiency. These devices are capable of up to a 9:1 speed reduction ratio (11 percent of full speed), and a 3:1 speed increase (300 percent of full speed).

Uses of motor speed controllers are easily identified in industry. Single speed induction motors are commonly used to drive industrial processing equipment, particularly centrifugal pumps and fans. The VFD takes advantage of a basic principle of mechanics of variable torque applications such as fans for pumps. The flow rate of water or air is directly related to pump or fan speed, while the motor HP output is related to the cube of the speed. This means that at 50 percent flow, or 50 percent motor speed, only (50%)³ or 12.5 percent of the original shaft horsepower is required.


Many fan and pump systems have a cost effective application for variable frequency drives. Quite often these systems have a variable flow rate using throttling devices such as valves and dampers to vary the flow. Throttling devices essentially waste excess energy to maintain a given frequency and the use of a VFD can be very cost effective in these situations. Typical examples of systems using throttling devices are: booster pumps for domestic water, process chilled or condenser water systems, and fan discharge dampers.

Other variable flow systems use mechanical or electrical methods such as inlet vanes, outlet dampers, eddy current clutches, hydraulic couplings, and variable pitch pulleys to vary the speed of the fan or pump. These are more efficient than throttling devices but not as efficient as VFD's.

Some fan and pump systems are currently constant flow but may be converted to variable flow systems by making modifications to the system. An example is a constant volume air distribution system. Adding variable volume boxes for each zone and a VFD to the fan will convert this system to a variable flow system and save considerable energy (and possibly improve comfort too). Another example is a chilled water system with three way valves to bypass chilled water around the cooling coil to maintain the desired air temperature. The three way valves can be replaced with two valves which vary the flow to the coil and allow a VFD to vary the pump speed as required to maintain system pressure. In this case, the chiller should be decoupled from the load or distribution piping.


The VFD is not without its shortcomings. As a result of the on • off switching done by the VFD, harmonic currents are produced. Harmonic currents cause slightly increased losses in electromagnetic equipment such as transformers and motors. Decreased motor efficiency and decreased capacity of the motor cooling fan at reduced speeds result in a slightly higher operating temperature, but motor overheating rarely results. If power factor correction capacitors are nearby, the circuit may have a resonating frequency, which is usually not a problem with smaller motors but should be considered in larger motors.

Power factor correction should also be considered as an option when specifying a VFD as some types of VFDs have a very poor power factor especially at low load.

Most medium to large size VFDs have capacitors for power factor correction and to protect power quality in the rest of the system.