Low oil prices are greatly affecting the investment and installation of new pumping equipment. Cost cutting on existing equipment will include using spares from idled equipment instead of buying new ones. While this may be necessary, it may not save money if the spare is improperly matched and installed. Focusing on the same horsepower (hp), revolutions per minute (rpm), frame size and voltage is not enough. Ignoring five motor application considerations discussed in this article will cause additional downtime faster than expected.
1. Motor enclosures & ingress protection
Matching a motor enclosure can be easy to visually confirm so that an open motor, where holes in the frame allow the free flow of air through the motor, does not get used as a replacement for a totally enclosed motor. This solves the primary safety concern to prevent objects, including human appendages, accessing the internal, rotating parts of the motor. Less obvious to the visual enclosure inspection is making sure the replacement motor has the same or better Ingress Protection (IPxx) level.
The ingress protection level is defined by two digits. The first designates the protection level against solid object ingress. The second defines the protection level against liquid ingress. Motor enclosures can improve ingress protection by adding gaskets, sealants in or around joints and bolt holes, enhanced frame drains, slingers, and seals on the motor shaft. It may be difficult to determine the protection level from a visual inspection of the motor features. Failure to protect the replacement motor from dirt and water ingress to the same degree as the original motor may result in bearing contamination and early failure or damage to the electrical windings or connections.
2. Frame drain locations
Drains play an important role in the motor’s ingress protection. A motor with open drain holes would not be able to replace one that was protected by T-drains. Removing the 90-degree bend provided by the T-drain would allow splashed liquids to enter the motor enclosure. Of equal importance to the drain type is making sure the drains are in the correct location on the motor enclosure for the mounting orientation. If a motor were originally floor-mounted — National Electrical Manufacturers Association (NEMA) F1 configuration — the drains would be installed in the 6 o’clock position on the frame. Gravity would allow any internal condensation to flow out of the motor. If this motor were then used as a replacement for one that was mounted on the wall with its shaft parallel to the floor (NEMA W1 configuration), the drains would now be in the 3 o’clock position. This would not be the lowest spot on the frame, and any internal condensation could accumulate inside the motor leading to early motor failure.
3. Motor slip for 3,600-rpm (two-pole) centrifugal pumps
Motor efficiency regulations are changing the motors available in the market. The Integral Horsepower Rule, effective June 1, 2016, in the U.S. requires many previously exempt 3,600-rpm motor designs to be more energy efficient. These motors are common on original equipment manufacturer packages for centrifugal pumps, fans and compressors. Higher efficient motors are designed with less slip, meaning the motor will have a higher full-load rpm. The affinity laws state that, in centrifugal applications, volume is proportional to speed and that pressure varies as the square of speed and hp varies as the cube of speed.
The result is that even seemingly small increases in the full-load rpm can turn into big increases in full-load hp that need to be accounted for in the design of the replacement motor. A premium efficient replacement motor with less slip and a higher full-load rpm could wind up being undersized compared to the standard efficient original motor with lower slip and full-load rpm. This undersized replacement motor will operate above full- load and draw more amps. This will cause additional internal heating in the motor windings, which reduces motor insulation and grease life.
4. Higher motor service factor
One might be tempted from the last centrifugal load application example to make sure the replacement motor has a higher service factor (SF) than the original to push through the higher load demands. This is easier to do in the marketplace today where some manufacturers are pushing 1.25 SF (versus typical 1.15 SF) as standard on their 230-/460-volt NEMA motors less than 100 hp. Unfortunately, this is not as good of an idea as it first seems. NEMA confirms this in its publication MG-1 Section 14.3: "A motor operating continuously at any SF greater than 1 will have a reduced life expectancy compared to operating at its rated nameplate horsepower. Insulation life and bearing life are reduced by the SF load."
Higher internal motor temperatures will result from operating in SF and thus reduce motor insulation and grease life. The reduced grease life is a much bigger concern if the motor has non-regreasible bearings because no additional lubrication can be added to the motor and early bearing failure may result.
5. Submersible motors
When replacing a submersible pump motor, match the horsepower, voltage, shafting and flange dimensions while also being mindful of the exterior shaft seal material. The exterior shaft seal will see that greatest wear as it is the first ingress barrier exposed to the pumped fluid. Selecting a seal material that will withstand the amount of debris and the abrasiveness of the fluid given the application is imperative.
In a water treatment plant, for instance, the water will be much cleaner with fewer abrasives as it gets closer to leaving the facility, so a submersible motor intended for use toward the end of the treatment process may not be ideal for an application at the beginning of the process. However, to reduce confusion and possible misapplications, maintaining an inventory where all units are fitted with seals suitable for highly abrasive applications is possible. Keep in mind that this approach will most likely add cost to each unit inventoried, but the reduction in downtime should far offset the added cost.
Given the harsh nature of submersible applications, submersible motors typically have multiple auxiliary devices to ensure safe operation and long life. These devices include thermostats, which protect the motor from overheating and help prevent motors from reaching unsafe temperatures when placed in applications in which flammable gases are present.
Another auxiliary device common among submersible motors is a space heater. Properly connected space heaters will help prevent condensation inside the motor body. This moisture can dramatically reduce the service life of the unit by damaging the electrical components and bearings.
One of the most important auxiliary devices used to prevent catastrophic failure in submersible motors is moisture probes. The purpose of these sensors is to detect exterior seal failure before water can reach the rotor and windings of the motor. These probes are typically placed in a cavity on the drive end of the motor that is filled with nonconductive oil. The probe leads should be connected to a control box that measures the resistance between the two probes. If water contaminates the nonconductive oil, the resistance between the two probes will rapidly drop. The control box will detect this rapid resistance drop and trigger a number of different notification responses, such as cutting power to the motor, sounding an alarm or setting off a flashing light. Proper connection of these auxiliary devices can mean the difference between simple service with replacement parts or a costly replacement of the entire motor.
Tyler Lensing is a product manager for Baldor Electric Company, a member of the ABB Group. He joined Baldor in 2005 and has an extensive NEMA motor background in mechanical design engineering, application and product management. Lensing is a regular presenter at conferences and training seminars.
George Weihrauch is a product manager at Baldor Electric Company, a member of the ABB Group. He has an extensive NEMA motor background in sales, applications and project management during his 25 years with Baldor. He has published several articles related to electric motor applications and energy efficiency and is a regular presenter at conferences and seminars.
