Keeping it simple: Steps to determine motor’s actual load

Oversized motors cost more to operate—sometimes a lot more. Fortunately, there’s a simple procedure for determining the actual hp required by a load, without expensive equipment or engineering.

10/20/2014


Figure 1: Percent load versus current. Source: EASAContrary to popular opinion, bigger isn’t always better—especially when it comes to electric motors. Plant maintenance and engineering departments like having a little extra power available “just in case,” so they sometimes specify larger motors than applications require. But oversized motors cost more to operate—sometimes a lot more. Fortunately, there’s a simple procedure for determining the actual hp required by a load, without expensive equipment or engineering. Bear in mind that loads should be determined when the motor is operating at its maximum load. Loads that vary widely are good candidates for variable-frequency drives (VFDs), which offer the added benefit of controlling rate of production.

Estimating actual load

As Figure 1 illustrates, percent load and current are essentially linear, from no-load up to motor’s nameplate current. An easy mistake, however, is to assume that zero load = zero current. That assumption will result in errors in determining hp, with the error inversely proportional to the load (shaded area of Figure 1). The biggest errors occur when considering motors most in need of “right-sizing” [i.e., 50% of full-load amps (FLA) does not = 50% load].

While it’s possible to determine the percent load a motor carries from the graph in Figure 1, the actual load of a motor can be approximated mathematically from good input data:



Operate the motor uncoupled and record the current at no-load (0% load). Don’t take any shortcuts here, because the measured current will be higher if the motor is coupled. To avoid errors, always use the uncoupled current.

Next, document the nameplate current and then measure the current at the motor’s actual load. Since an undersized motor presents other problems, it’s best to measure the current over the operating cycle of the process. If the load varies, record the current during peak load.

Cost of “safety margin”

The additional cost of operating seriously under-utilized motors often includes utility surcharges for poor power factor (more about that later). Cyclical power users may also be subject to demand charges based on the peak power usage. That means a single high-usage episode (e.g., across-the-line starting of a large motor) could trigger a higher kW/hour rate for an entire billing period. Identifying under-utilized motors offers many users an opportunity to reduce operating costs.

Hidden costs of oversized motors

Inrush current, the current a motor draws at the moment of starting is not load-dependent. For a motor of a given size, it is the same regardless of actual load. That means a 100 hp motor starting uncoupled draws the same initial current as it does when starting a 100 hp load. Because starting current is roughly six times nameplate current (depending on the NEMA code letter), it can be significantly higher for an oversized motor than a “right-sized” model.

A motor’s actual inrush current, also known as locked-rotor amps or LRA, can be calculated from the kVA/hp values associated with its NEMA code letter (Table 1):

LRA = CL x hp x 1000/1.732 x Voltage

(For CL, plug in the kVA per hp value from the table.)

Table 1: NEMA code letters for locked-rotor KVA (Reference: NEMA Standards MG1-2011, 10.37.2)

For example, the LRA for a 125 hp motor with the code letter G (5.6 – 6.3 kVA/hp) should fall between 878 and 988 amps:

5.6 x 125 x 1000/1.732 x 460 = 878 amps

6.3 x 125 x 1000/1.732 x 460 = 988 amps

Real life example

A 125 hp motor on a fan application had a nameplate rating of 148 amps but drew 44 amps (slightly less than 1/3 of FLA) when it was run uncoupled. Testing confirmed that, when operating at normal load, the motor drew only 63 amps. The actual hp required was calculated at less than 23 hp:

hp = 125 [1-(148-63/148-44)] = 22.8 hp

Substituting a higher efficiency 25 hp replacement motor dramatically decreased the starting current from 890 amps to 198 amps (at the same code letter G). The “full-load” current also decreased from 63 to 29 amps. Obviously, the plant had been paying for a lot of wasted electricity.

The power factor of the original 125 hp motor was also very low when operating a 22.8 hp load, which may have triggered surcharges from the utility.

Excessively high starting current can increase maintenance costs due to additional wear and tear on motor starters and contacts, but higher inrush current is also a feature of higher efficiency motors.

Note: Before “rightsizing,” be sure the lower hp motor can provide the required starting inertia of the load.

Power factor and efficiency

Power factor (PF) is important, because it can be used to determine efficiency. To calculate power factor, use the following formula:

PF = Input watts / [1.732 x Volts x Amps]

Efficiency can also be calculated if power factor is measured using one of several instruments available to today’s electrician. To calculate efficiency for a three-phase motor:

Efficiency = 746 x hp / [1.732 x Volts x Current x PF]

In the case of the 125 hp motor, an electrician measured a PF of 0.7, so its calculated efficiency when driving the 22.8 hp load was:

Efficiency = 746 x 22.8 / [1.732 x 460 x 63 x .7] = 0.48

The motor was operating at only 48% efficiency. Compare that to an NEMA Premium replacement motor with 95.4% efficiency. According to the Motor Master software available from the U.S. Dept. of Energy, the original 125 hp motor would use more than 457,700 kWH annually if it ran 24/7 (8,760 hours).A 25 hp EPACT replacement would use only 167,200 kWH. At 7 cents per kWH, the first year savings would top $20,000 (U.S.). Where do you want to spend your money?

Chuck Yung is a senior technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis, Mo.



JEFF , IL, United States, 10/28/14 10:09 PM:

Thanks. Great article.
OSCAR , Non-US/Not Applicable, Ecuador, 11/06/14 01:18 PM:

Great article, for sure it will be of help

thank you
SITAPATI , Non-US/Not Applicable, India, 11/26/14 02:29 AM:

One can expect more downward performance if power supply to motor is not sinusoidal. This condition can arise if there are plenty of rectifier using loads
James , KY, United States, 03/13/15 08:39 AM:

Very good info! Thanks
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
2016 Product of the Year; Diagnose bearing failures; Asset performance management; Testing dust collector performance measures
Safety for 18 years, warehouse maintenance tips, Ethernet and the IIoT, GAMS 2016 recap
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Safety at every angle, Big Data's impact on operations, bridging the skills gap
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role of plant safety and offers advice on best practices.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me