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Brush and Brushless DC Motor Control

Source: Author:He Visit:331 Release time:2005-02-12 21:17

The MC33033 is one of a series of high performance dc brushless motor controllers produced by Motorola. It contains all of the functions required to implement a limited–feature, open loop, three or four phase motor control system. Constructed with Bipolar Analog technology, it offers a high degree of performance and ruggedness in hostile industrial environments.The MC33033 contains a rotor position decoder for proper commutation sequencing, a temperature compensated reference capable of supplying sensor power, a frequency programmable sawtooth oscillator, a fully accessible error amplifier, a pulse width modulator comparator, three open collector top drive outputs, and three high current totem pole bottom driver outputs ideally suited for driving power MOSFETs.  Included in the MC33033 are protective features consisting of undervoltage lockout, cycle–by–cycle current limiting with a latched shutdown mode, and internal thermal shutdown. Typical motor control functions include open loop speed control, forward or reverse rotation, and run enable. In addition, the MC33033 has a 60°/120° select pin which configures the rotor position decoder for either 60° or 120° sensor electrical phasing inputs.

Three Phase Motor Commutation

The three phase application shown in Figure 1 is an open loop motor controller with full wave, six step drive. The upper power switch transistors are Darlington PNPs while the lower switches are N–Channel power MOSFETs. Each of these devices contains an internal parasitic catch diode that is used to return the stator inductive energy back to the power supply. The outputs are capable of driving a delta or wye connected stator, and a grounded neutral wye if split supplies are used. At any given rotor position, only one top and one bottom power switch (of different totem poles) is enabled. This configuration switches both ends of the stator winding from supply to ground which causes the current flow to be bidirectional or full wave. A leading edge spike is usually present on the current waveform and can cause  a current–limit error. The spike can be eliminated by adding an RC filter in series with the Current Sense Input. Using a low inductance type resistor for RS will also aid in spike reduction.

Three phase full wave brushless motor controller circuit


Fixed Off-Time Control

This circuit controls the current through the motor by applying an average voltage equal to zero to the motor terminals for a fixed period of time, whenever the current through the motor exceeds the commanded current. This action causes the motor current to vary slightly about an externally controlled average level. The duration of the Off-period is adjusted by the resistor and capacitor combination of the LM555 . In this circuit the Sign/Magnitude mode of operation is implemented (see Types of PWM Signals).

Brush Motor Control

Though the MC33033 was designed to control brushless dc motors, it may also be used to control dc brush–type motors.  An application of the MC33033 driving a H–bridge affording minimal parts count to operate a brush–type motor. Key to the operation is the input sensor code [100] which produces a top–left (Q1) and a bottom–right (Q3) drive when the controller’s Forward/Reverse pin is at logic [1]; top–right (Q4), bottom–left (Q2) drive is realized when the Forward/Reverse pin is at logic [0]. This code supports the requirements necessary for H–bridge drive accomplishing both direction and speed control.

The controller functions in a normal manner with a pulse width modulated frequency of approximately 25 kHz. Motor speed is controlled by adjusting the voltage presented to the noninverting input of the Error Amplifier establishing the PWM's slice or reference level. Cycle–by–cycle current limiting of the motor current is accomplished by sensing the voltage (100 mV threshold) across the RS resistor to ground of the H–bridge motor current. The over current sense circuit makes it possible to reverse the direction of the motor, on the fly, using the normal Forward/Reverse switch, and not have to completely stop before reversing.