Motor Load Matching – The Motor
Using motor application parameters to support root cause analysis
There is a machine in your facility whose motor fails every couple of years. Your job is to find out why. Conducting a root cause analysis of the failures is the place to start. You need an understanding of the factors that should be considered in choosing the correct motor for the application so you can tell if the motor is the right one for the machine.
The next several blog’s will attempt to introduce some of the major factors that could be causing the motor to fail. You can check each of these for your application as you search for a solution.
The motor
We need to understand some of the characteristics of motors; torque, horsepower, current, service factor, load profile among others.
- Torque and Horsepower
Motor horsepower is what we think of first, torque is next. The difference can be illustrated by imagining a wrench turning a shaft (figure 1).
Torque is the turning force applied to the shaft. It doesn't matter if the shaft actually turns or not. Torque is measured in foot pounds (ft-lb or lb-ft) and is independent of rotating speed.
Now if we let the shaft turn due to the torque, we introduce speed into the equation. This leads us to horsepower. Horsepower is how fast we are doing work on the shaft. If we double the turning speed while keeping torque constant, we double the horsepower.
By definition, the relationship between torque and horsepower is
- Electric motor torque curves
Figure 2 is a typical motor speed-torque curve. This one is for a NEMA Design B motor, probably the most common in industry. There are several points on this curve we need to understand if we are going to properly select a motor for our application.
Locked rotor torque
Also called Starting Torque, this is the torque applied to the shaft when power is first applied to the motor and before it starts to turn. It's usually expressed as a percentage of full-load torque.
Pull-up Torque
Also called accelerating torque, this is the minimum torque developed during the period from locked-rotor condition to breakdown. It is the minimum torque available to accelerate the load from zero speed to running speed and is a critical selection factor. Usually expressed as a percentage of full-load torque.
Breakdown torque
Breakdown torque occurs at the point where increasing load causes the motor to rapidly decrease in speed. Consider a motor with a brake. As we apply the brake tighter and tighter, a point will be reached where the motor slows and stops. That's breakdown. Expressed as a percentage of full-load torque.
Full-load torque
Full-load torque is that torque necessary to produce rated horsepower at full-load speed.
It's equal to:
Next we’ll look at the various motor current parameters.
Key takeaways:
- Understand the motor’s parameters to match it to the load.
- Horsepower tells how fast we’re doing work.
- A motor has several different torque values.
How do you select a replacement motor in your plant?
Do you consider these parameters?
Use the comments section to let us know!
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