Motors on solar positioning equipment orient panels to follow the sun daily and seasonally. There are four basic types of electric motors used in solar power applications: AC induction, stepper, and permanent magnet DC brushed and brushless. Jonathan Doyle, Application Engineer with Dunkermotor, shared some insight into motors and drives in solar applications.
Doyle says choosing a motor depends on the speed, torque and power requirements of the application as well as the communication and drive options required. Brushed motors are relatively efficient and inexpensive, but they also have a limited lifespan due to brush wear. Brushless motors are smaller, though more expensive, and have better torque and speed characteristics. They are also more efficient and have a virtually unlimited operating life. Brushless motors require high-level control, but reduce cabling complexity compared to other systems. A stepper motor can easily rotate by finite fractions. These are well-suited for accurate positioning and repeatability with a fast response to starting, stopping, reversing and speed control. Stepper motors also can hold loads steady once reaching a required position. AC induction motors will allow power to be used directly from the grid, but are typically not as controllable.
In the United States, Doyle says the term drives refers to electronic control, while in Europe it is a more encompassing term for the motor and control combination. So for AC motors, it’s variable frequency or an inverter. Stepper motors have a stepper drive. For brushed motors it could be a simple brush-type drive or more complex with a network. Options become even more diverse with brushless motors.
Motors on solar trackers must withstand extreme temperatures, twice that of a normal industrial setting, says Doyle. They also must have protection against water and dust. This can be achieved with sealing. However, sealing and changes in temperatures can lead to condensation that may result in corrosion. To prevent this, Dunkermotor uses a special membrane that allows the motor to breathe but maintains a high level of protection. A final challenge for solar motors is ultraviolet light that can damage the motor’s external components. Therefore Doyle says these must be UV hardened so they don’t degrade over time.
Doyle says one trend is to use networked motors, where Modbus, CANbus or other intelligence is built in. These allow pinpointing problems and resolving them faster. Also, because “slave” motors report to a “master” it’s easier to control tracking speed in strong winds or other harsh conditions. Another advantage of networked motors is that an industrial PC or PLC can be used instead of a purpose-designed drive.
Solar motors move large, heavy objects at a slow pace, so they may require as little as one to ten watts of output power during normal operations. Therefore, Doyle reports seeing high gear reduction in motors to primarily reduce tracking speed. The ratio is also needed to allow for the possibility of extremely high wind loading.
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