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Written Pole Motors

Lee Layton, P.E.

Course Outline

After reviewing the basic theory of electricity and motor operation, we will look at the types of conventional motors in use today as well as how phase converters can be applied to single-phase circuits.

We will then review the characteristics of electric motors including, speed, slip, torque, and efficiency.  The various methods of starting single-phase induction motors are also reviewed.

Next the construction of a written-pole motor is reviewed and the methods of writing the poles is discussed.  The three operating modes of a written pole motor: start, transition, and run mode are reviewed.  Finally the operational benefits of a written-pole motor are reviewed.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

After taking this course you should,

Intended Audience

This course is intended for electrical engineers and others who want to understand the benefits of this exciting new electric motor technology.

Benefit to Attendees

The design, benefits, and drawbacks of the Written-Pole® motor concept are explained in this course.  This should give the reader sufficient information to consider this type of motor for applications where normal three-phase motors are impractical.

Course Introduction

In the 1990’s, with support from the Electric Power Research Institute (EPRI), the Precise Power Corporation of Bradenton, Florida, developed a new concept in electric motors called the Written Pole® electric motor.  This new motor type dramatically reduces starting currents of single phase motors and allows the design of single phase motors up to 100 hp as compared to conventional single phase motors, which are generally limited to units of 15 hp and smaller. 

Electric motors are the backbone of an electrified society and electric motors are responsible for two-thirds of all electric energy generated in the United States today.  Most electric motors are small and only 2% of the motors in the United States are over 5 hp, but they account for over 70% of the energy used to drive electric motors. 

One fundamental problem with single-phase motors is the very high starting current required to get the motor up to speed.  A typical single-phase motor may have starting currents that are six to ten times their normal running current.  Newer motors with higher efficiency usually have even higher starting current inrush.  This presents a major problem in areas with limited electric supply or in rural areas where only single-phase lines are available. Because of high starting currents, single-phase motors are not generally available in sizes over 15 hp.  Many utilities restrict the use of single-phase motors to sizes under 10 hp for ordinary electric motors.  High motor starting currents on single-phase lines can cause line voltage dips that can cause problems for other loads.  

The motor developed by Precise Power Corporation, known as the Written-Pole® motor, uses an innovative approach of controlling the magnetic field of the motor to reduce the starting current requirements.  The result is a motor that has starting current requirements of only about twice the normal running current and, as an added benefit, the motors have efficiencies of 90% or better compared to conventional motors, which have efficiencies of around 85%.  The Written-Pole® motor has been developed with inherently low per-unit starting current and they can be applied in single-phase service areas to ratings of over 100 hp without exceeding the starting current limits of many utility systems.

In this course we will look at the operation of a typical induction motor, the characteristics of induction motors, the design features of Written-Pole® motors, and a review of the benefits of written-pole motors. 

Course Content

This course content is in the following PDF document:

Written Pole Motors

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Course Summary

The written-pole motor, developed by Precise Power Corporation, is a motor design that allows for the operation of very large single-phase motors.  This unique product has applications virtually anywhere that three-phase power is either not available, or is not economical to provide. 

The written-pole motor also has performance advantages over a conventional induction motor including low starting current, very high operating efficiency, and excellent power factor.  This motor technology has the additional benefit of being able to ride through brief service interruptions on single-phase power lines.  When compared to the total system costs, written-pole motors are an attractive alternative to convention three-phase induction motors.


Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.