DC Motor Selection and Replacement Guide

The selection of a D.C. motor for an initial installation or replacement can be an involved process requiring information1 about the load and torque requirements, operating environment, efficiency2, frame size, mounting configuration, enclosure type3, duty type4, among others. This guide will overview the typical D.C. motor selection and replacement process5. For specific or unique applications, this guide should be considered as a general guideline only.

Overview

In general, the selection of a D.C. motor consists of the following:

  • Determine whether the load will be driven at motor speed (direct drive) or at some other speed that will require gearing, coupling, pulley, belts, etc. based upon the maximum load speed.
  • Calculate load torque requirements.
  • Calculate motor horsepower, speed and full load torque requirements.
  • Based on the speed-torque and load requirements, select the motor type.
  • Determine the operational requirements: start/stopping, accel/decel open/closed speed loop, braking (and holding), type of controller, etc.
  • Select the motor enclosure based on environmental considerations and cooling requirements.
  • Select mounting configuration: floor- or flange-mounted.

Selecting a Replacement D.C. Motor: Motor Nameplate Identification

Selecting a D.C. motor is very much an investigative process. The time it takes to gather all the information depends on whether the motor selection is for an initial installation, (e.g., new machine design), or for a simple replacement. 6 For direct replacements, the motor nameplate identification7 can provide much of the information needed. It can also tell a lot about the motor application. If no other changes have been made to the application, the motor nameplate8 can very well be the only thing needed to properly select a replacement. The motor nameplate identification is located on the motor enclosure. The information contained on the nameplate varies with motor manufacturer, but it can contain the following information:

  • Motor Manufacturer
  • Model, Type or Catalog No.
  • Serial Number: this provides specific information about a model or general type of motor. For example, it may include a data code to indicate the manufacturing date, revision code or applicable drawings.
  • HP (Horsepower) or KW (kilowatts)
  • RPM (Revolutions per Minute): motor speed at full load
  • ARM. (Armature Voltage): maximum D.C. voltage applied to the armature. Generally, 24, 48, 90 or 180 VDC.
  • FLD. (Field Voltage) D.C. voltage applied to motor field winding. Generally, 100, 150 or 200 VDC.
  • A (Amps): full load current.
  • Fr (Frame): physical dimensions of the motor based upon NEMA standards9
  • Enc. (Enclosure): type of enclosure based upon NEMA/IEC standards10
  • CW (Clockwise Rotation) or CCW (Counter-Clockwise Rotation)
  • Maximum Ambient Temperature: Related to Insulation Class, this specifies the maximum operating temperature of the motor.11
  • Insulation Class: classifies motor winding insulation by maximum allowable temperature. Typical classes are B, F and H. 12
  • Duty: type of rated duty cycle (e.g., continuous, short-time, periodic, intermittent, etc.)13
  • Bearing Type: 14
  • Form Factor (Power Supply Code): NEMA classification of quality of D.C. drive power quality based on the ratio of ripple D.C. current to average RMS D.C. output. 15 Typical codes are A, C, D, E and K.
  • Winding Type: shunt, series or compound connections.
  • Connection Diagram

Basic Calculations: Sizing a D.C. Motor

Sizing a D.C. motor is the process of ensuring the motor is properly matched to the load requirements so the motor will perform normally. 16 This entails first calculating the mechanical characteristics of the application, (i.e., load torque.). The general case for calculating torque is:

Τ = Fd (F=Force; d= rsin θ = moment arm) 17

Calculating load torque for a specific application depends on a variety of factors, including type of drive mechanism (e.g., ball screw, pulley, belt, direct, etc.), motor speed, length of shaft (moment arm) and horsepower. 18 While all the permutations of load torque calculations are beyond the scope of this article, the general case for calculating motor full load torque is:

Motor Full Load Torque = Horsepower (HP) x 5252 / Speed (RPM) 19

Since torque is related to speed, to size a motor that can deliver the necessary torque for the application, the load curve of the application must be matched to the motor’s torque-speed characteristics. The load demand in terms of torque must match or overlap the torque-speed operating region of the motor. 20

Another useful value in sizing a motor is the Km constant. 21. It can be useful in sizing a motor because of its relationship to torque, power and line-to-line resistance. Other considerations in motor sizing are thermal losses. 22 The motor current and power dissipation of a motor is important because they are related to the motor’s temperature rise and affect motor efficiency. Motor losses in the form of heat dissipation (called I2R losses) are calculated:

Power dissipation = I2 x Winding Resistance.

Winding resistance varies with temperature based on the type of wire and its temperature coefficient, which defines how its resistance will vary with temperature. 23

Some Other Considerations in D.C. Motor Selection

Once a D.C. motor is properly sized in terms of torque, power and speed, there are several other factors to consider prior to making the final selection. Some of them include:

Wound Field vs. Permanent Magnet (PM) : PM motors have linear speed-torque characteristics while wound field motors are non-linear. 24 PM motors have good acceleration torque, run cooler and have a smaller frame size than wound field motors. 25

Brushed vs. Brushless: Brushless D.C. motors require less maintenance and can be operated at much higher speeds than brushed D.C. motors. However, they require a complex controller for electronic commutation; brushed D.C. motors can be operated with a simple control system. 26

Insulation Class: An insulation class gives temperature ratings above which would damage the insulation and, at some point, the motor. The most common classes are B, F and H, with Class H having the highest temperature rating. For D.C. motors being powered by variable speed SCR converters or PWM inverters, the higher insulation class ratings (F or H) are recommended. 27

Open vs. Closed Loop Control: Closed loop control of speed or torque provides better regulation than open loop control. However, closed loop systems require a feedback signal that is produced by a tachometer or optical encoder and adds to the cost of the motor.28

Selecting the Motor Enclosure: Environmental and Cooling Requirements

D.C. motors are operated in a variety of environments some of which can adversely affect normal motor operation. Some of these environments include corrosive, wet/humid, high ambient temperature, dust (air particulates) and explosive. Selecting a motor enclosure to prevent environmental contamination is a critical factor in the motor selection process. Conveniently, the National Electrical Manufacturer’s Association (NEMA) and the International Electrotechnical Commission (IEC) have classified motor enclosure types according the environmental protection. 29

Motor enclosures consist of two main categories: open and totally enclosed. Open enclosures consist of drip-proof, splash-proof, semi guarded, guarded, and weather protected. The totally-enclosed types include Totally Enclosed Non Ventilated (TENV), Totally Enclosed Fan Cooled (TEFC), Explosion Proof, Dust Ignition Proof, among others. In general, open enclosures have pathways in and around the motor for cooling air to flow by convection. These enclosures are selected for normal environments but are modified to adapt to minor environmental factors, such as raindrops, snow and airborne particles. 30

Totally enclosed types are selected for severe environments, such as corrosive, explosive, submerged and dust-ignition. They are force cooled by an internal or external fan/blower; however, there are water-to-air cooled types where air cools the motor and water cools the air via a heat exchanger. All totally enclosed enclosures are sealed from the outside environment to prevent contamination. 31

Explosion proof enclosures are totally enclosed with extra features that prevent the expulsion of explosive gases or vapors from the motor prior to them being cooled and no longer able to ignite. 32

Selecting a Motor Mounting Configuration

Selecting a mounting configuration is an essential part of the motor selection process because motors can be mounted in a variety of ways. The two basic mounting configurations are flange-mounted and foot-mounted. 33 Foot-mounting is designed for horizontal mounting on the floor, wall or ceiling. Flange-mounting also can be mounted in various orientations; however, they are designed to be mounted on the machine or driven load. 34. The mounting configuration is combined with the motor dimensions (shaft height) in the frame size code. The frame size is a numerical code with a letter suffix. In general, the number refers to motor shaft height; frame numbers increase with “increasing horsepower or decreasing speeds.” 35. The frame size suffix is a letter code that refers to the mounting configuration. 36.

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