During starting, higher than normal current draw can be expected. This momentary current rise is a function of the static friction within the motor and associated machine as well as any load the machine may be under at the time of startup. If starting current exceedes acceptable values, the motor is disconnected from the supply voltage to prevent damage to both the motor and drive devices. Control of this parameter is usually not seen in small motor applications, but can be significant on larger machines.
In many applications, it is unsafe or undesirable to allow a motor to coast to a stop. Dynamic braking can be used to eliminate this problem. In dynamic braking, the motor is loaded with an external resistor after electric power has been removed. This external resistance dissipates the mechanical energy of the motor to bring ti to a quicker slowdown. Dynamic braking is most effective when motor speeds are high. Regenerative braking is an alternate form of dynamic braking. In regenerative braking, mechanical energy is converted back to electrical energy and returned to the energy source instead of being dissipated. Regenerative braking is effective to speeds of zero.
Reversing motor rotational direction may be disirable in many applications. Depending on the type of motor, reversing rotational direction is easily accomplished. Exchanging power leads in an AC machine, reversing power polarity of a DC machine, or reversing input pulse sequences for a stepper motor will accomplish rotational reversal. In the first two cases, reversal can be inplemented through an extrta set of contacts in the controller while stepper reversal is software based. In many applications, this is incorporated within the dynamic braking system.
Velocity control is dependend on the type of motor being used. The velocity of DC motors can be controlled by varying the voltage at the terminals of the motor. Synchronous AC machines respond to changes in frequency of the power supply. Stepper motor velocity is controlled in the frequency of the imput pulses. There are two methods to implement speed control, open and closed loop. In closed loop, information about motor speed is feed into the control circuit which makes appropriate changes to regulate motor speed. Open loop control does not feed any information from motor to controller. Open loop control is most often seen in stepper motors.
To effectively drive a machine, all of the functions disguised above may need to be applied to each motor on a given machine. Not only do each motor need to be controlled, but also the realtionship between motors. As such, control systems may quickly grow in size and complexity.
Lindsay, J.F.Electromechanics and Electrical Machinery Englewood Cliffs, New Jersey:Prentice-Hall, Inc., 1986
Mobers, Gerald A. AC adn DC Motor Control New York:John Wiley and Sons, Inc., 1987
Kusko, Alexander Solid-State DC Motor Drives Cambridge, Massachusetts:The M.I.T. Press, 1969
Electro-Craft Corp. DC Motors Speed Controls Servo Systems Second edition Hopkins, Minn.:Electro-craft Corporation, 1973
Rexford, Kenneth Electrical Control for Machines forth edition Albany N.Y.;Delmar publishers, 1992