Monthly Archives: April 2019

Motor Run & Motor Start Capacitors

Electric motors exploit the interaction between two magnetic fields for rotating a shaft. The stator windings generate one of the fields, and the rotor windings provide the other. In some DC motors, permanent magnets replace one of the windings, while the commutator, whether brushed or brush-less, changes the direction of the current in the other winding to continuously alter the interaction between the two magnetic fields to allow the motor to rotate.

In three-phase AC motors, the interaction between the three incoming phases creates the rotating magnetic field in the stator windings, and this pulls the rotor along, making it rotate. The so-called single-phase AC motor is, in reality, a two-phase AC motor that is operated with a single-phase supply, with capacitors generating the second phase. These motors require two capacitors, one to start the motor, and the other, to keep it running.

A capacitor is a device to store charge. In the DC circuit, a capacitor will charge up and stay that way until allowed a path to discharge. In the AC circuit, where voltage and current change polarity regularly, the capacitor charges up to the peak voltage in one cycle, then discharges and again charges up to the negative peak in the next cycle, with the rate of charging and discharging dependent on the capacitor value and the impedance in the circuit.

Another important factor is the voltage on the capacitor does not follow the input voltage while it is charging and discharging—it lags behind. Even though the supply voltage may be at its peak, the voltage on the capacitor reaches the peak only after the capacitor charges. Likewise, as the supply voltage moves towards the negative peak, the capacitor voltage follows more slowly as the capacitor has to first discharge. This lag helps to create the second phase for the motor.

A motor starting from rest requires a high starting torque, but once it has started moving, requires a smaller running torque to keep it in rotation. That means a larger capacitor is necessary for starting the motor—providing it with a larger starting current. In fact, motors use a centrifugal relay to cut out the start capacitor from the circuit after the motor has reached a certain speed. The run capacitor, though, has to remain connected to the motor at all times.

As the run capacitor is engaged in the circuit continuously, they are oil cooled and in metal, cases to allow heat dissipation. As they face peak to peak voltages all the time, their voltage rating tends to be on the higher side—typically, 1.5 times the line voltage, although the capacitance value may be low, ranging between 5 µF and 45 µF. On most 240 V systems, run capacitors are likely to be rated 370-440 VAC, and in 480 V systems, 600 VAC capacitors are more common. Run capacitors are rated for 100% duty cycle.

Start capacitors, being of larger capacity, are physically larger as well. As the start current does not need to be very precise, start capacitors are available as 8.3 µF, 15 µF, 43 µF, 60 µF, and above. Common voltage ratings for start capacitors are 110, 125, 165, 220, 250, or 330 VAC.

Storing Data with Light

Currently, we have several methods of storing and transmitting data. The most common method followed is storing data on a magnetic hard disk in the form of bits. Here minute magnetic domains form a North and a South pole pair, with the direction of the magnetization of the poles defining whether the stored bit is a digital 1 or a 0. Writing the data means physically switching the magnetization of the relevant bit, one at a time.

Switching or changing the magnetization of each bit requires the application of an external magnetic field, which forces the alignment of the poles to change to either up or down for representing a digital 1 or a 0 respectively.

However, it is also possible to use light to flip the magnetization. Two things are required here, one, very short laser pulses of femtosecond wavelength, and the second, synthetic ferrimagnets that respond to these laser bursts. Using laser and ferrimagnetic material makes data storage far faster compared to what can be achieved by magnetization alone. Ferrimagnets are materials that work with spintronics with the application of a femtosecond laser pulse, and that makes the whole process extremely fast and energy-efficient.

As such, light offers the most energy-efficient method of sending and receiving information. However, storing light is not an easy task. That is the reason data centers all over the world prefer to use magnetic storage methods such as tapes and disks, even though these methods consume a lot of energy to operate. This hybrid technique of storing information using lasers and electric current, developed by researchers at TU Eindhoven at the Institute of Photonic Integration, was presented in the journal Nature Communications. The new method combines the advantages of the high speed of light and ease of magnetic storage. They are using ultra-short pulses of light to write the information directly on a magnetic medium, the result is highly energy efficient and the speed of operation matches the speed of light.

Scientists using the above method of data storage with lasers, have another trick up their sleeve. They combined this optical switching with race-track memory. Here, the data is stored inside a magnetic wire and transported using an electric current. Now, as soon as the bit is stored at one end of the wire using the laser burst, it can be efficiently transported along by the current, freeing up space and thereby allowing the laser to write the next bit.

This efficient on-the-fly operation with the help of lasers and current using magnetic race-tracks does not require any intermediate electronic step. In fact, the physical analogy the scientists offer for this method is of a person jumping back and forth between two high-speed trains moving alongside each other, instead of using a station to change over from one train to the other. The laser and current method, therefore, represents faster speed and higher efficiency.

Obviously, the wires are actually micro-wires. The scientists who designed the system plan to reduce the wires to nano-scale in the future to enable them to be integrated inside chips. They are also working on reading information using optical methods.

Contactless Magnetic Angle Sensing

Contactless magnetic angle position sensors are now giving optical encoders a run for their money. This was recently demonstrated by Monolithic Power Systems at Electronica 2018. They had on display a unique non-automotive-focused electric vehicle, mCar, with motion control and angular sensors. According to MPS, their mCar demos two main functions—motor control elements, and angular position sensors.

As Quitugua-Flores, the mechanical engineer and primary designer of the mCar at MPS explains, the steering of the car is a complete drive-by-wire concept, and there is no mechanical connection between the tires and the steering wheel. A magnetic angle sensor detects the angle of the steering wheel and converts the signal to control the tire angle necessary for the various steering modes. The magnetic angle sensor provides visible feedback via a blue LED mounted on the dashboard, with the LED lighting up when the driver turns the steering.

An electronic system takes in the magnetic angle sensor information and feeds it wirelessly to the rest of the car, thereby instructing the wheels to turn. The angular sensor, along with the board and antenna for sending the wireless signals is attached to the steering column.

The throttle and brake pedals use similar rotary magnetic angle sensors and send their signals wirelessly just as the sensor on the steering wheel does. Pivots on the brake and acceleration pedals house the angular sensors, and they measure the angle of depression of the pedals.

However, MPS has made the mCar as an R&D application, and they have not yet approached the National Highway Traffic Safety Administration (NHTSA) for compliance with their safety regulations.

According to Quitugua-Flores, another aspect of the mCar is its driver seat pivots freely. The front and rear suspension modules keep the seat suspended such that when the mCar enters a curve, the seat tilts into the turn just as it happens in a motorcycle or a plane. This keeps the driver firmly in the seat in a turn, rather than being literally pushed out of it.

An angle sensor attached to the seat detects the rotational position and sends the information to the suspension control. The shock absorbers in the mCar come with individual integrated BLDC motors that can change the length of the shock absorbers independently. Therefore, the suspension has complete control over camber or the vertical tilting of each wheel. As the frame of the mCar tilts when turning, the suspension changes such that each tire tilts in a corresponding direction—just as a four-wheeled motorcycle does.

Shafts suspending the driver cockpit also have angular sensors attached to them. This allows the driver to enjoy a smooth ride by controlling the behavior of the suspension.

According to MPS, the mCar is only a demonstration for the effective operation of a sensing and motion control for a demo Electric Vehicle but is not a high-precision application. For systems requiring high-precision applications, MPS has demonstrated a robotic arm that allows seven degrees of freedom.

With sixteen angular sensors inside it, the arm demonstrates the capabilities of the current generation of MPS angular sensors for precision applications.