Variable Frequency Drives

Starting a motor across the line causes the motor to draw up to six times the normal operating amp draw for an instant. Power companies monitor the peak amp draw and charge customers for the availability of that power for the entire billing cycle even though the peak draw may have been momentary.

Across the line starting of large motors also places tremendous stress on the different drive components including gearboxes, shafts, belts, blower wheels, impellers, couplers and sheaves.

A variable frequency drive (VFD) is an adjustable speed drive for alternating current motors. Variable speed driven motors have many advantages over a non-driven motor.

The variable frequency drive allows the motor to start slowly and reduces stress on the motor and drive assemblies. It allows automatic adjustment of the speed of a pump or blower as load changes. For example, when the load changes, speed increases or decreases depending on the process.

The power consumption of a motor varies greatly depending on the speed of the motor. As a rule of thumb, a motor running at 50% speed will consume 25% of the energy required to run a motor at full speed. In other words, the power required to operate the motor is reduced by the square.

Energy Management Example Using VFD - Created by The Service Manager

30 Hp Blower Motor Operates 24/7.

30 Hp continuous operation for one year=236,323,951 watts /1000=236,323.95kw X .133 kw/hr=$31,431.00 annual operating cost at full speed without drive.

30 HP Blower Motor Operates 24/7 at 50% speed.

30 HP Blower motor operating at 50% speed for one year=59,080,988 watts/

1000=59,080.99 Kw X $.133 kw/hr=$7,857.77 annual operating cost at 50% speed.

Consider that the blower or pump may need to run at full speed (60HZ) all the time. Also consider the savings if the drive only had to operate at 100% for 30% of the time and the rest of the time the motor could operate at 25% of rated speed at 1/8 full speed power consumption.

30 HP Blower Motor operating at full speed for 30% and 1/2 speed 70% of the time for a year.

[($31,431.00 annual cost at full speed X .30)= $9,429.30]+[($31,431.00/8 )X .70=$2,750.00]=$12,179.30 annual operating cost at full speed 30% of the time and .25 full speed for 70% of the time. Please consider the added benefit of reduced peak amp draw and drive wear.

Please contact an HMI professional about your VFD application or energy management concerns.