Introduction to Motor Efficiency

How to improve motor efficiency?

This efficiency of the motor is affected by a small amount of losses, including resistance losses, mechanical losses caused by friction, losses caused by the dissipation of magnetic energy in the core, and different losses depending on the type of material used. To make the motor more efficient, we must reduce the losses in the motor. Here are some methods or tips that can be adopted to improve the efficiency coefficient of the motor.

1. The function of the motor frame is to provide mechanical protection for the windings. It is also responsible for the interface mounted by the legs. The motor frame plays a vital role in its thermal performance as it is responsible for transferring the heat generated inside the motor to the surface of the frame where the air blown by the fan will promote heat dissipation. This will reduce heat losses.

2. Another basic tip is to maintain a minimum gap between any walls close to the back of the fan cover to allow air intake. Keep the area around the motor clean and regularly check for any air blockages that may reduce the performance of the cooling system. Therefore, the cooler the motor runs, the longer it will last.

3. The stator is the main component of the motor and causes 60% of the losses, so in order to reduce these losses, the mass of the stator winding must be kept large because the increase in mass reduces the resistance. High efficiency motors contain 25% extra copper compared to motors designed for standard efficiency models.

4. Rotor losses are also considered a minor source of losses, mainly caused by the degree of slip the motor shows. Slip is actually the difference between the speed of the magnetic field (RPM) and the actual RPM of the rotor and shaft at a given load. Among them, slip is obtained by subtracting the speed of the loaded motor from the speed of the unloaded motor and dividing it by the speed of the loaded motor. Therefore, in order to reduce these losses, slip must be reduced, which is done by increasing the conductivity of the rotor. Copper must be used in large quantities because of its high conductivity. But the copper must be die-cast because a process for die-casting copper has been developed recently.

5. Core magnetic losses are derived from hysteresis, and 20% of the total losses are caused by core eddy currents and saturation. By using high-quality materials and quality control, losses can be minimized, thereby improving efficiency.

6. In order to reduce the effects of hysteresis and saturation, steel containing a small amount of silicon will be used for lamination instead of low-cost carbon steel. In this way, core losses can be eliminated. By reducing the lamination thickness and increasing the lamination length, the flux density and core loss can be minimized.

7. Eddy current losses can be minimized to a large extent by ensuring proper insulation between the sheets or laminates used.

8. The lubrication interval is a function of the motor installation rated speed, bearing size, grease type and temperature rise. Therefore, please be careful when using lubricants. Do not mix grease types, even if they use the same elements. If other greases are used in the application, this will directly affect the motor performance.

How to reduce motor losses?

Since the loss distribution of the motor varies with the power size and the number of poles, in order to reduce the loss, we should focus on taking measures for the main loss components of different powers and poles. Some ways to reduce the loss are briefly described as follows:

1. Increase effective materials, reduce winding loss and iron loss

According to the similarity principle of motors, when the electromagnetic load remains unchanged and mechanical loss is not considered, the loss of the motor is approximately proportional to the cube of the linear size of the motor, and the input power of the motor is approximately proportional to the fourth power of the linear size. From this, the relationship between efficiency and effective material usage can be approximated. In order to obtain a larger space under certain installation size conditions so that more effective materials can be placed to improve motor efficiency, the outer diameter size of the stator punching becomes an important factor. Within the same machine base range, American motors have greater output than European motors. In order to facilitate heat dissipation and reduce temperature rise, American motors generally use stator punchings with larger outer diameters, while European motors generally use stator punchings with smaller outer diameters due to the need for structural derivatives such as explosion-proof motors and to reduce the amount of copper used at the winding end and production costs.

2. Use better magnetic materials and process measures to reduce iron loss

The magnetic properties (magnetic permeability and unit iron loss) of the core material have a great influence on the efficiency and other performance of the motor. At the same time, the cost of the core material is the main part of the cost of the motor. Therefore, the selection of suitable magnetic materials is the key to designing and manufacturing high-efficiency motors. In high-power motors, iron loss accounts for a considerable proportion of the total loss, so reducing the unit loss value of the core material will help reduce the iron loss of the motor. Due to the design and manufacturing of the motor, the iron loss of the motor greatly exceeds the value calculated according to the unit iron loss value provided by the steel mill, so the unit iron loss value is generally increased by 1.5~2 times during design to consider the increase in iron loss.

The reason for the increase in iron loss is mainly because the unit iron loss value of the steel mill is obtained by testing the strip material sample according to the Epstein square circle method, but the material is subjected to great stress after punching, shearing and laminating, and the loss will increase; in addition, the air gap caused by the presence of the tooth slot leads to the tooth harmonic magnetic field causing no-load loss on the core surface, which will lead to a significant increase in iron loss after the motor is manufactured. Therefore, in addition to selecting magnetic materials with lower unit iron loss, it is also necessary to control the lamination pressure and take necessary process measures to reduce iron loss. In view of price and process factors, high-grade silicon steel sheets and silicon steel sheets thinner than 0.5mm are not used much in the production of high-efficiency motors. Low-carbon silicon-free electrical steel sheets or low-silicon cold-rolled silicon steel sheets are generally used. Some manufacturers of small European motors have used silicon-free electrical steel sheets with a unit iron loss value of 6.5w/kg. In recent years, steel mills have launched Polycor420 electrical steel sheets with an average unit loss of 4.0w/kg, which is even lower than some low-silicon steel sheets. The material also has a higher magnetic permeability.

In recent years, Japan has developed a low-silicon cold-rolled steel sheet with a grade of 50RMA350. A small amount of aluminum and rare earth metals are added to its composition, thereby maintaining a higher magnetic permeability while reducing losses. Its unit iron loss value is 3.12w/kg. All of these are likely to provide a better material basis for the production and promotion of high-efficiency motors.

3. Reduce the size of the fan to reduce ventilation loss

For larger power 2- and 4-pole motors, wind friction accounts for a considerable proportion. For example, the wind friction of a 90kW 2-pole motor can reach about 30% of the total loss. Wind friction is mainly composed of the power consumed by the fan. Since the heat loss of high-efficiency motors is generally low, the cooling air volume can be reduced, and thus the ventilation power can also be reduced. The ventilation power is approximately proportional to the 4th to 5th power of the fan diameter. Therefore, if the temperature rise is allowed, reducing the fan size can effectively reduce wind friction. In addition, the reasonable design of the ventilation structure is also important for improving ventilation efficiency and reducing wind friction. Tests have shown that the wind friction of the high-power 2-pole part of a high-efficiency motor can be reduced by about 30% compared with that of an ordinary motor. Since the ventilation loss is reduced by a large margin and does not require much additional cost, changing the fan design is often one of the main measures taken for this part of high-efficiency motors.

4. Reduce stray losses through design and process measures

The stray losses of asynchronous motors are mainly high-frequency losses generated by high-order harmonics of the magnetic field in the stator and rotor cores and windings. To reduce the load stray loss, the amplitude of each phase harmonic can be reduced by using a Y-Δ series-connected sinusoidal winding or other low harmonic winding, thereby reducing the stray loss. Experiments show that the use of sinusoidal winding can reduce the stray loss by more than 30% on average.

5. Improve the die-casting process to reduce rotor loss

By controlling the pressure, temperature and gas discharge path during the rotor aluminum casting, the gas in the rotor bars can be reduced, thereby improving the conductivity and reducing the rotor aluminum loss. In recent years, the United States has successfully developed copper rotor die-casting equipment and corresponding processes, and is currently conducting small-scale trial production. Calculations show that if a copper rotor replaces an aluminum rotor, the rotor loss can be reduced by about 38%.

6. Apply computer optimization design to reduce losses and improve efficiency

In addition to increasing materials, improving material properties and improving processes, computer optimization design is used to reasonably determine various parameters under the constraints of cost and performance, thereby achieving the maximum possible improvement in efficiency. The use of optimized design can significantly shorten the time of motor design and improve the quality of motor design.