The Machine Tools Motors Requirements are:
- The motors must be reliable and low cost, requiring less maintenance.
- They must be capable of speed control. Some applications may require operation at fixed speeds. Sometimes stepless and smooth speed control is required for better machining timings and surface finish.
- Starting torque may vary from about 10% to 250% of full load torque depending upon the type of Machine Tools Motors Requirements.
- The acceleration of the motor should be sufficiently fast to avoid motor heating during starting. The braking must also be effective and fast. This is because frequent starting and stopping are required. In view of safety reasons, braking must be such that exact stopping of the tool is assured.
- The duty cycles are specified for the machine tool operations. The design of the Machine Tools Motors Requirements depends upon the duty cycle.
- Peak loads of short duration may alternate with light load in certain operations. A smaller size motor may be selected if a flywheel is used. The motor should have sufficient speed regulation to make use of flywheel. High slip induction motors or cumulatively compound motors are used.
- Variable speed operation with constant torque at all speeds may be required in machine tools, such as grinder, planer, polishing, rapid reversing, etc. Variable speed operation with constant power also finds applica
- Some Machine Tools Motors Requirements very high speeds of operation. These are high speed grinders.
- Numerically controlled machine tools are being preferred to conventional machine tools. These have increased utilisation of the machine. The production rate can be increased. These are costly at present, but are expected to become economical.
- The requirements of the drive motor are fast response, wide range of speed control, low vibrations, better thermal capacity, low maintenance, To have fast response the inertia must be low. They must give precise positioning.
A choice of the motor must be made to meet the above requirements.
Due to the simple, economical and robust construction, realiability and less maintenance, squirrel cage motors are suitable for driving Machine Tools Motors Requirements. Till the advent of thyristor power converters which are capable of providing variable voltage, variable frequency supply, the speed control of these motors was a problem. Using power transmission equipment , such as gears, speed caused affect the accuracy of the output. A four pole or six pole motor may be used. A motor with larger number of poles has poor power factor, smaller starting torque and lesser efficiency. Two pole motors are noisy and have vibrations.
Pole change multi speed motors are available when definite stepped speed operation is allowed. These connections are available for constant torque as well as constant power operation. They provide high and low speeds.
When smooth speed control is required for better finishing and machine timing, suitable controls are necessary. The advent of thyristors and associated power converters has paved the way for the smooth speed control of ac motors. Both constant torque as well as constant power operations are possible. The variable frequency converter used for speed control may also be used for starting purposes. Variable frequency starting has advantages and imparts a very good starting behaviour. The starting or accelerating torque can be controlled. These are also costly at present. The availability of thyristors at reduced prices and in higher ratings may make these drives popular in future.
The kW rating of the motor is decided by the duty cycle. A suitable rating of the motor must be selected, having sufficient mechanical overload as well as thermal overload. The motors for duty cycles S1-S6 are listed.
S1 Hydraulic pump motors, lubrication pump motor, coolant pump motor.
S2 Rapid transverse motor.
S3 Main motor for gear shaper, and for drilling machine.
S4 Main drive motor in lathes without clutch in the drive, work head motor in grinding machine, main drive motor in gear nobbing machine, coolant pump motor with frequent starting and stopping.
S5 Work head motor in grinding machine with electric braking.
S6 Main and feed drive motors with clutch in the drive.
The frequency of starting and stopping decides the type of starter as well as the braking method. Slip ring induction motors with rotor resistance starting are desirable. If plugging is employed for frequent stopping the motor gets thermally overloaded. The motor must be disconnected at zero speed. DC dynamic braking can also be employed. This takes more time for braking compared to plugging, but the heating is decreased. For safety reasons, along with electrical braking, mechanical brakes must also be provided as a standby.
High slip motors are required for use with a flywheel, to drive a load which alternates with no-load conditions. When the load is applied the speed should fall by 8-10% so that the flywheel can provide a part of power required by the load from its stored energy. During no-load condition the speeds goes up and flywheel stores this energy. Slip ring motors with additional rotor resistance are advantageous. The resistance causes necessary slip. The motor does not get heated, The inertia must be specified. When dc motors are employed compound motors with cumulative compounding are used.
Motors for numerically controlled Machine Tools Motors Requirements may be specially designed dc motors, inverter fed motors or permanent magnet motors.