AC motive power takes off
For decades, the ac induction motor has been recognized as a highly reliable and cost-effective power source for stationary equipment and machine tool applications such as pumps, compressors, fans, electric doors and conveyors. AC motors have a simple motor design with fewer moving parts than their dc counterparts, yielding unparalleled reliability while keeping maintenance requirements to a minimum. This simple motor design, coupled with the accessibility of ac power through standard wall sockets, has made it the standard in stationary machinery for many years.
The evolution of ac motor technology into mobile equipment began in Europe in the late 1970s, first in trains and later in electric automobiles. Today, ac motors represent a major trend in mobile industrial equipment, especially electric-powered lift trucks.
AC motors for lift trucks and mobile applications
The ac motors and controllers that are used today in lift trucks and other mobile industrial machines first appeared three decades ago in Europe and Japan as part of their evolving train motor technology. AC-powered lift trucks have been prevalent throughout Europe and Asia for about 10 yr, and have only become widely available in the United States in recent years. This increase is due primarily to advancements in controller design that have reduced both size and cost.
To understand the reasons for the delay of ac-powered lift trucks and other mobile industrial equipment in the United States, it is important to examine the differences in voltage standards. Both Europe and Asia use higher voltage/lower amperage motors and controller systems. Conversely, the United States uses a lower voltage/higher amperage system. Initially, commercially available ac controllers were very large, very expensive, and did not support standard U.S. voltage requirements. This created a delay in the development, use and acceptance of ac-powered lift trucks in the U.S.
Therefore, ac power faced a significant challenge in the U.S. lift truck market. It would not become a viable option until a lower voltage/higher amperage controller could be developed. In addition to being reliable and affordable, a new controller would have to be small enough to fit within the same compartment of a standard dc controller, due to the compact design of lift trucks, while delivering comparable power.
Recognizing the outstanding performance and maintenance benefits of ac power, major manufacturers of both lift trucks and motor controllers shifted their research and development efforts toward the creation of commercially viable ac motors and controllers, eventually overcoming cost, size, and voltage challenges. Now, with the advent of reliable and affordable electronic components, ac power is readily available in the U.S. and widely accepted as a superior alternative to dc power when reliability, performance, efficiency and reduced maintenance are the primary criteria.
DC motor construction and theory
While they operate using different principles and mechanics, both ac and dc electric motors have the same purpose — to convert electrical energy into mechanical energy.
DC motors of an equal power rating are larger in size because they require extra room to accommodate brushes and a commutator. In a dc motor, permanent magnets are located on the stator field coil, and the armature winding is located on the rotor. A “fixed frequency” motor current passes through motor brushes that are in direct contact with the commutator as the rotor spins. The friction caused by this contact means that dc motors do not spin as fast as their ac counterparts and therefore do not generate the same level of torque.
The motor brushes are wearable parts, which must be monitored and replaced as part of regular scheduled maintenance. Even though failure to properly maintain brushes can result in serious damage to the motor, they are often ignored because their location on the motor mounting is not readily accessible.
As the state of charge of a lift truck’s battery drops during prolonged use, climbing ramps or pushing loads, additional heat on the commutator is generated as ampere usage increases. This heat can distort the surface of the commutator, potentially causing damage and premature brush failure.
AC Motor Construction and Theory
The ac induction motor owes its superior performance to its simple design, with fewer moving parts and no wearable components. The rotor is the only moving part, and there is no physical point of contact, since it does not contain brushes or a commutator. This also means that the size of an ac motor can be greatly reduced, while providing significantly more power than a comparably sized dc motor (Fig. 1).
An ac lift truck motor operates using a dc battery and dc current for all circuits except ac motor control. For motor current, a controller or inverter changes the dc current into a manufactured three-phase, variable-frequency, alternating sine wave, which is used to pulse the three field coils in a sequenced circular motion. The three field coils represent the three phases in an ac motor. Each of these three field coils covers 120 deg (or one third) of the circular stator core. Combined, they cover the complete 360-deg rotation of one complete rotor turn (Fig. 2).
The three field coils are mounted on the motor housing, or stator core, and remain stationary. Magnetic fields are created and pulsed through these three field coils electronically in a rotational sequence. When current flows in a fixed direction in the stator core, the magnetic field generates a rotational field and spins the rotor. In simple terms, the rotor is “chasing” the changing magnetic poles of the field in a sequence that causes the rotor to spin.
The motor’s output shaft is supported on bearings at both ends of the motor housing. There is no physical contact between the rotor and the stator. An air gap exists which allows the rotor to spin freely at a very high rate. This design is inherent in ac motor technology and allows the motor to spin up to three times faster than conventional dc motors, permitting increased gear reduction, providing greater torque to the wheels, and yielding greater acceleration and higher top-speed capability.
DC/AC controller theory
Conventional dc motor controllers use high-speed switching to rapidly turn the flow of current on and off. Changing the ratio of on-and-off time regulates speed and acceleration. There are two common types of dc technologies that accomplish this high-speed switching: silicon controlled rectifiers (SCR), and the newer, transistor-controlled pulse width modulation (PWM). Each of these types of dc controllers turns the flow of electricity to the motor on and off and at a constant, fixed frequency. The longer the dc controller is on, the more current flows to the motor. The motor can then produce greater torque and operate at higher speeds. The longer the controller is off, the lower the flow of electricity to the motor, reducing speed and torque.
In contrast, an ac motor controller converts dc battery current into a “manufactured” three-phase alternating current. The motor’s performance is then controlled by varying the frequency of the sine wave pattern of the three-phase current. More simply, a broader range of control is possible because the frequency can be varied (Fig. 3).
DC motor characteristics are determined by the output voltage of the controller. Therefore, performance will decline as the state of the battery charge deteriorates, presenting a significant challenge for lift trucks and other industrial equipment.
With an ac power system, the ranges of control and performance are expanded. The motor can adjust between a variety of ac current and frequency combinations in order to maintain a near-constant level of performance even as the battery charge depletes. The operator will experience a much more even level of performance, and will gain up to 25% improvement in efficient operation time. This performance translates into higher levels of productivity.
Future of ac technology in lift trucks
Currently, electric lift trucks comprise over 60% of the market, replacing many internal combustion (IC) engine lift trucks. AC systems are the first to provide electric power that is comparable to IC engines while eliminating emissions, thereby meeting or exceeding strict government environmental requirements. This trend will grow as businesses continue to recognize the significant performance advantages and cleaner operation, combined with lower maintenance costs and improved productivity of ac-powered trucks.
David Morzella is Electric Product Specialist for Toyota Material Handling, U.S.A., Inc., and has been instrumental in the development of ac drive systems for industrial lift trucks. He can be reached at 949-474-1135 or firstname.lastname@example.org . Article edited by Rick Dunn, editor, 630-288-8779, email@example.com .
Merits of ac power systems
Simple motor design. AC systems do not require brushes or directional or lift contactors. Because they contain fewer wearable parts, they provide increased reliability.
Less heat generation. AC motors have no point of contact between the rotor and stator and therefore do not cause friction, which produces heat. Additionally, since the armature winding of an ac motor is located on the stator, which is connected to the motor house, the heat generated by the armature current can be radiated to the ambient air via the motor housing. In a dc motor, the armature current runs in the rotor and does not have thermal contact with the ambient air. Therefore, dc motors have an increased risk of overheating.
Greater power output yielding better torque and speed. Because there is no contact with the armature, an ac motor shaft can spin up to three times faster than a conventional dc motor allowing for greater gear reduction and providing greater torque to the wheels. The results are greater acceleration and higher top-speed capabilities.
Reduced maintenance costs. Absence of brushes and directional or lift contactors eliminates some inspections and maintenance (Fig. 4). Lower routine maintenance requirements mean reduced downtime, both planned and unplanned, and increased productivity.
Energy regeneration. Regeneration is a process by which the motor is used as a generator to recharge the battery. AC systems recover a substantial amount of battery energy by using three forms:
Coasting (when the accelerator pedal is released)
Switch back (when the directional lever is operated in a back and forth motion).
The inertia energy created by these actions and the lift truck’s momentum is recovered and returned to the battery, significantly extending the overall operating time per charge.
Smaller motors. An AC induction motor is smaller in overall size, when compared to a conventional dc motor of equivalent power output, permitting significant ergonomic improvements in the operator’s area of the lift truck.