Ohio Magnetics Frequently Asked Questions

A malfunction can occur in any of the seven components of a lifting magnet system. Isolating the problem to the defective component must be achieved before the defect can be corrected.

Create a Benchmark to Compare Against

Being familiar with the equipment, its rating, performance, operation and instructions is a "must" before value judgments can be made on the equipment.

Keep a technical folder on the equipment on each crane. The information should include: wiring diagrams; operating instructions; ratings such as power (in watts), current (in amps), resistance (in ohms), lifting capacities, and ground resistance; and a history and dates of prior problems.

When the system is first installed or known to be operating well, record readings of the system for future reference. For example: Record magnet resistance and ground resistance (check against the supplier's test records); record DC voltage at full load and no load; record current, at initial set-up and when "hot;" etc.

System Trouble-Shooting

The following guide assumes that the system includes a voltmeter and ammeter panel. A solution for both rectified and generated power supply is included:

Problem

Defective Area & Remedy

Low Voltage;

Low Current;

(Poor Lift)

General:

Excessive line loss -- wiring too small
Loose connections
Cable reel brushes worn

DC Generator:

Too small -- overloaded
Low speed -- belt slipping or improper pulley ratio
Adjust rheostat

Rectified Power Supply:

Low AC voltage
One AC fuse blown
Defective diodes
Capacity too small -- overloaded

No Voltage;

No Current;

(No Lift)

General:

Broken wire or connection
Cable reel brushes worn

DC Generator:

Worn brushes or broken spring
Broken belts or drive coupling
Open rheostat
Open armature or field winding
Loss of residual magnetism

Rectifier:

Push reset or ON button
No AC voltage
Blown AC fuses
Defective AC contactor
Defective diodes

Low Voltage

System short-circuit or low resistance. Check:

Short or ground in cable reel
Malfunction of controller
Observe for proper opening and closing of devices
Short or ground at the magnet -- check magnet terminals or coil for low resistance or low ground readings

High Voltage;

(Good Lift)

Check DC generator for:

Rheostat adjustment
Overspeed -- excessive engine speed or improper pulley ratio

Fluctuating Voltage or Excessive Voltage Drop from No Load to Full Load;

(Poor Lift)

General:

Excessive line loss -- wiring too small
Loose connections
Cable reel brushes worn

DC Generator:

Too small -- overloaded
Engine too small
Engine compression poor -- requires overhaul
Engine governor defective
Belts slipping, loose or insufficient in quantity

No DC Amperes;

DC Volts OK;

(No Lift)

Controllers:

Check master switch and controller for operation

Cable Reel:

Check for worn-out brushes

Electromagnet:

Check for broken terminal connections
Check magnet resistance for open condition

General:

Check power cables and connections from controller to cable reel and magnet
Check ammeter

Low DC Amperes;

DC Volts OK;

(Poor Lift)

General:

Defective ammeter or shunt

Electromagnet:

Check magnet resistance for partially open (high resistance) and/or grounded condition
Check magnet terminals for high resistance connections

High DC Amperes;

DC Volts OK;

(Good Lift)

General:

Check for grounds and shorts in the generator, controller and cable reel

Electromagnet:

Check for low resistance, (partially shorted) coil and/or terminals

Check ground resistance from each terminal to ground for a short- circuit to ground

Poor Lift Characteristics

General:

Check voltmeter and ammeter for correct readings and isolate the problem with one of the above conditions

Poor Drop Characteristics

Controller:

Check controller for malfunction in the drop circuit. Refer to factory instruction sheets

Electromagnet:

Partly shorted and/or ground coil. Refer to factory instruction sheets

DC Volts OK;

High Current;

(Poor Lift)

Electromagnet:

Check magnet for shorted turns

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Inspecting a Magnet

Maintenance Form Checklist (To print, set top, bottom, left and right margins to 0.5" or 12 mm.)

To determine the condition of a magnet, the resistance of the coil and insulation resistance to ground must be measured.

To be meaningful, these readings should be compared with figures obtained from the factory for your particular magnet, at the ambient temperature in which it is operating.
It can take two days for the magnet to reach ambient temperature after the magnet has been taken out of service. Preliminary readings can be taken immediately, however, and this will indicate if the magnet is grounded, shorted or open. When readings are taken off of a "hot" magnet, consideration must be given to the values for the heated condition.
The coil resistance should be within 5% of the original value when the magnet is at room temperature (25�C). The resistance to ground on a new magnet at room temperature should be 10 M ohms (???) or more. When the magnet is hot, the coil resistance can increase to 70% of its value and the ground resistance can drop to 0.2 M ohms (???).

Obtain a resistance bridge or meter with a 0.01 M ohm resistance accuracy, and a 500 V minimum ground megger.
Unplug the cable reel leads from the magnet and inspect the magnet leads.
If the magnet leads are in good condition, then connet the meter to the ends of the leads for measurement purposes.
If the measurement indicates a faulty condition, remove the leads from the terminals and measure directly at the terminals.
If the measurement at the terminals indicates a faulty condition, then the problem is inside the box: possibly the internal leads, the coil, or the magnet terminals.
Refer to the terminal replacement instructions and remove the coil leads from the terminals at the interior of the terminal box.
Inspect the leads for burned conditions and splice a new section if this appears to be a problem. Measure the coil; if it is faulty, the magnet should be returned to the factory for further inspection and service.
Inspect the terminals and measure the resistance to ground on each one. If one checks or appears bad, replace both of them.

In addition to checking the coil, inspect the magnet for:

worn chain or chain pins;
worn center pole shoe;
loose center pole bolts;
worn, broken or cracked case;
loose outer pole bolts

The chain and center pole can be replaced in the field, but if problems with the magnet case are evident, the magnet should be returned to the factory for further inspection and service.

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Troubleshooting a Magnet Controller

Although the basic functions of magnet controllers are common, the major manufacturers of magnet controllers use different circuitry and techniques to perform these functions. Each has its advantages and disadvantages, and personal preferences and economics usually dictate the selection of a particular product.

The following problems are common to all controllers, and can be used as a guide in troubleshooting your magnet controller:

Problem	Defective Area & Remedy
Controller Does Not Operate	No or low DC voltage -- check power supply Defective master switch on push button station Burned out or open operating coil Polarity reversed -- L+ must be positive, L- must be negative
Controller Operates; No or Low Magnet Current	Main contact tips worn or burned-out Contact tip spring broken Contact shunt burned or broken Lift contactor armature not closing completely
Controller Operates; Load Dribbles from Magnet	Controller and magnet not properly sized Partly shorted magnet Reverse circuit malfunction -- check components Polarity is reversed; CR defective and/or bypassed Diode DM1 is open
Excessive Arcing of Main Contacts; (Billowing Arc)	Open in discharge circuit -- check components, contact and wiring Diode DM1 is open Polarity is reversed; CR defective and/or bypassed
Excessive Arcing of Reverse Contacts	Shorted reverse circuit resistors Contact tips worn or contact spring bad
Reverse Contactor Does Not Operate	Open reverse contactor coil or reverse circuit component Diode DM1 open Defect in push button or master switch (for manual controllers only)
Lift & Drop Come in Together; Resistors Overheat	Diode DM1 shorted

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Checking Diodes

On newer units employing diodes instead of the auxiliary contact, there is no maintenance required. However, should the operation of the drop cycle be faulty, the diode may be defective. The diode can be checked using a digital multi-meter (DMM) with a diode check function:

Make sure all power is off.
Disconnect the two wires from the diode terminals, isolating the diode from the rest of the circuit.
Locate terminals 1, 2 and 3 on the diode; they are impressed into the diode molding.
Place the red lead of the DMM on terminal 1 and the black lead on terminal 2. The reading should be low. Reverse the leads; the reading should be high.
Place the red lead of the DMM on terminal 3 and the black lead on terminal 1. The reading should be low. Reverse the leads; the reading should be high.
If the readings are good, then the diode is OK. If readings show a low reading (or a high reading) in both directions, then the diode is defective and should be replaced.

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Ohio Magnetics, Inc., 5400 Dunham Road, Maple Heights, OH, 44137 - 3687 U.S.A.

phone: 216 / 662 - 8484 or 800 / 486 - 6446, fax: 216 / 662 - 2911 or 216 / 662 - 9526, email: sales@ohiomagnetics.com

standard business hours: 8 AM to 5 PM EST, Monday through Friday