Power Quality and Drives LLC

Skullduggery Afoot!
Copyright © 2003 Francis J. Martino

Below are three industrial applications which were open for bid. Sales
techniques that were attempted with each application are also given. The
applications demonstrate the need for a consultant to be utilized rather
than maintaining a dependence upon a selling vendor.

Power Quality and Drives LLC offers consulting services. When the
service is utilized, the equipment specified will be generic so that, in
most cases, any manufacturer can meet the specifications. The equipment
may then be purchased from the client’s normally used vendor.

Application #1

Replacement of a 250 HP, 1725 RPM wound rotor motor with a new
1750 RPM DC motor and control was required. The requirement included
the use of field weakening with the new motor in order to drive the
machine at 2100 RPM or higher if possible.

The maximum speed capability of the copper tube drawing machine was
not known, but the maximum speed would be determined by test with the
new drive.

The user had three considerations by which the new motor would
be selected:

a) A constant torque application would require a 20% increase in
horsepower in order to gain a 20% increase in speed. Therefore, the user
would use a new motor rated at 300 HP.

b) An eighty year old, 250 HP mill motor is quite capable of developing
275 HP on a bad day. The designers of machinery were aware of the full
capabilities of the mill motors and, consequently, often took advantage of
the service factor which was inherent within the motors even though the
service factor was never indicated on the motor nameplate. If it were
indeed the case that the existing motor was required to develop
horsepower in excess of 250, the user would need a 400 HP motor.

c) Meter the existing motor for current draw when the machine is
operating at its maximum load. The current reading would allow an
estimate of the maximum horsepower requirement at 1725 RPM. If the
machine required 20% less than 250 HP, then the new motor could be rated
250 HP. If the motor was developing 250 HP or higher, then the new drive
should be sized for an additional 20% horsepower. Metering is the
preferred method of selection.

The sales personnel reasoned as follows:

"If I quote a 300 HP drive, the price of the project may be so high that the
user may not spend any money and, therefore, I won't make a sale.

"If I quote both a 250 HP and a 300 HP drive and try to explain to the user
why he may need the higher horsepower motor, he may get confused and
then buy the drive from some other distributor who doesn't confuse him.

"Therefore, I will quote a 250 HP motor and, if he complains later that the
motor can not run fast enough, I will tell him that we were asked to
replace the motor and that we did so on the basis of the motor nameplate."
See Torque Capabilities of DC and AC Drives in the Constant Horsepower
Range Using a Wound Rotor Motor Replacement as a Typical Application Example.

Application #2

A 2 HP variable frequency drive was powered from a 750 kVA upstream
transformer. Forty feet of pipe and wire and 150 feet of bus duct were
utilized. Upon application of power to the terminals, the inrush charging
current caused the input devices of the drive to immediately burn out. Had
an input line reactor been used as per the drive instruction manual, the
drive would have been protected from the distribution system short
circuit capacity. See Variable Frequency Drives: Source Impedance and
Line Reactors.

The sales personnel reasoned as follows:

"I am competing against the bearing and mechanical equipment
distributors who also sell drives. They are knowledgeable about
mechanical devices but don't know what a line reactor is. If I attempt to
sell a reactor with the drive, my price will be higher than the other
distributors who do not sell reactors and I will not make a sale.
Therefore, I will never discuss short circuit capacity with my customers
and will quote only the drive which the customer has requested. If the
customer later has a problem with the drive, I will send in a serviceman."

Here is the scenario that follows:

Some manufacturers will replace a small drive under warranty if
the unit fails due to a line reactor not being used. However, they do not
want to see the second unit come back. The distributor knows that the
drive may be replaced under warranty and therefore sells the drive
without a reactor.

The serviceman visits the customer and examines the burned out drive.
The customer is then advised that a reactor is needed. The service call
will cost the customer $500. The reactor, which would have cost $150 if
quoted for purchase with the drive, will cost $200 because, at the present
time, a competitive price is not needed. The serviceman returns the next
day to install both the reactor and the new drive for an additional $500
service charge.

The customer has now paid $1200 for a $150 reactor in order to
protect a $400 drive. In addition, one day of production has been lost.

Application #3

It was desired to save energy by reducing the speed of a centrifugal
pump on an oil filtration unit. According to the affinity curve, a 20%
reduction in flow will allow a 49% reduction in energy. The affinity curve
gives the performance of the pump at the points of greatest internal
efficiency when pumping against zero static head. However, all pump
systems will have a static head which the pump must work against. In
most applications, the static head will reduce the energy savings by
one-half to one-third of what the affinity curve indicates. See Elusive
Energy Savings: Centrifugal Pumps and Variable Speed Drives.

In addition to the existing fifteen feet of observable vertical head, the
oil filtration unit received the incoming oil and distributed it through
many smaller pipes that utilized ninety degree turns along the journey to
the filter. The piping presented additional head within the system.

The oil filter was changed at the beginning of each month. When new, a
filter could present a typical three to fifteen pounds of head to a system,
depending upon filter design. At the end of the month the head would
increase significantly as the oil path became increasingly obstructed by
the contaminates which accumulated in the filter. In this application,
without metering motor current, an estimated energy reduction would
be 15%.

The sales personnel reasoned as follows:

“I will advise the customer that the 20% reduction in speed will give a
50% savings in energy based on the affinity curve, which is the data that
is used by the drive manufacturers in all applications and is accepted as a
close approximation by the utility company in most applications.”

Here is the scenario that follows:

The utility gives the incentive to pay for the cost of the new drive and
the customer makes a purchase. Shortly after installation, the facility
personnel will discover that the payback time will be much longer than
that estimated by both the vendor and accepted by the utility.

When the drive needs replacement or repairs during the next eight to
ten years, the facility personnel will not have retained knowledge of why
the drive was initially installed. At that time the drive will be replaced
without an incentive from the utility.

Upon replacement, the maintenance personnel will be happy because
the drive can be replaced quickly. The manufacturer and the selling vendor
will be happy because another drive will be purchased. The utility will be
happy because there will be a continued 15% energy reduction. The
production manager will be happy because the equipment returned to
operation. However, the low return of perhaps 4% per year or less on a ten
or twenty thousand dollar drive investment is not discernible and,
therefore, management does not realize that the money may have been
better spent in some other area that would have increased production. It is
a win-win-win-lose situation. The company loses. See Payback Analysis
for Variable Frequency Drives.

It is often said that a consultant is not necessary because of the good
service that is available from the vendors. The necessary question that
arises is simply:

Do you want a skilled consultant sitting on your side of the table and
looking out for the best interests of your company, or would you prefer the
service of the above noted vendors who are sitting on the other side of the
table and trying to sell what there is to sell?

For our non-American friends who visit this web page:
skullduggery is “treachery” and afoot means “walking about.”

Power Quality and Drives LLC

Power Quality and Drives LLC
P.O. Box 83
Middlebury, CT  06762
Phone: (203) 217-2353
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