Cyndi Nyberg
Former EASA Technical Support Specialist
When manufacturers design a motor, there are many variables. For a given stator, the winding has to conform to some fairly rigid rules, and there is not a lot of variance, even among different manufacturers. However, the rotor design is wide open. The variables in the rotor design include: the number of bars, bar material, bar shape, end ring design, skew, air gap, and construction (cast or fabricated). All of these will ultimately have an impact on the speed torque characteristics of a motor.
Unfortunately, because of the rotor design, it is difficult to alter the basic shape of the speed torque curve of a motor in for repair by modifying the stator winding. Modifications can be made to the rotor such as changing the bar material or size, and changing the end ring design, but it is difficult to determine the actual effect they will have on the operating characteristics.
The NEMA Design letter of an induction motor is defined by the torque, current, and speed characteristics. The rotor resistance and reactance, along with the amount of stator flux, determine the speed-torque characteristics of the motor.
These include starting, breakdown, maximum, and pull-up torque, as well as starting current. The minimum torques and maximum starting current for each Design letter can be found in NEMA MG-11998, Section 12, and the EASA Electrical Engineering Pocket Handbook.
Design A & B
Design A and B motors have the same minimum torque requirements. These motors are capable of starting most constant and variable torque loads. Design A motors have the same minimum torque requirements as Design B motors, but in reality, they tend to have higher torque values. Design A motors are normally more efficient and have less slip than Design B motors since by design they are normally stronger (higher flux). However, the tradeoff is higher starting current. Both Design A and B motors are suitable for loads where the starting torque requirements are relatively low.
Design C
Design C motors are considered high torque designs, typically with a considerably higher starting torque than a Design B. The rotor material for a Design C motor will have a higher resistance than a similar Design B, or it may often have a double cage. The higher resistance material in the bar (or the outer bar for a double cage design) will give the motor high starting torque and lower starting current characteristics. (In other words, a Design C motor develops more torque per amp.) Design C motors are suitable for loads that require moderate starting torque like conveyers, compressors, and reciprocating pumps that are started already loaded. Design C efficiencies are typically lower than a Design A & B.
Design D
Design D motors are high torque, high slip motors (typically 5-13% slip depending on the application). The rotor design of these motors typically uses a high resistance alloy material that causes high starting torque, low starting current, and a lower efficiency than Designs A, B, and even C. Punch presses, elevators, pump jacks, and hoists are the type of load associated with a Design D motor.
Wound rotor motors
Wound rotor motors can use variable resistance supplied to the rotor to attain the desired speed-torque characteristics. These motors are often used with high impact loads where the speed-torque characteristics resemble a Design D motor. However, when a wound rotor motor is used as an induction motor, with the slip rings shorted, the starting torque is low and the starting current is high.
Motor replacement
When a motor comes in for repair, we don’t always know its application. But if a Design D motor needs replacement, a “standard” Design B motor may not be a good choice. The Design B motor may not develop the starting torque necessary to drive the load, or have the proper slip. In applications where slip is not an issue, a Design B motor properly sized for the torque requirement might be a good choice because it will have a higher efficiency.
Variable frequency drives
Variable frequency drives are being used more than ever with motors. Special care must be taken to select the right motor to use with the drive. Normally, a Design A or B motor would be used with VFD applications for a number of reasons, including their higher efficiency, but Design C and D motors can be used as well.
New NEMA document
Speaking of variable frequency drives, there is a new document available for download at the NEMA Web site. It is titled “Application Guide for AC Adjustable Speed Drive Systems” and is available at http://www.nema.org/publications/appguides.html. This document has some information about the basic types of motors used with drives, as well as the types of controls, types of loads, and performance information. This is a very good reference document for users and suppliers of motors and drives.
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