Drives for Web Handling

By Clarence Klassen 
 
Most induction motors used in VSD applications including Web Handling are constructed with a “Squirrel Cage” rotor. The conductors in the rotor have the shape of the rotating hamster cage exercise machine as in “The wheel is turning, but the hamster is dead."

The rotor conductor geometry is determined by cutouts or slots in the steel laminations which are stacked on the motor shaft to form the rotor. Once the laminations are accurately stacked and pressed, the rotor conductors are cast in place. Usually, aluminum is cast into the rotor. High-efficiency motors may cast copper. The casting includes rings connecting all the conductors at each end. Casting conductors is a very cost-effective way of forming them in specific geometries. See the attached diagram showing a slotted rotor lamination and the resulting squirrel cage.

The steel laminations are designed for high resistance and have little effect on the electrical conduction of the rotor.

The shape and cross-sectional area of the conductors in the rotor determine the torque available to the motor at various levels of rotor slip. This affects starting and steady state torque. Once the conductor design is settled, the characteristics of the motor are determined. That includes the important variables including cost, starting torque, inductance, losses, and difficulty of manufacturing.

Rotor slots have a great deal of variety.

The link to the Wolfram Demonstrations Program AC Induction Motor Rotor Design shows the torque curve with changes made to the rotor resistance, reactance and blocked rotor voltage. Watch the Web Preview.

By Clarence Klassen 
 
I occasionally encounter Adjustable Speed Drives (ASDs) running with only the basic drive setup. That includes motor nameplate data and scaling for correct RPM at top line speed. This is not adequate for web handling. We need to have our drives tuned. Drive vendors try to make this easy for the commissioning personnel, but the procedure is different for each vendor.

Every drive has a number of regulators which must be tuned in sequence. AC ASD drives typically have a torque loop, a speed or velocity regulator and may have a tension or position regulator.

The drive vendor usually specifies the requirements for the torque loop. Motor nameplate data is required. In most cases, a torque loop tuning procedure makes accurate measurements of the motor characteristics affecting torque and uses these in tuning the torque regulator. This may be called “motor identification," “motor tests," "vector tuning," “motor data optimization," etc. This test applies power to the motor but generally does not rotate it. The torque loop is tuned for fast response or high bandwidth usually measured in radians per sec (rad/sec). Many VSD’s provide 300 rad/sec or higher response for the torque loop.

The speed or velocity loop is tuned next. The desired response or bandwidth may be selected for the application. We want a fast response. This may be 100 rad/sec for servo applications, 5 rad/sec for most web handling equipment, and 1 rad/sec for large paper mill equipment. Autotune for the speed loop generally accelerates the motor to measure the load inertia. Before running this test, enter the maximum safe speed, accel rate and decel rate, and torque limits. Keep the area safe with caution tape and watchers since the normal stop buttons may not be functional while tuning.

The tension regulator is tuned last. Generally, there is no autotune for the tension regulator – it must be tuned manually. The line has to be threaded to tune the tension regulator. The response of the tension loop must be slower than the response of the speed regulator unless special control engineering calculations have been made.

Tuned drives will provide much better tension control for web handling.

By Clarence Klassen 
 
Which is the best soft drink? Which is the best car? These are really questions with no answers.
If the product is still on the market, there are satisfied customers.

The subjective answer will need to be based on price, performance, delivery, and service.
We know a lot of the brand names (in alphabetical order):

• ABB
• Danfoss
• GE
• Hitachi
• Mitsubishi
• Parker
• Rockwell A-B
• Siemens
• Telemecanique
• Toshiba
• TMEIC
• WEG

I am sure there are many others. I have worked with most of those listed above. The surprising thing is that all of these vendors have drives suitable for web handling. Of course, we need to procure the vector, sensorless vector or servo ac VSD offerings for web handling applications.

Predictably that the larger companies charge more for their product. In exchange, they may offer more in the way of service and support and range of product (small to high power, communications options, training, etc.). Large customers are comfortable purchasing from large suppliers. Is post sales assistance provided in purchasing and configuring your drive?

Performance for a drive system may mean different things. For a continuous process, the most important item is high availability (long mean time between failure). For a roll to roll line, cost and excellent tension control will be most important.

Fast delivery is important for all suppliers.

Service means people. Experts in the industry cost $2K-$3K per day (more than the cost of a small drive). Are you happy with your sales and service? If you are, continue purchasing the drives you are using. If not, get a good reference in your region before switching suppliers.

By Clarence Klassen 
 
Most motors used with Variable Speed Drives have some degree of shaft current induced in normal operation. That is the spinning rotor in cutting lines of flux, induces some voltage on the shaft. Stray capacitance puts voltage onto the shaft each time the VSD switches. If a loop through ground exists, a shaft current will flow. This happens with dc motors, and ac induction motors. Although I have not verified this, I suspect Permanent Magnet motors are not immune. About 1% of drive applications have damaging shaft currents.

The shaft current typically passes through one or more of the bearings (usually a motor bearing) to ground. Each pulse of the VSD (4000 Hz or higher for ac drives) pits a few atoms of the bearing race. Because the timing of the pulses and the speed of the motor is usually constant, the bearings repeatedly pit in the same spots. The damage manifests as a unique pattern (fluting) on the bearing race and can be readily identified after the damage has occurred.

Symmetrical design of the rotor is the strongest tool in reducing induced shaft current. In the 1980s, we were working with one 1000-HP dc motor which had severe shaft current problems. Tests with a cable and clamp-on ammeter drew an arc and we read shaft current over 100 Amps at low frequency. Needless to say the bearings failed within a week. In this case, a design flaw put rotor stabilizing bars on only one of two armature windings, resulting in an asymmetry.

The higher frequency shaft currents due to VSDs have a number of solutions. Special cables are available for VSDs. These have three grounding cables symmetrically arranged with the current carrying cables and surrounded by a properly grounded shield. The motor frame should be properly grounded (high frequency). I have seen one instance where motor cables heated all metal through which they passed to the point thin metal burnt away. Shaft currents were also a problem here.

Other steps that may be required in problem situations are:

• Insulate the motor coupling
• Shaft grounding brush
• Insulated motor bearings with grounding brush on the driven end of the motor
• One insulated motor bearing with grounding brush (location specified by engineering)

A failed grounding brush or one installed on the wrong end of the motor can move the problem from the motor to the machine bearings.

By Clarence Klassen 
 
I recently had a call about a winder shutting down due to oversized rolls. This happened only when orders for large rolls were run. The rolls were not actually oversized. The diameter calculator was calculating incorrectly.

Before discussing the cause and cure for the incorrect diameter, I want to mention some of the other problems associated with incorrect diameter calculation.

• Accurate diameter calculation is required to determine the correct spindle RPM.
• Accurate diameter calculation is required to set the motor torque which establishes tension.
• Accurate diameter calculation is required to provide the correct inertia compensation for speed changes.
• Accurate diameter calculation is required if rolls are sold by diameter.

These other problems had been observed but were given a lower priority than the winder shutting down due to oversized rolls.

Most diameter calculators in web handling divide line speed (measured with a tacho) by spindle RPM. On this turret winder, both spindles were showing incorrect diameter. It is unlikely that both spindles would read incorrect RPM. The problem was that the web was slipping past the traction point used by the diameter calculator. In fact, the problem turned out to be a stripped gear.

I cannot stress enough the need for accurate roll diameter calculation in web handling.