Is your company “going green?” Green has become the latest catchword to include actions such as recycling containers, reducing pollution output, and using products twice — for instance, using old oil for heating. Reducing electricity usage is another method of going green, and a clutch motor is one more tool for engineers to consider in the quest to save energy.
Go green by reducing electricity use
With energy costs rising, companies are looking at ways to save energy as well as the environment. One way to is to stop using “brake motors” (basically, motors with brakes mounted on them) and start using “clutch motors” on many cycling applications. Clutch motors may not be familiar to every engineer, but they are used in a way similar to how brake motors are applied.
First some background: Several manufacturers offer brake motors, which are suitable in certain applications. These motors were originally designed as a convenient solution for applications that presented a safety concern when the motor wasn't running, such as inclined conveyors. When transferring packages along an assembly line with a conveyor that runs up or downhill, it's smart to put a brake motor on the conveyor to hold the product when motion is stopped. Without the brake, the weight of the product may start the conveyor moving even when the motor is off, causing a safety risk.
However, as brake motors have become more common, they are often used in the wrong applications, such as a conveyor feeding into a filler or palletizer. Here, an indexing conveyor may move a box into position, stop to allow filling, and then start to move the next box into position. Instead of letting a regular motor coast to a stop, a brake motor is used to allow faster loading and more precise stopping. The problem is that brake motors don't do this job very efficiently because standard motors are designed to run continuously.
If low cycle rates are required (say, less than 5 to 10 per minute) a brake motor may work well; however, at higher cycle rates (of more than 10 per minute) they become inefficient. Unfortunately, many applications fall into this category, becoming even less efficient as horsepower increases. First, there is a current inrush required to start the motor. Next, the motor rotor (high inertia) needs to be accelerated, so the rotor or disc stack of the brake needs to be accelerated, as well as accelerating the load. The motor contributes maximum power to accelerate the load as quickly as possible; when it stops, the brake must not only stop the load, but also the high inertia of the motor rotor and the brake rotor.
In contrast, let's look at what happens when using a motor clutch — a clutch brake unit connected to a motor. The motor will run continuously, the most efficient condition for a standard electric motor. When it's time to index, the clutch engages, connecting the rotating motor rotor to the load. Stored energy in the spinning rotor is transferred to the load, helping to accelerate it. Reduced current is required, as only the clutch rotor (with low inertia) and the load is accelerated — the same principle used in some hybrid car designs.
Following is a comparison showing the difference between using a brake motor and a clutch motor:
Load inertia: = 2.5 lb-ft2
Motor inertia: 15 hp 254T frame = 1.18 lb-ft2
Brake inertia: 75 lb-ft = .089 lb-ft2
Total inertia to be started: = 3.769 lb-ft2
Acceleration time: = 0.372 sec (approximate time to accelerate motor brake and load)
Load inertia: = 2.5 lb-ft2
Clutch inertia: 03 Posidyne clutch motor = 0.10 lb-ft2
Motor inertia: = 0 (actually a contributor)
Total inertia to be started: = 2.6 lb-ft2
Acceleration time: = 1.00 sec (adjusted for softer start)
Using the formula T = WK2 × rpm change/308 × time acceleration (sec)
Motor brake-only torque requirement (no external load) = 59 lb-ft
Motor clutch-only torque requirement (no external load) = 15 lb-ft
The motor clutch drive reduces the power requirement by approximately 75%, with savings greater for larger hp drives, and smaller in lower hp drives.
This month's tips were provided by Stan Porter of Force Control Industries. For more information, visit www.forcecontrol.com.