Optimized Variable Speed Drive Cuts Operating and Design Costs in Appliances; Saves Energy

In the extremely competitive appliance marketplace, costs associated with motor-driven applications are a key driver. Overall costs depend on a number of factors including:

• Motor speed control method
• Power consumption
• Required hardware
• Required software (if any)
• System development costs
• Time-to-market
• Maintenance and repair considerations

A major cost factor for both manufacturer and user is motor drive speed. Unlike fixed speed motors that run at selected fixed speeds, variable speed drives (VSD) can vary their speed to match an application’s requirements using computer techniques that automatically control the speed according to the requirements of its load. Because power consumption varies directly with the motor speed, significant energy savings can be made. For example, the VSD motor in a cooling system runs at low speed when the temperature is low, shifting to a faster speed in order to provide more cooling when the temperature increases.

VSDs can generally be categorized into two systems: constant torque and variable torque. The energy saving potential of the variable torque system is greater than that of the constant torque system. Typical variable torque loads include centrifugal pumps and fans commonly used in appliance applications. In such variable torque applications, the required torque varies with the square of the speed while the horsepower varies with the cube of the speed. Therefore, even a small reduction in speed can result in a substantial reduction in horsepower, resulting in lower energy consumption: a motor will consume only 25% as much energy at 50% speed that it will at 100% speed.

As global energy costs increase, the savings from the use of variable speed motors can be significant. Data from EPRI (Electric Power Research Institute) shows that about half of the electricity consumed in the U.S. can be attributed to electric motors. Therefore, widespread use of variable speed motors in HVAC (Heating, Ventilating and Air Conditioning) and similar applications can have a major impact on energy consumption, so while the cost of energy does not directly affect the appliance manufacturer, lower energy costs are a significant marketing tool as consumers seek maximum energy efficiency from their appliances.

FIGURE 1

Equipment life and maintenance are other cost factors for motor-driven appliances. Manually controlled speed starts the motor abruptly, subjecting the motor to a high starting torque and powerline current surges that can be up to 10 times higher than the normal amount. In contrast, VSDs can be designed to gradually ramp up the motor speed, reducing mechanical and electrical stress. This gradual ramp up cuts maintenance and repair costs and extends the usable lifetime of the motor and drive system. Fig. 1 shows a block diagram of a VSD that operates from the ac powerline. This system employs power semiconductor devices optimized for the motor control application. The powerline input to the motor control system includes an EMI filter, power factor correction (PFC) circuit and dc power supply. The EMI filter prevents powerline noise from affecting motor control performance. The PFC circuit prevents harmonics generated by the power supply circuit from entering the powerline, which may affect other equipment on the same powerline.

The iMOTIONTM integrated design platform (Fig. 2) comprises a development system and mixed-signal chipset, which when co-designed together simplify motion control designs and bring energy-efficient, cost-effective solutions to market faster.

FIGURE 2

The digital control stage comprises a mixed signal chipset that uses the IRMCK201 or IRMCK203. Unlike traditional motor drive controls that rely on complex digital signal processers (DSPs), or microprocessor control units (MCUs), the IRMCK201 and IRMCK203 contain a unique Motion Control EngineTM (MCE), which eliminate software programming. The new devices have a wide range of high performance applications in the industrial and automation industries, such as dental drills, semiconductor wafer-handling equipment, pick and place machines and other precision motion control applications.

The analog chipset is the “bridge” between the digital control engine and the power stage in the integrated design platform. High Voltage ICs (HVICs) in the analog control stage provide gate drive and current sensing functions. The chipset consists of the IR2175 linear current-sensing IC and the IR2136 three-phase inverter-driver IC.

The IR2175 provides current feedback information and features a fast frequency output (FO = 130kHz) for the higher bandwidth and faster acceleration required for demanding appliance servo motor applications. The IR2175 has a 2.0µs overcurrent shutdown feature with a wired-OR connection to communicate directly to the microprocessor or DSP. The IR2175 is designed for 230V three-phase ac or brushless dc industrial drive applications. Motor drive circuits using the IR2175 eliminate external opto- or Hall Effect sensors, reducing circuit size, simplifying design and increasing reliability.

The IR2136 has a dead-time as low as 250ns, and a typical turn-on/turn-off time of 400ns, to match the IR2175. The output drivers feature a high pulse current buffer stage for minimum driver cross-conduction, and include cross-conduction prevention logic to eliminate short-circuit conditions. Matched propagation delays for all six channels in the single IC package ensure consistent high frequency operation. An overcurrent function terminates all six outputs. An open-drain fault signal to the main controller indicates that an overcurrent or undervoltage shutdown has occurred.

FIGURE 3

The motor drive power output stage utilizes integrated power module technology. For example, the 16A-rated IRAMX16UP60A module (Fig. 3) is designed for 750W to 1.2kW VSD applications that include in-room air conditioners, commercial refrigerators and large capacity washers. The IRAMX16UP60A combines low loss, non-punch-through (NPT) short circuit-rated IGBTs with a three-phase, high-voltage, high-speed driver IC and over 20 individual parts into a single unit. EMI emissions are minimized due to shorter connection routing, optimized component layout and internal shielding.

An open emitter configuration facilitates the implementation of several motor algorithms. Using the IRAMX16UP60A module enables multi-shunt current feedback for a sophisticated vector control loop, with no circuit layout limitations. A 10A-rated module, the IRAMS10UP60A is also available. Co-designing the analog and power stage hardware with the high voltage gate drive and current sense ICs enables fast parallel processing architecture.

Included with the iMOTION design platform is a Windows®-based configuration tool, ServoDesigner™, that maps the internal registers to configure motor type, motion peripherals, control mode, tune control parameters, and monitor and diagnose internal signal waveforms.
 

Technical Papers - VFD Basics