Motors & Drives: Tips and tools for efficient motor management, Part 2
Michael Lyda, motor and drive engineer with Advanced Energy Corp., explains tips and tools for efficient motor management in this transcript from a December 2020 webcast.
Michael Lyda, motor and drive engineer with Advanced Energy Corp., explains tips and tools for efficient motor management in this transcript from a December 2020. Part two focuses on motor applications. This has been lightly edited for clarity.
Tips & tools for motor applications
Now, we’ll transition to motor applications. A few of the topics we’ll be covering in the motor application section are recommended operating conditions, selection and proper sizing of motors. And lastly, we’ll review the application affinity laws. First, let’s talk about ideal operating conditions for your motors.
Ideally, you need balanced voltage from your utility. For instance, if you have three-phase 480-V supply at your location, those phases should be balanced to within at least 1% or better. The more out of balance your phase-to-phase voltage may be, the more detrimental to your motors’ efficiency and long-term reliability. Notice that most low voltage motors are rated for 230-, 460- or 575-V operation, even though typical low voltage distribution levels are 240-, 480-, or even 600-V.
You may be wondering about the difference. The 10, 20, 25-volt difference accounts for the voltage drop in the distribution lines into your facility and then within your facility. Slight over-voltage at your motor leads is all right. It’s not generally that much of a problem. Heavy over-voltage can be a problem. But what you don’t want to have is under-voltage at your motors.
If voltage at your motor terminals is under the nameplate rating, you definitely want to troubleshoot to determine the cause. Second, motors should be aligned to the load to have as little mechanical vibration as possible. This is achieved typically by good shaft alignment. There are various tools out there. Having a laser alignment tool may be a good addition to your toolbox, especially if you do a lot of motor installs in-house. Otherwise, verify that the service provider is using some type of precision alignment tooling.
Third, you want a clean environment for motors. I understand this is kind of engineer talk. This isn’t always possible given the application. But generally, just make sure a motor is kept free from contaminants. If the motor has a fan, if it’s a fan-cooled machine, keep the fan trout unblocked so the air can properly move across the motor for good cooling.
Clean the fan blades every so often if you’re in a high dust particle area. Of course, make sure your motor is de-energized before doing anything like that. Think lock out/tag out. Fourth, the motor’s ambient temperature is important. If the motor name plate states the maximum ambient temperature, you should respect that. If you see the ambient temperature consistently above the rating on the motor name plate, then it’s likely the motor should be derated for that application.
You can always reach out to the motor manufacturer for more details. Or, if you’re consistently running the motor at too high of an ambient, you may be dealing with a failed motor earlier than expected. And then finally, the last point on this slide, motors are optimally designed to be ran at their rated load. For instance, if you have a 10-horsepower motor and you run it at five horsepower, the efficiency will be poor and the power factor will be very low.
Poor efficiency and low power factor are going to cost you in the long run and motor oversizing is a common problem across the industry. Purchasing folks, not to say anything against, but purchasing personnel sometimes may believe if the application calls for a 15-horsepower motor, for instance, purchasing a 20 horsepower motor may make the motor last longer, but this isn’t typically the case.
Heat kills motors. If heat kills motors, this begs the question, what causes motors to heat up? These include high ambient temperature, blocked fans, blocked cooling vents, overloading the motor above rated load. And ff course, power quality issues at the motor terminals.
Typical power quality issues include unbalanced voltage, single phasing, drop in a phase, or relatively close in proximity nonlinear loads that will lead to voltage and current distortion, known commonly in the industry as harmonics.
Let’s look at some operation and maintenance best practices. For maintenance folks, one of your main goals is to prevent failures.
Limiting downtime with proper motor maintenance can alleviate stress.
First we’ll talk about the mechanical conditions listed. Even if a motor is aligned well at installation, various factors can lead to misalignment during operation. If the motor itself starts making some loud or strange noise, shut it off, verify the alignment. You can verify parallel alignment with a simple straight edge. To check for angular misalignment, you might need a telescoping gauge, which not everyone will have in their standard toolbox. But they can be purchased or ordered very quickly.
They even come in packages covering many size ranges. What I have is a pack of five or so telescoping gauges that cover many different motor sizes. Additionally, you want to keep the motor bearings well lubricated. Bearing failures are the leading cause of motor failures.
Grease the motor bearings at the recommended manufacturer interval and be sure to use the proper type and amount of grease recommended. Grease both the drive end and non-drive end bearings. Make sure the grease nubs are clean before adding the grease so no contaminants get in the grease.
Most motors today use Polyrex EM. You can’t assume that for all motors. That’s just typically the most common grease for new motors. Keep some tubes of that around if you don’t already have some. Also keep your fans checked for obstruction and make sure no stray debris get inside the fan shroud at any time. And then for some electrical problems, keep a multimeter around to verify your phase-to-phase voltages. Check for the balance voltages if you have access to the motor terminals.
If you have the motor on a contactor, which is likely, check the contactor terminals periodically for carbon or contaminant buildup, especially if this is an on-again, off-again application. Think conveyors or press. Also keep an eye out for arcing at the contactor or pitting over time. You can also use an amp meter or ammeter to verify amperage at the motor leads if you have access to those.
Once you have the amperage readings, you can compare those to the motor name plate full load amperage. Additionally, with the motor de-energized, you can check winding resistance of the motor. The motor should have balanced phase-to-phase resistances. If the motor is hot and maybe you want to compare winding resistance to the original manufacturer values, then you’ll need to let the motor cool down for quite a bit to have an apples-to-apples comparison.
If the resistances are out of balance, then you may want to run some dielectric test with a Megger or something somewhere. And of course, with all of this, only professionally trained personnel should be completing the maintenance procedures and using any of the equipment mentioned.
Next we see the leading causes of failures and motor failures. This is taken from the Institute of Electrical and Electronics Engineers (IEEE).
Again, we see bearing failures very high on the list due to the issues mentioned earlier. Also listed here, insufficient lubrication, even excessive lubrication, too much grease, the wrong grease, parallel or angular misalignment, vibration or overheating of the bearing.
The next leading cause of motor failure is stator winding issues. This is characterized by short circuit or open circuit. These will be caused by repetitive overloading and subsequent overheating of the motor causing some issues in the windings. External conditions are right there with the winding issues. These include heat, humidity, contamination problems.
Motor build quality will absolutely play a major role in motor reliability and long-term life. As a maintenance personnel, you can do everything right, but if the motor has quality issues from the factory, it can still fail prematurely.
Here’s a general overview of what we’ve discussed in this section.
One of the most basic, useful preventative maintenance techniques is to use your senses. Look for part discoloration of motor leads and wires. Feel the motor for strong vibration and excess heat on the case. Smell for strange odors or a burning smell coming from the machine. Finally, listen for excess or unusual noises.
Keep the exterior surfaces and cooling passages clean and clear. Keep bearings properly lubricated. Keep the motor alignment good and any belts or chains at the proper tension amounts. Keep the motor terminal lugs at the proper tightness and on down the line at your bus bar or motor control center, keep those with the proper tightness as well.
What you don’t want is have your motor terminals or motor wires falling out of a VFD, for example. On the right side of the screen, you see some more advanced preventative maintenance measures listed, including the vibration analysis, infrared thermography, current signature analysis and remote monitoring, if you have that capability. A lot of folks won’t have remote monitoring.
Let’s look at a few examples of motor loads before we get into the energy savings portion of the presentation.
Generally, there are two types of loads: centrifugal and linear.
Centrifugal fans includes the spinning inertial loads where the load will tend to decrease with speed. Some examples are pumps, fans and blowers. Linear loads are more direct loads like pressures, elevators, or conveyors. And typically linear loads require some constant torque application independent of the speed.
For the remainder of the material today, we’ll be focusing mainly on centrifugal loads. Two types of motors are used for these loads, fixed speed and variable speed. For a fixed speed motor, the output air or liquid flow, the fan or pump must be regulated mechanically by valves or dampers. There are quite a few innate losses in this type of system. However, if you can use a variable speed motor for the same system, the output air or liquid flow can be regulated by motor speed. This provides potential energy savings based on the affinity laws.