Common Premium-Efficient Motor Misconceptions

How NEMA (National Electrical Manufacturers Association) Defines Premium Efficiency

As part of the EISA initiative, the NEMA Premium efficiency motor standard was introduced to:

1. Reduce energy operating costs of electric motors on industrial & commercial buildings
   
2. Help purchasers find higher efficient motors that save money & improve system reliability
   
3. Optimize efficiency in motor systems regarding power supply & utility deregulation issues
   
4. Reduce energy consumption & pollution associated with electrical power generation

NEMA Premium standards bring a new question to electric motor purchasers: Are the motors you are buying as efficient as they could be? Or are your motors wasting energy and money?

1. Efficiency automatically equals savings.

Not necessarily; it depends on the application. Current Australian MEPS (Minimum Energy Performance Standards) requirements mandate that new motor efficiencies be as a minimum IE2 High Efficiency motor or IE3 Premium Efficiency with efficiencies based on AS1359.5:2004, which “roughly” aligns with the international IEC60034:30 standard on induction motor energy efficiency. The efficiency levels are typically based upon 80-100% loading with the motor connected to constant power source (i.e. across the line). So purchasing a High or Premium Efficiency motor from one manufacturer will very likely give you a motor with basically the same efficiency as a High or Premium Efficiency motor purchased from another manufacturer.

If you only install a premium-efficient motor, you are not automatically saving all the money you could be saving. There are multiple reasons why this might be possible, as discussed below:

• Your new motor may only be a few percent more efficient than your previous motor. Therefore, in cycling or intermittent duty applications, the savings you recognize are so small that they are outweighed by the higher cost of the new motor.
• Your new motor may not be well-suited to saving energy in your type of application, e.g. high-cycling applications.
• Your new motor may be over-sized for the application, yielding much less efficiency than what the nameplate says.
• Other parts of your drive-train may be much less efficient, causing higher-than necessary energy consumption from your efficient motor.

While a premium-efficient motor is important, it’s critical to evaluate your entire drive-train for efficiency and to realize that the motor is just a single part of the overall equation.

2. Replacing my motor will give me the best bang for my buck.

It depends. A motor is only one component in the drive-train (and, truth be told, motors for some time have been comparatively efficient). Each component in a system will inherently have some inefficiency and these energy losses multiply together to provide an overall system efficiency. Just one component with poor efficiency will quickly drag down the rest of the system. Consider the following theoretical example where every component has an almost-impossible efficiency of 99%:

You’ll see that even in this example with 6 components of ideal efficiency, you are still losing almost 6% of the energy.

Now, to consider two more realistic examples:

You can see in both examples that you are losing over 42% of the energy going into the system. You can also see that replacing your motor with a premium-efficient model will save you just over 4% efficiency, even though the new motor is 7% more efficient that the old motor. That is because the other, less efficient components in your drivetrain are still wasting energy. Therefore, the investment you have made in a premium-efficient motor will take longer to recoup than you had planned.

3. Replacing my motor will automatically make my line more efficient.

Well, yes- but by less than you can expect. However, replacing some of the other components along with your motor can provide some very substantial efficiency gains. Consider, for instance, that you replace the gear unit as well as the motor. Worm gear units, which are installed in many manufacturing environments, are inherently inefficient, as the gears are essentially sliding against one another causing heat (energy loss). Sure, there are instances on which worm drives are necessary for the application (e.g. withstanding heavy shock loads, or providing back-driving resistance). But, in many applications a helical-bevel gear units, which operates with rolling contact, will be much more efficient.

Take the previous “real world” example and replace both the motor AND the gear unit.

Now you are quickly recognizing substantial, double-digit efficiency grains- nearly 20%- and your line begins to become much more efficient.

To gain even more efficiency, consider changing or eliminating your transmission elements. Replacing a v-belt with a direct drive or use a shaft-mounted gear unit. Shaft-mounted units, such as the TorqLOC® from SEW-EURODRIVE, offers a keyless, taper hollow shaft with a shrink disc that has a liberal tolerance so it installs easily. It can even retrofit onto an existing keyed shaft.

4. A premium-efficient motor is an appropriate energy-saving choice for all applications.

Again, it depends. Most premium-efficient motors used in continuously-running applications will begin to show at least modest energy savings (depending, as previously shown, on the other elements in the drive-train).

But, motors used in high-cycling applications may never recognize the entire efficiency gain of a premium-efficient motor, due to the start-and-stop nature of the application fighting against the higher rotor inertia of many premium-efficient motors. Hence, the extra investment in a high-efficiency motor may not ever be completely recouped.

However, some Premium Efficiency motors, such as the DRP motor from SEW-EURODRIVE are engineered to make them more efficient in high-cycling applications. These motors are designed with low losses, and less heat accumulation in the windings, which increases efficiency and provides a very high number of starts and stops per hour.

Be sure that you consider all the options available to you, and be careful to choose the premium-efficient motor that is best suited to your need.

5. Adding a variable frequency drive (VFD) will automatically make my line more efficient.

Maybe. As the efficiency equation shows, a VFD is a load. It produces heat (losses) from electricity conversion, switching frequency, and harmonics. So by itself, it will decrease your system efficiency. What’s more, many VFD’s have an adjustable carrier frequency that reduces audible noise during operation. Unfortunately, when the carrier frequency increases, so does the heat. In fact, the heat produced as a high carrier frequency can be so significant that a room full of VFD’s may require a substantial increase in air conditioning.

Thus, the key to energy savings is to use a VFD to reduce other losses in the system (i.e. smart control), as in the following applications.

• Regenerative Energy: When a motor is trying to stop a high inertia load or lower a load, it acts as a generator. All of the kinetic or potential energy stored in the machine has to be removed. All of the kinetic or potential energy stored in the machine has to be removed. Typically, it is wasted as heat through a braking resistor. But, a regenerative VFD can put the energy back onto the grid. Some even allow the energy to be directly given to another VFD as it accelerates, such as in a storage retrieval system.
• HVAC: Typical systems used in HVAC contain mechanical dampers with motors that run continuously. Using a VFD to turn off the motor or to optimize the motor speed is much more efficient, especially since the load decreases more than four times at half the speed.
• Soft Start: Using a VFD to control the acceleration on a cycling application lowers the motor starting current. So, the motor runs cooler since less energy is converted to heat.
• Motor Efficiency Correction: Motor nameplate is usually rated at 80% loading. Therefore, when a large motor is applied to a small load (e.g. 1 kW used instead of 0.25 kW), its actual efficiency decreases considerably. Using a VFD with vector control (or VFC technology) optimizes the motor efficiency, regardless of the loading conditions.

The bottom line? If properly used, VFDs can have some big efficiency benefits when added as part of a complete drive-train efficiency solution.