In this article we will discuss about:- 1. Selection of Electric Motor 2. Installation of Electric Motor 3. Maintenance.
Selection of Electric Motor:
There are three main considerations according to which electric motors are selected:
1. According to the horse power
2. According to the starting requirements
3. According to working condition
1. Selection According to hp Required:
The motors of common farm use are of 1/4,1/2, 3/4, 1, 2, 3, 4 and 7.5 horse power.
There are a few thumb rules according to which sizes of motor may be chosen:
(a) If an equipment can be successfully operated by a main, 1/4 hp electric motor is chosen.
(b) If one hp petrol engine is to be replaced, 3/4 hp electric motor can be used easily. If 2/3 hp electric motor is not available, the next standard size 3/4 hp may be used.
(c) Sometimes proper sizes of motors are selected by referring some standard table and manufacturers catalogues.
2. Selection According to the Starting Requirements:
There are some equipment like vacuum pump, milling machine, meat grinder, feed grinders, refrigeration compressor and piston type water pumps etc., which require very high starting torque. Hence those motors which give high starting torque should be selected for these machines.
3. Selection According to the Working Condition:
The selection according to the working condition includes the items such as:
(a) Enclosures
(b) Bearings
(c) Type of base and
(d) Method of overload protection
There are several types of housings for enclosing the motors. It may be fully enclosed or open roof type enclosures. Usually ball bearings are used where lubrication is difficult. Sleeve type bearings may also be used because of its cheapness.
Bases may be rigid type or non-rigid type. For less than 3 hp motors, any of these two types of bases may be used.
Overload protection is usually of four types:
(a) Built in over load protection
(b) Time delay fuse
(c) Manually operated switch
(d) Magnetic switch
The first two types are used up to 1 hp motor, the third type is usually used for 1 to 3 hp motors and the last is recommended for 3 to 5 hp motors.
Installation of Electric Motor:
While installing a motor, the main points to be considered are:
1. Base for mounting the motor
2. Type of drives
1. Base for Mounting:
Motor is installed on suitable base fixed on a levelled surface. Slots or bolts are provided for adjustment of motor bases. The motor is held in perfect tight and secured place on a suitable mounting.
2. Type of Drives:
The motor is connected to the load either by pulleys and belts or by direct coupling. When the load is connected with the motor by a direct coupling, the load is driven at the same speed as the motor. Speed of the motor being pretty high, all the farm machines are not directly coupled with the motor because some farm machines work on low speeds also. Centrifugal pumps, grinder wheels, fans etc. are directly coupled with the motor. Pulley and belt drive is commonly used for farm work.
There are several arrangements of belt drive:
(a) When the motor is very close to the equipment or mounted on the equipment V-belt drive is used.
(b) Flat belt drive is used when the equipment has already a flat pulley or when the equipment pulley is large enough to get proper speeds.
When the motor is located at some distance from the driven machine, flat drives are recommended.
Selection of Pulleys and Belts:
While selecting pulley, the factors to be considered are:
i. Sheave should be large enough to prevent excessive bending.
ii. Size of the sheaves and bushings should be specified.
iii. The number of belts used should provide adequate facilities to transmit the required hp.
Pulley diameter for driven machine= diameter of motor pulley x motor speed / speed of the driven machine
Length, width and thickness of belt should be selected after consulting some standard tables.
Maintenance of Electric Motor:
For proper care and maintenance of motor, the points to be considered are:
i. Voltage:
The voltage available at the motor terminals should be within 10% of the rated voltage of the motor on load.
ii. Frequency:
The variation in frequency should be limited to 5% above or below the standard frequency.
iii. Temperature:
The ambient temperature should not exceed 40°C for motor of standard design.
iv. Atmospheric Condition:
Motor should be protected from dust, moisture and excessive humidity.
v. Drive:
Drive should be selected carefully. It should be maintained in proper condition.
vi. Load:
The load should not exceed the rated load; otherwise it may harm the motors to a great extent.
vii. Grounding:
The motor should be grounded, if required in accordance with the electric supply code.
viii. Lubrication:
Proper lubricants should be used at specified intervals. Dust and dirt should be cleaned first and then lubricants be used according to requirement.
ix. Overload and Low Voltage Protection:
Usually motors get damaged due to overloads as well as low voltage supply and hence proper arrangement of overload protection must be provided. The over current device may be a time delay fuse, a current breaker or a heater coil.