Motor mayhem
If you’ve been following along with the blog, you’ll know that we’ve covered heat and we’ve looked at motors, but at this point, motors and heat is uncharted territory. So, guess where we are headed today?
When we were talking about heat and humidity a couple of weeks ago, we gave you a general rule of thumb for temperature increase and resistance measurement. If you’re curious, check it out here. Great news though, we are going to give you a new rule today! Listen up.
When it comes to excessive heat, experts agree that rapid deterioration of a motor’s winding insulation is imminent. To be more specific, the general rule is that for every 10°C (or 18°F) of additional heat to the windings, a motor’s insulation life is cut in half. Well, that’s not great news for your motor. Want to hear an extreme, yet probable, example?
Imagine, you are working with a motor that should last about 20 years – a decent lifetime. However, it’s running its day-to-day tasks at 40°C above the ideal rated temperature. Ouch. Your motor is only going to last about 1/16 of its anticipated life span. In a little over a year, you will be purchasing a new motor. Goodbye, precious money!
Still not convinced? Out of all motor failures, 30% are the result of insulation failure. Of those, 60% of insulation failures are caused by excessive heat in the motor. That’s a pretty big chunk of all motor failures.
So, why does a motor overheat?
It’s all about HOPES.
- High Effective Service Factor
- Overload
- Poor Power Condition
- Environmental Influences
- Stops and Starts
Before we get into the HOPES factors of excessive heat in motors, let’s look at the basic set-up of a motor, specifically an AC motor. If you look at the figure below (obtained from Galco Industrial Electronics), you’ll see the two parts of a motor – the rotor and the stator.
The stator is stationary (easy, right?), and it is made up of a group of individual electro-magnets, arranged in a cylinder. Whereas the rotor is the rotating electrical component (again, makes sense!). The rotor is also composed of a group of electro-magnets, arranged around a cylinder. The rotor’s magnet poles face toward the stator poles, which are pointed towards the center of the cylinder. When these two components of the motor are put together, the rotor is fixed in the stator and then both parts are installed in the motor’s shaft. If you’ve ever played with magnets as a child, you know that opposite magnetic poles attract, whereas like poles repel. The basic principle of a motor is built on that simple fact. If you can progressively change the polarity of the stator’s magnetic poles, so that the combined magnetic field will rotate, then the rotor will follow and rotate too! And, that’s how a motor works. Check out the gif below for another great visual representation of an electric motor.
High Effective Service Factor
To calculate the effective service factor, you’ll need to look at the currents and voltages and estimate the load level using a highly accurate dynamic motor analyzer. Then, you can calculate the effective service factor using the equation below.
Is it useful? Yes! By combining a real-life application (load) with professional standards (NEMA), the effective service factor returns an accurate determination of the stress on your specified motor-load application. If your motor’s service factor is greater than 1.0, it is stressed out. A service factor above 1.0 indicates that your motor is only capable of running at an over-loaded capacity for short periods of time. If you are performing longer, steady-state operations at this level, your motors health will rapidly deteriorate. If you work with motors on the regular, you probably know that poor voltage conditions are common and result from a multitude of different reasons. Luckily, NEMA will tell you exactly what load level is permitted under poor voltage conditions, so you can still accurately calculate the effective service factor.
Overload or Excessive Voltage
You’ve been briefly introduced to the concept of load in the previous section, as well as the composition of a motor. It’s time for some more new vocabulary. Meet, stator current! This is a commonly used measurement of load level in an electric motor, which can be easily masked by an over voltage (or excessive load) condition. Many electricians and technicians make the same mistake of operating their motors at an over-voltage, just to reduce the stator current. While this is done with the good intention of reducing the heat on the motor, this is not the case. Excessive voltage may cause the current of the motor to vary but will not result in a decrease in heat or a reduction in losses (of energy). Let’s look at some numbers. A motor (ranging from 10 to 200 horse power) operating at a 10% over voltage will only experience a decrease in losses of about 1-3%. That’s nothing!
Additionally, relying on just the stator current to determine the load conditions is not your best bet. Often, the stator current analysis will detect mild over-loading, while the true values are significantly higher. Your motor’s health is critical to the financial success of your organization, and the costs of downtime, repairs, and replacement will quickly accumulate. While perfect voltage conditions are few and far between, preventing loss and excessive heat in the motor with over-load is imperative, as your motor’s winding insulation and bearing is constantly at risk of deterioration.
Poor Power Condition
Power conditions are usually poor to begin with in industrial factories and manufacturing plants. To compensate, we derate our electric motors (using NEMA standards) to maximize their useful life. The figure below, from NEMA MG-1, specifies the allowed level of percentage load, based on the voltage quality, as a function of balance and distortion. Basically, the higher the level of unbalance, the lower the load you should be running your motors at.
You can use the formula below to define the NEMA derating curve from above.
Have you ever heard of a variable frequency drive (VFD)? A VFD runs an AC motor at different speeds by adjusting the frequency of the motor, which in turn, increases or decrease the rpms (rotations per minute). Unfortunately, the poor conditions of power within manufacturing facilities, combined with the increased use of VFDs can mean bad news for your motor.
Environmental Influences
This is probably the most obvious of reasons why your motor may be heating up. If the ambient temperature is high, your motor will be hot too! Duh, right? Clogged ducts, chemically abrasive substances in the air, humidity, and high-altitude operations may also increase the temperature in your motor. Thermography (infrared imaging) is often used to determine the heat that an electric motor is experiencing from non-electrically induced temperature stress.
Stops and Starts
Are you stopping and starting your motor frequently? Stop that! This places unnecessary stress on your motor. In a motor’s lifetime of operation, the stress of a startup is the worst experience you can put your motor through. We know, however, that these start ups are often inevitable and may occur daily or even hourly. To avoid failure, it is important that you are closely monitoring the quantity and duration of your start-ups with on-line monitoring equipment to ensure that you are meeting (or exceeding) the professional motor standards and guidelines.
