Power Quality and Drives LLC

Product Tip of the Month:
Energy Savings - But not the way you think!
Copyright © 2011 Francis J. Martino

Variable Frequency Drive manufacturers and utility companies have repeatedly advised
that a 20 per cent reduction in speed on a centrifugal pump or blower application will
allow a 49 per cent savings in energy.  

The below linked article tells why that doesn't happen:

Elusive Energy Savings: Centrifugal Pumps and Variable Speed Drives - Part I

You have bought the VFD, installed it, followed the manufacturer's recommendation for
programming the drive for variable torque with a centrifugal pump or blower application, and
you are running the motor at a reduced speed.

You now notice that the motor full load amp draw is greater than it should be and, in some
applications, greater than the motor Full Load Amp rating. You notice that motor Slip RPM
is also greater than it should be and the measured running RPM may be lower than the
nameplate rated RPM. The energy savings you expected isn't there.

When the system and pump curves are analyzed, observe the following:

The torque requirement of the affinity curve is a cubed function. When programmed for 
variable torque, the drive will allow a torque output of a squared function which indeed yields
all the torque needed for a cubed function.

Introduce static pressure or static head and you will find that the system curve will cross the
variable torque curve with just a very small reduction in speed. The petrochemical industry is
noted for designing systems so that the system curve begins operation to the right of the affinity
curve. At that point the system curve allows a much larger reduction in speed before crossing
the variable torque curve.

The Slip RPM of the motor is too high for the following reason:

"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."
See footnote 3 of  Energy Savings: Solid-State Reduced Voltage Starters vs. VFDs

The increased slip of the motor causes needless energy loss.

Product Tip of the Month:
Always program your drive for constant torque on all variable torque applications.

Product of the Month:
Across-the-Line Starter with Timed Automatic Restart after
Undercurrent,  Power Failure,  Over or Under Voltage, or other protective trips.
Copyright © 2011 Francis J. Martino

The Auto-Restart feature was originally designed for submersible pump applications on municipal
well field systems. The undercurrent protection would operate when the well was pumped dry and
would shut off the pump to keep it from being damaged. The adjustable timer would be set to
restart the pump after time was allowed for the well to refill.

The following specifications can be changed to fit your application:

Full Voltage, Non-reversing, for 3/60/480 power input,
Control Power Transformer for 120 VAC Control,
with Hand-Off-Auto Selector switch in door, UL label.

Overload protection will be by Class 10 bimetallic relay
with adjustable trip amperage range, manual reset button
will be on the overload relay. Overload protection will
have manual reset only.

The following protective functions will be included with 
electronic protective relays and will have an automatic reset
with an adjustable timed restart from 10 seconds to 24 hours:
Undercurrent (3 second trip)
Phase Failure (3 second trip)
Phase Unbalance (5 second trip)
Phase Reversal (within 0.1 second)
Stall (5 second trip)
Locked Rotor (within 0.5 seconds)
Undervoltage (−30% to +25% of rated input voltage)
   (adjustable trip 0.1 to 30 seconds)
Overvoltage (−30% to +25% of rated input voltage)
   (adjustable trip 0.1 to 30 seconds)

An alarm output from the bimetallic overload relay will be a
N.O. contact that will be wired to a terminal strip for customer access.

The Model CDP electronic protective relay must be adjusted to maximum
overload amps and Class 10 trip to allow only the bimetallic relay to
provide overload protection.

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