|Determining Electric Motor Load and Efficiency|
|Written by USDOE Motor Challenge|
|Thursday, 10 June 2010 15:01|
DETERMINING ELECTRIC MOTOR LOAD AND EFFICIENCY
Most likely your operation’s motors account for a large part of your monthly electric bill. Far too often motors are mismatched—or oversized—for the load they are intended to serve, or have been rewound multiple times.
To compare the operating costs of an existing standard motor with an appropriately-sized energy- efficient replacement, you need to determine operating hours, efficiency improvement values, and load. Part-load is a term used to describe the actual load served by the motor as compared to the rated full-load capability of the motor. Motor part-loads may be estimated through using input power, amperage, or speed measurements. This fact sheet briefly discusses several load estimation techniques.
Reasons to Determine Motor Loading
Most electric motors are designed to run at 50% to 100% of rated load. Maximum efficiency is usually near 75% of rated load. Thus, a 10-horsepower (hp) motor has an acceptable load range of 5 to 10 hp; peak efficiency is at 7.5 hp. A motor’s efficiency tends to decrease dramatically below about 50% load. However, the range of good efficiency varies with individual motors and tends to extend over a broader range for larger motors, as shown in Figure 1. A motor is considered underloaded when it is in the range where efficiency drops significantly with decreasing load. Figure 2 shows that power factor tends to drop off sooner, but less steeply than efficiency, as load decreases.
Overloaded motors can overheat and lose efficiency. Many motors are designed with a service factor that allows occasional overloading. Service factor is a multiplier that indicates how much a motor can be overloaded under ideal ambient conditions. For example, a 10-hp motor with a 1.15 service factor can handle an 11.5-hp load for short periods of time without incurring significant damage. Although many motors have service factors of 1.15, running the motor continuously above rated load reduces efficiency and motor life. Never operate over- loaded when voltage is below nominal or when cooling is impaired by altitude, high ambient temperature, or dirty motor surfaces.
If your operation uses equipment with motors that operate for extended periods under 50% load, consider making modifications. Sometimes motors are oversized because they must accommodate peak conditions, such as when a pumping system must satisfy occasionally high demands. Options available to meet variable loads include two-speed motors, adjustable speed drives, and load management strategies that maintain loads within an acceptable range.
Determining if your motors are properly loaded enables you to make informed decisions about when to replace motors and which replacements to choose. Measuring motor loads is relatively quick and easy when you use the techniques discussed in this fact sheet. You should perform a motor load and efficiency analysis on all of your major working motors as part of your preventative maintenance and energy conservation program. Use Attachment A, “Motor Nameplate and Field Test Data Form,” to record motor nameplate data and field measurements.
We recommend that you survey and test all motors operating over 1000 hours per year. Using the analysis results, divide your motors into the following categories:
Determining Motor Loads
Input Power Measurements
When “direct-read” power measurements are available, use them to estimate motor part-load. With measured parameters taken from hand-held instruments, you can use Equation 1 to calculate the three-phase input power to the loaded motor.You can then quantify the motor’s part-load by comparing the measured input power under load to the power required when the motor operates at rated capacity. The relationship is shown in Equation 3.
Line Current Measurements
The current load estimation method is recommended when only amperage measurements are available. The amperage draw of a motor varies approximately linearly with respect to load, down to about 50% of full load. (See Figure 3.) Below the 50% load point, due to reactive magnetizing current requirements, power factor degrades and the amperage curve becomes increasingly non-linear. In the low load region, current measurements are not a useful indicator of load.
Nameplate full-load current value applies only at the rated motor voltage. Thus, root mean square (RMS) current measurements should always be corrected for voltage. If the supply voltage is below that indicated on the motor nameplate, the measured amperage value is correspondingly higher than expected under rated conditions and must be adjusted downwards. The converse holds true if the supply voltage at the motor terminals is above the motor rating. The equation that relates motor load to measured current values is shown in Equation 4.
The Slip Method
The slip method for estimating motor load is recommended when only operating speed measurements are avail- able. The synchronous speed of an induction motor depends on the frequency of the power supply and on the number of poles for which the motor is wound. The higher the frequency, the faster a motor runs. The more poles the motor has, the slower it runs. Table 1 indicates typical synchronous speeds.
The actual speed of the motor is less than its synchronous speed with the difference between the synchronous and actual speed referred to as slip. The amount of slip present is proportional to the load imposed upon the motor by the driven equipment (see Figure 4). For example, a motor running with a 50% load has a slip halfway between the full load and synchronous speeds.
By using a tachometer to measure actual motor speed, it is possible to calculate motor loads. The safest, most convenient, and usually most accurate tachometer is a battery powered stroboscopic tachometer. Mechanical tachometers, plug-in tachometers, and tachometers which require stopping the motor to apply paint or reflective tape should be avoided. The motor load can be estimated with slip measurements as shown in Equation 5 and the following example.
The speed/slip method of determining motor part-load is often favored due to its simplicity and safety advantages. Most motors are constructed such that the shaft is accessible to a tachometer or a strobe light.
The accuracy of the slip method, however, is limited. The largest uncertainty relates to the 20% tolerance that NEMA allows manufacturers in their reporting of nameplate full-load speed.
Given this broad tolerance, manufacturers generally round their reported full-load speed values to some multiple of 5 rpm. While 5 rpm is but a small percent of the full-load speed and may be thought of as insignificant, the slip method relies on the difference between full-load nameplate and synchronous speeds. Given a 40 rpm “correct” slip, a seemingly minor 5 rpm disparity causes a 12% change in calculated load.
Slip also varies inversely with respect to the motor terminal voltage squared—and voltage is subject to a separate NEMA tolerance of ±10% at the motor terminals. A voltage correction factor can, of course, be inserted into the slip load equation. The voltage compensated load can be calculated as shown in Equation 6.
An advantage of using the current-based load estimation technique is that NEMA MG1-12.47 allows a tolerance of only 10% when reporting nameplate full-load current. In addition, motor terminal voltages only affect current to the first power, while slip varies with the square of the voltage.
While the voltage-compensated slip method is attractive for its simplicity, its precision should not be overestimated. The slip method is generally not recommended for determining motor loads in the field.
Determining Motor Efficiency
The NEMA definition of energy efficiency is the ratio of its useful power output to its total power input and is usually expressed in percentage, as shown in Equation 7.
By definition, a motor of a given rated horsepower is expected to deliver that quantity of power in a mechanical form at the motor shaft.
Figure 5 is a graphical depiction of the process of converting electrical energy to mechanical energy. Motor losses are the difference between the input and output power. Once the motor efficiency has been determined and the input power is known, you can calculate output power.
NEMA design A and B motors up to 500 hp in size are required to have a full-load efficiency value (selected from a table of nominal efficiencies) stamped on the nameplate. Most analyses of motor energy conservation savings assume that the existing motor is operating at its nameplate efficiency. This assumption is reasonable above the 50% load point as motor efficiencies generally peak at around 3/4 load with performance at 50% load almost identical to that at full load. Larger horsepower motors exhibit a relatively flat efficiency curve down to 25% of full load.
It is more difficult to determine the efficiency of a motor that has been in service a long time. It is not uncommon for the nameplate on the motor to be lost or painted over. In that case, it is almost impossible to locate efficiency information. Also, if the motor has been rewound, there is a probability that the motor efficiency has been reduced.
When nameplate efficiency is missing or unreadable, you must determine the efficiency value at the operating load point for the motor. If available, record significant nameplate data and contact the motor manufacturer. With the style, type, and serial number, the manufacturer can identify approximately when the motor was manufactured. Often the manufacturer will have historical records and can supply nominal efficiency values as a function of load for a family of motors.
When the manufacturer cannot provide motor efficiency values, you may use estimates from Attachment C. Attachment C contains nominal efficiency values at full, 75%, 50%, and 25% load for typical standard efficiency motors of various sizes and with synchronous speeds of 900, 1200, 1800, and 3600 rpm. Attachment C indicates “industry average” full- and part-load performance for all standard efficiency motors currently on the market.
Three steps are used to estimate efficiency and load. First, use power, amperage, or slip measurements to identify the load imposed on the operating motor. Second, obtain a motor part-load efficiency value consistent with the approximated load either from the manufacturer or by interpolating from the data supplied in Attachment C. Finally, if direct-read power measurements are available, derive a revised load estimate using both the power measurement at the motor terminals and the part-load efficiency value as shown in Equation 8.
For rewound motors, you should make an adjustment to the efficiency values in Attachment C. Tests of rewound motors show that rewound motor efficiency is less than that of the original motor. To reflect typical rewind losses, you should subtract two points from your standard motor efficiency on smaller motors (<40 hp) and subtract one point for larger motors. Shops with the best quality-control practices can often rewind with no significant efficiency degradation.
Computerized Load and Efficiency Estimation Techniques
There are several sophisticated methods for determining motor efficiency. These fall into three categories: special devices, software methods, and analytical methods. The special devices package all or most of the required instrumentation in a portable box. Software and analytical methods require generic portable instruments for measuring watts, vars, resistance, volts, amps, and speed. These need to be instruments of premium accuracy, especially the wattmeter that must have a broad range including good accuracy at low power and low power factor.
Washington State University Cooperative Extension Energy Program, in partnership with the Oregon State University Motor Systems Resource Facility, recently conducted lab testing of several efficiency-measuring methods. These included three special devices: the Vogelsang and Benning Motor-Check, the ECNZ Vectron Motor Monitor, and the Niagara Instruments MAS-1000. Their efficiency readings were carefully compared to “true” efficiency, measured by a dynamometer and precision lab instruments per IEEE testing standards. From 25% load to 150% load the special devices tended to hold an accuracy within 3%, even in adverse conditions of voltage deviation and unbalance on old, damaged, or rewound motors. In less challenging test conditions, they tended to operate within 2% accuracy. These instruments require a skilled electrician or other personnel trained in the safe connection of electrical equipment in industrial power systems plus about a day of training and practice. The motors must be temporarily unpowered for a resistance test and temporarily uncoupled for a no-load test, i.e., running at normal voltage unloaded. Uncoupling in-situ is rarely convenient, but the no-load test can be run at times such as receiving inspection or following service at the shop. No-load performance does not tend to change significantly over time in the absence of a failure/repair event.
Software and analytical methods were also tested in the lab research described above. When measurement of input data was made with precision lab instruments, the accuracy of methods requiring a no-load test approached that of the special devices’ performance.
The Oak Ridge National Laboratory has developed ORMEL96 (Oak Ridge Motor Efficiency and Load, 1996), a software program that uses an equivalent circuit method to estimate the load and efficiency of an in-service motor. Only nameplate data and a measurement of rotor speed are required to compute both the motor efficiency and load factor. The program allows the user to enter optional measured data, such as stator resistance, to improve accuracy of the efficiency estimate. Future refinements of ORMEL96 are expected to create a more user-friendly product.
Finally, motor load and efficiency values are automatically determined when measured values are entered into MotorMaster+ software’s motor inventory module. MotorMaster+ contains a database of new motor price and performance, and features many motor energy management capabilities including replacement analysis, maintenance logging, inventory control, energy and dollar savings tracking, and life cycle cost analysis. MotorMaster+ is available at no cost to Motor Challenge Partners.
Instantaneous power is proportional to instantaneous voltage times instantaneous current. AC voltage causes the current to flow in a sine wave replicating the voltage wave. However, inductance in the motor windings somewhat delays current flow, resulting in a phase shift. This transmits less net power than perfectly time- matched voltage and current of the same RMS values. Power factor is the fraction of power actually delivered in relation to the power that would be delivered by the same voltage and current without the phase shift. Low power factor does not imply lost or wasted power, just excess current. The energy associated with the excess current is alternately stored in the windings’ magnetic field and regenerated back to the line with each AC cycle. This exchange is called reactive power. Though reactive power is theoretically not lost, the distribution system must be sized to accommodate it, which is a cost factor. To reduce these costs, capacitors are used to “correct” low power factor. Capacitors can be thought of as electrical reservoirs to capture and reflect reactive power back to the motor.
AC voltage rises positive and falls negative 60 times per second, so how do you state its value? Industry practice is to quote the RMS voltage. RMS is a value 70.7% of the peak positive voltage. An RMS voltage will produce exactly the same heating rate in a resistive load as a DC voltage of the same value. RMS is the acronym for the mathematical steps used in its derivation. Square the voltage at all moments in an AC cycle, take the mean of these, and then take the square root of the mean. For reasons lost in obscurity, the steps are stated in reverse sequence, Root Mean Square.
Following the voltage, the power derived from an AC source peaks and falls to zero 120 times per second. This causes torque pulsations in motors, creating noise, vibration, and higher shaft stresses. Though minimal in fractional-horsepower household motors, this would be intolerable in larger motors. Larger motors usually re- quire three-phase power, which avoids the problem. Three conductors supply AC voltage, offset in time or phase, so that they peak not simultaneously but at equally spaced intervals. This produces constant smooth torque from a motor, because at all moments, the sum of power from all three phases is constant. Three-phase power also eliminates the need for special starting windings required in single-phase motors.
Voltage is a differential parameter; it is always measured between two points. There are two ways to measure three-phase voltage—between two of the three lines or between a line and neutral or ground. Service and motor voltages are quoted as line to line, unless otherwise noted. Because of the phase difference, line-to-line readings are 1.73 times line-to-neutral readings.
Inside a three-phase motor there are three windings, one for each phase. The easiest three-phase motor connection to visualize is with each of the three windings connected line to neutral. This is called wye because, schematically, it looks like the letter “Y”. A more common connection eliminates the neutral tie and connects the three windings from line to line. This is called delta because, schematically, this looks like a triangle or the Greek letter Delta. The winding experiences 73% higher voltage when connected line to line, so it must be designed for the type of connection it will have. Even if a motor’s windings are internally wye connected, its nameplate voltage rating is the line-to-line value