Matching servo amplifiers to brushless DC motors


A servo system consists of a number of elements (see diagram). One aspect of the system integration process involves matching the servo amplifier to the brushless dc motor. This may seem as simple as selecting the amplifier and motor from the same manufacturer. Unfortunately, that isn't always possible or desirable.

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Several factors will influence the way a brushless dc motor and a servo amplifier interact:

Motor winding type-- Brushless dc motors generally have a three-phase winding with a wye connection and produce trapezoidal torque characteristics. (Motors that produce sinusoidal torque format are commonly called 'ac servomotors.') If you've selected a brushless dc motor for an application, the servo amplifier should be designed for this type of motor and not an ac servomotor or a brush-type servomotor.

Commutation sensor configuration-- Most brushless dc motors use three hall effect sensors for commutation. These sensors read the motor's rotor position andenable the amplifier to switch the three winding phases on and off in the proper sequence to produce rotary motion. Confirm that the servo amplifier you're considering is designed to accept hall sensor feedback and can be configured for your motor's sensor spacing (60 or 120 degrees).

DC supply voltage-- Once a brushless dc motor that operates within your application's required speed range has been selected, calculate the supply voltage needed to power the motor at maximum speed and corresponding load using:

E = T L T + K E

where, E = supply voltage, T L = load torque, R = motor winding resistance, K T = motor torque constant, K E = motor voltage constant, N = motor speed at full load. Take as an example a 3/8 hp brushless dc motor (4-in. frame), with motor constants K T =75 oz-in./A, R=3.4 {OMEGA}, and K E =42 V/krpm. If we assume T L =150 oz-in. and the desired speed is 2,500 rpm, the above equation yields E=114 V for supply voltage.

Current limits-- Servo amplifiers have two adjustable current limits: continuous and peak. Continuous current limit should be at least as high as the rms current of the motor in your application. Peak current limit should be at least as high as the current drawn from the motor during peak loading conditions.

If current limits of the amplifier are higher than the ratings of the brushless dc motor, the amplifier's adjustment should be turned down accordingly in order to prevent overloading the motor.

Mode of operation-- Most servo amplifiers can be configured for three modes of operation: current (torque) mode, velocity mode, and open-loop mode. Current mode is generally used for positioning involving a digital motion controller and encoder feedback. Velocity mode is typically used where four-quadrant speed control is needed, such as an inclined conveyor with an overhauling load. Open-loop mode is generally used only during the initial set-up of the servo amp.

Motor-to-amp connections-- Connecting a brushless dc motor and servo amplifier made by different manufacturers can often be confusing. One reason is that no industry standard exists for labeling the three motor phases.

It's likely that the manufacturer of your servo amplifier has some experience in properly connecting the brushless dc motor you're using. The motor manufacturer is another possible source for the appropriate connection diagram. If a diagram is unavailable, you will have to experiment.

First, connect the hall sensors. After the sensor power connections are made, it's the relationship between the winding connections and sensor connections that matters--not how the three winding connections are actually made.

With the sensors connected, six possibleways exist for connecting the three winding leads. Four of these will cause the brushless dc motor to not operate at all. That leaves only two connections, one of which is correct. The incorrect connection will result in erratic operation of your motor.

When all of the above factors are evaluated, you should have a properly matched brushless dc servo system.

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