Cycle-by-Cycle: Current Limiting Eases Design of Motor Drives

Nov 1, 2008 12:00 PM
By Sam Robinson, Marketing and Applications Manager, Cirrus Logic - Apex Precision Power, Tucson, Ariz.

The integration of this protection feature within brushless dc motor-driver ICs protects the motor driver against high inrush currents without adding to pc-board space requirements or driver complexity.

Brushless dc motors have the advantages of high reliability and smaller size, but the absence of brushes can pose challenges. One challenge is the requirement for electronic commutation and control, but the plummeting cost of processing power reduces the financial impact of this increased complexity. An additional challenge is the elimination of brush resistance, which causes inrush current to be substantially higher than average-run current.

For example, a 1-A continuous motor current might require a drive amplifier to deliver well over 10-A peak current to accommodate the initial inrush startup current. Low-inertia brushless motors can have a peak-to-average current ratio above 30, which leaves a designer with the choice of either selecting a driver that can safely handle the large inrush current or installing adequate current limiting.

The high current solution requires a look at the rise in back electromagnetic force (EMF), which corresponds to the motor velocity and the current (Fig. 1). Clearly, an unlimited current motor accelerates faster and can be a benefit in some applications. But in many cases, acceleration is not the critical measure of performance, so the drive circuit size can be cut by limiting the current.

Rotor Dynamics

Startup is a stressful time for a brushless motor-drive circuit. When at rest, the motor generates no back EMF (V_BEMF), thus the drive voltage and passive motor characteristics are the only contributors to motor current. When the motor spins up, it generates back EMF, easing the current demands on the drive circuit. The red line in Fig. 1 depicts V_BEMF behavior.

Applying voltage to the motor causes the rotor to begin turning, generating V_BEMF as governed by this calculation:

V_BEMF = K_B × speed, (Eq. 1)

where K_B equals the motor voltage constant (volts/1000 rpm) and speed equals revolutions per minute (expressed in thousands).

Therefore, motor current is:

where I equals the motor current in amperes, V equals the applied drive voltage in volts, t equals the time in seconds, R equals the stator resistance (winding pair) in ohms and L equals the stator inductance (winding pair) in henries.

These equations apply to both brush and brushless motors, but the absence of brush resistance makes the exponential R/L term in Eq. 2 a more-significant factor for brushless motors. Although the inrush current may last only for a brief moment, a drive stage without current limiting must use an output stage capable of safely handling the large inrush current. Whether the output drive stage is an IC or a discrete collection of MOSFETs, a high current output generally implies an increase in package size, pc-board layout area and overall cost.

Current-Limit Functionality

As with many motor-drive discussions, the concept of current limiting in the context of a pulse-width-modulated (PWM) circuit is straightforward. Simply measure the current in each motor phase or each drive leg, and turn off the drive transistors when the current reaches some programmable threshold. Then let the current decay in the motor for the rest of the PWM cycle, begin a new PWM cycle and repeat the process. The PWM voltage and current waveforms are similar to those in Fig. 2.

A look at Fig. 2 reveals the cycle-by-cycle current-limit behavior. During the first PWM pulse, no current limiting occurs, because the pulse ends before the rising current reaches the current-limit threshold. At the end of the first pulse, there is a short decay before the second PWM pulse begins and the current resumes its rise.

During the second PWM pulse, the current reaches the limiting value before the PWM input pulse ends. The PWM output pulse is shut off early in its cycle. Then the motor current decays until the third pulse is applied, which once again causes the current to rise.

This behavior continues until the motor rotation begins to generate sufficient back EMF for the current to fall below the current-limit threshold, as shown in Fig. 1b. Notice that the rise and fall of the motor-current waveform depends on the resistance and inductance of the motor winding and is asynchronous with the PWM frequency. It is common for the waveforms during current limiting to create a periodic function with a frequency in the audible range. A chirp during startup or in response to a sudden mechanical load change is not unusual. This is generally called a subcycle oscillation and is not a cause for alarm.

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