An Introduction to Electric Drive in Non-Road Mobile Machinery (NRMM)

Induction Motor vs PMAC Motor

The two most common motor types are the induction motor (asynchronous motor) and the PMAC motor (synchronous motor). The main difference between these two types lies in how the magnetic fields in the motor are generated, which also gives the motor many of its characteristics.

The PMAC motor provides the best efficiency and is the most common choice in electric cars where low energy consumption is very important.

The induction motor on the other hand is simpler to manufacture and does not contain expensive permanent magnets, which has made it the most common choice in forklifts and other similar machines for many years. The efficiency of a motor does not only affect how long the battery lasts but also how much heat the motor generates. Losses cause the motor to heat up, and a cheaper motor can lead to a need for more expensive cooling solutions.

The induction motor also has something called a lower power factor, which, in simple terms, means that additional current that is not really used to perform any work will circulate through the system and must be accounted for in the design.

Component Performance

Since mobile machinery rarely is used continuously with a constant load, the performance of the components is usually specified based on “short-time duty – S2” or “intermittent duty – S3” according to the IEC 60034-1 standard.

Short-time duty S2 is always specified with a time indication, commonly S2-2min or S2-60min, for example, a motor can be rated for 5 kW (S2-60min) and 15 kW (S2-2min). This means that the specified power can be drawn for 2 or 60 minutes without the motor overheating, provided that the system starts at an allowed ambient temperature. The IEC standard specifies a maximum ambient temperature of 40°C, but for motors intended for use in mobile machinery a higher ambient temperature is sometimes assumed. The manufacturer also specifies under which other conditions the rating is valid, such as air-, or coolant flow.

The S2 cycle can be relevant for, for example, propulsion, where the S2-60min rating can be compared to the average power required for longer movements and S2-2min corresponds to the worst case when the machine needs to overcome a temporary obstacle or climb its maximum slope.

Intermittent duty S3 is instead specified with a percentage, for example, a motor may be rated 10 kW (S3-30%). The motors are tested against a duty cycle where first a constant power, in this case, 10 kW, is applied, after which the motor is allowed to rest for 70% of the cycle. Unless otherwise specified, the entire cycle is assumed to be 10 minutes long, i.e., power on for 3 minutes and off for 7 minutes in our example. S3 duty can be useful to compare to, for example, a lifting function, where the machine moves around and periodically performs heavy lifts.

Choice of a Rotor Position Sensor: SinCos, Hall Effect or Resolver

Common solutions for measuring the rotor position are analogue absolute position sensors (SinCos sensors) or digital Hall effect sensors. The hall effect sensors are a simpler solution but provide a lower resolution signal and less precise control. In larger motors and in some more critical applications, a sensor type called a resolver is also used. The resolver provides better performance and is more resistant to interference but is significantly more expensive and requires more advanced electronics in the motor controller, which is why it is an uncommon choice in smaller systems.

The consequences can be severe if the sensor signal from the motor does not provide the correct value; in the worst case there is even a risk of a runaway motor. Therefore, it is important to choose fully compatible motor controllers and sensors. It is also very important that the motor controller is configured correctly to monitor the sensor signals and detect any fault conditions.

Motor Controller

To drive an AC motor from a battery that delivers DC voltage, a motor controller is needed. Motor controllers are often referred to as inverters or drives. Each motor controller can control one motor, so if you have two motors, you need two motor controllers or possibly a so-called dual variant, where two controllers are combined into one unit.

Although there are now large battery-powered excavators and loaders on the market, weighing many tens of tons, most electric mobile machinery is still significantly smaller and operates at relatively low voltages.

As a rule of thumb, 24V is suitable for an average power up to about 5 kW, 48V up to 15–20 kW, and 80–100V is needed if you need up to 30–35 kW. The peak power that can be drawn from the systems is then two to three times higher than the average power.

Voltage levels are not absolute, and different battery types have varying maximum and minimum levels. A so-called “48V” system may in fact have a nominal voltage of around 52 Volt and a maximum level of over 56 Volt. Therefore, it is important to make sure that all components are selected according to the actual conditions, and that the motors will always deliver the expected power.

Above these power levels, higher system voltages need to be considered, and most components are then designed for between 400V and 800V.

Regulation and Communication

A mobile motor controller, such as the ACS series from Inmotion, has the main task of regulating voltage and electrical frequency to the connected motor. However, this is usually not something a machine developer needs to pay much attention to. What is more often of interest when building a system is the regulation of the motor’s speed, torque, or position. In many cases, several different types of regulation are combined to achieve the desired behaviour.

One common way to control a motor is to let the motor controller listen to a speed command sent to it via CAN. The setpoint is often combined with a value for the so-called ramping, i.e., how quickly the change in speed should occur. The motor controller then ensures that the right amount of torque is generated to reach and maintain the desired speed. Communication on the CAN bus can take place with different “dialects”, the most common being CANopen and J1939. Since there is quite a bit of freedom within the different dialects, the motor controller software often needs to be adapted somewhat to make the integration as smooth as possible.

Another common design in simpler systems is for the speed command to come from a pedal, knob, or joystick connected directly to the motor controller’s input pins.

Adaptation for Specific Applications

The most common functions for electric motors in mobile machinery are propulsion and pump control. For propulsion, much of the customization lies in how the machine should react in different situations. Examples of questions include how quickly the machine should accelerate, whether it should be allowed to accelerate faster downhill, or if that poses any risks. What should happen after the machine has braked to a stop on a slope? Should the motor continue to try to keep the machine stationary with the risk of eventually draining the battery? Another factor to consider is how powerful the machine should be allowed to be in different situations. A customized torque curve affects how the machine feels to drive and can also save energy by avoiding unfavourable operating points.

A pump application has other challenges. Sometimes it is enough to ensure that the motor maintains a desired speed, but the motor controller also has the capability for more advanced control with outputs to control hydraulic valves. Motor regulation can be based on signals from, for example, pressure sensors, position sensors in a cylinder, or angle sensors in an articulated steering system to create more advanced standalone distributed hydraulics.

Fortunately, much of this functionality is prepared for in the motor controllers, but there is almost always some need for configuration or adaptation based on the conditions in the specific application.

Batteries

Batteries are the most expensive component in an electric machine and can also be the most difficult to give general advice about. Lead-acid batteries are now a rare sight in mobile machinery; they simply become too large and heavy in most cases, and the choice is usually between different types of lithium batteries.

The global demand for batteries is driven by the automotive industry, and according to the International Energy Agency (IEA), in 2022, lithium-nickel-manganese-cobalt oxide (NMC) was the most common type of lithium battery with a market share of 60%. This was followed by lithium iron phosphate (LFP) with almost 30% of the market, and nickel-cobalt-aluminium oxide (NCA) with about 8%.

NMC and LFP, which are most common in cars, are also the most common in mobile machinery. LFP is chemically more stable and therefore also safer but has, in turn, a lower energy content per kilogram. LFP can handle more discharge cycles while maintaining performance, but the thousands of cycles an NMC battery can handle are often more than sufficient compared to the machine’s lifespan. In many cases, NMC or NCA may be the only option to fit the amount of energy the machine needs to perform its task. The cost of LFP is generally lower than NMC today, although prices have leveled out somewhat in recent years as the prices of the raw materials have developed at different rates.

Individual battery cells are small, and several thousand cells are often needed to build a battery pack that can power a whole machine. Since it would be a challenging task to try to develop standardized battery packs that would work in all mobile machinery, the battery manufacturers instead combine cells into building blocks, so-called battery modules. The modules are easier to integrate into packs and also add an additional level of safety and control.

This overview is intended to provide an initial insight into the components included in an electric drive system and some of the different factors to consider when choosing components.

Feel free to contact us for expert help. We have long experience with electric drive systems and will help you choose the right solution for your application.

Explore Further 

For more information about our system, explore: Regamotion Electric Drive System

Read about an example of the choices you can make regarding electrification in the customer case Electric Forestry Machine.

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