Selection of Motors: 7 Considerations | Machine Tools | Industrial Engineering

Several considerations involved in the selection of motors are given below:

Consideration # 1. Power Supply:

In most of cases where A.C. is available, it is quite suitable to use A.C. motors. However, if a wide range of speed is required by a number of machinery, it is advantageous to use. D.C. motor and interpose suitable conversion apparatus like motor generator sets or rectifiers. For machine tool work, it is better to employ low voltages (below 600 V) for reasons of safety.

Consideration # 2. Group or Individual Drive:

Modern practice favours the use of individual drive for each machine. This results in greater flexibility of operation and layout and enables the motor and control equipment to be tailor made for the drive. Multi-motor drives and motorised machines are the order of the day, as their advantages outweigh the increase in capital costs as compared to the group drives.

Consideration # 3. Mechanical Features:

i. Mounting:

Motors are made available in several types of mounting; foot-mounted, face- mounted, flange mounted, skirt-mounted, spring-mounted etc., for horizontal or vertical application.

ii. Enclosures:

Depending upon the environment, the proper type of enclosure can be chosen from among the many available such as open, screen protected, drip-proof, splash proof, dust proof, fire proof, totally enclosed with or without ventilation, pipe or duct ventilated or enclosed heat- exchanger ventilated machines. For machine tool service, the totally enclosed fan-cooled motor is the most popular choice.

iii. Transmission of Drive:

Direct with rigid or flexible coupling, flat or V-belt, rope drive and chain drive are several alternatives. Direct coupling or belt drive is preferred for machine tools. Where low speeds are required, advantage can be taken by using gear motors which occupy less space and are cheaper than other types.

iv. Permissible Vibration:

Motors should be chosen with due discretion, particularly in machine tool application, as any vibration will directly reflect upon the finish obtainable from the machine.

v. Noise Level:

Though not quite critical, it is desirable to be kept down to a minimum; but a quieter motor means more cost.

vi. Special Features:

The ratio of length to diameter can often be varied to satisfy specific requirements of inertia, space limitation or appearance; Pan-cake motors are a familiar instance. Shell Motors are skeleton motors without shaft, body or cover, suitable for mounting directly on to a machine shaft resulting in integrated design.

Consideration # 4. Electric Features:

i. Starting Characteristics:

The motor must have sufficient starting torque to start and accelerate the load to the required speed within the required time, without drawing unduly heavy starting current. A knowledge of the inertia of moving parts is necessary for this purpose. The number of starts and stops per hour has a decided bearing on the size of the motor.

ii. Running Characteristics:

The torque speed characteristic of the motor should match that of the load. The running losses, power factor and magnetising current should have reasonable values. The pull-out torque of the motor should cater to the maximum load torque.

iii. Speed Control:

(a) Constant Speed with Slight Regulation:

Induction motor with low slip or D.C. shunt- wound motor (Shun characteristic).

(b) Varying Speed:

The speed fluctuates widely with load (series characteristic) e.g., Series Motor (A.C. or D.C.) and Repulsion motor Multi-speed: The machine can be operated at one or more definite speeds, each being practically independent of load e.g., Induction Motor with several pole-groupings.

(d) Adjustable Speed:

The speed is variable over a considerable range, but when once adjusted remains practically unaffected by the load, e.g., D.C. Shunt motor with field control, Scharge Motor,

(e) Adjustable Varying Speed:

The speed can be varied gradually, but when once adjusted, changes appreciably with load, e.g., D.C. Compound wound motor with field control or a wound-rotor induction motor with rheostatic speed control,

(f) Reversible Type:

The direction of rotation is reversible. Most of the motors can be made suitable for this service. Whenever speed control is involved, it is necessary to specify whether it requires constant H.P., constant torque or variable torque.

iv. Braking:

Braking may be either mechanical or electrical or a combination of both types. The number of brakes per hour is an important feature of a drive. Electrically there are several systems of braking such as plugging, dynamic braking, capacitor braking, regenerative braking and D.C. braking.

In machine tool industry, braking is employed to reduce cost and non-productive work-time as in traverse or carriage drives, to effect quick tool changes, to effect rapid and easy shifting of gears and to prevent damage to work or machine or operator in case of wrong operation.

Consideration # 5. The Capacity of Motor:

The size of electric motor is determined by the cutting speeds, feeds and types of tools employed as also the duty cycle of the drive. Due consideration should be given to the starting duty, nature of overloads, number of starts and stops per hour and the method of braking in addition to the running power consumption.

Two basic considerations are involved in fixing the size of the motor:

(a) The motor should be capable of delivering the load torque requirements,

(b) The temperature rise of the motor should be within the safe value for the insulation class.

As stated above, the capacity of motor is selected on the basis of power spent on cutting operation or on the basis of rise in temperature, or over loading whichever results in higher capacity.

On Basis of Power for Cutting:

ƞ = efficiency of all individual transmissions involved in transmitting motion from motor to the operative element. Its value may be of the order of 0.8 to 0.85 for rotary cutting motion and 0.6 to 0.7 for reciprocating cutting motion.

On the Basis of Over-Loading:

Fig. 11.86 shows cycle of operations in a cycle where the motor is alternately on and off. During on position, temperature rises and it falls during off position. Finally motor attains the ambient temperature. If P₁, P₂…………… be power requirements for periods t₁, t₂…………… etc., then power of motor

If temperature at the end of cycle attains a stable value above ambient, then it is possible to select a motor used for continuous loading based upon the equivalent power rating.

Intermittent duty motors with ratio of on time and (on and off time) of 15, 25, 40 and 60 are available. If actual ratio in a cycle differs from these standard values then motor rating

Consideration # 6. Cost:

In addition to the capital cost, the running costs which include the cost of losses, penalty for low power factors (if any), the cost of maintenance and the cost of controls have to be given proper consideration.

Consideration # 7. Appearance:

Pleasing appearance is a good aspect of design, obtained by harmonious blending of the motor finish, mounting and dimensions in line with those of the driven machine.