Motor Drives–Direct Drives and Pulley Drives

Objectives

After studying this unit, the student will be able to:

• State the advantages of direct and pulley drives

• Install directly coupled motor drives and pulley motor drives

• Check the alignment of the motor and machine shafts, both visually and with a dial indicator

• Install motors and machines in the proper positions for maximum efficiency

• Calculate pulley sizes using the equation:

Drive revolutions per minute Driven Pulley Diameter

Driven revolutions per minute Drive Pulley Diameter

DIRECTLY COUPLED DRIVE INSTALLATION

The most economical speed for an electrical motor is about 1800 revolutions per minute. Most electrically driven constant speed machines, how ever, operate at speeds below 1800 rpm. These machines must be provided with either a high speed motor and some form of mechanical speed reducer, or a low-speed, directly coupled motor.

Synchronous motors can be adapted for direct coupling to machines operating at speeds from 3600 rpm to about 80 rpm, with horsepower rat ings ranging from 20 to 5000 and above. It has been suggested that synchronous motors are less expensive to install than squirrel cage motors if the rating exceeds one hp. However, this recom mendation considers only the first cost. It does not take into account the higher efficiency and better power factor of the synchronous motor. When the motor speed matches the machine input shaft speed, a simple mechanical coupling is used, preferably a flexible coupling.

Trouble-free operation can usually be obtained by following several basic recommendations for the installation of directly coupled drives, and pulley or chain drives. First, the motor and ma chine must be installed in a level position. When connecting the motor to its load, the alignment of the devices must be checked more than once from positions at right angles to each other. For exam ple, when viewed from the side, two shafts may appear to be in line. When the same shafts are viewed from the top, as shown in figure 60-1, it is evident that the motor shaft is at an angle to the other shaft. A dial indicator should be used to check the alignment of the motor and the driven machinery, figure 60-2. If a dial indicator is not available, a feeler gauge may be used.

During the installation, the shafts of the mo tor and the driven machine must be checked to insure that they are not bent. Both machine and motor should be rotated together, just as they ro tate when the machine is running, and then re checked for alignment. After the angle of the shafts is aligned, the shafts may appear to share the same axis. However, as shown in figure 60-3, the axes of the motor and the driven machine may really be off center. When viewed again from a position ninety degrees away from the original po sition, it can be seen that the shafts are not on the same axis.

To complete the alignment of the devices, the motor should be moved until rotation of both shafts shows that they share the same axis when viewed from four positions spaced ninety degrees apart around the shafts. The final test is to check the starting and running currents with the con nected load to insure that they do not exceed spec ifications.

There are several disadvantages to the use of low-speed, directly coupled induction motors. They usually have a low power factor and low ef ficiency. Both of these characteristics increase electric power costs. Because of this, induction motors are rarely used for operation at speeds be low 500 rpm.

Constant speed motors are available with a variety of speed ratings. The highest possible speed is generally selected to reduce the size, weight, and cost of the motor. At five horsepower, a 1200 rpm motor is almost 50 percent larger than an 1800 rpm motor. At 600 rpm the motor is well over twice as large as the 1800 rpm motor. In the range from 1200 rpm to 900 rpm, the size and cost disadvantages may not be overwhelming factors. Where this is true, low-speed, directly coupled motors can be used. For example, this type of mo tor is used on most fans, pumps, and compressors.

PULLEY DRIVES

Installation

Flat belts, V-belts, chains, or gears are used on motors so that smooth speed changes·at a con stant rpm can be achieved. For speeds below 900 rpm, it is practical to use an 1800 rpm or 1200 rpm motor connected to the driven machine by a V-belt or a fiat belt.

Machine shafts and bearings give long service when the power transmission devices are properly installed according to the manufacturer’s instruc tions.

Offset drives, such as V-belts, gears, and chain drives, can be lined up more easily than di rect drives. Both the motor and load shafts must be level. A straightedge can be used to insure that the motor is aligned on its axis and that it is at the proper angular position so that the pulley sheaves of the motor and the load are in line, figure 60-4. When belts are installed, they should be tightened just enough to assure nonslip engagement. The less cross tension there is, the less wear there will be on the bearings involved. Proper and firm po sitioning and alignment are necessary to control the forces that cause vibration and the forces that cause thrust.

The designer of a driven machine usually de termines the motor mount and the type of drive to be used. This means the installer has little choice in the motor location. In many fiat belt or V-belt applications, however, the construction or main tenance electrician may be called upon to make several choices. If a choice can be made, the motor should be placed where the force of gravity helps to increase the grip of the belts. A vertical drive can cause problems because gravity tends to pull the belts away from the lower sheave, figure 60-5. To counteract this action, the belts require far more tension than the bearings should have to withstand. The electrician should avoid this type of installation, if possible.

There are correct and incorrect placements for horizontal drives. The location where the mo tor is to be installed can be determined from the direction of rotation of the motor shaft. It is recommended that the motor be placed with the di rection of rotation so that the belt slack is on top. In this position, the belt tends to wrap around the sheaves. This problem is less acute with V-belts than it is with flat belts or chains. Therefore, if rotation is to take place in both directions, V-belts should be used. In addition, the motor should be placed on the side of the most frequent direction of rotation, figure 60-6.

Pulley Speeds

Motors and machines are frequently shipped without pulleys or with pulleys of incorrect sizes. The drive and driven speeds are given on the mo tor and machine nameplates, or in the descriptive literature accompanying the machines.

Four quantities must be known if the machinery is to be set up with the correct pulley sizes: the drive revolutions per minute, the driven revolutions per minute, the diameter of the drive pulley, and the diameter of the driven pulley. If three of these quantities are known, the fourth quantity can be determined. For example, if a mo tor runs at 3600 rpm, the driven speed is 400 rpm, and there is a four-inch pulley for the motor, the size of the pulley for the driven load can be deter mined from the following equation.

By cross multiplying and then dividing, we arrive at the pulley size required:

If both the drive and driven pulleys are miss ing, the problem can be solved by estimating a reasonable pulley diameter for either pulley and then using the equation with this value to find the fourth quantity.