This is the second half of a two-part article discussing principles and application of the AC induction motors that provide motive power for most of the equipment HVACR engineers specify. In March, we covered basic principles, application conditions, motor ratings, and nameplate data. This month, we dig a bit deeper into starting methods, VFD application, efficiency, and international standards.

Starting Methods

The simplest way to start a motor is to connect it directly to the voltage source. EEs call this a full-voltage start; it is also referred to as “across the line” starting. A full-voltage start provides the highest starting torque and shortest acceleration time of the motor and load but has consequences that may sometimes be undesirable.

If the motor is large relative to the capacity of the electrical system, the high starting current may cause the voltage to dip during the acceleration period, adversely affecting other loads. Full starting torque applied instantaneously may over-stress the driven equipment, belts, or coupling. Either or both starting current and transient torque can be reduced through one of the commonly used methods listed in Table 1, which compares their relative effects on starting current and torque to a full voltage start.

Table 1. Effect on starting current and torque of common motor starting methods.

The need to reduce starting current is more common than the need to reduce transient torque and the first three methods use techniques that reduce current by reducing the voltage applied to the motor during starting. As explained in the previous article, motor torque varies with the square of voltage, and the effect of this on the load must be considered. Figure 1 presents the same motor and load speed-torque characteristics we considered in Part 1, but it shows the variation of motor torque with starting voltage. Clearly, a reduced voltage start decreases the margin of torque that is available for acceleration, and if it causes the motor torque curve to cross the load torque curve, acceleration will stop at that RPM and the motor will run below rated speed until the starting current causes an overload trip.

Reduction of starting torque with voltage does not present a problem for most centrifugal fan and pump loads that start in an unloaded condition. For these loads, the solid state reduced voltage starter, also referred to as a “soft” starter, is the most popular option because of its ability to ramp the voltage up gradually and eliminate transient torque during starting. These starters use power electronics to control the voltage during starting and close a bypass contactor when the motor reaches full speed to eliminate the losses and associated heating in the power electronic components while running. They have the advantage of being able to re-engage the power electronics for a “soft” stop as well, which can reduce check valve wear and water hammer in piping systems. Both the solid state and resistor or reactor type starters reduce the starting current proportionally to the reduction in motor voltage.

Equipment like positive displacement pumps or conveyors that start under load, and centrifuges or large diameter fans that have high inertia, may not tolerate the exponential reduction in starting torque with voltage of a standard reduced voltage starter. The autotransformer starter reduces the starting current drawn from the electrical system by the square of the voltage applied to the motor, which allows it to maintain a higher starting torque for the same degree of current reduction than the solid state and reactor type starters. The Wye-Delta starter provides an equivalent ratio of maintained starting torque to starting current reduction by changing the way the three-phase motor windings are connected between starting and running. This starting method is popular for factory-wired starters for equipment such as hydraulic elevators and chillers, but it is not commonly used when the starter is located in an MCC because it requires six conductors to the motor instead of the usual three.

As discussed in Part 1, standard service conditions for motors include full-voltage starting. If the size of the motor or other considerations dictate an alternate starting method, the manufacturer must be informed of that in your specification, even if the starter is furnished separately from the equipment.

Variable Speed Operation

Varying the frequency of the applied voltage, which changes the speed of the rotating magnetic field, is the most common method of AC motor speed control. VFDs use power electronics to convert 60 Hz AC voltage into DC voltage and then convert the DC back into an AC voltage of the desired frequency. Characteristics of the output voltage of VFDs and operation below rated speed can have adverse impacts on the motor that must be considered. Most designers are familiar with the term “inverter duty” as indicating that a motor design has taken these factors into account and can be safely applied with a VFD.

Requirements for inverter duty motors are defined in MG1 Part 31. This requires additional insulation to protect against voltage spikes and neutral shift, as well as defining temperature rise and speed-torque characteristics under VFD operation. The standard condition is assumed to be load torque varying with the square of speed. Since current decreases with decreasing torque, less heat is produced in the motor at lower speeds, which is important since the shaft-mounted fan provides less cooling. Turn-down capability for a motor with a variable torque load on a VFD may be as high as 10-to-1. If load torque does not drop off as quickly with speed, available turn-down is lower and operation at the bottom end of the speed range may require an external blower to provide cooling air independent of rotor speed.

A motor on a VFD will run hotter than the same motor on sine wave power under the same load and ambient conditions, due to additional heat produced by harmonic components of the VFD output voltage. A motor with a 1.15 SF on standard sine wave voltage thus has a 1.00 SF on a VFD, and it is common to specify thermal margin in the insulation system (typically Class F insulation with Class B design temperature rise) for VFD applications.

Load speed-torque characteristic also affects specification of the VFD itself. As with the motor, the base condition is a centrifugal load, for which a Normal Duty VFD is appropriate. A non-centrifugal torque characteristic may require specifying a Heavy Duty VFD capable of providing more current at low frequencies. Normal Duty and Heavy Duty have supplanted the previously-used terms Variable Torque and Constant Torque, but the principle remains the same.

A final consideration for specifying motors for VFDs is protection against bearing currents. High-frequency switching used to create the motor voltage can impose a high-frequency voltage to ground on the motor shaft. If this voltage is high enough, it will break down the normally-insulating layer of lubricating oil between the balls or rollers and the bearing race. When breakdown occurs, a small spark of current passes through the oil and erodes material from the ball and the race.

A continuous series of such breakdowns has the same effect as electrical discharge machining, eventually damaging the bearing surface enough to lead to premature failure. While this effect has been well documented and proven in situations where damage has been observed, there is no good way to predict in advance whether it will occur for any particular motor-VFD application. Recommended protection for large or critical motors consists of an insulated or ceramic bearing on one end of the shaft and a brush assembly to continuously ground the other end of the shaft. For small and non-critical motors, it is common not to provide protective measures unless a problem develops.

Incidentally, although not discussed in the section on starting methods, the VFD makes an excellent reduced voltage starter. By maintaining a constant ratio of voltage to frequency as the motor accelerates, it allows the motor to produce rated torque while drawing only rated current. The cost and efficiency penalty does not justify its use in a constant-speed application where a standard reduced voltage starter will perform satisfactorily, but it is a good solution if a significant reduction in starting current is required and high starting torque must be maintained.

Efficiency

A 2011 International Energy Agency report estimated that electric motors account for between 43% and 46% of global electricity consumption, making efficiency an important parameter for facility owners concerned about the cost and environmental impact of their operations. In the U.S., national standards for motor efficiency have existed since the enactment of the Environmental Policy Act (EPACT) in 1992. The current requirements, effective on June 1, 2016, state that most electric motors must comply with minimum efficiency levels that NEMA has identified as Premium Efficient in MG1 Table 12-12.

EPACT-mandated efficiencies are defined by a Nominal Efficiency value on the motor nameplate that represents the average efficiency of a group of motors of the same HP rating, RPM, and type. Some motors will have higher efficiencies and some lower, but any motor must operate at not less than the Minimum Efficiency associated with its Nominal Efficiency value. In general, the required efficiency increases with size and for any particular horsepower rating is highest for 4-pole (1,800 RPM) construction.

Motor efficiency is tested at full load under standard conditions including horizontal operation at 25ºC ambient and standard deep groove ball bearings. If the actual operating conditions are different, the motor efficiency may be different. The manufacturer’s motor datasheet will specify efficiencies at 25, 50, 75, and 100% load, which are more accurate for predicting actual power consumption. These datasheets also illustrate that efficiency varies with load and peak efficiency typically occurs in the neighborhood of 75% load. This is a good thing since the limited number of standard HP ratings and the improbability of a pump or fan’s BHP requirement landing neatly on a standard HP rating means that most motors operate at less than their full-load rating. 

International Motors

While motors manufactured in the U.S. are governed by the NEMA Standard MG1 described in Part 1, motors manufactured in Europe and most of the rest of the world are governed by standards of the International Electrotechnical Commission, or IEC. We will not discuss these standards in detail, but try to provide a comparison of IEC and NEMA terminology that will help you navigate the nameplate of a motor sourced from outside the U.S.

IEC motors are rated in kW rather than HP and the conversion is based on the standard 0.746 kW per HP. Use of a rating that we associate with electrical and not mechanical loads carries a caution; this is still output shaft power and cannot be used to calculate the motor input current without taking into consideration efficiency and power factor.

NEMA combines the integrity of the enclosure with the cooling method as in Open Drip Proof (ODP), but IEC provides separate designations. The enclosure integrity is defined by an IP code (IP stands for Ingress Protection) similar to those used for other electrical equipment in which the first digit defines the degree of protection against solids and the second against liquids. IP22 for example would closely correspond to a NEMA drip-proof guarded enclosure and IP54 to a totally enclosed motor. Cooling is defined by a separate IC code, such as IC00 for an open machine and IC411 for a fan-cooled machine. Thus an IEC equivalent of a TEFC motor would be IP54/IC411.

IEC motors also use design letters to define their torque characteristics. An IEC Design N torque characteristic is similar to a standard NEMA design B characteristic, and an IEC Design H characteristic is similar to a NEMA high-torque Design C characteristic. IEC motors use the same insulation classes as NEMA motors and, although the permitted design temperature rise for each class varies slightly, a motor with Class F insulation and Class B rise should provide similar insulation life expectancy in either NEMA or IEC construction.

The ratings of an IEC motor for use in the U.S. should match one of the standard NEMA voltages and be at 60 Hz. Many motor nameplates list multiple voltage ratings, so don’t be alarmed if you see both 380V at 50 Hz and 460V at 60 Hz; as long as the winding connections for the appropriate system voltage are used, the motor will perform satisfactorily. However, a motor rated only for 50 Hz operation should not be used on a 60 Hz system unless the manufacturer has determined that the motor can safely operate at the 20% higher speed and the driven equipment load at that speed is within the motor’s torque capability.