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FAQs: Motor control design

Posted: 17 Oct 2008 ?? ?Print Version ?Bookmark and Share

Keywords:FAQ? motor control? frequency optimal switching?

What is the optimal switching frequency I need to drive a motor?
The optimal frequency is the highest frequency that gives acceptable efficiency, current ripple, and EMI performance and at the same time keeps the driver devices in the safe operating region. The switching frequency is generally chosen to be above the audible range.

What is the difference between center-aligned and edge-aligned PWM signals?
In edge-aligned multi-phase PWM control all phases switch on at the same time but switch off at different times depending on the duty cycle of each phase. Multiple high current phases switching simultaneously cause a much larger EMI transient than if the phases always changed at different times. Center-aligned switching solves the EMI problem by activating the PWM outputs such that the centers of all the active periods are aligned. For each phase output, half the duty cycle occurs before the center alignment reference followed by the other half.

What is dead time control?
Dead time is the delay measured from turning off the driver switch connected to one rail of the power supply to the time the switch connected to the other rail of the power supply is turned on.

Why do I need complimentary outputs?
Complementary outputs are required to provide alternate drive to the high side and low side switches. The drive current in each phase of a BLDC motor reverses direction twice during each electrical rotation of the motor. This requires the drive voltage at the phase terminals to reverse. Voltage reversal is accomplished by connecting each phase terminal to both the positive and negative outputs of the power supply through separate MOSFET or IGBT drivers. When one driver is on, the other is off or, in other words, they are complementary drivers requiring complementary drive inputs.

What is a 3-phase or 4-phase PWM?
3-phase pulse width modulation (PWM) consists of three outputs (or three pairs of outputs in the case of complementary drive) each providing a separate pulse width modulated signal. Separate outputs makes emulation of an analog 3-phase system possible where the effective voltage level at each output is the average voltage determined by the pulse duty cycle. 4-Phase PWM is the 4 output equivalent of 3-phase PWM.

Can the A/D be synchronized with events in the 3-phase PWM system?
Yes. To make it easier to design sensorless applications, Microchip provides the capability to perform an ADC conversion at a specific time in the PWM period.

What is meant by "sensorless"?
Sensorless motor applications are not actually sensorless, but they eliminate one or more mechanical feedback sensors by deriving position and velocity information from other feedback sources. Sensorless control is an option for brushless DC motors, for example. A sensorless BLDC application might the voltage induced across an unpowered winding of the motor to derive the rotor position.

How do I sense the motor's position or speed and how many sensors are needed?
Position and speed can be sensed with optical encoders, resolvers, and other devices like hall effect sensors. Usually one sensor is used on each motor that requires feedback.

Where can open loop control be used?
Open loop control can be used in systems where motor velocity is not critical, and where accurate positioning is not necessary. Open loop control can also be used with stepper motors to control their position, provided they are properly sized for the load.

What is meant by open loop vs. closed loop?
Open loop is control without feedback. Closed loop control compares motor current, velocity, or position feedback with a desired value and generates a modified command to make the error smaller.

What is the difference between Microchip's motor control family vs. standard MCU devices?
Most of Microchip's MCU devices can be used to control a motor, but the motor control MCU family has peripherals which are designed to simplify the implementations of controlling various types of motors. Current peripherals offered are I/O ports driven via firmware to turn on/off, sequence, or generate PWM signal to the motor, CCP's, ECCP's, & 3-Phase PWM modules to generate PWM signal to the motor.

What peripherals do I need for motor feedback?
Depending on the controlled parameter and sensor technology, Microchip microcontroller families provide several choices of peripherals. To see examples of peripheral usage by various control types, please refer to the motor block diagrams. The list below shows the available peripherals today:

  1. I/O ports with firmware timing to measure motor speed for low speed applications.

  2. Timers, CCP, or ECCP modules configured in Capture mode to sense motor speed.

  3. QEI module to sense motor speed, and absolute rotor position

  4. A/D module to sense motor voltages, currents or resolver outputs.

Source: Microchip Technology Inc.

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