This paper examines the current situation and development trend of travel motors used in electric skid steer loaders.

Electric skid steer loader is an important equipment for engineering construction, agriculture, animal husbandry, forestry and so on all over the world, the reliability of its motor travelling system directly affects the vehicle performance and operation efficiency. The travelling motor is the transmission terminal of the travelling system, and its external characteristics and adaptability to the working conditions are the key indexes in the system's design.

Combining the characteristics of different motors, the operating environment, the actual operating conditions, and the requirements of the electric travel system on the motor, we analyse and study the current situation and development trend of travel motor applications in electric skid steer loaders.

Electric skid steer loader is a kind of miniature or compact skid steer loader with a relatively small volume, which is mainly used for the occasions of narrow working sites, uneven bottom surfaces and frequent changes of working content. Travelling motors are electric motors used in locomotives, on-road vehicles and off-road vehicles, whose function is mainly to provide the required driving force for vehicle operation and the required braking force for vehicle deceleration to achieve the travelling function of the vehicle.

Travelling motors, as the transmission terminal equipment of the travelling system, are mainly divided into DC motors, switched reluctance motors, AC asynchronous motors, and permanent magnet synchronous motors.

The travelling motor and reducer of the four-wheel drive electric skid steer loader are integrated into a travelling drive assembly, which drives four travelling wheels respectively. The output end of the travelling motor is connected to the input end of the reducer, and the output end of the reducer is connected to the wheels to achieve the travelling drive. As a new type of power travelling device, the travelling motor reducer of the electric skid steer loader effectively replaces the traditional hydraulic driving travelling device by setting up a two-stage planetary gear transmission structure.

This paper analyses and researches the application status of different travel motors in the field of electric skid steer loaders and the development trend of travel motors for electric skid steer loaders according to the operating conditions of skid steer loaders and the characteristics of different travel motors.

1, Electric skid steer loader on the technical requirements of the travelling motor

Electric skid steer loader is mainly used for the operation site is narrow, the bottom of the undulating uneven, the operation content the frequent change of the scene, to adapt to the needs of complex and changing working conditions, while ensuring the reliability of the electric skid steer loader and production safety, the electric skid steer loader travelling motor mainly has the following requirements.

(1) Lightweight design.

The travelling motor of the electric skid steer loader, as a vehicle-mounted component, has strict weight index requirements to achieve a lightweight vehicle design and reduce vehicle operating fuel consumption.

(2) Efficient constant power area.

To ensure the power performance of the mine car, its design speed is generally not less than 20km/h, but the operating conditions are variable, to ensure the safety of the operation in the complexity of the multi-state, the load is even lower than the operating speed of 14 km / h within. Therefore, the travelling motor of the electric skid steer loader works in the low-speed zone.

 In order to ensure the transmission efficiency of the travelling system, when the design of the travelling motor is carried out, it must be ensured that the motor enters the constant power point in the low-speed zone and has a large constant power multiplier to ensure the travelling performance in the high-speed section.

(3) Large torque at low speed.

In order to ensure the travelling performance of an electric skid steer loader when working in complex working conditions, the travelling motor must have the ability of low speed and large torque output to achieve the travelling performance of the whole vehicle.

(4) Large torque and small current.

The loss of the travelling system mainly includes the loss of transmission parts, line loss, motor body loss, etc. Under the same output torque, the smaller the motor current is, the lower the system heat loss is. When designing the travelling motor, it is necessary to match the relationship between the motor output torque and motor current, especially in the low-speed and large-torque speed range, to improve the comprehensive energy efficiency of the travelling system.

(5) Environmental adaptation.

Due to the harsh operating environment of the electric skid steer loader operation site, dust and other particulate pollutants, greatly affect the performance of the motor heat dissipation, insulation and so on. Therefore, in the design of the motor, its protection level needs to meet the requirements of the dusty working conditions.

2, Electric skid steer loader travel motor application status quo

2.1 DC traction motors

As DC motors need mechanical commutation when working, there are brushes, commutators and other components, their structure is complicated, they need to be maintained, and their reliability is poor, which limits their development in the field of electric skid steer loader travelling system. With the maturity of power electronics technology, DC traction is gradually withdrawn in the field of electric construction machinery.

2.2 Switched Reluctance Motors

Due to the switched reluctance travelling motor has the characteristics

of large torque pulsation, high noise, and poor low-speed operation performance, which greatly limits its application scope, and it is mainly used in the field of general industry, household appliances, and a small range of electric vehicles. The control mode of the travelling motor of the electric skid steer loader is mostly torque control and vector control, which requires high torque characteristics of the travelling system to ensure the travelling characteristics of the whole vehicle and higher comfort of the operators.

2.3 AC asynchronous motors

The AC-DC system uses mature power electronics technology to achieve frequency conversion speed regulation of asynchronous travelling motors, which in turn achieves the travelling brake of the electric skid steer loader. At the same time, to meet the needs of electric skid steer loader travelling systems with large torque, and wide speed range, AC asynchronous travelling motors are usually designed for low-speed large torque, a wide constant power range of external characteristics. After nearly half a century of development, AC asynchronous motors have the advantages of simple design and manufacture, reliable technology, excellent performance, large applications, and cheap price. However, based on the physical characteristics of asynchronous motor rotor winding cutting stator rotating magnetic field, AC asynchronous motor has the following shortcomings:

(1) The rotor winding cuts the stator rotating magnetic field, there are rotor copper consumption, iron consumption, low efficiency;

(2) The low speed when the load is working causes the travelling motor to work in the low-speed inefficient zone for a long time, which further reduces the efficiency of the travelling system;

(3) AC asynchronous motor power density is low, the motor volume and weight, increasing the vehicle weight, limiting the design of the reducer;

(4) AC asynchronous motor protection level is mostly IP20, with weak adaptability to the harsh working environment.

2.4 Permanent magnet synchronous motors

The rotor of a permanent magnet synchronous motor adopts permanent magnet material, which has the characteristics of high efficiency and high power density, and the efficiency of the full-speed section is higher than that of an asynchronous traction motor. Under the same volume and weight, a permanent magnet motor has better torque characteristics, which can effectively strengthen the power performance of the whole vehicle, and at the same time, it can reduce the requirements of electric skid steer loader for the reduction ratio of the wheel side reducer. With the maturity of permanent magnet material technology and permanent magnet synchronous motor control technology, permanent magnet synchronous motors have been applied in large quantities in traction fields such as rail transport, electric buses and ship propulsion. Combined with the actual operating conditions of electric skid steer loaders, permanent magnet synchronous motors applied to electric skid steer loaders have the following advantages

(1) The rotor permanent magnet cuts the stator magnetic field, and the rotor does not heat up, eliminating the rotor copper consumption and iron consumption, and improving the working efficiency;

(2) The high-efficiency zone of the permanent magnet synchronous motor is much higher than that of the AC asynchronous traction motor, especially in the low-speed zone of the motor. When a permanent magnet synchronous motor is applied to the electric skid steer loader, it can give full play to its high-efficiency characteristics in the low-speed section, and improve the efficiency of the electric drive system;

(3) The power density of a permanent magnet synchronous motor is higher than that of an AC asynchronous traction motor, in the same output torque, and constant power speed range, the volume and weight of a permanent magnet synchronous motor are significantly smaller than AC asynchronous motor, easy to achieve the lightweight design of the vehicle, and at the same time reduce the difficulty of designing the wheel side reducer;

(4) Permanent magnet synchronous motors are mostly designed as fully enclosed motors, which are more adaptable to various operating conditions, and they are maintenance-free during the whole life cycle, which improves the adaptability and reliability of the system.

3, The development trend of electric skid steer loader travelling motor

Electric skid steer loader walking system with lithium iron phosphate batteries and motors as a power source, the selection of its motor is closely related to the application of working conditions and motor characteristics. The DC motor has poor reliability, and the brush must be maintained every 500 hours on average, so it is not recommended. The switched reluctance motor is limited by its torque pulsation, poor low-speed performance, complex control mode and other characteristics, it is not recommended for use. AC asynchronous motor is not recommended due to its low efficiency, large volume, large weight and low protection level. Combined with the actual operating conditions of the electric skid steer loader, the permanent magnet synchronous motor is adopted.

With the promotion of green environmental protection, resource conservation and utilisation, as well as the implementation of energy saving and emission reduction policies, the industry continues to explore new types of transmission methods to enhance the energy efficiency of electric construction machinery and reduce its whole life cycle energy. Permanent magnet synchronous motors with their high efficiency and high power density, small size and weight, maintenance-free and other advantages, have become the development direction of the electric construction machinery field.

Electric skid steer loader is one of the branches in the field of construction machinery, A  permanent magnet synchronous motor with its leading technological advantages, will certainly become the future development direction of the electric construction machinery industry.