Power Quality and Drives LLC

Protect Pumps from loss of flow with undercurrent sensing using Model CDP Motor Protection Relay.

Pump Control and Protection
Copyright © 2004 Francis J. Martino

Pump and pump system protection is available through a variety
of solid-state control solutions. The primary considerations presented are
thermal rise within a motor, mechanical stress, cavitation, and
overload protection. The solutions given here are not necessarily the
only solutions that are commercially available.

Application challenges, including applying pumps to generator power,
are also addressed.

Limiting thermal rise within a motor will allow both energy savings and
longer lifetime of the motor.

Resistance of the stator windings and rotor bars will increase as motor
temperature rises. Thus, the increase in resistance will cause higher
energy losses within the motor.

With a full voltage motor starter, the high inrush current upon
starting the motor will contribute to temperature rise at the "hot
spots" within the motor winding. The hot spots are those points of a
motor winding at which there will be higher temperatures than other
points in the same winding. The higher temperatures are caused by
irregularities in materials and construction of the motor. Motor
windings often fail at a hot spot due to degradation of the insulation
caused by excessive heat at that particular spot.

Lower temperatures can be obtained by using a reduced voltage start
rather than full voltage. Technologies currently used for reduced
voltage starting are: an auto-transformer starter which allows one or
more steps of voltage on start, wye-delta and part-winding starters
which allow one step of voltage on start (and which require motors that
are designed for wye-delta or part-winding starting), a wound rotor
motor, an eddy current drive, a solid state reduced voltage starter,
and a variable frequency drive.

Preferred for smoothness of acceleration, the ability to be used on
standard induction motors, and lower cost are the solid state
reduced voltage starter and the variable frequency drive.

The link below addresses energy considerations in choosing between a
reduced voltage starter and variable frequency drive. The primary
concern is the harmonic generation caused by the non-linear loads that
are presented by both types of drives.

If energy savings can not be obtained by varying the speed of the load,
then use a reduced voltage starter with a bypass contactor that will
place the motor across-the-line when it achieves full speed. Keep in
mind that a variable frequency drive may also be built with a bypass
contactor that will also place the motor across-the-line when it
achieves full speed.

Energy Savings: Solid-State Reduced Voltage Starters vs. VFDs

Limiting thermal rise within a motor by avoiding common misapplications: Constant Torque Programming vs. Variable Torque Programming.

The affinity curve for centrifugal pumps, fans and blowers indicates a 50% energy savings with a 20% reduction in speed. The affinity curve, however, is based on a pump or fan that is operating with zero static head or static pressure. When introducing the system curve, which takes into account the weight of water in a a vertical pipe and also the pipe or duct resistance to flow, it will be found that a 20% reduction in speed will never give a 50% savings in energy. It will also be found that many systems, when operating with a 4 to 20 mA pressure or flow signal, will not allow the motor and pump speed to operate with a speed as low as a 20% reduction. See:

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

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

Payback Analysis for Variable Frequency Drives

When programming a drive that is operating with a static head or static pressure that is a high percentage of the maximum system capability, the drive must be programmed for constant torque operation and not variable torque. See the second paragraph of footnote 3 in:

Energy Savings: Solid-State Reduced Voltage Starters vs. VFDs

Limiting thermal rise within a drive by avoiding common misapplications: Constant Torque Ratings vs. Variable Torque Ratings.

A typical drive may be rated for use at 125 HP, 183 Full Load Amps with a constant torque load. The same drive will also be rated for 150 HP, 228 Full Load Amps when used on a variable torque load.

Notice that the drive has two current ratings. The higher current rating that is given for a variable torque load assumes a greater thermal dissipation capability of the drive due to the reduced torque requirement of a variable torque load.

In most variable torque applications, the variable torque rating will allow proper heat dissipation and, therefore, a reasonable life-time of the drive.

However, let us consider a pump that is located on the first floor of a building and which is pumping water to a chiller that is located on the thirtieth floor. If the working static head is a minimum of 350 feet at the lowest pump speed that is allowed by the system requirement, and the maximum system capacity is 360 feet at the maximum pump speed, then the drive will effectively be seeing a constant torque load.

A variable torque rating for a drive in that application will allow the drive to operate properly throughout the warranty period. However, a constant torque rating on that application will allow much more heat dissipation, cooler operating temperatures, and ultimately a greater and reasonable life-time of the output devices and electronic circuitry.

Thus, in the application, use a typical constant torque drive rating of 150 HP, 223 Amps. That typical drive will have a nominal variable torque rating of 200 HP, 264 Amps.

Always consult the system Head vs GPM curve before sizing a drive.

Variable Torque vs. Constant Torque: Ratings and Programming

Mechanical stresses due to surges of pressure or flow may be resolved with either a reduced voltage starter or a variable frequency drive.

a) A reduced voltage starter that will give a timed current controlled voltage acceleration and a timed voltage controlled deceleration is a standard type of control and will be the lowest cost.

b) A reduced voltage starter may give a ramped acceleration and an S-Curve deceleration. The S-curve deceleration is used to reduce hammer head on pump deceleration and will give better performance than the traditional voltage controlled deceleration.

c) A reduced voltage starter that utilizes a torque control algorithm which will give a linear torque acceleration and a linear torque deceleration. Torque control will give the smoothest control available for pump control.

d) A reduced voltage starter or variable frequency drive controlled by tachometer feedback will allow a smooth speed ramp. When used for conveyor control, the mechanical stress reduction will give a longer conveyor belt life.

e) A variable frequency drive will have timed acceleration and deceleration ramps.

Cavitation from loss of fluid may be protected against by:

a) loss of flow or pressure detected by a flow or pressure meter.

b) loss of water detected by liquid level sensors.

c) loss of water detected by an undercurrent trip of a protective relay. (See Model CDP)
Under current sensing eliminates the need for low level electrodes and
the associated required maintenance.

Quick acting overload protection that is adjusted for the specific application can mean the difference between a one thousand dollar bearing replacement or a six thousand dollar repair that includes not only bearing replacement but also a rewind of the motor stator and extensive mechanical repair that may be casued by bearing collapse.

Applying pumps and other loads to generator power can be problematic due to the high impedance of the generators. A common error to avoid is specifying a reduced voltage starter to have a capability of starting a motor with a 150% inrush current.

Many reduced voltage starters may be adjusted for a 150% full load amp maximum. However, most loads require 300 to 350% inrush current in order to develop enough torque to start when used with a reduced voltage start.

If an inrush current of 150% is absolutely essential then a variable frequency drive must be used. A variable frequency drive gives a reduced frequency as well as a reduced voltage on start-up. The torque characteristics of the motor are such that both reduced frequency and voltage will allow a start-up with a 150% inrush current.

See Solid-State Reduced Voltage Starters and Starting Torque

Exposure of remote pump houses to lightning strike.
See Transients: Lightning Strike
and Surge Suppressor Technology.

A note on cavitation.
See A Problem with Pump Cavitation

Protect Pumps from loss of flow with undercurrent sensing using Model CDP Motor Protection Relay.

Power Quality and Drives LLC
7 Yale Avenue
Middlebury, CT 06762

(203) 217-2353


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