Variable Speed Control – by Randy Baker

By Randy Baker

In today’s security-conscious environment, automating larger and heavier doors and gates has become an increasingly common occurrence for many dealers. Several manufacturers are now marketing anti-terrorist crash-deterrent gate products that are necessarily extremely large and heavy. Military bases, utilities and countless federal, state and municipal government sites are candidates for anti-terrorist security upgrades using these heavy crash-deterrent products. This trend, coupled with a related desire for increased gate speeds, puts a heavy burden on the operators used to move these loads. One very effective solution to automating heavy doors and gates is the use of variable speed operators that require significantly less force than a fixed speed operator to smoothly control the motion of a heavy door or gate. To thoroughly grasp the mechanics of this solution requires a brief understanding of the physics involved.

Physics 101

When a challenging job crops up, it is often remarked: “It’s not rocket science,” implying that what we do on a daily basis in the business of automating doors and gates is somehow far less scientific than the daily routine of the rocket scientists in Houston or Cape Canaveral. In reality, many of the same classical physics equations are equally useful in designing an effective door or gate automation system as an inner-planetary rocket voyage. The scientific foundations for modern rocketry, as well as for gate movement, were laid by the great English scientist Sir Isaac Newton.

Newton organized his understanding of physical motion into three scientific laws. Newton’s Second Law and its derivatives are the ones we are most concerned with for this discussion. It states: The acceleration of an object is in the direction of the net external force acting on it. It is proportional to the net external force and is inversely proportional to the mass of the object. Mathematically it’s defined as F=ma.

What this equation means to us is if the mass of the gate increases and the acceleration stays the same, the force required will increase proportionally. From this basic equation, physicists have developed considerably more sophisticated equations to explain and describe the characteristics of physical motion. One such equation that illustrates the huge advantage of using variable speed operators on large gates is the Linear Impulse Formula. Mathematically it’s defined as:

Force x Time =(Weight/Gravity=Mass) x (Velocityfinal – Velocity initial) This equation can be simplified for our example purposes to: Force = (Mass x £Velocity) / Time

Assume we are automating a 2,000- pound gate that will move at 2 feet per second. Most gate and door operators on the market today use fixed speed A/C induction motors. These motors are designed to spin at a single designated speed to produce optimum torque. When power is switched on, the motor spins up to the designated speed within a fraction of a second. If we apply our force equation to this operator, we can determine that it will require 1,242 pounds force to move this 2,000-pound gate at 2 feet per second.

Gate Weight = 2,000 lbs Gravity Constant = 32.2 Final Velocity = 2 feet per second Initial Velocity = 0 Time = .1 second 1242 = (2000/32.2) x (2-0)/.1

Now, take the same scenario using a variable speed operator designed to ramp the speed of the gate up from 0-2 feet per second over a 3-second time interval instead of the tenth of a second with the fixed speed operator.

Gate Weight = 2,000 lbs Gravity Constant = 32.2 Final Velocity = 2 feet per second Initial Velocity = 0 Time = 3 seconds 41.4 = (2000/32.2) x (2-0)/3

Because of the exponential relationship between velocity and time, this small increase in time has a huge effect on the force required to achieve the same velocity. By allowing an extra 2.9 seconds to achieve final velocity, the force required is reduced by 97 percent!

This dramatic decrease in force has two significant advantages for the gate system. First, and perhaps most significant, it greatly reduces the shock load on the system, which means less wear and tear on the gate and the operator resulting in longer life and less maintenance. Second, it reduces the amount of force required to move a large, heavy door or gate, allowing a standard A/C induction motor to be used in most applications.

Making the Choice

Now that we have shown the exponential advantage of variable speed operators in moving large gates or doors, the next logical question is what options are available to implement variable speed characteristics in door and gate applications. Many gate dealers instinctively think of hydraulic operators when faced with large or heavy gate installations.

While hydraulic operators do often incorporate variable speed capabilities using valves that restrict the flow of hydraulic fluid, it is by no means the only way to realize the advantages of variable speed. A select group of operator manufacturers are using an alternative technique for achieving variable speed control of doors and gates that’s rapidly gaining popularity. This technique involves the use of a variable frequency drive controller coupled with a polyphase A/C induction motor in a standard mechanical reduction chain-drive operator.

The use of variable frequency drives is not a new technology. It has been used successfully for decades in the process control, HVAC and material-handling industries to control pumps, blowers and conveyor drives. Applying this proven technology to variable speed gate and door operators offers several distinct advantages over competing variable speed technologies.

One of these advantages is simple economics. Because these operators use standard A/C induction motors and mechanical reduction and drive components, they are considerably more cost-effective to purchase and install than hydraulic operators. The modern variable frequency drives are very sophisticated and extremely reliable. They’re completely programmable, which allows each installation to have a customized speed profile and ramp-up and ramp-down characteristics.

The poly-phase motors used with the variable frequency drives are also universally considered to be the most practical and simplified electric motor designs available. The rotating magnetic fields used in these motors eliminate the failure-prone start windings and auxiliary switches found in single-phase induction motors.

Variable speed operators offer considerable advantages—especially when automating large or heavy doors or gates. By accelerating the mass over a few seconds rather than in a fraction of a second, substantially less force is required; therefore the gate system is subject to substantially less shock load. This results in longer life and less maintenance for the gate and operator as well as the ability to smoothly move the mass at considerably higher speeds.

While several methods for achieving variable speed are possible, the flexibility, simplicity, reliability and cost-effectiveness of using variable frequency drives in door and gate operators makes this technology a great option.

This article reprinted from Professional Door Dealer