Technical solution of variable speed drive using transmission

Variable speed drives (VSD) can change the torque and speed of electric motors very effectively, and are widely used in heavy-duty applications such as electric motor drives, servos and heat, ventilation and air conditioning (HVAC). Before VSD was adopted, AC output power could only be applied at the line frequency of grid power, and mechanical braking was usually used when full speed was not required. Therefore, adjusting the speed according to demand can not only reduce energy consumption, but also extend the working life of the motor.

Author: Zhou Jinchang

Variable speed drives (VSD) can change the torque and speed of electric motors very effectively, and are widely used in heavy-duty applications such as electric motor drives, servos and heat, ventilation and air conditioning (HVAC). Before VSD was adopted, AC output power could only be applied at the line frequency of grid power, and mechanical braking was usually used when full speed was not required. Therefore, adjusting the speed according to demand can not only reduce energy consumption, but also extend the working life of the motor. One of the most popular devices for this purpose is the converter-inverter-brake (CIB) module. Figure 1 shows the basic outline of the CIB module. The module circuit is composed of three parts: frequency converter, frequency converter and brake. The initials of these parts-C, I and B-are well known. During normal operation, the input of the converter stage (R/S/T in Figure 1) draws three-phase power from the grid and regulates AC power to DC power.

There are two commonly used three-phase voltages. 240V class and 400V class; according to the voltage, it is recommended to use 650V CIB type module or 1200V CIB type module. After the converter stage, the capacitor will be connected to the DC bus immediately to eliminate the voltage ripple from the inverter caused by dynamic power usage. Then, the inverter stage chops the DC input to AC output to power the motor. This can be achieved by turning on and off the 6-IGBT in this part of the module. The output voltage/current is controlled by pulse width modulation; the signal is structured to generate the power required to drive the motor at the required speed and direction. When ON semiconductor® defines the ampere rating of the TMPIM power module, the current refers to the IGBT rating in the inverter section. As a guide, 1200V 25A TMPIM CIB module will provide 5kW motor power; 35A TMPIM will output 7.5kW; 50A can provide 10kW, 15kW and 20kW power. It is important to note that the kilowatt output power rating is usually provided. If the application uses different control and cooling settings, this power rating may vary greatly.

Therefore, the maximum output power is defined by the design of the power module and how to control and cool the module. ON Semiconductor’s motion control online simulation tool can help you choose the most suitable module. When the motor stops and decelerates, its operation will switch to regeneration mode. The power generated by the motor is transferred back to the DC bus capacitor. When the generated power is too large, it may overcharge and damage the capacitor. In this case, the brake IGBT is turned on, leading the excess current to the external brake resistor connected in series with the IGBT. This arrangement will dissipate excessive regenerative power and keep the capacitor voltage at a safe level.

In applications with fans, pumps, and heater drives, the brake can be removed when the regenerative power is not important. In this case, the module is called CI module, which stands for converter inverter module.

Technical solution of variable speed drive using transmission
Figure 1: The basic configuration of the converter-inverter-brake (CIB) module

Innovative packaging of power integrated modules

The general CIB/CI module uses a gel-filled package, which encapsulates the power components in the housing. This method involves a multi-stage manufacturing process, but perhaps more importantly, it inherently combines an extra layer of non-uniform materials and interfaces, which weakens the module and reduces its robustness. ON Semiconductor has challenged this specification by developing a Transmission Molded Power Integrated Module (TMPIM). As the name suggests, the development process is a single-stage packaging technology that can use the same material to create the packaging and the medium that surrounds the component.

The transfer mold process eliminates the need for multiple materials, including plastic boxes that are commonly used to hold components, glue, and sealants that surround power devices. In addition to the overall more efficient manufacturing process, transfer molding can also provide ten times the temperature cycle, thereby directly improving efficiency. This provides greater flexibility in the size and shape of the final product, and provides higher reliability and greater power density.

So far, ON Semiconductor has adopted its TMPIM process to develop and release many modules for applications with power requirements between 3.75 kW and 10 kW, including six 1200 V rated currents of 25 A, 35 A and 50 A. CIB module. These devices are provided in a DIP-26 package and include CBI and CI variants. Now, ON Semiconductor will expand its product line to provide 1200 V CBI modules, which provide 75 A and 100 A current output, and launch a series of 650 V modules with rated currents between 35 A and 150 A. These devices will be able to meet applications with power requirements up to 20kW and will be provided in the QLP package profile. The DIP-26 package has terminals on both sides, while QLP is a quadrilateral lead frame package with terminals on all four sides.

Package enhancements provide higher power density

In order to adapt to higher output power levels, ON Semiconductor has further developed its TMPIM process, thus introducing a standard version and an enhanced version. The enhanced version has an advanced substrate with a thicker copper layer, eliminating the need for a substrate, thus keeping the external dimensions of the two package types the same. This makes it easier for manufacturers to migrate between the two based on their power requirements. Compared with comparable modules, removing the bottom plate can reduce the volume of the module by about 57%, and at the same time, it can increase the thermal conductivity by 30% compared with the standard TMPIM package.

Technical solution of variable speed drive using transmission
Figure 2: ON Semiconductor’s standard and enhanced TMPIM packages

Longer life

By increasing the thickness of the copper used, the package has lower thermal resistance and higher thermal quality, and the advanced substrate further improves the reliability of the module.

As mentioned earlier, the entire assembly, including the chip, lead frame and bonding wires, is encapsulated in the same epoxy as the package. In the DIP-26 package, both CBI and CI modules share the same pin assignments. In the CI module, the brake terminals are not internally connected.

Analysis of ON Semiconductor’s own competitors shows that modules manufactured using its transfer molding process can provide ten times the high temperature cycle, the power cycle can be increased by three times, and at the same time have better thermal conductivity and overall efficiency.

in conclusion

In motor drive, servo and HVAC applications, VSD usually uses the power module in the CIB or CI circuit. Through innovative TMPIM technology to develop power integrated modules, ON Semiconductor is now able to provide higher efficiency and higher power density in a smaller package.

The Links:   NL10276AC30-42C RM100DZ-24

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