Spindle Motors

A spindle motor is a small, high-precision, high-reliability electric motor that is used to rotate the shaft or spindle used in machine tools for performing a wide range of tasks like drilling, grinding, milling, high spped routing for woodworking, aluminum cutting or plastic cutting etc. All machine tools consist of motors which provide a mechanical means of transferring motion to the cutting tool and balance control. It is the motor which drives the spindle shafts. All spindles either have an external motor or an internal motor to run. There are some spindles which are driven by an integral motor spindle design and such spindles are known as motorized driven spindles. The motor provides power and torqure to the spindle. It is an integral part of the spindle shaft and housing assembly. It is with the help of the motor that a spindle can rotate at high speeds.


Types of motors used in spindles

• AC motors: Alternating Current motors as the name suggest requires an alternating current. They are usually sized in horsepower. They come with various speed ratings, which are specified as rotations per minute (RPM) at no load condition. As the AC motor is loaded, the speed will slow down. When the AC motor runs at its rated power draw, the speed of the rotating shaft is the full load speed. AC motor are widely used in any applications requiring variable speed and low cost and small size.
  
  
• DC motors: Direct current motors as the name suggest requires a direct current to run. These type of motors are electric motors that use electricity and a magnetic field to produce torque, which turns the motor. A simple DC motor requires two magnets of opposite polarity and an electric coil. This serves the purpose as an electromagnet. As the magnets within the motor attract and repel one another, the DC motor turns.
  
  
• Universal motors: These motors can use both DC and AC current and they operate at high speed but are not used continuously.
  
  
• Brushless DC Motors: They are the preffered motors in spindles because they last longer, more efficient than AC motors and do not produce excessive heat.
  
  
• PM DC Motors: They are small motors that produce approximately 50% greater torque than other comparably sized motors.
  
  
• Single-phase AC induction motors: This type of motor has a rotating magnetic field to produce the starting torque.
  
  
• Three-phase AC induction motors: These are used in high power machine tool applications.
  
  
• Single-phase AC synchronous motors: They rotate in a synchronous manner with the main current frequency. They have magnetized rotors and do not need an induced current.
  
  
• Spindle Servo motors: They are special types of geared motors, providing more power and have finer controls. They are used in spindles of accurate CNC machine systems. Major types of servo motors are DC and DC.

Favorable features of spindle motors
An ideal motor for a spindle should consists of the following features:

• Small size
• Low power consumption
• High reliability
• Ability to run many hours
• Tolerate hundreds of start and stop cycles without failure
• Minimal wobbling
• Minimal vibration
• Low heat output
• Minimal noise output

Characteristics of spindle motor
Some important characteristics of a typical spindle motor are as follows:

• Small size
• Minimal noise output
• Low power consumption
• High reliability
• Minimal wobbling and vibration
• Low heat output

Calculation of motor speed
The speed of a spindle motor is determined by the following formula:
Speed (RPM) = (Frequency in Hz x 120) / (number of motor poles).
This means that a two pole spindle motor, with top speed of 30,000 RPM, needs a drive with the functionality to provide full motor voltage at an output frequency of 500 Rz. If this motor has four poles then a maximum frequency of 1000 Hz is needed.

Some examples: A diagrammatic representation
Example :1 In a bear or a gear driven spindle, there is space for a large motor because the motor can be located outside the spindle as seen in the first part of the diagram below. On the other hand, as seen in the second part, an integral motor can be fitted inside the existing bore.


Example:2 The following diagram shows a spindle with an overhanging motor which places the motor at the back-behind the rear bearing set. This spindle design allows for a larger motor, if the machine design accommodates it.