[Big Caveat: I’m going to take a stab at answering these questions, but I’ll make sure I eventually find someone with more electrical and controls talents to contribute to this section. -tjw]

What is a drive?

Definition: A drive is an electronic device to provide power to a motor or servo.

You can divide the world of electronic motor drives into two categories: AC and DC. A motor drive controls the speed, torque, direction and resulting horsepower of a motor. A DC drive typically controls a shunt wound DC motor, which has separate armature and field circuits. AC drives control AC induction motors, and—like their DC counterparts—control speed, torque, and horsepower.

(From “What is a drive?” web page of IEEE Kansas City chapter.)

How is motor size determined?

Motors are sized for torque and speed in units of horsepower or watts (usually kilowatts). James Watt invented the unit of horsepower to describe the work over time of ponies lifting coal. From a rough estimate, he decided a good rough estimate was a single horse could haul 33,000 pounds of coal up one foot in one minute. This estimate continues in use today in any motor’s HP rating. To find power in the metric units of watts or kW, simply divide the work (N-m) by the time to get it done in seconds (1 W = 1 N-m/s).

To calculate your power needs, there are two approaches.

  1. Use force and velocity (in feet/min or m/s)HP = (Tw)(V)/(33000)     or    Power, P in watts = (T in N/m)(w in m)(V in m/s)

    T = Tension in lbs/in or N/m
    w = Width in inches or meters
    V = Velocity in feet/minute or m/s

  2. Use torque and speed (in rpms or rad/s)HP = (M)(n)/5250       or    Power, P in watts = (M in N-m)( w in rad/s)

    M = Torque in ft-lbs or N-m
    n = Speed in revolutions per minute (RPMs)
    w = Speed in radians per second

I recommend using the second approach since tension isn’t the only need for torque in web handling. The torque of a web handling drive point is the tension differential x radius + inertia x acceleration + torque losses in transmission or from rolling resistance.

For a winder or unwinder, the tension differential is the full web tension, a negative value for unwinding.

In an intermediate drive roller (what I usually call a pull roller), the tension differential is the difference between the upstream and downstream tension setpoints. This can be a negative or positive value. If the upstream tension is higher, the pull roller is in power mode, assisting the downstream tension to oppose the higher upstream tension. If the downstream tension is higher, the pull roller is in regeneration mode, assisting the upstream tension to oppose the higher downstream tension. If the upstream and downstream tension are equal, the motor has little work to do except overcoming rolling resistance.

Motor size can also be viewed in terms of actual physical size. One of my expert controls resources once told me that motor physical size (and cost) is more a function of torque than power. This is one reason many web processes use a speed reducer; to increase motor efficiency and reduce costs. However, there may be unintended consequences of limiting your control options (forcing the use of speed control over-torque control), increased torque losses, increase mechanical complexity, and less responsive performance.  He argued that yes, this will increase motor cost, but also improve performance. (More on this later.)

What type of motors are used in web handling?

Many types of motors are used in web handling, including both DC and AC motors. One feature that any web handling motor should have is four-quadrant operation, which means they can run in any combination of rotating in forward or reverse and in power or regeneration mode.

Is it better to control motor speed or torque?

For pacer sections, motors are speed controlled. In open-loop tension control, motors are used in either speed (or draw) or torque control depending on what is best for the process. In closed-loop tension control, there is a growing debate on the best approach.

For many years, I have been taught that most multi-zone processes with closed-loop tension control adjust motor speed, trimming +/- 5-10 percent of the pacer’s speed reference. More recently, I have heard good cases made that new drive technology works better in torque control mode. The central argument is that motors are torque generators. Anytime you use a motor in speed mode, you are adding a level of complexity.

How fast does a tension loop respond?

When I first asked this question in the early 1990s, the answer was about 2 Hz or updating 2 times per second. This goes along with how the closed-loop control is set up. The 2 Hz answer came from a consideration of the three nested loops in tension control. The first loop is the motor’s internal torque loop, operating at 200 Hz. The next loop is the motor speed control. Since the speed loop is outside the torque loop, it must be slower, running around 20 Hz. Lastly, the tension loop is outside the speed loop and must run slower yet at 2 Hz.

Since the early 1990s, motors and drive technology has changed and I’m told tension loops are running at closer to 20 Hz or an update every 50 ms.

Is 2 Hz or 20 Hz fast enough? Consider the tension upset from an out-of-round roller or unwinding roll. If the roll or roller diameter is 150mm (6″), the circumference is about 0.5 m (20 inches). At 60 m/min (200 fpm or 40 in/s), the upset frequency is 2 Hz. At 600 m/min (2000 fpm), the upset frequency is 20 Hz. Like any control loop, there is a high-frequency limit where the control loop is too slow to respond.

Moral of this analysis: Don’t expect your tension control loop to eliminate tension upsets from out-of-round roll or rollers, bad bearings, gear teeth, or other high-frequency problems.

How is a tension control loop tuned?

Instead, let me talk about some of the issues here. I’m most familiar with the speed-trimmed tension loops that are probably the most common closed-loop tension control method in use today.

These systems get tuned with a PI loop (usually no D or derivative in the PID loops). The proportional variable is a function of the web and tension zone length. If you want to increase the tension from 1 PLI to 2 PLI, this may represent a small speed change (0.01 percent in Aluminum), a modest speed change (0.1 percent in many papers or polyester), or a significant speed change (1 to 5 percent in stretchy film and nonwovens). So the tension loop must be set up for a given web spring constant. If the web is set up and tuned for one material, you may find it is unresponsive or hyper-responsive for another material. In these wide ranging processes, the solution is either to change the tension loop tuning for extreme product changes or switch to a torque-based tension control that doesn’t care about speeds.