Dos and Don'ts for A-S drive systems

Feb. 1, 2000
Problems with A-S drive systems may be more a result of insufficient application analysis then a fault with the drive. Before you select or replace a drive, review these dos and don’ts to ensure top performance

Two business trends affect the selection of drives in motion control applications: Downsizing and engineering specialization. Neither trend makes your task easier. Instead, these factors make it more important than ever to thoroughly analyze a motion control application before specifying and installing a drive. A thorough analysis can help you avoid some of the common pitfalls we frequently see in the field.

Paying for what you don’t need. Manufacturers are building many new features into drives, such as microprocessors and sensors that monitor the motors that in some, eliminate encoders and resolvers for feedback. But not every application needs the latest drive feature or capability. When vector drives came out, for example, many bought these devices even though the application only needed adjustable-speed operation over a 2-to-1 or 3-to-1 speed range.

Even on applications involving an overhauling (regenerative) load at low speeds, a vector drive may be overkill. (For more on regeneration, see PTD, “Regeneration with AF Drives,” 6/95, p. 21). However, vector drives are best for certain kinds of situations — such as operating at the low end of a motor’s speed range.

New features typically cost 20 to 30% additional, so proper selection is important. As a general rule, before installing the latest products, ensure that operators can operate it, maintenance personnel can maintain it, and the application justifies the expense.

Keep it cool. Often we see situations where the A-S motor is improperly cooled. It runs hot and develops problems that are blamed on the drive. If the motor operates slower than 60% of base speed when it’s nearly fully loaded, it’s best to use a separately powered fan to cool it. The motor fan mounted on the motor shaft will not keep the motor cool enough, because as the speed of the motor slows, the speed of the motor fan slows, reducing the amount of air blowing over the motor. For motors of 5 hp and smaller, an alternative is to select a larger motor or a totally enclosed, nonventilated (TENV) motor.

As long as the motor is properly cooled, it’s possible to obtain zero speed from ac and dc motors. Many ac vector drives can operate at near zero speed with full torque, but there are still limitations.

Mixing drives with existing motors. Drives, especially the newer ones, work best when connected to compatible motors, which are usually the newer, highefficiency designs. Often, however, a new drive is put on an old motor, leading to costly results, because the devices’ compatibility was not examined thoroughly.

An inexpensive ac motor on an ac drive will only offer full pull-out torque in limited situations. If you’re going to operate a vector drive over an extended speed range, you’ll need a motor designed to go with that vector drive. A conventional PWM A-S drive also needs a motor capable of operating with it, because all solidstate A-S drives introduce harmonics into the motor. We recommend that if you’re going to invest in an A-S drive, which can cost anywhere from $400 to $1,000, then spend a few extra bucks to invest in a compatible high-efficiency motor.

At one site, for example, the engineers had an existing gear train that worked fine at a constant speed. They assumed that if they put an adjustable-speed drive on it, it would continue to work equally well. But the old gears used a splash lubrication system. When the gearbox was slowed, less lubricant was splashed onto the gears. The gearbox eventually burned out, and the blame was put on the drive. One of the safest ways to avoid this, and similar types of problems, is to buy the motor, speed reducer, and drive from the same manufacturer.

Calculate all the loads and motion parameters. Don’t guess. Too often we see applications where important factors were not included in the analysis. Varying loads from the driven equipment including the starting and operating torques, the possible effects of new materials within the motion system, and even how the driven equipment is maintained are frequently left out when determining the best drive for an application.

The following example of a motor with a belt-drive illustrates this common error. The problem was not the drive, it was unfamiliarity with changes in belt materials.

Twenty years ago, belts had cords in them that would stretch under tension. Today’s belts use fiber glass for the cords so they no longer stretch. The belt manufacturer indicates how much tension the belt supports, which has no bearing on how much overhung load the motor shaft, or shaft driven equipment, will take.

At one installation, a new fiber-glass belt was tightened to the specifications of the previous stretchable belt. The new belt, though, did not relax. So the motor worked with an excessive overhung load that never eased up. Bearing failure from the load eventually destroyed the motor, however, the users blamed the drive. To prevent such a problem, the engineers and maintenance staff should have looked at the manufacturer’s recommended maximum tension on the belt, then looked at the driven equipment and the motor shaft, and configured the system to produce a usable overhung load.

Loads are not hard to calculate, even in an integrated system. If unsure about the total load, you can use a clamp-onammeter to measure the motor current. This gives a starting point for determining the percentage of the motor’s capacity while operating at one speed.

Size properly. It’s important to look at the total environment of the motor and drive when sizing them. Many drive manufacturers recommend that if you increase the size of the motor, then increase the size of the drive. Most people ignore this advice. A 20-hp drive might work when it’s running at 1750 rpm. At 600 rpm, though, the application may need a 30-hp motor with a 25-hp drive, because it’s now operating under different conditions.

If it is known that a particular size motor overheats in an application, a solution is to oversize the motor. Other solutions include a separately powered fan for the motor or selecting a TENV motor.

But engineers shouldn’t oversize because they are unsure of the load conditions. They should oversize to accommodate certain operating conditions. We’ve had engineers tell us that they thought they could get by with a 5-hp motor, but they were unsure. They knew they could work with a 7.5-hp motor, but just to be safe, they went with a 10 hp. Now they need a 10-hp drive, which increased the cost of the installation. This sizing might be necessary, but it might not.

The real solution is to analyze the problem, not guess.

The importance of training. The companion trends of downsizing and the incorporation of intelligence into drives makes it crucial that operating, production, and maintenance personnel be trained in the drive operation. Most drive manufacturers offer tutorial programs for their more complex systems.

Many drive problems occur because no one taught these people how to interpret the diagnostic messages that may appear on the operator interface monitor. For example, at a three-shift operation plant, a recently installed drive kept shutting down without warning around 5:30 or 6:00 PM every day. An over-voltage message would come on the operator screen, but when the operator was asked what was wrong, he could never explain why the drive kept shutting off. We suggested that maintenance personnel put a recording voltmeter on the system to see what was happening during that time period. Before 5:30, the line voltage was running close to 480 V. After 5:30, the voltage jumped to 535 Vrms, not peak, an 11% overvoltage. It seems all the other nearby plants stopped operation around 5:00 PM. The power surge would run down the line to this still-operating plant, and the over-voltage would cause the drive to shut itself off. It was only defending itself. Properly training the operator, however, would have saved much time and effort because the diagnostic message on the operator screen indicated “Bus overvoltage.” But those words meant nothing to the operator.

Another problem that training can help solve involves fear of job loss. Today’s drives are capable of executing many commands that operators used to give the drives. Unnecessary tweaking, or even sabotage, can be avoided if the operating personnel understand the equipment they are using.

Insist that drive and motor manufacturers supply enough manuals to give everyone a copy.

Stick with the basics. Making major projects out of rather simple situations is another way engineers produce problems. If they stick with the basics, they are less likely to get into trouble. For example, fully understand the speed-torque requirements of the load and the speed torque capabilities of various types of A-S drives, Figure 1. Not understanding these curves generally guarantees a drive misapplication. Be sure to ask enough questions about the application or about why there’s a problem. Be thorough. And remember, each application has to be examined by itself.

Getting help. Sometimes the task of installing a drive falls to an engineer with little drive experience. Even though this engineer has specialized knowledge in another field, he or she may not know how to thoroughly analyze an entire motion control system. But there are resources available, including the manufacturer of the drive and the distributor.

Tom Karones is product manager at Norman Equipment Co., Bridgeview, Ill.

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