Alternating current motors
AC motors are used worldwide in many residential, commercial, industrial, and utility applications. Motors transform electrical energy into mechanical energy. An AC motor may be part of a pump, fan, or other form of mechanical equipment. AC motors are found in a variety of applications, from those that require a single motor to special applications that require several motors working in concert.
All AC motors are made up of a magnetic circuit formed by a stationary member called a stator and a rotating member known as rotor. The stator and rotor are separated by an air gap. The stator has primary windings that are connected to the power source and develop a rotating magnetic field. The rotor has secondary windings that rotate in the magnetic field created by the stator windings. This causes currents to flow in the secondary windings and causes development of a secondary magnetic field. How the rotors are designed and how the currents are made to flow determines the type of motor and its performance characteristics. Two types of AC motors are widely used in the oil and gas industry: induction and synchronous.
The National Electrical Manufacturers Association (NEMA) sets the standards for a wide range of electrical products, including motors. NEMA is associated primarily with motors used in North America. NEMA’s standards (see NEMA Standard Publication No. MG 1) represent general industry practices and are supported by manufacturers of electrical equipment. The NEMA standards might not apply to some large AC motors that are built to meet the requirements of a specific application. Such motors are referred to as "above-NEMA." The American Petroleum Institute (API) and the Institute of Electrical and Electronic Engineers (IEEE) also have standards for NEMA-sized and above-NEMA motors.
Principle of rotation
To see how a rotor works, use a magnet mounted on a shaft in place of the squirrel cage rotor, as shown in the upper image in Fig. 1. Energizing the stator windings establishes a rotating magnetic field. The magnet has its own magnetic field that interacts with the rotating magnetic field of the stator. The north pole of the rotating magnetic field attracts the south pole of the magnet, and vice versa. As the rotating magnetic field rotates, it pulls the magnet along, causing it to rotate. Motors that use this design are known as permanent-magnet synchronous motors.
The squirrel-cage rotor acts essentially the same as the magnet. When power is applied to the stator, current flows through the winding, causing an expanding electromagnetic field (emf) that cuts across the rotor bars.
When a conductor, such as a rotor bar, passes through a magnetic field, it induces a voltage (an emf) in the conductor. The induced voltage causes a current flow in the conductor. The current flows through the rotor bars and around the end ring, producing magnetic fields around each rotor bar. In an AC circuit, the current continuously changes direction and amplitude. The resultant magnetic field of the stator and rotor continuously change. The squirrel-cage rotor becomes an electromagnet with alternating north and south poles.
The lower image in Fig. 1 illustrates one instant in time during which current flow through winding A1 produces a north pole. The expanding field cuts across an adjacent rotor bar, inducing a voltage. The resultant magnetic field in the rotor tooth produces a south pole. As the stator magnetic field rotates, the rotor follows.
The speed of the rotating magnetic field is referred to as the synchronous speed (Ns). Synchronous speed is equal to 120 times the frequency (f), divided by the number of poles (P):
If the frequency of the applied power supply for the two-pole stator is 60 Hz, then the synchronous speed is 3,600 rev/min:
Synchronous speed decreases as the number of poles increases. Table 1 shows the synchronous speed at 60 Hz for different numbers of poles.
The relative difference in speed between the rotor (N) and the rotating magnetic field (Ns) is called slip. There must be some slip because if the rotor and the rotating magnetic field were turning at the same speed, no relative motion would exist between the two, such that no lines of flux would be cut and no voltage would be induced in the rotor. Slip is necessary to produce torque and depends on load—an increase in load will cause the rotor to slow down, ergo increase the slip. Conversely, a decrease in load will cause the rotor to speed up, and so will decrease slip. Slip (S) is expressed as a percentage and can be determined by:
where N = rotor speed, rev/min. For example, a four-pole motor operated at 60 Hz has a synchronous speed of 1,800 rev/min. If the rotor speed at full load is 1,765 rev/min, then slip is 1.9%:
Factors such as voltage and frequency variation, altitude, and temperature can affect the operation and performance of an AC motor enough to lower its rated capability, and should be considered when selecting a motor.
AC motors are designed to operate on standardized voltages and frequencies. A small variation in supply voltage can affect motor performance significantly. Fig. 2 shows that when the supply voltage is 10% below the rated voltage of the motor, the motor has 20% less starting torque. This reduced voltage might prevent the motor from getting its load started or keep it from running at its rated speed. A 10% increase in supply voltage, on the other hand, increases the starting torque by 20%, which might cause damage during startup (e.g., a conveyor might lurch forward at startup). Voltage variation causes similar changes in a motor’s starting amperage, full-load amperage, and temperature rise.
Variation in the frequency at which a motor operates causes changes mainly in its speed and torque. For example, a 5% increase in frequency causes a 5% increase in full-load speed and a 10% decrease in torque (Table 2). AC motors should operate successfully at their rated load with a combined variation in voltage and frequency of up to 10% above or below the rated voltage and the rated frequency, provided that the frequency variation does not exceed 5%; however, performance within this combined variation range might not be the same as the standards established for operation at the rated voltage and frequency.
Altitude and temperature
The NEMA standards for allowable temperature increase for motor winding insulation discussed in the Insulation Classes section of this chapter is based on motor operation at or below an altitude of 3,300 ft and at a maximum ambient temperature of 40°C. For operation at altitudes of > 3,300 ft (at 40°C ambient), most motors must be derated because of temperature increase in the windings, as shown in Table 3. Some motors (Class A or B insulated) can be operated successfully at altitudes of > 3,300 ft in locations where a decrease in ambient temperature compensates for the increase in temperature rise. Also, motors with a service factor of 1.15 or higher will operate satisfactorily at a 1.0 service factor at a 40°C ambient temperature at altitudes of between 3,300 and 9,000 ft.
Matching AC motors to load
One way to evaluate whether the torque capabilities of a motor meet the torque requirements of the load is to compare the motor’s speed/torque curve with the speed/torque requirements of the load.
Load characteristics tables
Use a load-characteristics table to find the torque characteristics of various types of loads. 'Table 4 is an example of such a table, although it contains only a partial list of load types. NEMA MG 1 is one—and a complete—source of typical torque characteristics.
Calculating load torque
The most accurate way to obtain torque characteristics of a given load is to obtain them from the equipment manufacturer.
When a motor accelerates a load from zero to full-load speed, the amount of torque it can produce changes. Throughout acceleration, the motor must produce more torque than required by the load. Fig. 3 graphs speed/torque curves for a NEMA B motor with a centrifugal-pump load. The pump load curve shows that the centrifugal pump only requires approximately 20% of full-load torque to start. The torque dips slightly after the pump is started, then increases. This typically is defined as a variable torque load. The pump will operate at the speed where the torque required by the pump equals that furnished by the motor.
Screw down actuator
Fig. 4 graphs speed/torque curves for a NEMA-B motor with a screw-down actuator load. The actuator-load curve shows that the starting torque of a screw-down actuator is approximately 200% of full-load torque. Comparing the load’s requirement with the NEMA B-design motor of equivalent horsepower shows that the load’s starting torque requirement is greater than the motor’s capability. The motor therefore will not start and accelerate the load.
One solution would be to use a higher-horsepower NEMA B motor. A less-expensive solution might be to use a NEMA D motor of the same horsepower requirements as the load. A NEMA D motor would start and accelerate the load easily, as shown in Fig. 5.
The motor selected to drive the load must have sufficient torque to start, accelerate, and run the load. If ever the motor cannot produce the required torque, it will stall or run in an overloaded condition. This will cause it to generate excess heat and typically to exceed current limits, causing protective devices to disconnect the motor from the power source. If the overload condition is not corrected, or the proper motor not installed, the existing motor eventually will fail.
|rotor speed, rev/min
|synchronous speed, rev/min
|number of poles
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