Electric Motors

Electric motors are classified both by their function or application and by their electrical configuration. Some functional classifications (described below) are gearmotors , servomotors , and stepping motors . Many different electrical configurations are also available, independent of their functional classifications. The main electrical configuration division is between AC and DC motors, though one type, the universal motor is designed to run on either AC or DC.

AC and DC refer to alternating current and direct current respectively. AC is typically supplied by the power companies and, in the U. S., alternates sinusoidally at 60 hertz (Hz), at about 120, 240, or 480 volts (V) rms. Many other countries supply AC at 50 Hz. Single-phase AC provides a single sinusoid varying with time, and 3-phase AC provides three sinusoids at 120 ° phase angles. DC current is constant with time, supplied from generators or battery sources and is most often used in vehicles, such as ships, automobiles, and aircraft. Lead-acid batteries are made in multiples of 2 V,† with 6, 12, and 24 V being the most common. Both AC and DC motors are designed to provide continuous rotary output. While they can be stalled momentarily against a load, they can not tolerate a full-current, zero-velocity stall for more than a few minutes without overheating.

† Other battery types have different cell voltages. Carbon-zinc batteries are 1.5 V/cell, alkaline batteries are 1.3 or 1.55V cell, and nickel-cadmium batteries are 1.2 V/cell.

DC MOTORS These motors are made in different electrical configurations, such as permanent magnet (PM), shunt-wound, series-wound, and compound-wound . The names refer to the manner in which the rotating armature coils are electrically connected to the stationary field coils—in parallel (shunt), in series, or in combined series-parallel (compound). Permanent magnets replace the field coils in a PM motor. Each configuration provides different torque-speed characteristics. The torque-speed curve of a motor describes how it will respond to an applied load and is of great interest to the mechanical designer as it predicts how the mechanical-electrical system will behave when the load varies dynamically with time.

PERMANENT MAGNET DC MOTORS Figure 9.20a shows a torque-speed curve for a permanent magnet (PM) motor. Note that its torque varies greatly with speed, ranging from a maximum (stall) torque at zero speed to zero torque at maximum (no-load) speed. This relationship comes from the fact that power = torque x angular velocity . Since the power available from the motor is limited to some finite value, an increase in torque requires a decrease in angular velocity and vice versa. Its torque is maximum at stall (zero velocity), which is typical of many electric motors. This is an advantage when starting heavy loads: e.g., an electric-motor-powered vehicle needs no clutch, unlike one powered by an internal combustion engine that cannot start from stall under load. An engine’s torque increases rather than decreases with increasing angular velocity.

Figure 9-20b shows a family of load lines superposed on the torque-speed curve of a PM motor. These load lines represent a time-varying load applied to the driven mechanism. The problem comes from the fact that as the required load torque increases, the motor must reduce speed to supply it . Thus, the input speed will vary in response to load variations in most motors, regardless of their design.* If constant speed is required, this may be unacceptable. Other types of DC motors have either more or less speed sensitivity to load than the PM motor. A motor is typically selected based on its torque-speed curve.

* Synchronous AC motors, servomotors, and speed controlled DC motors are exceptions.

SHUNT-WOUND DC MOTORS These motors have a torque speed curve like that shown in Figure 9-21a. Note the flatter slope around the rated torque point (at 100%) compared to Figure 9-20. The shunt-wound motor is less speed-sensitive to load variation in its operating range, but stalls very quickly when the load exceeds its maximum overload capacity of about 250% of rated torque.