Motors and Drives: Repair or replace?

03/01/2009


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There are usually two reasons why we think about replacing motors: either our existing motors are humming along just fine but we want better energy efficiency, or they fail. And when they fail, we have to determine whether it makes better sense to repair them or buy new.

First, let's consider energy savings. According to the U.S. Dept. of Energy (DOE), motor-driven equipment accounts for 64% of the electricity consumed in the U.S. industrial sector. Motors used in the HVAC environment represent a high potential applications group with much untapped, relatively easily obtained energy savings potential.

If you have any standard-efficiency motors (anything manufactured before October 1997) with substantial operating hours installed in your building, don't wait for a failure; replace them immediately. Many studies have shown that the first cost of an ac motor represents only 2% of its total lifetime operating cost. An ac motor's average lifetime is currently estimated at 28 years, and the energy used to operate the motor over its lifetime far exceeds its first cost. There are many free calculation programs/estimators available to determine what percentage of efficiency increase will return an acceptable payback for your currently installed, standard efficiency motors.

Another way to save energy is to ensure that your motor/drives are not oversized—many are. Quite often, it's possible to decrease the motor and drive size by at least one frame. To determine if the motors/drives for a particular application are oversized, compare the motor full-load running amps to the actual nameplate amps.

For example, a 25 hp ac motor is probably nameplate-rated for somewhere around 34 amps. If, when measuring motor amps, you find that the motor pulls a maximum of 27 amps or less at full speed, it may be possible to replace the 25 hp motor and/or drive with a 20 hp motor and/or drive. Published motor efficiency data is the efficiency of that motor at full load and full speed. Motor efficiency drops substantially if the motor is operated at partial-load conditions.

There are numerous reasons why motors are often oversized in the HVAC environment. Many systems were over-engineered in the first place, with large safety factors in place during the design load calculations. Some of these systems were designed when electricity was relatively inexpensive, so operating costs may not have been high on the designer's priority list. Or perhaps your building has undergone a lighting retrofit or other energy conservation measures that have decreased the load on the HVAC system. Finally, changes to office layouts, occupancy, and other physical system changes from the original design may have decreased full-load requirements.

If it appears that you can, in fact, downsize motors and/or drives, keep the building's occupancy in mind. If you expect it to remain fairly steady, it makes sense to move forward. In that case, pay attention to motor rpm. Centrifugal fans and pumps are very dynamic by nature. Replacing a 1,750 rpm motor with a new 1,765 rpm motor will have an effect on fan cubic feet per minute and input power required. You should match as closely as possible the full-load rpm of the replacement motor to the existing motor nameplate rpm.

Horsepower requirements from a fan increase as the speed cubed, just like they decrease as the speed cubed. An increase of 10% in operating rpm will require a 33% increase in operating power [(1.1)3 = 1.33]. The good news is that, typically, it is fairly easy to re-sheave the application if the new, smaller, high-efficiency motor has a higher nameplate base speed than the old, less efficient motor.

WHEN A FAILURE OCCURS

When a motor fails, try to determine why before making any repair-or-replace decisions.

Failed bearings account for more than 50% of all ac motor failures. If a given facility is having a rash of premature motor bearing failures, there may be a systemic reason that will not be cured by rewinding or replacing.

I once encountered a facility with an extraordinarily high premature motor/drive bearing failure rate; the belts were being massively over-tensioned during motor installation. The extreme belt tension was causing overhung load issues on the motors and other drives, and this overhung load was causing the premature bearing failures.

Another possible cause of motor failure is using the wrong motor for the application. For example, open drip-proof motors may not be a wise choice for outdoor, cooling tower motor application in a facility near an ocean. Totally enclosed, fan-cooled motors or other chemical-duty rated motors may be a better choice for this application.

Motors also fail due to high-temperature issues. It is estimated that a motor operating-temperature increase of 18 F decreases motor life by 50%. Motors operating near boilers, or for some other reason operating at high temperatures, will not last as long as motors operating at lower temperatures, regardless of whether the motor is new or rewound. For these applications, choose a motor with a high-temperature-rise insulation class or purchase the proper motor enclosure and options such as external motor-cooling blowers.

If you've ruled out systemic reasons for motor failure, the repair-or-replace decision-making process begins. In the past, the old rule of thumb was that if a rewind would cost more than 57% of the price of a new motor, you should buy a new motor. Today, that figure is around mid-60%. Why the increase?

  • Electricity has become more expensive.

  • Technology advancements have improved the rewind process. In the past, a rewound motor typically became 1% to 5% less efficient. This is no longer true.

  • The 57% figure was based on an average of 2,000 or more hours of motor operation per year. Today, most motors in the HVAC environment operate at least 3,500 hours per year.

In addition to using the mid-60% rule of thumb, consider the following:

  • The hours of operation per year of the application

  • The local cost of electricity/kWh

  • Whether any tax credits are available

  • Whether any rebates or other incentives are available.

In general, it is probably better to rewind than replace if any of the following conditions exist (these assumptions are based on motors 15 hp and larger):

  • The existing motor is a high-efficiency motor and little gain in efficiency is possible with a replacement motor.

  • The rewind shop can guarantee and verify that there will be no degradation in motor efficiency.

  • The motor has a low duty cycle (runs very few hours per year).

  • The motor is 150 hp or greater and also is high-efficiency or low duty cycle.

On motors below 15 hp, it often costs equal or less to replace the motor than to rewind it.

CONSIDERATIONS FOR VFDs

Many motors presently being controlled by variable frequency drives (VFDs) are standard National Electrical Manufacturers Assn . (NEMA) design B motors. Thousands of these motors have been running on VFD-produced pulse-width modulated (PWM) waveform power for many years with no motor degradation issues. Standard NEMA design B motors are designed according to the publication NEMA MG 1.

Part 31 of this publication provides guidelines for designing inverter duty motors, which are manufactured using special high-insulation stress-withstand conductors and end-turn phase insulation. Part 31 requires the motor insulation to be able to withstand peak voltage levels of 1,600 V. It also requires the motor insulation system to be able to handle the very steep voltage rise typically seen when using modern PWM VFDs. These steep voltage pulses are often defined using the change in voltage divided by the change in time (dV/dt). This is typically expressed in volts per microsecond.

The steep PWM pulses typical on the output waveform of PWM drives can set up a phenomenon known as voltage reflection or standing waves. When making motor repair-or-replace decisions, consider replacing standard NEMA design B motors with inverter duty motors. These motors should seriously be considered on any VFD application where the run of VFD to motor conductor wire is greater than a certain number of feet.

Unfortunately, the motor wire lead length that will cause the voltage reflection phenomenon to present itself varies depending on the VFD switching algorithm, motor size (surge impedance), and, to a lesser extent, the type of VFD-to-motor conductor used in the installation. For 460 V motors under 50 hp, the conservative approach would be to specify inverter duty motors whenever the motor cable run is 25 ft or greater. For 460 V motors greater than 50 hp, specify inverter duty motors whenever the motor cable run is 50 ft or greater.

Always consult the VFD manufacturer for specific guidelines. Many manufacturers' guidelines are 50 ft and 100 ft, respectively, for the sizes listed above. The 25/50 ft rule of thumb is a conservative estimate for the hardest-switching PWM drives available on the market.

VFDs are fairly high-tech devices, and technological advancements happen frequently. In some cases, the repair-or-replace decision will already be made for you, because the VFD manufacturer will say it can no longer get the parts to support the installed technology.

This is probably not an excuse to sell you a new VFD; microprocessor technology changes all the time. For example, when a silicon chip or other electronics manufacturer upgrades from one microprocessor design to a newer microprocessor design, it discontinues the older device.

Often, the rights to manufacture the older microprocessor design are sold to a smaller silicon chip manufacturer. The new manufacturer will increase the price of the microprocessor to the VFD manufacturer but continue to sell it the older design device. Later, the smaller manufacturer may approach the VFD manufacturer for a “last buy.” In other words, the smaller chip manufacturer no longer finds it profitable to make the device and is planning to stop manufacturing it. If this occurs, the VFD manufacturer often will attempt to buy the amount of parts it thinks will be required for the next 7 to 10 years of installed base product support parts.

However, there will come a time when the VFD manufacturer can no longer source certain parts for replacement boards of drives it manufactured in the past. This is simply a fact, and virtually all electronics and VFD manufacturers will experience this challenge sooner or later.

Other VFD repair-or-replace considerations include the following:

  • Are any older-style variable voltage inverters (VVIs) installed? These are typically 3% to 5% less efficient than modern, PWM-based VFDs. Also, many of the older VVI-style drives have a decreasing power factor (the power factor reflected to the utility decreases as the motor speed decreases). Therefore, if you run a motor on a VVI-style drive at 50% speed, the power factor of that drive-motor combination reflected to the utility will be 50% or less. If the local utility has power factor penalty charges, these charges can quickly cover the first cost of purchasing a new drive to replace the older VFD technology.

  • Harmonics. If you're experiencing high harmonic distortion in a certain area of your facility, you may want to consider the newer, harmonic-signature VFDs now on the market.

  • Bypass or no bypass? VFDs have become much more reliable over the past 20 years; a bypass on an HVAC application is an investment that needs to be analyzed closely.

Finally, the simple answer on repairing or replacing VFDs: When it costs more to repair the existing VFD than it does to purchase a new VFD, it's time to purchase a new VFD.


Author Information

Olson is manager — HVAC applications at


Develop a policy

It's best to have a replace-or-repair policy in place before motors fail. Gather as much information as possible about the motors in your facility, including:

Motor nameplate data

The date the motor was purchased new

Whether the motor has been rewound and, if so, when

The motor's maximum operating amps. Measure the amps when running at full speed at maximum possible load. Include worst-case scenarios, such as a very hot day.

You may find that you're able to determine a simple rule of thumb for the existing motors in your facility, given their hours of operation and your cost of electricity/kWh. In any case, having a policy will enable you to make sound choices when you need to act quickly.

Online tools

There are many useful resources on the Internet that will enable you to perform a lifecycle cost analysis on motor repair-or-replace decisions rather than just a simple first-cost analysis. One particularly useful tool is the U.S. Dept. of Energy's MotorMaster+ software, available for download at



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