Understanding Motors for Automatic Drives

Automatic drives, particularly those used in industrial applications like rolling mills, paper machines, and elevators, require motors that offer smooth, precise, and wide-range speed control, often coupled with high starting torque. Different types of electric motors possess varying speed-torque characteristics and control capabilities.

Analyzing Motor Types for Automatic Drives

Let's examine the options provided and evaluate their suitability for automatic drive applications:

• Synchronous motors: These motors run at a constant speed determined by the supply frequency and the number of poles. While efficient for constant speed applications, they are not suitable for automatic drives requiring significant speed variation and dynamic control.
• Squirrel cage induction motor: This is a very common and robust motor. Its speed is relatively constant, slightly below synchronous speed, and depends on the load. While modern variable frequency drives (VFDs) can provide speed control for induction motors, the basic squirrel cage motor without advanced control is not the preferred choice for high-performance automatic drives needing very fine and wide speed adjustments and high starting torque control.
• Ward Leonard controlled dc motors: The Ward Leonard system is a classic method for controlling the speed of a DC motor. It involves a motor-generator set to provide a variable voltage DC supply to the armature of the main DC motor. By varying the voltage, the speed of the main DC motor can be smoothly and precisely controlled over a wide range, from zero up to rated speed and even beyond. DC motors naturally offer high starting torque and relatively easy speed control by varying armature voltage or field current. The Ward Leonard system enhances this control significantly, making it highly suitable for demanding automatic drive applications.
• Repulsion motor: A repulsion motor is a type of single-phase AC motor that has good starting torque. However, it is primarily used for applications requiring high starting torque but not typically for sophisticated, wide-range automatic speed control systems like those mentioned above. Its speed control characteristics and efficiency are not comparable to controlled DC drives for general automatic drive purposes.

Preferred Choice for Automatic Drives

Based on the need for smooth, wide-range speed control, precise speed and torque regulation, and high starting torque capability, the Ward Leonard controlled DC motor system stands out among the given options as historically and technically preferred for many high-power automatic drive applications where such characteristics are paramount.

Therefore, the Ward Leonard controlled DC motor is a highly preferred choice for applications requiring precise and smooth automatic speed control over a wide range.

Revision Table: Motors and Automatic Drive Suitability

Additional Information on Ward Leonard Control

The Ward Leonard control system is essentially a method of controlling the speed of a DC motor by varying the armature voltage. It uses a motor-generator set: an AC or DC motor drives a DC generator, and the output voltage of this generator is supplied to the armature of the main DC motor whose speed is to be controlled. The generator's field current (and thus its output voltage) can be easily varied using a rheostat or electronic means, providing smooth voltage control from zero up to the maximum. This variable voltage then drives the main DC motor, providing smooth speed control from zero to full speed. While now often replaced by power electronic drives (like thyristor-based DC drives or VFDs for AC motors), the Ward Leonard system was historically significant for high-power applications requiring precise variable speed control.