The history of forklift-the main parts of electric forklift (Part 4)

The history of forklift is divided into seven parts. This issue is the fourth part of the main parts of the electric forklift.

the main parts of electric forklift — Forklift motor

As we all know, it is the motor that provides direct power to the electric forklift. At present, the global electric forklift industry without exception shows that the use of AC motor has become a trend. However, until the 1990s, electric forklifts have been using the drive DC motor. Given the technology at the time, there was no alternative.

After 1990, two new types of motors were introduced into the industrial production of forklifts almost at the same time. Low voltage alternating current (AC motor) and separately excited motor (SEM) were used. Separately excited motor belongs to DC motor. It means that excitation coil and armature winding are separated, excitation current is provided separately, it’s independent of armature current. The excitation winding and armature winding of separately excited motor are supplied by two power sources respectively.

Due to the use of separate excitation power supply, the equipment of separately excited motor is more complex. However, this kind of motor has a wide range of adjustment and transportation, and is mostly used in the main engine drive. There is no directional contactor for separately excited motor. It is no longer necessary to weaken the excitation to accelerate, and the braking is regenerative. In manufacturing, the independent excitation motor is almost the same as the conventional DC motor.

Therefore, in the traditional forklift manufacturing industry, AC motor is subject to technology to some extent. However, since the late 1990s, designers have again focused on this design of electric drivers. In fact, AC motor is advanced in manufacturing technology and has many specific advantages.

Since AC motors do not have front / rear reversing contactors, there are no brushes or commutators. It’s smaller, lighter and spins faster. It’s no longer necessary to check and replace carbon brushes regularly. The most critical problem is that the forklift motor needs little maintenance during its entire service life. As a result, forklifts are more reliable and stable. In addition, there is no need to reserve the maintenance space when designing the forklift. Because the motor is completely in the sealed pile, the structure of the forklift is more compact. The non-contact brake can reduce the wear of the brake, which is much simpler than the traditional brake. In addition, when the forklift driver uses the brake to slow down or stop the vehicle or change direction.

The motor is like a generator, which converts electromagnetic torque into braking torque. This means that brake pad wear will be minimal. This greatly reduces maintenance costs and minimizes operating costs. In addition, the operation and braking efficiency of AC motor is higher. When braking or changing direction, energy will be generated. Strong braking will produce more regenerative energy. The energy will return to the battery to extend its operating time. Although some DC-driven forklifts also have regenerative braking, they are only activated during very strong braking. This means that a lot of energy is transferred to heat during regenerative braking. However, the AC frequency converter can generate regenerative energy under almost all working conditions and until the forklift stops completely. Obviously, the energy regeneration efficiency is higher than that of DC motor.

(1) DC and  (2) AC motors

The main parts of electric forklift — Electronic system

Engine control technology has also changed completely. From the 1920s to the 1950s, forklifts were produced using carbon stacks – a control system. The system consists of several carbon rings as resistors, which are included in the power supply circuit of the moving engine in turn. When you press the speed pedal, the carbon ring is pressed, reducing resistance and increasing machine speed.

Then, from the late 1940s to the early 1960s, it was the drum controller. When the speed pedal is pressed, the controller will close some metal resistance parts. Due to the change of resistance, the rotor speed of the engine is adjusted. Another technology solution that was applied at the end of the 1950s is. A system consisting of many switches.The starting resistor is switched on and off in various steps by means of a microswitch on the speed pedal. At the end of the 1960s, forklift trucks were manufactured using transistors on household appliances as their electronic control systems or load switches and SCR controls (silicon-controlled rectifiers).SCR control systems are cumbersome because the main transistor has to be turned on or off.

In1980s, another important development was introduction of microprocessors, which were invented by faggin, Hoff and mazor in 1971. With the introduction of microprocessor, the control becomes more compact and fast. Microprocessors can also quickly detect abnormal conditions in the engine, thus making necessary adjustments more quickly. A load switch bipolar transistor was also created. In general, the transistor was invented by Bardeen, brattain, and Shockley in 1947. But until then, he couldn’t switch the high voltage of the traction battery. The new load switch is easier to operate than SCR control, and it doesn’t require complex circuits to turn on the main transistor. Control systems with bipolar transistors are also smaller and cheaper.

Microprocessors are not only easier to use, but also run very fast with no energy loss.It can also be connected in parallel or even regulated, which means it can be used on heavy forklifts. The MOSFET switch is suitable for adjusting battery voltage up to 96 V.Currently, another technology based on IGBT is switching this high voltage. It works almost as easily as MOSFET and as reliably as SCB control.