kenneth edmiston said:
Rpm of the motor is 650/1950 at 500v arm and 150v field.
If this 650/1950 RPM is from the nameplate, 650 RPM shaft speed will be at full armature voltage at full shunt field current, and 1950 RPM at full armature voltage at minimum (fully field weakened) shunt field current.
650 RPM is "base speed" (full armature voltage at full shunt field voltage). Up to this point in the speed range a motor operates in a 'constant torque/variable power' regime. Multiply nameplate armature full load current by armature voltage from 0 to 500V, and you'll see available motor power follows armature voltage. To this point, shaft speed also follows armature voltage.
At base speed and above, armature voltage is limited to maximum nameplate value (500V). In order to run faster than base speed it is necessary to weaken the shunt field. This regime of operation above base speed is called 'constant power/variable torque'. If the shunt field controller is operating properly, shunt field voltage (hence, field current) will begin to fall off at base speed (650 RPM/500V armature), and continue to drop until the motor shaft is turning at the maximum speed (1950 RPM) specification.
Chicago Slitter has a brief overview of a typical recoiler application.
https://chicagoslitter.com/resources/tech-tips/recoiler-motor-drives/
kenneth edmiston said:
Summary: DC Stab Shunt Motor. HELP please!
I am uncertain if the drive contains field loss safeties so I’m worried if it looses shunt field the motor will take off down the highway.
Double and triple check this. I'm not aware of any commercially available DC drive designed for field weakening control that doesn't have safety interlocks, and for the reason you've stated. If shunt field excitation drops too low at low shaft loading, the motor will overspeed, and quite possibly spin so fast that it rips itself apart.
What make and model drive is used? Can you post a legible photo of the motor nameplate?
kenneth edmiston said:
I’ve verified another motor in a separate line is wired identical (open series field)
You've indicated this is a stab shunt motor, which means the series (stab; speed stabilizing) field was designed to provide just enough additional magnetic flux to overcome speed reduction caused by increased shaft loading.
https://chicagoslitter.com/resources/tech-tips/recoiler-motor-drives/
I've never worked on a recoiler, but if the stab shunt isn't used on either the properly functioning or problem-causing installation, it could be the speed stabilization it provides acts at cross-purposes in this application.
By 'open', do you mean it isn't connected? If it were wired into the armature circuit but open, the motor wouldn't run at all. If disconnected, are the stab field leads taped off? If they can come into contact with one another (either directly, or through conduit box steel) I'm not certain what would happen, but doubt it would be good.
kenneth edmiston said:
I was also told that the motor is dropping rpm uncommanded which led me to believe the drive is sending incorrect outputs, or the load was too high and torque too low.
Is speed control open loop (armature feedback) or closed loop (tach/encoder feedback)?
If open loop, shaft speed sags as mechanical load increases, which is why closed loop is often preferred.
If closed loop, tach feedback ought to maintain set point speed. If it doesn't, the question usually becomes "what other limits is this controller hitting up against?" Armature current limit is often implicated.
Another possibility is improper loop gain settings. A typical DC drive controller has two loops:
1. Speed loop (tach feedback compared against speed command)
Also known as "major loop", and "outer loop".
2. Current loop (current feedback compared against current command)
Also known as "minor loop", and "inner loop".
It depends on the manufacturer and when the drive was designed (digital drives usually have full PID (Proportional-Integrating-Derivative) options; older analog drives were often PI only) but if gain on either loop is set too high the controller will become unstable. Gross instability is rather easier to spot due to continually large shifts in output with little or no change in command. If gain is marginally too high (particularly current loop gain), the drive will run without excessively bad behavior when presented with low magnitude transient overloads, but commands massive current output on larger transient overloads.
kenneth edmiston said:
... constantly blows line and load fuses. It is on a DC drive. The drive has been replaced and the motor has been rewound.
If it was blowing line fuses only it might suggest misfiring SCRs, or other power bridge-related issue. Load fuses clearing suggest high armature current.
Nailing this down is, at least in part, an exercise in minutiae.
First, get the drive and motor nameplate data, and find the drive and motor installation and operating manuals. It is difficult to troubleshoot a problem if you don't know what the manufacturer documented their equipment is capable of doing.
Second, measure, measure, measure!
This sounds like an intermittent problem, so if a suitable data recorder is handy (one capable of handling high voltage inputs; a 0-10 V recorder input connected to a 500V armature is a bad idea) it's a good time to set it up. If the drive features a communication link, monitor and log relevant performance data.
Suggested monitoring points
- Speed command
- Speed feedback (tach)
- Armature voltage
- Armature current
- Shunt field voltage
- Shunt field current
Let's say high armature current is involved. There are many ways high armature amps can occur. A few, off the top of my head:
- physical load with intermittent binding (a bearing just beginning to seize, etc.)
- uneven feed into recoiler (metal pulling taut upon occasion)
- tach to motor coupling slipping (causing motor to speed up, pull metal taut, and trigger armature overload).
- loose wire connection in shunt field (suggested if rapid and intermittent field voltage increases are observed in conjunction with matching reductions in field current).
- loop gain(s) set too "hot".
Another option, and a good one, is to call in a field tech from the recoiler manufacturer, and have him or her dig into the issue. Provided they have worked in the trenches for awhile they'll be experienced with both common and uncommon problems encountered in this application, and will make sense of it rather quickly. Have your best in-house tech unobtrusively bird-dog the factory service tech to follow what they are doing and pick up pointers (stress "unobtrusive" - nobody thinks straight if their elbow is continually joggled).
I'm assuming proper drive setup (particularly, loop gains) are critical in this application, and a good factory tech ought to rapidly ferret out any such issues.
