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Constant torque vs variable torque motors

Constant torque or variable torque – The answer depends on the application.

Inverter duty motors typically have both constant torque and variable torque capabilities. To understand how a motor operates, we will first need to review what makes a motor inverter duty.

A motor’s inverter duty capability is directly related to how it is built. First, and possibly most importantly, is the insulation system and process included in the design and manufacturing processes. The components inside the motor, which are not seen from the outside, are very important to the overall inverter duty capability. Withstanding voltage peaks and minimizing or eliminating air pockets further impacts the overall performance. Typically, copper is used to generate the magnetic field, and this material needs to be spike resistant to withstand voltage spikes that commonly occur when inverters (also known as variable speed drives – henceforth referred to as “drives”) convert sine wave power to inverter power. The copper wire forms the motor’s windings, which are installed into the stator and typically have insulation paper lining the slots and separating phases. This prevents phase-to-phase shorts, which could cause damage to the motor performance. Finally, an insulation system is completed by applying varnish to the completed stator. This is the final critical step to mitigate the generation of corona, which is an electrical discharge caused by air pockets near the windings.

There is another portion of the equation that needs to be discussed, which is the turndown or speed ratio. When a motor operates on a drive, the drive is typically feeding voltage and frequency to the motor. As the voltage decreases, the frequency decreases, which means the motor speed is reduced. The inverse is also true. We will come back to this topic after we discuss some motor design considerations.

These two pieces of information are important when we review the capabilities of the motor; however, there is one more that is very important: Will the motor be able to cool itself during operation? That answer is directly related to the motor enclosure. There are multiple enclosures available, but they fall into one of two categories: open and enclosed frames. Typically, open frames have slots or openings in the frame to allow ambient air to cool the stator, which allows the heat to be dissipated and the motor to operate in an acceptable temperature range. Enclosed motor frames are totally enclosed from the environment. Some enclosed frames do not have any cooling methods (totally enclosed non-ventilated - TENV), while others have fans mounted on the shaft (totally enclosed fan-cooled - TEFC) or external blowers to push air across the motor to provide cooling. Most common is the TEFC, which require the motor shaft to be rotating at a specific speed to provide appropriate cooling to dissipate the heat generated naturally inside the motor. If additional cooling is needed for the motor to perform to specifications, external blowers can be added to push air over the motor to keep the skin temperature lower, which decreases the internal temperature of the motor as well.

Now that we have discussed the internal components and the cooling options for motors, let’s move to the focus of this post: What is the difference between constant torque and variable torque?

Variable-torque applications
These applications are most often centrifugal loads, such as pumps, fans and compressors, that take advantage of affinity laws. As the motor speed is reduced, the torque required in the application is reduced, and vice/versa. Affinity laws, regarding motors, are fairly simple:
Flow is proportional to shaft speed
Pressure is proportional to the square of the shaft speed
Power is proportional to the cube of the shaft speed

That means if the motor speed is reduced by 30 percent...
The volume is reduced by 30 percent
The pressure is reduced by 50 percent
The power is reduced by nearly 65 percent

Therefore, there is a large amount of energy consumption reduction potential in variable-torque applications. When a fan or pump is running at full speed, the system is inefficient. Running the fan or pump at the optimum design speed, according to the manufacture’s curves, can save nearly 40-60 percent of energy costs.

Constant-torque applications
These applications, such as conveyors, mixers and extruders, require constant torque when the speed is reduced. There are different ranges needed, but the application defines the range. The common phrased used is “turndown ratio,” or the ratio between the standard base speed and the reduced speed. Whether you talk in speed (RPM) or frequency (Hz), the ratio is calculated the same way

Standard/reduced value = turndown ratio
60 Hz / 30 Hz = 2:1 ratio
1750 RPM / 1450 RPM = 1.2:1 ratio
60 Hz / 8 Hz = 7.5:1 ratio

Motors have different capabilities. Many motor manufactures can provide this information.

To summarize, the differences between constant-torque and variable-torque motors are as follow:
The motor insulation system must be adequate to operate on a drive
The motor must have the appropriate cooling to operate as needed at the reduced speed
•The application drives the motor requirements. There are many different types of motors available to accommodate nearly any requirement

Contact your local ABB representative for assistance or additional information.



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