
Energy Savings: SolidState Reduced Voltage Starters vs. VFDs
Copyright © 2002 Francis J. Martino
Application #1: A load that requires operation at rated motor
speed after a ramped acceleration.
A variable frequency drive (VFD) is often chosen to provide a rampup
and rampdown to eliminate hammerhead on centrifugal pump systems,
eliminate shock loading on machinery, or reduce kW Demand. The VFD will
then run the motor at full rated speed continuously.
The problem that is created with the application is that a VFD will
introduce a nominal 5% harmonic current into a motor in addition to the
motor’s normal running amperage. The harmonic current will generate 5%
more heat within the motor than will be generated with a pure sinusoidal
60 HZ waveform, thus decreasing the motor efficiency by 5%. Therefore, to
maintain full load torque, the motor will draw an additional 5% kW input
when operated on a VFD. [1]
In comparison, a solidstate reduced voltage starter will also allow a
rampup and rampdown and give a reduction in kW demand. However, the
reduction in kW demand will be less than that of a VFD.
A reduced voltage starter, when utilizing an acrosstheline bypass
contactor after accelerating the motor to rated speed, will not introduce
harmonic current into the motor. Harmonic current will only be generated
by the solidstate starter during rampup and rampdown. For that reason,
the solidstate starter with bypass will operate the motor 5% more
efficiently than a VFD that is running a motor at full speed. A VFD may
also be specially designed to include a bypass contactor for use at full
rated speed. The variable speed feature of the VFD will not be able to
be utilized once operating at full speed.
Application #2: Energy savings by adding a variable frequency
drive (VFD) to an existing centrifugal pump system.
According to the affinity law for pumps, a 20% reduction in both speed
and flow will yield a 49% reduction in energy. The affinity law defines the
natural flow of a pump along the path of least internal friction while
pumping against a zero static head. Using the 20% flow point on the curve
as a typical example, review the pump manufacturer’s curves and
introduce the static head of the system into the curves. You will see that
the system may indeed allow operation at a 20% reduction in flow,
however, the motor speed will be reduced by far less than 20%. The
nonattainability of both the projected motor speed and the projected
energy savings will hold true for any point on the system curve. [2]
The energy savings of an existing VFD may be determined by measuring
the motor RPM with a handheld tachometer. In the above example of a 20%
reduction in flow, a 1750 RPM motor must be operating at 1400 RPM in
order to gain a 49% reduction in energy. If the measured RPM is 1600 at
the point of a 20% reduction in flow then your energy savings is 23.6% and
not the 49% as shown by the affinity curve. [3] Instead of a two to three
year payback, the payback will be four to six years. [4] Keep in mind that
there will be no dollar incentive from the power company for a
replacement unit when the VFD eventually fails. [5]
If the review of an existing application indicates both a low energy
savings and no need for constant flow or pressure regulation, then, when
the time comes for repair or replacement, consider a lower cost solid
state reduced voltage starter that will allow reduction of shock loading
and water hammer, offer kW demand savings, and will have a lower cost
than a VFD.
[1] For a discussion on the adverse affects of harmonic generation on
motors and machinery, see IEEE Std 5191992, Section 6.2, pages 3536.
For a discussion of the adverse affects of VFD generated harmonic
voltages that are caused to other motors that are fed from the same
power system that feeds a VFD, see P. G. Cummings, “Estimating Effect of
System Harmonics on Losses and Temperature Rise of SquirrelCage
Motors,” IEEE Transactions on Industry Applications, Vol. IA22, No. 6,
pages 11211126, Nov/Dec 1986.
[2] See Elusive Energy Savings: Centrifugal Pumps and Variable Speed Drives.
[3] Energy savings may also be determined by reading the kW meter
function that is available on the drive keypad. Compare the keypad reading
to the calculated kW which the motor would normally consume if operated
at full speed and at rated frequency. kW = V x A x 1.732 x PF. The drive
uses a mathematical algorithim to determine the kW reading. Therefore, to
be sure the reading is accurate, check the drive programming for the
proper settings for the actual (measured) incoming system voltage and the
motor nameplate data of HP, RPM, voltage and full load amps.
The squared volts per hertz of the variable torque programming will give
a more efficient operation than that of the linear volts per hertz of the
constant torque function when operating a system in which the system
curve has its maximum RPM point to the right of the affinity curve, a
nominal flow of 80% of maximum or greater, and has a low static head.
With a high static head, the system curve will cross to the left of the
squared volts per hertz curve. The motor will then be starved for voltage
and its torque capability will be reduced. As a result, the motor will
increase its slip to develop adequate torque. The increased slip
will cause an increase in both motor current and heating which will lower
motor efficiency. In such a case it is necessary to program the drive
output for the constant torque linear volts per hertz function.
[4] As was seen in Application #1, the harmonic generation by the VFD
will cause 5% higher losses in the motor. Also, there will be a higher kWH
billing based on the VFD raising the line power factor to .96 rather than
the motor’s original .86. With both harmonic generation and power factor
taken into account, the payback period may be at six years or longer.
[5] In considering the replacement cost of a drive, in the State of
Connecticut where the kWH rate is near 4.7 cents per kWH, a long term
dollar savings may be realized with larger VFDs. An overall dollar savings
may not be available with a low horsepower unit. In the State of
California where the kWH rate is 30.0 cents per kWH, there will be a
dollar savings on both small and large horsepower units.
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