Tom Bishop, P.E.
EASA Technical Support Specialist 

The speed-torque characteristics of three-phase motors are an important consideration when selecting a replace­ment. If an incorrect design type is chosen for the new motor, the motor may not start the load, or the motor may draw excessive starting current and consequently trip overload protective devices. When it comes to repair, at times a squirrel cage rotor is determined to have an open circuit and will need to be rebarred. 

It is the rotor bar and end ring material and physical shape that have the greatest effect on starting as well as running performance. Simply put, the rotor design determines motor Association) standards fall into four major categories, or design types. These are identified on the motor nameplate as design A, B, C and the less common D. There was a design E designation that NEMA withdrew a few years ago. What are the differ­ences between the design types? We will define the starting characteristics and examine these different designs. Note: IEC (International Electro-technical Commission) motors do not have design type designations. 

Starting characteristics 
The primary characteristics associated with three-phase motor starting are locked rotor current, locked rotor (starting) torque, pull-up (minimum) torque and breakdown (maximum) torque (see Figure 1). The torque developed by the motor during the period from standstill to the speed at which breakdown torque occurs. Finally, breakdown torque is the maximum torque that a motor will develop without an abrupt drop in speed. 

Before we begin examining motor designs, a few words of caution. Motors built to NEMA standards use alphabetical letter codes to designate a number of alternating current motor characteristics. These characteristics are the locked rotor kVA code, design, and insulation class. Make certain that the letter code taken from the name­plate is the design letter and not the kVA code or insulation class. 

Design comparisons 
We will use classifications of starting current, as well as torques, to compare the different designs to each other. The magnitudes of various torque and current values versus rated values vary with horsepower rating and speed. Therefore, rather than attempt to quantifynumerous horsepower and speed ratings, we will simplify the ratio of the parameter value versus rated by using relative terms such as medium, high and very high. 

Classifying locked rotor current magnitude as mediumstarting torque and current characteristics much more so than or high, design A has high the stator winding design. In this article we will describe some key characteristics associated with the design letter designation, and physical differences in the rotors of the different designs. 

The starting speed-torque charac­teristics of many three-phase squirrel cage motors manufactured to NEMA (National Electrical Manufacturers rated values for a motor all apply at rated voltage and frequency. Locked rotor current is the steady state line current with the rotor at standstill. Locked rotor torque is the minimum torque that a motor will develop at standstill for all angular positions of the rotor. 

Continuing with the definitions, pull-up torque is the minimum starting current, whereas designs B, C and D have medium starting current. NEMA standards prescribe limits for locked rotor current for designs B, C and D. However, there is no standard limit for the locked rotor current of a design A motor, though it is typically up to 20% higher than the limits for designs B, C and D. 

This characteristic could lead to tripping of motor starting protec­tion devices if a design A motor replaces a design B or C motor. The potentially higher design A locked rotor current could be the cause. 

Starting torque and current 
Locked rotor torque can be classified as medium, high or very high. Design A and B locked rotor torques are comparable, rated medium. Design C has higher locked rotor torque than A or B, and can be classified as high. Design D motors have an excep­tionally high starting torque and can be classified as having very high locked rotor torque (see Figure 2). Design A and B motors are suited to loads such as pumps, smaller fans, unloaded conveyors, unloaded compressors and low inertia loads. Design C may be needed for high inertia loads such as larger fans and blowers, loaded conveyors, and compressors that start partly loaded. Very high inertia loads and loads that vary greatly are often best suited for Design D. Examples of these are punch presses, hoists and elevators. 

While a motor may have the horsepower rating, actually the required torque, to operate a load at rated speed, it may not necessarily be able to start the load. For example, a compressor that must start with some head pressure on the discharge will probably need the high starting torque associated with design C. Inadvertently installing a design B motor in place of a design C on such a compressor could result in the motor not being able to accelerate to full rated speed and either tripping overload devices or damaging the motor, or both. 

Pull-up or minimum torque of design A and B motors are compa­rable and would be rated medium. Design C pull-up torque can be rated high, and design D does not 
have a pull-up torque rating. That is because the speed-torque curve of design D motors peaks at locked rotor and gradually diminishes as the rotor acceler­ates as illustrated in Figure 2. There is no minimum starting torque rating because the “mini­mum” value coincides with full load, i.e., there is usually no dip in the speed torque curve. 

The breakdown or maximum torque of the design B motor, the most commonly used design, can be rated medium, and designs A and C rated high. The maximum torque of a design D motor occurs at locked rotor, and would be rated very high in comparison to the other designs. The design A breakdown torque level may be needed in applications such as a molding machine, where the extra torque is used to make certain the mold is completely full before the machine pressure is released. 

Note: An interesting side point is that when purchasing some consumer items such as vacuum cleaners and bench grinders we may see the motor rated as “maximum horsepower” or “peak horsepower.” The manufacturer in those cases is rating the motor based on the breakdown or maximum torque. For example, if the motor was actually 1 hp, but had 200% break­down torque, the manufacturer would rate it “2 peak hp”. 

Construction comparisons 
Having described the different designs, and how they are related to starting torque and current, a question that arises is: What is different in the motor construction for each design? The answer lies in the rotor, most notably in the rotor bar construction. The other parts of the motor, e.g., the stator, do not normally affect the starting torque and current characteristics directly. The term “normal” in the last sentence is a significant qualifying factor. If the stator winding is changed to make it magnetically too strong or too weak, the operating as well as starting characteristics will be affected and will deviate from rated values. 

Generic rotor configurations for the four different designs are illustrated in Figure 3 and explanatory descriptions follow. From Figure 3 it can be seen that, in general, a design A rotor bar shape is fairly rectangular and not as deep as that of a design B. Design B rotor bars tend to be similar to design A bars, but deeper and narrower. The cross section of the design A bar is typically larger than that of design B, resulting in lower resistance (for the same material conductivity) and therefore higher locked rotor amps. The deeper bar of a design B increases the resistive effect to being magnetized (reactance) and results in a lower breakdown torque compared to designs A and many design C motors. 

Rotor cage 
The bar shape for design C is effectively a double bar, or indepen­dent bars if there are two sets of bars and end rings. Incidentally, the combination of bars and end rings are termed the rotor cage. Referring to the example bar in Figure 3, the top part of the bar is of smaller conductor area compared to the bottom part. 

At startup, the rotor has line frequency current induced in it, and the current flows almost entirely in the top part of the bar. The reason for this is an effect termed “depth of penetration,” the higher the fre­quency, the more the magnetic flux is concentrated near the surface. The frequency in the rotor is inversely proportional to speed. Thus, at startup with the rotor not turning the fre­quency is line frequency, typically 50 or 60 Hz; and when the rotor attains running speed the frequency is typically only about 2-3 Hz. 

At startup the current flows almost entirely at the tops of the bars and as the rotor frequency is reduced with increasing speed, the current pen­etrates deeper into the bars. Because the starting current crowds into the top cage, it is the top cage that determines the starting characteristics. When the rotor is up to speed, the lower cage becomes dominant as it has the larger area and higher conductivity. Some rotors may have two separate sets (cages) of top and bottom bars, or there can be a physical “bridge” between them as in the case of die cast rotor construction. The bridge between top and bottom bars has very little effect on rotor performance. If the bottom cage is separate, it will have bars of much higher resistivity than the upper cage. 

The other rotor bar configuration is design D. The bars are usually located near the rotor core surface and may be wider than they are tall. Compared to the other designs overall bar area, the bar area of design D is generally smaller. The smaller size can be accommodated because design D motors are usually applied in short-time duty applications, i.e., they are not normally used for continuous duty. The material is relatively low conduc­tivity, i.e., high resistance, which results in low starting current and high starting torque. 

The difference in rotor bar shape and conductivity has been shown to have an enormous effect on starting torque and current. That is the reason that rebarring rotors must be done with care.

For example, changing the bar material from 50% conductivity die cast aluminum to 100% conductivity copper would reduce the rotor resistance. 

That would then probably cause starting current to be unacceptably high, and starting torque to be inad­equately low. Inserting fabricated rectangular bars in the rounded and tapered shape slots of a die cast rotor will inherently leave air space between the bars and sides of the slots. The air space increases reactance, and reduces physical support of the bars. The end result will almost certainly be a motor with incorrect starting torque and current, and that may fail prematurely due to the lack of physical integrity of the bars in the slots. 

Categories: Design/theory/application

Tags: Motor design

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