The market for industrial compact motor drives has grown faster than other types of drives. The reason for this is that industrial compact motor drives provide higher power density and can fit easily in narrow and tiny cases, which are in demand today. Moreover, in the motor control loop, key feedback information on phase current, bus current and voltage are required to achieve smooth control. As a result, miniature isolation amplifiers with built-in safety insulation have been designed to fulfill this application need at a much better price/performance than traditional current transducers.
Compact motor drives are forecast to continuously lead the industrial drives market. Among other industrial drives, the market for compact AC drives (power less than 25 kW) has achieved double digit annual growth rate in recent years. Compact AC drives are used primarily in low-power variable torque applications such as fans, pumps, heating and air conditioning. They are mainly driven by the trend of replacing fixed speed solutions with variable speed ones, which leads to substantial improvements in energy efficiency. Energy saving varies from 30 percent (typical) to 80 percent, depending on the motor usage.The servo drive market is another high growth segment. There is a trend of higher integration of motor and drive units. This type of integrated servo drive integrates a complete servo system, including motor, sensor, encoder, power supply, driver and controller, and communication modules into a single compact unit. Some of its advantages are space saving in the control cabinet, cabling reduction, lower installation cost, and much simpler service.
In this discussion, compact drives refer to, but not limited to, AC drives, servo/integrated servo drives, and stepper motor drives that are packed in narrow and compact cases. Various types of this drive feature lower power ratings and are supplied from 60/80V DC, or 120/230V AC.
OPTIONS FOR SENSOR
A big challenge in motor drive applications is the sensing of motor phase current, bus current and bus voltage, and feeding back to the controller. When designing a compact drive, a designer needs to address additional difficulties due to the physical package limitation. Therefore, a viable sensing solution should provide:
• Sufficient accuracy with reasonable bandwidth to accurately capture the signal
• Very low temperature drift to sustain gain accuracy when temperature goes up because the sensor is placed close to power devices (heat sources) in the crowded case, where temperature rise is considerable
• Safety insulation to isolate the hazardous voltage from the non-hazardous zone. According to IEC 60950-1, a voltage >42.4Vpeak (or 30Vrms) or 60V DC is hazardous. A reliable double insulation from hazardous live is required to guarantee a touch-safe system.
• High common mode rejection ratio (CMR), so that the feedback signal is not distorted by the rapidly-changing voltage levels on the high-voltage side
• A low package profile, which critical for compact drives
• Solution cost and flexibility are often important considerations as well
With these requirements in mind, a designer may consider the following options:
1) Traditional current traducers ─ e.g., Hall-effect sensors:
• Open-loop Hall-effect sensors offer lower cost, but suffer significant inaccuracy due to hysteresis-related errors including offset error and nonlinearity.
• Closed-loop Hall-effect sensors mitigate the hysteresis problem by use of an amplifier and driver circuit, which creates a current flowing through a secondary winding on the core to cancel the original magnetic flux density induced by the primary current. However, their price may be prohibitive for cost-sensitive applications.
• Hall-effect sensors are not immune to external magnetic field noise, and sometimes a shield is necessary, which results in design complexity and additional cost.
• The large profile of Hall-effect sensors makes them difficult to incorporate onto high-density circuit boards. They are incompatible with standard-sized pick-and-place fixtures, which further means hidden cost for manual handling.
• Large temperature drifts and poor CMR.
2) Non-isolated op-amps: These can be used to measure the low side bus current with a shunt resistor. It costs less and has no safety isolation, as the solution name suggests. Adding in another isolator requires extra cost and effort.
3) Isolation amplifiers: These are also shunt-based solutions, but the sensors come with isolation. Among them, the ACPL-C78A/C780/C784 miniature isolation amplifiers from Avago offer an ideal choice for compact drive designs.
MINIATURE ISOLATION AMPLIFIER
The ACPL-C78X sensor is available in a stretched small outline-8 (SSO-8) package with 30 percent smaller footprint than a standard DIP-8 package, and it takes a fraction of the space for its Hall-effect counterpart. Occupying merely 11.5mm × 5.85mm, this tiny package stays a very low profile of 3.18mm (Figure 1). Moreover, it offers a robust galvanic isolation and accurate measurement, including:
• IEC/EN/DIN EN 60747-5-2 safety approval of working insulation voltage of 1,140V
• UL 1577 double protection rating 5kVrms/1 min
• CSA recognition
• 8mm clearance and creepage
• 15kV/s high common-mode rejection provides precise and stable control
• 1 percent high gain accuracy (ACPL-C78A)
• 0.004 percent extremely low nonlinearity feeds back high fidelity signal
• DC to 100kHz wide bandwidth
CURRENT SENSING
The ACPL-C78X uses advanced sigma-delta A/D converter technology to allow accurate measurement of motor phase currents across an optical isolation barrier. Figure 2 shows an overview of a motor drive system using the ACPL-C78X series for current sensing and voltage sensing.
As shown in Figure 3, using the isolation amplifier to sense current can be as simple as connecting a shunt resistor to the input and get the differential output. By choosing an appropriate shunt resistance, any range of current can be monitored, from less than 1A to more than 100A. In operation, currents flow through the shunt resistor and the resulting analog voltage drop is sensed by the ACPL-C78X. A differential output voltage is created on the other side of the optical isolation barrier. This differential output voltage is proportional to the current and can be converted to a single-ended signal by an op-amp or sent to the controller’s ADC directly.
SHUNT RESISTOR
One of the benefits of using an isolation amplifier is that one sensor can fit in all the models with the shunt changed accordingly. Designers can focus on optimizing sensor performance and can easily port over the design to other models.
Selecting a shunt is easy. For example, if a compact motor has a maximum current of 10 Arms and can experience up to 50 percent overload, then the peak current is 21.1A (= 10 1.414 1.5). Assuming that the sensor input voltage is 200mV for optimal performance, then the shunt resistance would be about 10m. The maximum average power dissipation is about 1W (see Reference 4). Various shunt resistors are available to fulfill this type of application need. They are offered in case size of 2512 or similar at inexpensive price, featuring 3W power rating, decent tolerance and temperature coefficient.
The ACPL-C78X can also be used to sense signals with large amplitudes by use of a resistive voltage divider (R1 and R2) at its input (Figure 4).
The market demand for compact drives continues to grow, so does the demand of small size current/voltage sensors. The competitively priced ACPL-C78X miniature isolation amplifiers deliver the reliability, small size, superior isolation and over-temperature performance motor drive designers need to accurately measure current/voltage at a lower price compared to traditional current transducers.
References
1. IMS Research Report.
2. IEC 60950-1.
3. “Isolation Circuits for Current and Voltage Sensing,” Avago Technologies, Publication No. 5965-8207E.
4. “ACPL-C78A, ACPL-C780, ACPL-C784 Data Sheet,” Avago Technologies, Publication No. AV02-1436EN.
Click here for the illustrations: Figure 1, Figure 2, Figure 3, Figure 4 |
