[Design View / Design Solution]
Drive And Control Electronics Enhance The Brushless Motor's Advantages
With the appropriate driver IC and microcontroller, designers can exploit the brushless motor's declining price, smaller size, lighter weight, and minimal maintenance.
DESIGN EXAMPLE As an example, choose a low-inertia, brushless motor that delivers 55 oz-in. of torque at 5000 rpm, such as the Galil Motion Control BLM-N23-50-1000-B. With this motor, any stator-winding pair exhibits a resistance (Rm) of 1.2 Ω and an inductance (Lm) of 2.6 mH.
The torque constant (Kt) of the motor is 12.1 oz-in./A, and the voltage constant Kb is 8.9 V/1000 rpm. The first step is to ensure that the SA305's 12-A maximum current capability isn't exceeded, which would cause the IC and the drive circuit to shut down.
If V/R < 12 A regardless of the other parameters, the current can never reach this value. This can be seen in Figure 6, where both the first and all succeeding pulses approach the value of V/R. Another way of looking at it is that the current in any L-R circuit can never exceed V/R. Consequently, the applied voltage, which includes any instantaneous ripple, will never shut down the SA305.
If V/R > 12 A, then several factors in our design must be considered, including Rm's value of 1.2 Ω If we assume a 60-V drive, then V/R = 60/1.2 = 50 A. When we apply the initial voltage to the motor, the current ramps up as explained in Equation 2. As the back EMF builds, the current tapers off (Fig. 7).
We may never see the maximum current in normal operation because of the back EMF. The motor's torque constant and the inertial load will govern the rate at which the motor comes up to speed. If the motor has a particularly low L/R time constant relative to the mechanical time constant, the current can reach the maximum well before the motor builds any back EMF.
Note that in this example the simulation in Figure 7 shows that current will never exceed 8 A—well below 12 A. If the current were to exceed the limit of the driver, adding external series resistance or inductance would limit the peak current and di/dt, respectively, but each would adversely affect system performance.
We can safely accelerate the motor if we control the startup current with a PWM drive by limiting the duty cycle of each pulse so as not to exceed the maximum peak current rating of the driver. The SA305's current monitor feature makes this type of feedback relatively simple to implement.
By employing a microcontroller and monitoring the instantaneous currents in all three phases, we can develop a closed-loop algorithm for startup purposes, which would hold the peak current near 12 A without exceeding it. Actually, a small amount of headroom makes sense, so program it for 11-A motor current.
The advantage of this approach is that it optimizes the run up, keeping the current as high as possible so the acceleration is as high as possible. In such an approach, the duty cycle would be modulated based on the current sensed in the three phases (Fig. 2, again). References 3, 4, and 5 offer even more information about using microcontrollers to drive brushless motors.
References:
1. Motion Control Primer by David Palombo, www.aveox.com
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