Mike Dupuis
Monelco, Ltd.
Our service center location in Southern Ontario, Canada, is considered ground zero for automotive stampings. One of the more common prime movers employed on stamping presses in our area is the dependable eddy current drive. Most units our technicians encounter are in the 50 hp plus category, which makes for a large piece of equipment.
Removal of these units from a stamping press is no small task, so taking steps to more positively identify a problem with the actual drive unit as opposed to the controller or load is highly advisable. Our company provides on-site service and troubleshooting to our customers, which allows us to inspect the application firsthand before removal of the unit is performed.
On-site tests
Our technicians have a few basic on-site checks they perform to help identify the cause of the problem. The tests they employ are typically done on the direct coupled eddy current drives, as opposed to the larger stand alone, belt-driven constant energy system (CES) drives. The following troubleshooting procedures are basic and fairly quick, and could help to reduce the possibility of unnecessary removal of a large eddy current drive.
Fact-finding mission
The on-site service call for our technicians starts out as a fact-finding mission upon arrival. Discussions with the plant maintenance crew are always beneficial, but we’ve also found some of the best information comes from the machine operators. The press operator can be your best source of information such as abnormal noises, speed changes, and differences in machine performance. Their job usually has them operating the press for an 8 hour shift, so they can provide great input when their equipment is “not quite right.”
Basic questions to ask deal with the initial start up of the press. Could they hear the main motor start up normally, or was there a delay in the motor coming up to speed? Eddy current drives are fairly high maintenance pieces of equipment, and contamination is a major failure point of these units. We’ve seen the units arrive for repair with both the drum and spider completely seized to each other from a buildup of contaminants.
The design of the eddy current unit allows for the main motor to be started independently of the eddy current rotor and load, with the motor coming up to full speed before the rotor is excited with DC to bring the load up to speed.
Effects of contamination
If the operating clearance between the drum and eddy current spider has been reduced due to contamination, the resulting drag could cause a delay in the motor coming up to full speed. Another result of the contamination could be the main motor overloads tripping before the motor reaches full operating speed. Sometimes a visual check from the ground is enough to confirm this, as the flywheel of the press could attempt to turn during main motor start up. Contamination to this degree requires removal for cleaning and overhaul in shop.
Another question to ask is about press speed. Does the press appear to slow down after it’s been in operation for a while? While not definitive, this could indicate a mechanical problem with the press, and not a problem with the eddy current drive unit. Mechanical drag after prolonged operation is a fairly common fault and could be mistaken for a problem with the eddy current unit itself. Further testing could include infrared temperature readings of the rolling elements on the press and removal of the flywheel belts for free operation of the eddy current clutch unit.
Verify DC portion
Once it has been determined the main motor is starting properly, the next thing to verify is the DC portion of the eddy current unit. Without DC excitation of the eddy current rotor coil, there would be no movement of the flywheel. Testing of the electronic controller is the first step to verify proper DC output, but intermittent failures might not be present during testing. A simple test to assist in determining if the problem is occurring in the controller or at the eddy current unit itself involves the use of a separate DC power source. A simple DC powerstat would suffice for basic testing of the eddy current drive, as we’ve found the current draw to be usually less than 10 amps on these units.
The DC coil leads are removed from the electronic controller circuit and connected to your separate DC supply, leaving all other connections intact. The press is started normally to bring the main motor up to full speed. After the motor is running at rated speed, DC excitation is applied slowly through the powerstat to the eddy current drive. If the eddy current drive is operating properly, the flywheel will begin to move, up to rated speed, if enough DC is supplied. Keep in mind all safeguards relating to press operation must be left in place to ensure safety of both the operator and the technician. If no movement is observed, further inspection of the eddy current unit and associated wiring is required. A trip up to the top of the press might indicate worn out brushes on the eddy current slip rings (if the unit under test has a rotating DC coil with slip rings). Such a fault is easily corrected on site, enabling the press to be put back in operation with very little downtime. Damaged or broken cabling to the eddy current coil can also be electrically and visually checked prior to the labor-intensive removal of the complete drive unit.
Tachometer signal
The next question is: Does the press speed run away upon supply of DC excitation to the eddy current unit? This can be an indication of a loss of tachometer feedback from the eddy current unit, or a problem with the associated wiring. A quick check involves verifying presence of the tachometer signal from the eddy current unit to the drive controller. The tachometer output is usually an AC linear output, directly proportional to the speed of the eddy current rotor. Loss of this signal will result in the controller increasing the DC output to the eddy current unit until the proper speed is “seen” via the AC voltage feedback. This loss of signal can be defective press wiring, or a defective tachometer on the eddy current unit. If the wiring to the tachometer is satisfactory, the tachometer is the likely suspect.
Has job been changed?
Another question to ask both the operator and set up crew is: Has the job been changed for this particular press? On more than one instance the answer to us has been yes. An eddy current drive not only has a maximum speed, but also a minimum speed at which it can operate properly. Trying to operate the unit below its minimum speed will result in overheating, and possible binding of the drum and spider of the eddy current unit. This binding will not only drive the flywheel to main motor operating speed, but can also do major mechanical damage to both the eddy current unit and driven press components. Corrective action to prevent this includes running the eddy current drive within nameplate parameters and possibly changing pulley ratios to achieve required output speeds.
The end result of the above quick tests and questions might still require the removal of the eddy current unit for proper repair in your service center. We’ve also found that at times we were able to get our customers back into production sooner, with a lot less work and costs involved.
Related Reference and Training Materials
Print