April 2013

Combined capacitor and choke for motor-run applications

Economic and efficient control of bidirectional motors

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A design by STMicroelectronics for the control of bidirectional AC motors features the EPCOS LCap, a combined motor-run capacitor and choke in a single rugged package. Thanks to LCap’s narrow tolerances over time, compact size and reduced assembly complexity, it is designed for use in economical and energy-saving drive controls for washing machines, dryers, garage door openers, gate openers, power shutters and more.

Standard bidirectional asynchronous induction motors used in appliances such as economy washing machines have two windings: the primary winding and the auxiliary. A motor capacitor is needed to start the motor in the required direction by shifting the phase to the necessary winding. Conventionally, electromechanical switches have been used to control the motor (on-off, direction of rotation). Electromechanical switches, however, have obvious drawbacks. They produce noise, have a higher breakdown risk, and they have a shorter average operating life than electronic components. There is a large demand for electronic control solutions to replace electromechanical designs (relays) with the objective of increasing operating life and reliability, reducing power consumption and making the drives quieter.

Premium appliances such as top-of-the-line washing machines and dryers with sophisticated programs employ variable speed drives that are controlled by frequency converters. These solutions achieve a very high degree of variable motor control and energy efficiency. The electronics, however, are complex and require a large number of electronic parts. The complexity and costs of the subsequent assembly processes are correspondingly high.

Reliable and elegant solution

Together with TDK, STMicroelectronics developed and qualified an elegant and economical electronic control system for bidirectional AC motors (Figure 1). The design consists of the following circuit components:

  • STM microcontroller unit (MCU)
  • STM triacs (ACST, BTA and High Tj Triacs series) to switch the power to the required winding
  • EPCOS LCap combined motor-run capacitor and choke to produce the phase shift for the auxiliary winding and protect the switches from overcurrents, respectively.

Figure 1: Electronic motor control

The MCU drives two triacs for the primary and auxiliary windings of the motor. The EPCOS LCap combines the motor-run capacitor and the series choke.

Compared to circuits that use electromechanical switches, the design offers several benefits:

  • Higher efficiency because triacs are operated by a single pulse, thus eliminating wasted power consumption from a continuous flow of current through the relay coil
  • Increased reliability because triacs have a longer operating life than relays
  • Reduced EMI thanks to spark-free operation
  • Noise reduction through the elimination of mechanical relays
  • Suitable for use in single-speed bidirectional drives for applications such as garage door and gate openers or power awnings and shutters

The latest generation of triacs is well suited for this application. They offer fast switching and are driven by gate currents as low as 5 mA. Moreover, the triacs, for example, have a very high surge current capability (ITSM). A single 16-A triac can withstand a surge current of 140 A for 20 ms and a repetitive dI/dt of 100 A/μs. The phase angle can be easily controlled, which would be hard to achieve with mechanical relays.

Effective overcurrent limiting with integrated choke
A choke is needed in the electronic circuit with two triacs to protect the switches in the event of current overload. EMI in the circuit, for example, can force both switches to the “on” state, which would discharge the capacitor without any current limitation. The current overload resulting in such an event can damage the switches. To prevent this, an inductor is inserted in series with the capacitor to limit the current supplied by the capacitor during the period of current overload.

A triac current of more than 1000 A (Figure 2) is possible for the first peak lasting 25 μs when both triacs are forced to the “on” state simultaneously (red curve). This value is typical for a bidirectional asynchronous 230 V, 50 Hz induction motor with a rated power of 150 W that is driven by a circuit with two triacs using a 10 μF phase shift capacitor. This high current exceeds the maximum value allowed for the triacs.

Figure 2: Peak current with and without a choke in series with capacitor

When both triacs are forced to the “on” state simultaneously, a peak current of –1073 A passes through the switches for 25 µs if the circuit is not protected by a choke (red). With a choke the peak current is limited to –232 A for a period of 110 µs (green).

A series choke effectively limits the current peak. The dI/dt rate will be shifted to values that triacs can withstand without damage. As shown in Figure 2, an inductance of 80 μH reduces the first peak current to less than 250 A (green curve) with a duration of 110 μs, keeping the current within the safe operating area.

Tests at the STMicroelectronics laboratories have shown best results when using an EPCOS LCap, an integrated capacitor and choke that combines the phase shift capacitor with the current limiting choke in one package. During washing mode, the drum is rotated in both directions by turning on each triac alternately. One of the two windings of the induction motor is supplied directly by the mains voltage. The other is supplied through the motor-run capacitor providing a phase shift and a high voltage across the capacitor, which can reach a peak value of 650 V. Only one triac is turned on during spin mode, since the drum rotates at maximum speed in one direction only.

Two components, one package

The EPCOS LCap, which combines the motor-run capacitor and choke in one package, offers a number of performance and logistics benefits compared to a solution with discrete components:

  • Low ESR
  • Long-term stability with narrow tolerances
  • Highly rugged and maintenance-free
  • Compact size
  • Two connections instead of four
  • Reduced assembly time

The integrated capacitor and choke in one package offers tighter tolerances than a discrete solution because the air-core coil choke typically exhibits higher tolerances as a result of mechanical influences that can physically change the winding. In addition to LCap's ideally matched electrical characteristics, the combined component offers measurable benefits for assembly in production lines. Moreover, the lower component count means that fewer assembly operations are necessary, resulting in improved failure rates. Thanks to these advantages, LCap technology is already being used successfully by global market leaders for economical and easy-to-mount solutions.

Table 1: Key data of the EPCOS LCap combined motor-run capacitor and choke

Rated capacitance [µF]3 to 50
Rated voltage [V AC]230 to 450
Tolerance [%]± 5
Rated inductance [µH]5 to 100
Tolerance [%]± 10

The EPCOS LCap is available in a wide range of rated capacitances and inductance values. Types with further values, for example, for small motors are available on request.

Table 2: Typical combinations of LCap and triacs

Motor power EPCOS LCap (B32350*)TRIAC 1)
[W @ 230 V, 50 Hz]Capacitance [µF]Inductance [µH]Type

1) From STMicroelectronics



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