Chapter 3: AC and DC Motors - AC Motors: IEC Ratings

IEC Ratings
At this point, it would be helpful to briefly review IEC motor ratings and then compare IEC with NEMA. The motor market today has become more global, with IEC rated motors on equipment exported from Europe.

IEC is the acronym for the International Electrotechnical Commission. IEC, like NEMA, establishes and publishes mechanical and electrical standards for motors. Many IEC standards have been nationalized for a specific country, such as Germany, Great Britain, or France.

Though NEMA and IEC standards use different terms, they are essentially similar in ratings and in many cases are interchangeable. NEMA standards are probably more conservative, which allows for interpretations in design. IEC standards are more specific and categorized. They are typically more precise.

Both IEC and NEMA use letter codes to indicate mechanical dimensions. They also use code letters to indicate general frame size. The NEMA and IEC dimension codes are not interchangeable, nor are the frame sizes (exception being the 56 frame, which is the same in NEMA and IEC).

IEC lists two numbers: the first number indicates protection against solid objects; the second number indicates protection against water entry. The enclosure letters "IP" indicate ingress protection. (Example: IP55. The first "5" indicates complete protection, including dust-tight, and the second "5" indicates protection from water sprayed from a nozzle from any direction. This type of motor would be considered wash-down duty.)

NEMA would list the enclosure type to indicate the particular cooling method employed in the motor. IEC, however, would use a letter and number code to designate how a motor is cooled. (Example: IC40. The "4" indicates frame cooling, while the "0" indicates convection cooling with no fan.) The temperature insulation class ratings are identical, whether NEMA or IEC.

IEC motors are listed as "50 Hz" rather than the NEMA "60 Hz." A 50-Hz IEC motor will normally operate satisfactorily on 60 Hz, as long as the voltage is increased by the same ratio as the frequency. (Example: 50 Hz at 380 V to 60 Hz at 460 V) The motor speed would be 1/6 higher than at 50 Hz. However, operating a 50-Hz motor at the lower U.S. voltage of 230 V may not operate satisfactorily without derating (requiring the motor to deliver 15 or 20% less torque at nameplate rating, due to motor heating).

When applying an IEC motor instead of a NEMA motor, it is always sound practice to consult a motor rating table for comparisons. NEMA ratings include a factor for overload, whereas IEC strictly rates motors with little to no overload capability.

Wound Rotor
The speed and torque characteristics of an AC induction motor are essentially defined by the design, number of poles, and line power applied. In contrast, the wound rotor version of an induction motor does have controllable speed and torque characteristics. Different values of resistance are inserted into the rotor circuit to obtain various performance options.

Wound rotor motors are normally started with a secondary resistance connected to the rotor circuit. The resistance is reduced to allow the motor to increase in speed. This type of motor can develop substantial torque, and at the same time, limit the amount of locked rotor current. The secondary resistance can be designed for continuous operation at reduced speeds. Special consideration is required for heat dissipation at reduced speeds because of reduced cooling effects and high inertia loads. Figure 3-40 indicates a wiring diagram of a wound rotor motor.

The advantages of this type of motor include a lower starting current (less than 600%) with a high starting torque. This motor type also provides for smooth acceleration and easy control capability.

A disadvantage of this type of motor is that efficiency is low. The external resistance causes a large drop in rpm, based on a small change in load. Speed can be reduced down to 50% of rated value. Another disadvantage is that the relative cost of this motor may be substantially higher than an equivalent three-phase induction motor.

Synchronous Motors
The three-phase AC synchronous motor is a unique and specialized type of motor. Without complex electronic control, this motor type is inherently a fixed-speed motor. This type of motor is used in applications where constant speed is critical. It is also in cases where power factor correction is desired, since it can operate at leading or unity power factor. The synchronous motor is a highly efficient means of converting AC electrical power to mechanical power.

The synchronous motor could be considered a three-phase alternator, operated backward. Direct current is applied directly to the rotor to produce a rotating electromagnetic field. The stator windings are connected in either a wye or delta configuration. Figure 3-41 indicates a diagram of the synchronous motor.

It should be noted that the synchronous motor has a "wound" rotor that is connected to a brush assembly system connected to DC power. In reality, synchronous motors have little to no starting torque. An external device must be used for the initial start of the motor.

Devices such as an auxiliary DC motor/generator or damper windings are typically used to initially start the synchronous motor. The motor is constructed such that it will rotate at the same speed as the rotating stator field. At synchronous speed, rotor and stator speed are equal, and therefore, the motor has no slip. With a load on the shaft, slip increases and the

motor responds with more torque, which increases the speed back to "synchronism."

Synchronous motors in sub-fractional ratings are usually self-excited using damper windings. High-horsepower synchronous motors are usually DC excited using an external DC motor/generator.

Multiple Pole Motors
Multiple pole motors could be considered multiple-speed motors. As stated earlier, the speed is a direct result of the number of pole pairs. At 60 Hz, a four-pole motor would have a synchronous speed of 1800 rpm. At the same 60 Hz, a two-pole motor would have twice the synchronous speed-3600 rpm. Typically, an AC induction motor has only one set of pole pairs-2, 4, 6, or 8 poles, or more. However, specially designed multiple speed motors would be wound for two different pole pair connections.

Most of the multiple pole motors would be dual-speed or two-speed design motors. Essentially, the conduit box would contain two sets of wiring configurations: one for the low-speed and one for the high-speed windings. The windings would be engaged by a two-position switch or electrical contacts. The switch or contacts would connect either the low- or high-speed winding to the three-phase power source.

This type of motor configuration provides a certain amount of flexibility in manufacturing. Perhaps the low-speed winding would be used for a production process taking place on a feed conveyor. Once the process is complete and a limit switch is closed, that same conveyor would move the product at high speed to the packaging and labeling section. There are many other industrial, packaging, food processing, and HVAC applications where two-speed motors could be an advantage. The possible disadvantage of this type of motor is the additional cost of some type of external switch control.

IEC Ratings
At this point, it would be helpful to briefly review IEC motor ratings and then compare IEC with NEMA. The motor market today has become more global, with IEC rated motors on equipment exported from Europe.

IEC is the acronym for the International Electrotechnical Commission. IEC, like NEMA, establishes and publishes mechanical and electrical standards for motors. Many IEC standards have been nationalized for a specific country, such as Germany, Great Britain, or France.

Though NEMA and IEC standards use different terms, they are essentially similar in ratings and in many cases are interchangeable. NEMA standards are probably more conservative, which allows for interpretations in design. IEC standards are more specific and categorized. They are typically more precise.

Both IEC and NEMA use letter codes to indicate mechanical dimensions. They also use code letters to indicate general frame size. The NEMA and IEC dimension codes are not interchangeable, nor are the frame sizes (exception being the 56 frame, which is the same in NEMA and IEC).

IEC lists two numbers: the first number indicates protection against solid objects; the second number indicates...