Energy-efficient sensorless motor control, variable speed motor control

Sensorless speed control motor and variable speed motor control technology are key energy-saving technologies.

By Wu Kang November 9, 2011

Sensorless speed control motor and variable speed motor control technologies are gaining market awareness. Connected low-efficiency motors can gain about 60% energy savings. In addition to energy savings, speed control motor control technology makes less audible and electrical noise, and the motor can work under lower vibration to increase the reliability and realize more precise motor control. More functions can be integrated into target applications.

Mid-sized or small-sized motors can be found in refrigerators and washing machines in the house, or in industrial pumps, fans, air conditioners, and compressors. Most such application waste power, but if compressor motor speed can be controlled, peak energy consumption and average energy consumption can be reduced significantly. Finding methods to improve the design of motor speed control can increase effectiveness. A modular, rugged design can decrease cost.

Permanent magnet motor

Using a traditional direct-drive permanent magnet motor in a washing machine increases costs and the complexity of the control problem. Control requires rotor position information. Although Hall Effect sensors are used, they are driven by a rotor magnet, providing necessary feedback for each axis. However, traditional trapezoid current converting will bring torque jitter at the switching point. Torque jitter will be amplified by the external rotor, leading to more noise, preventing the drive torque curve from matching a real washing application. High torque at low speed and low torque at very high speed become difficult. Therefore, interpolation technology between Hall switching points is necessary to realize sine current control to reduce acoustics noise and smooth the torque at high speed and low torque. Reliability of this technique is difficult for the manufacturer and adds cost for the consumer.

Digital control variable frequency

Figure 1 shows the design platform for digital control variable frequency speed control motor solutions and the design platform for speed control sensorless motor. This platform includes a digital control chip and integrated power module for special equipment, such as air conditioners, washing machines, or water pumps. It offers designers an integrated and systematic method to realize energy savings and variable speed sine current control without position sensors. The resulting design is effective, with less noise, fast reaction, and less cost.

Digital control chip function

A digital control integrated circuit (IC) includes all necessary control and analog interface functions required by the speed control PMSM (permanent magnet synchronous motor), adapting direct current bus (dc bus) current measurement. Analog function modules on this integrated circuit include differential amplifier, double sampling and holding circuit, and 12-bit A/D converter required by dc bus shunt low-voltage signal sampling. This variable frequency power module (integrated power module) integrates high-voltage grid IC with 6 IGBT switches and dc bus shunt used in motor current measurement and power module protection. The motor control algorithm uses a special motion control engine (MCE) for digital control IC. Application software runs on the integrated 8-bit processor independently, which is the main processor for the system, taking care of the load switching, speed distributing, and external communication.

Application of sensorless motor control technologies

Sensorless field-oriented control (FOC) algorithms can be found in high-end industrial transmission devices for permanent magnet ac motor control and give PMSM variable speed control a high cost performance ratio and excellent dynamic torque control. Meanwhile, motor efficiency is increased. The torque is very smooth because of the sine motor current, so noise and mechanical vibration are effectively reduced. Applying PMSM variable speed control function requires using some control method to avoid a rotor position sensor, which is common in industrial transmission devices.

The digital control chip can take advantage of a special motion control processor to realize sensorless FOC algorithm. The motion control engine (MCE) contains a sequencer inside, which is used to connect motor control ASIC function in the MCE library. This kind of technology combines the flexibility of a programmable system with the speed and efficiency of the special ASIC. The control chip also integrates analog amplifiers and AD converters required by motor phase measuring.

Motion control engine (MCE)

The MCE library consists of a PI compensator, limit function, and vector revolve function, which are widely used in motor control algorithms. Graphic edit tools can be used to configure the algorithm; no software code is required. Executing the speed of the algorithm may be one or two times faster than RISC or DSP, because time control calculations are done by special hardware.

Control parameters and system variables are stocked in shared data RAM and are also accessed by an integrated 8-bit microprocessor. This can help washing machine application software to easily change the set value (such as target speed) or monitor control variables (such as torque current). The software can be developed on an independent 8-bit microprocessor with C language, which makes application development easier.

Sensorless field-oriented control (FOC)

Sensorless BIDC FOC motor control technology does not require expensive sensors, but still offers low noise, so it is increasingly accepted. This technology features digital control chips.

A dsPIC digital signal controller (DSC) makes the process of adding digital signal processing ability to an embedded motor control design very easy. A dsPIC DSC integrates the computing ability and throughput capacity into a high-performance 16-bit flash single-chip microcomputer, which includes a 40-bit accumulator and a single loop 16 x16 MAC for double operand fetch operation. Operating speed is up to 40 MIPS. An advanced on-chip peripheral is available. Figure 2 shows the structure of the digital signal controller.

High-performance electric apparatus

Speed control motor control chips to support high-performance electric apparatus also are available. Typical examples include an off-line switch PeakSwitch (36 W / 72 W) type PKS606Y that has peak output power characteristic and a CiPOS 600 V/8-22 A control integrated power system. Figure 3 shows how PKS606Y is applied in the design of speed control motor drives.

Analysis of design plan

The output power of PKS606Y is 36 W (72 W maximum), input voltage is 90-265 V ac, and output voltage is 12 V, with the flyback topology. A simple single-level circuit is used to replace the double-level power supply and a switch. This design eliminates the use of a dc motor speed control circuit. Motor speed is controlled by a tiny potentiometer or a variable dc voltage from 3.6 V to 10 V. The number of components used is reduced to 47, and efficiency is more than 77% (36 W load), meeting the requirements for transmission EMI specified in EN55022B. The ON/OFF mode keeps stabilized in the whole range of motor speed (output voltage).

Operation mode

Figure 3 also shows that a flyback converter uses a component U1 (PKS606Y) to drive a 35 V motor, and gives 75 W peak output power during starting and load jitter. There are two methods to change the speed of the motor: one is to use a potentiometer R20 (connected to J3); another method is to use an external 3.6 V -10 V dc voltage source (connected to J4) to adjust motor speed by changing the output of the voltage source.

Feedbacks from the output ports are internally controlled to open or cut off the integrated MOSFET. By cutting off bridging across the switch cycle, output voltage can be detected from VR2 and LED (parallel with R13) of U2. When the output increases to the break-over threshold voltage of VR2, current goes through the LED of U2, and Q3 is open. When Q3 draws current from the EN/UV pin of U1, the switch cycle is bridged over, so very little energy flows to output. Once the output voltage decreases, the switch cycle will be enabled again.

Offset winding of a transformer (T1 4 and 5 pin) is rectified and filtered by DT and C6, offering operating current to U1 through RT. The under voltage lock-out (UVLO) and lock-out cutoff function of U1 are enabled by the intelligent ac detecting circuit formed by D5, C7, R5, and R6. Shielded winding inside T1 and 2 small Y capacitor (C10, C19) bridging over T1 together decreases the transmission EMI. So placing one common mode inductor, one small X capacitor (C3), and two small Y capacitors (C1, C2) is enough. Limitation from EN55022B contains a surplus capacity of 12dBµV. With the clamping effect of RCD zener diodes (R3, C5, D6, and VR1), the drain voltage peak value can be kept below the integrated MOSFET’s rated voltage of 700 V.

If JP3 is removed, the external variable resistor (R20) can adjust the output voltage by adjusting the voltage across R12. The speed adjusting voltage (3.6 V -10 V) of the external motor can be effectively controlled by adjusting the node voltage between R12 and VR2. If the external adjusting voltage is lower than 3.6 V, diode D12 will prevent current from flowing through R19.

Control integrated power system

New CiPOS series modules are integrated into several power and control components inside one package, which increases the reliability of the design and decreases the size and cost of the PCB. Figure 4 shows a block diagram of a control integrated power system.

The main features are an insulated package, outstanding thermal resistance Rth=3K/w, minimized saturation voltage of TrenchStop IGBT: VCEsat=1.5V, reliable SOI gate driver technology, resistance to transient negative voltage: -50V≤Vs≤600V, overall protection (under voltage lockout, overheating protection, overcurrent protection, and straight coupling interlock function), and bridge current measuring by emitter open circuits.

This module can be used in an ac motor variable frequency speed control driver of a washing machine, air conditioner, compressor, or vacuum cleaner. This module’s package is suitable for power conversion applications that require perfect heat conduction, EMI control, and overload protection.

– Wu Kang for Control Engineering China

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