Balancing by Force

Constant speed a-c motors with variable-speed transmissions are used with floating drive frames. Where speeds are changed only infrequently, a mechanical rigging between the transmission adjusting screw and fixed frame causes speed to vary with load owing to speed differences. Hand wheels can disconnect the balance rigging so that a base speed can be set on each drive simultaneously. Drives are adjusted for best operation. Mechanical balancing makes necessary speed adjustments.

Floating drive frame position can also be used to operate a potentiometer slider to provide a signal for servo motor electronic control of variable-speed transmission to change speed in response to varying chain loads and also by remote control. One drive is made a master whose speed is changed by manual switch. Tachometer generators on variablespeed shafts of drives provide velocity signals. Follower drives match speed and preset drive load relationships to master drive.

High-slip (8 to 13ro) a-c squirrel cage motors supplied by a variable frequency 3-phase alternator may be used. The alternator is driven by mechanical speed changer from a constant-speed motor. Voltage varies approximately with frequency. Standard 220-volt, 60-cycle motors can be operated over a range of 20 to 100 cps. All motors are connected in parallel with alternator through thermal overload relays. Conveyor starts and stops with alternator. Drives must be located for nearly equal loading. Care is required in matching motor size to load. Best results are obtained when motors are nearly fully loaded.

Direct-current motor drives require a motor generator set and operator's panel for each conveyor to supply variable armature voltage. Motors are compound wound for 10 to 20ro slip or are shunt wound with armature voltage dropping resistors to vary speed with load. Excitation for generator and motor field is supplied by belt-driven generator, electronic tube, or dry type rectifier. The generator proyides for safe stopping on power failure when dynamic braking is used. Motor armatures are connected in parallel with each other and in series with generator armature and d-c contactor contact. A thermal overload relay and ammeter is provided for each motor.

Motor shunt fields are connected in parallel to the exciter. Each is provided with a series vernier rheostat for adjusting drive balance. Increasing field resistance causes motor to tend to .run faster, forcing it to take more load to hold its speed down to that of the other drives. Ammeters sh9W drive loads and show operator wh~m correct adjustment is made. Experience will determine if conveyor "runs best with equal or unequal drive loads.

Conveyor speed is controlled by a rheostat in series with the generator field. This controls armature voltage, and speed is approximately proportional. vVhen more exact speeds are required, generator field may be controlled electronically. Motor and generator sizes must be carefully considered. Motors operate in the constant-torque range below base speed theoretically, but when shunt field rheostats are used, base speed increases and torque decreases. This can be compensated for by increasing ratio of V -belt drive between motor and speed reducer. In calculating drive speed ratio use motor base speed plus 10 to 15% when conveyor travels at maximum design speed. This will assure ample torque and permit slow down to minimum speeds.

Note that motor manufacturers may indicate an 8 to 1 speed range but that continuous operation at minimum speed and full load is not recommended. At extremely slow speeds, regulation is very poor and heating becomes a problem. Best results are obtained when normal probable speed is near base speed. Since cost of motors and generators rises rapidly with size, there is sometimes pressure to keep them as small as possible. This leads to overloading and poor operating conditions develop. Many companies now specify that motor generator and control be large enough to supply at least one additional drive. Otherwise if conveyor requires an extension, new equipment would be required.

Eddy current clutch motors with electronic excitation can be used. Electronic control matches output speed as indicated by builtin tachometer generator with reference voltage from manually set potentiometer. Torque-limiting and sensitivity circuits permit matching drive loads as indicated by motor current ammeters. Each control is kept electrically separate. Reference potentiometers for the drives are ganged. Flanking potentiometers provide individual adjustment. One control is adjusted for close speed regulation. Other drives are set with lower sensitivity. Two sets of control wiring are required. One is used for motor starters with interlocks with electronic control panels. The other, interlocked with control panel time delay relays, controls the on-off energization of clutches.