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DYNAMIC HOIST TESTS

Thomas D. Barkand
William J. Helfrich
Mine Safety and Health Administration
Mine Electrical Systems Division
Cochrans Mill Road, P.O. Box 18233
Pittsburgh, Pennsylvania 15213

ABSTRACT

An increasing number of hoists are being commissioned at the request of coal mine operators and MSHA enforcement personnel. The commissioning procedure consists of testing the hoist controls and mechanisms prior to placing the hoist into service. These tests are conducted as a cooperative effort to reduce the potential for hoisting accidents. This paper will discuss the protective devices tested, and the method used to determine the hoist braking capacity using electrical parameters.

INTRODUCTION

Today mine hoists are subjected to severe duty cycles. In many instances they are required to function as production, service, and escape hoists. These multifunction hoists require a complex control system to reliably operate in all of these intended modes. In addition, safety features incorporated in the control system design must provide reliable protection for all of the various modes of hoist operation.

The first step in achieving a safe hoist system, is to design the system properly. Designers should not only evaluate the schematic diagram to determine safety but must also examine the physical layout of the system. This would include such things as interconnection diagrams, limit switch mounting, proximity of the hoist house to the hoist, and susceptibility of the equipment to damage.

After the system has been carefully designed, it must also be installed properly. Installation of the hoist should include careful attention to such details as isolation of low level signal wires from power conductors, accessibility of bypass switches and isolation of critical control circuits.

Once the design and installation have been carefully completed, the total system should be checked out and certified that it performs to the design specifications. The hoists controls should be tested at all operator stations, and recordings should be made of the hoist's performance. These checks should anticipate multiple fault modes to determine if any safety devices would be defeated. Recordings of current, voltage and speed should be made under various operating conditions such as full load, no load, and the various rated speeds. Tests of the normal and emergency stopping systems should be made and recorded at these various loads and speeds. Brake cars on slope hoists should also be tested to verify the brake cars operation and to determine its braking capacity.

Analysis of these recordings and tests should then be made to determine if the hoist performs as it was designed. This documentation should be kept so that when future tests are made on the hoisting system, comparisons can be made to determine if any problems exist.

Because of problems that have occurred in the past with hoists, MSHA's Mine Electrical Systems Division located in Bruceton, Pennsylvania has been providing a commissioning service to both MSHA's district offices and the industry. In addition to the commissioning of new hoists, the Mine Electrical Systems Division has been providing a safety check and test service for existing hoists. This paper details the tests that the Mine Electrical Systems Division performs on hoists and the analysis work that is accomplished.

TEST PROCEDURE/ANALYSIS

Instrumentation

A mobile test laboratory is used when conducting hoist investigations. The mobile laboratory contains shock mounted relay racks that are equipped with instruments capable of measuring and recording the hoist's electrical parameters. The test laboratory is driven to the mine site and positioned adjacent to the hoist house on the side nearest the control cabinets. High voltage test leads are used to connect the isolation preamplifiers in the test lab, to the electrical transducers in the control cabinets. The isolation preamplifiers condition the electrical signals to a level compatible with the input requirements of the recording equipment.

Four hoist motor parameters are continuously monitored and recorded during each test, they are, armature current, armature voltage, field current and motor speed. The armature current and field current are measured using shunts located inside the control cabinets. These shunts are incorporated into the circuit to provide current feedback signals for the regulator. The armature voltage is measured directly across the motor armature circuit at an accessible location. The motor speed signal is available from a tachometer mounted on the motor shaft which supplies the speed feedback signal to the regulator.

Circuit Protective Devices

A thorough study of the hoist's electrical prints is conducted to determine if any control malfunction could occur and to gain a knowledge of how the control system functions. From this study, a test procedure is developed which lists the functional tests of the control system that should be performed. Normally there are three stops used to protect the hoist. They are in order of importance, Emergency Stop, Protective Stop, and Normal Stop. Each of these stops will be initiated by various protective features built into the hoist control. Briefly an emergency stop open the loop contactor and sets the mechanical brakes, a protective stop allows the hoist to complete the trip and then does not allow the hoist to be restarted until the fault has been cleared.

To prevent slack rope from occurring during rapid stops while raising the load, the upward force generated by the moving load cannot exceed the downward force imposed by gravity and the retarding force of friction. This relationship is independent of the load weight, as expressed below in simplified form.

a
g
< sin �  +  u cos �

where a
g
°
u
=
=
=
=
braking deceleration
acceleration of gravity
angle of shaft inclination
coefficient of friction

Next, the emergency stop tests are performed when lowering the load. The emergency stopping times when lowering the conveyance are load dependent. The first test is conducted with a minimal load on the conveyance. Then the load is increased to the rated load for each progressively slower hoist speed, until the maximum load is reached.

After each speed and load test is conducted, the actual braking capacity is calculated from the test data to verify that the drum brake is performing as designed. In addition to the test data, certain mechanical parameters related to the hoist construction and physical layout must be known. The following is a list of the data and parameters necessary to calculate the actual braking capacity.

    1. Motor speed test data.
    2. Weight of the conveyance and rope.
    3. Angle of the shaft inclination.
    4. Inertia of the drum and motor.
    5. Gear ratio of the speed reducer.
    6. Diameter of the drum.
The motor speed signal is converted into the hoist speed by the following expression.

v    =   (RPM)   x   (GR)   x  (piD)       (2)

v  =  hoist speed 
RPM  =  motor speed     
               GR  =  speed reducer gear ratio
  D  =  drum diameter

Once the hoist speed is known, the braking deceleration is calculated from the slope of the speed curve. A typical emergency stop speed curve is shown in figure 1.

Fig. 1. Typical E-Stop Speed Curve

The braking deceleration is,

a   =   delta v         (3)
t      
where              delta v   =   change in velocity            
     delta t    =   change in time

Secondly, the force of the hoist load is calculated. For shaft hoists, the load is simply the weight of the conveyance and rope. For slope hoists, only a component for the load's weight is directed parallel to the slope. The force is related to the weight of the load by the following expression. The derivation of this relationship is given in the following section discussing electromagnetic brakes.

F   =   W sin �      (4)

where    F   =   force applied to the hoist
          W   =   total weight of the load
             ÷    =   angle of shaft inclination

Finally, the rotating interia or flywheel effect of the hoist system needs to be considered. The rotating inertia is a predominant factor influencing the emergency stopping time of the hoist. The values of rotating inertia are usually represented as Wk�, and given in units of 1b-ft�. For mechanical analysis, the value of Wk�is dependent on the rotational base speed. This is similar to the manner that impedance is dependent on the voltage base for power system analysis. The equivalent Wk can be calculated for a different base speed by the following relationship between speed and Wk fFor constant stored energy.

The M-G set overspeed is very difficult to check. It is normally only activated to check to see if it is wired in the control. It is also the least critical of the above overspeeds.

All of the above overspeeds normally work into the emergency stop circuit. Therefore, recordings of these tests should all appear similar to the E-stop button stops.

Overtravel

Detection of overtravel is normally made in the shaft way or slope, by way of mechanically operated limit switches, or in the hoist house by the Lilly or the programmed limit switch. Each of these can easily be checked by running the hoist into the appropriate overtravel. Normally either the shaft way, or slope overtravel limit switch is the first to be activated with the programmed switch and the Lilly following. Therefore, it is necessary to defeat the shaft way overtravel to check the programmed limit switch and then defeat the shaft way and the programmed overtravel to check the Lilly. Checking these overtravels is normally done at very slow speeds and under manual operation. The overtravel should cause an emergency stop of the hoist and recordings of these tests should reflect this.

AC Power Loss

Due to the frequent power outages incurred by mining company properties, the hoist control must be able to respond to these outages and safely stop the hoist. Therefore, one of the dynamic tests is designed to evaluate how the hoist responds to a loss of AC power. To perform this test the hoist is allowed to accelerate to full speed and then the main power is tripped off. Recordings of the hoist are made to determine that the hoist safety comes to a stop. For M-G set controls, it normally does not matter whether the hoist motor is motoring or regenerating. However, with static drives if is important to make sure that the hoist is motoring to prevent damage to the static drive package. When the power is lost during regeneration there would be no AC power system to accept the reverse power and therefore may cause a failure of the static power supply. Stops under AC power loss appear the same as an emergency stop and the recordings should reflect this.

Loss of Motor Field

On hoists incorporating a DC motor, it is very important that some means of stopping the hoist be provided if the motor field is lost. This means is normally a current relay in series with the motor field. When the field is lost, the hoist should be emergency stopped. This is accomplished by placing the contacts of the field loss relay in the same fault stop string that contains the overspeed, overtravel, and E-Stop buttons. Normally the easiest and safest means of checking out this feature is to verify that the fault string will drop out when the field loss relay is activated. This test is performed when the hoist is stopped. The next item to verify is that the field loss relay is activated when field current is lost. The detection of field loss does not require the motor to be running. Therefore, this test should not be performed when the hoist is running since a loss of motor field may cause dangerous overspeeding of the motor. Normally the motor field current is supplied from a separate power source that can easily be deenergized. The only action required is to deenergize the field supply and verify that the field loss detection activates, indicating a loss of motor field.

Loss of Tachometer Feedback

Hoists are normally designed as a speed regulated drive with an inter-loop current regulator. The primary quantity to be controlled is speed. In order to allow the regulator to sense speed, a tachometer coupled to the motor is used to provide the speed feedback signal. If this signal is lost, the drive could start running uncontrolled and would not decelerate or stop. The tachometer signal is very important to the safe running of the hoist and is therefore normally monitored. This is usually done with a voltage sensitive relay that monitors the tachometer feedback signal. It is not advisable to check this circuit by disconnecting the tachometer while the hoist is running. The normal method is to disconnect the tachometer and then see if the hoist will start up. It should emergency stop and then not restart until the drive has been reset and the tachometer signal is reconnected.

Slack Cable

Slack cable detection is normally accomplished by stretching a cable between two mechanically operated limit switches that are located below the hoist rope. This should be checked by blocking the hoist from movement and then checking to see that when the hoist is called to move that the slack rope switches are activated and cause an emergency stop of the hoist. This detection method should be checked when the conveyance is at the top and the bottom of the shaft way, since the weight of the rope could prevent the slack rope switch from activating during a slack rope condition. This usually occurs when the cage is at the bottom and the weight of the rope is enough to keep it from going slack. While doing dynamic testing of the hoist, the slack rope switches will occasionally be activated due to the emergency stopping of the hoist.

Load Tests

Load tests are conducted to determine if the hoist brakes are capable of stopping the fully loaded platform, cage or other device when traveling at rated speed. The load tests also provide the data required to calculate the actual value of braking force delivered by the mechanical or electrical braking system.

The three categories of hoist brake systems tested are shown below.

    1. Mechanical - drum brake.
    2. Electromagnetic - slop car brake.
    3. Electrical - dynamic brake.

The test procedure and analysis for each of the three types of braking systems will be discussed in the following sections.

Mechanical Brake

Hoists used to transport personnel or materials should be equipped with a drum brake which is capable of stopping and holding its fully loaded cage, platform, or other device at any point in the shaft, slope, or incline.

The drum brake is set during two different hoist operating conditions. Primarily, the drum brake is set near the completion of each normal stop. During a normal stop, the conveyance is brought to creep speed by the hoist motor through regeneration. Once the hoist is slowed down electrically, the drum brake is set to stop and hold the conveyance stationary.

Secondly, when a critical condition is detected by either the hoist operator or control system, the drum brake is set immediately to produce an emergency stop. During an emergency stop, the hoist motor's braking capacity is defeated by opening the armature loop contactor, and the mechanical drum brake is set.

Selecting the proper braking capacity requires careful consideration of the load rating and physical layout of the hoist. The drum brake normally applies a constant brake force to stop the conveyance. However, the loads on the hoist vary considerably from personnel loads to material loads and the braking requirements vary depending on the direction of travel and the amount of load imbalance. Therefore, the optimal braking capacity needs to be selected which will stop the maximum load within a safe distance, while not stopping a lighter personnel load too abruptly.

Once the drum brake capacity is selected, it is tested to verify proper design and installation by performing an emergency stop while raising and lowering the conveyance at rated load and speed. Multispeed hoists are tested for each speed at the rated load for that respective speed.

The drum brake tests begin with performing an emergency stop while raising the conveyance. For unbalanced hoists, this is the least severe direction of travel for testing the brake system. Consequently, the fastest deceleration rate occurs since gravity assists in retarding the motion of the conveyance. The deceleration rate is calculated from the recorded speed curve and analyzed to determine if the conveyance is being stopped too abruptly. The metal, nonmetal, and sand and gravel federal regulations have established safe limits on the deceleration rates. The maximum emergency braking deceleration should not exceed 16 ft/s2

The maximum deceleration available while raising a load in a slope is limited by the gravitational force acting on the conveyance. The gravitational deceleration is inversely related to the slope angle. If the hoist drum is stopped at a faster rate than the component of gravity can retard the movement of the conveyance, slack rope will develop. When this occurs, the hoist rope will be damaged as the brake or supply cars run over the slack rope. Therefore, the rope is observed during the emergency stop tests in the up direction to see if any slack rope develops. The normal stop allows the hoist to decelerate and go into creep speed with the mechanical brakes finally stopping the hoist. Normal stops occur during each trip of the hoist whereas a protective stop or emergency stop occur only under fault conditions.

Each of these stops have various mechanisms or circuits that cause the stop to be initiated. For example, a normal stop is usually initiated by a push button, limit switch, or part of the programmed control. However, an emergency stop or protective stop can be a push button, an overspeed sensing device or a motor field current sensing device. The following will discuss the test procedure used to determine if the sensing device is working properly.

E-Stop Buttons

The E-Stop button is normally a red colored mushroom type push button. It is activated by manually depressing the button. This feature is normally tested by running the hoist up to test speed and activating the E-Stop button when the cage is close to mid-shaft. The hoist should come to a quick stop. To determine if the sequence of events occurred properly, recordings of speed, current, and voltage should be made during the test. The speed curve will show an abrupt change in speed from full speed to zero speed, also the current and voltage should abruptly drop to zero. This indicates the armature loop opened, the power supply for the motor was suicided, and the brakes were set.

There are normally several E-Stop buttons, and each of these should be activated to prove that the controls are operating properly. Since the hoist is subjected to a more severe duty when an emergency stop occurs, it is usually not necessary to cause the hoist to emergency stop from each E-Stop button. All of the buttons normally activate one control relay. Therefore, it would only be necessary to check that the E-stop relay is activated by each button and that an emergency stop occurs when the E-Stop relay is activated.

Overspeed

There are several overspeed sensing devices that are associated with hoists. For example, a typical shaft hoist will have a Lilly overspeed to detect if the cage is overspeeding, a motor overspeed device to detect if the motor is overspeeding, and when the hoist incorporates a motor generator (M-G) set, overspeed protection is provided by a M-G set overspeed. A slope hoist, in addition to the above overspeeds, will also have a relay fly weight on the brake car to detect an overspeed condition.

It is very important that each of these overspeeds be activated by an actual overspeed of the device it is protecting to determine if it is functioning properly. The motor overspeed can be easily and most safely checked before the hoist is roped. In this way, the motor can be safely run into overspeed without the cage or car being subjected to the overspeed condition.

The two most critical overspeeds are the Lilly and the brake car. The Lilly and brake car overspeed test must be performed once the hoist is roped. The Lilly overspeed must also be checked at its deceleration and acceleration areas. Checks of these devices must be done under closely controlled conditions that prevent hoist damage.

(Wk2)2   =  ( N21   �  N22)   (Wk2)1                        (5)

where                   (Wk2)n   =   inertia at base speed n                                                          
Nn   =   rotational speed n                              

A physical reference point needs to be established for the mechanical analysis. For the purpose of discussion, the hoist drum is selected as a reference. The motor inertia is transferred to the drum side of the speed reducer by using equation 5 to find the equivalent WK2, as shown below.

Wk2em   =   Wk2m   (GR)2                        (6)

where                   Wk2m   =   motor inertia

                                                                      Wk2em   =   equivalent motor inertia on the drum side

The total weight of the rotating parts referred to the drum radius is;

Wh   =   (Wk2d  +   Wk2em)  �  r2                 (7)

where                    Wk2d   =   drum inertia                                                                   
r = drum radius                                                

Wh is the total equivalent weight of the hoist if all the mass were concentrated at the drum radius. This weight usually exceeds the maximum rated load of the hoist. The total systems mass, mt, with respect to the drum is;

mt   =   (Wc  +  Wr  +  Wh)  �   g

where        Wc   =   weight of conveyance                          
Wr   =   weight of the rope              
g   =   accelerational gravity       

The total system mass is given by equation 7 and the braking deceleration was calculated from equation 3. Therefore, the decelerating force, Fd, is;

Fd   =   mta                               (9)

In addition to the decelerating force, there is also a static force necessary to hold the load stationary. This holding force, Fh, is calculated using equation 4.

Fh   =   W   sin  �                    (10)

A small portion of the hoist decelerating force is attributed to friction. Retarding forces occur in the form of windage, brush, bearing, gear and rolling friction. The contribution of friction is usually 2 to 5 percent of the total decelerating force.

Therefore, the actual braking force of the hoist drum brake is given by the following expression.

Fb   =   Fd   +   Fh   -   Ff            (11)

where                 Fb   =   actual braking force                                           
Ff   =  frictional force                          

Once the actual braking force is calculated, it is compared to the manufacturer's brake specifications to identify any discrepancy which may be a result of improper installation, malfunction, or mechanical wear.

In addition to the drum brake, the hoist may also be equipped with a mechanical motor brake. The motor brake applies a braking force directly to the motor shaft and is adjusted to set after the conveyance is stopped by the drum brake. The motor brake is an independent parking or holding brake and serves as a backup protection for the drum brake. Therefore, the motor brake does not normally contribute to the dynamic braking capacity of the hoist.

The motor brake is tested by analyzing the hoist motor current necessary to overcome its braking force. The test is conducted by holding the motor brake on, while the drum brake is defeated, and then attempting to manually hoist the conveyance. The recorded motor current usually reaches the current limit set point before the motor brake is overpowered. If the motor begins to turn before the rated motor current is reached, then the motor brake may be unable to stop a fully loaded conveyance.

Electromagnetic Electromagnetic Brake

The second type of brake system tested is electromagnetic. Hoists used for transporting personnel should be equipped with safety catches or other no less effective device that acts quickly and effectively in an emergency. On slope hoists, this personnel protection is typically provided by a brake car. Brake cars derive the braking force from electromagnetic brake shoes which magnetically clamp the rail in the event of an emergency. Magnetically clamping the rail provides a brake force which is independent of the brake car's angle of inclination.

The data needed to quantitatively determine the braking force of the electromagnetic brakes is obtained in two steps. First, the motor armature current is recorded while hoisting the brake car with the electromagnetic brakes set. Next, the motor armature current is recorded while hoisting the brake car, at the same speed as the previous test, with the electromagnetic brakes released.

To calculate the actual value of the braking force given the armature current values, a generalized relationship needs to be developed between the mechanical power output and electrical power input. For separately excited direct current shunt motors, the armature current is related to shaft torque by the relationship shown below.

  T   =   K   �   I (11)
 
where   T   =  shaft torque
K  =  proportionality constant
�   =  field flux
I   =  armature current
The relationship between the normal and braking operation can be derived by dividing equation 11 for the case where the brakes are set, by the normal operating torque. When this is done, the constant of proportionality, K, drops out of the equation. The field flux is constant for motor speeds less than base speed, therefore, also becomes unity. The resulting proportionality is shown below.

            Tb   �   Tn  =  Tb   �   Tn (12)
where Tb   =   torque when the brakes are set
Tn   =   normal operating torque
Ib   =   armature current when the brake are set
In   =  normal operating armature current

Equation 12 shows the change in armature current is directly proportional to the change in motor torque. The normalized current term, Ib/In, is the increase in torque required to hoist the brake car when the brakes are set. To convert this factor into an actual braking capacity, reference should be made to the static force diagram of a slope hoist shown in figure 2. This dynamic test can be analyzed statically because the conveyance is moving at constant velocity, thus no torque is being consumed for accelerating the conveyance. In addition, frictional rope and bearing losses are neglected.


Fig. 2. Slope Hoist Static Force Diagram

�  =   angle of shaft inclination
   N   =   normal component of force
W   =   weight of cars and rope  
   F   =   force along shaft inclination

The following relationship can be derived from the force diagram.

F   =   W   sin   �                                  (13)

The force, F, is directed downward at the angle of shaft inclination. The force is related to the normal torque, Tn of equation 12 as a function of the drum radius, r.

Fn   =   Tn  �  r                                    (14)

When the above expression is substituted into equation 12, the constant drum radius, r, term drops out leaving the expression defining the braking force as a function of the normal force and the normalized braking current.

Fb   =  (Ib  �   In)  Fn                                  (15)

where            Fb   =   force when the brakes are set                                        
Fn   =   normal operating force                              

The normalized braking current Ib/In is obtained by the test procedure previously mentioned. The normal force value is calculated from the static force diagram knowing the weight of the brake car, supply cars and rope and given the shaft angle of inclination.

The value of Fb calculated from equation 15 is based on hoisting the brake car. However, the electromagnetic brakes are usually applied when the car is traveling down the shaft in an overspeed condition. Therefore, the force of the cars and rope weight should be deducted from Fb in equation 15 to obtain the actual value of brake force delivered by the electromagnetic brakes when traveling down the shaft, as shown below.

Fbf   =  ((Ib  ÷  In)  -  1)  Fn                       (16)

where            Fbf   =   actual electromagnetic brake force                             

The maximum load the brake car can safely stop is directly related to the actual braking capacity. As a general rule, the actual braking capacity should be at least 150% of the maximum load, to maintain a safe stopping distance. The maximum load that can be safely stopped is also inversely related to the incline of the shaft as shown below.

Wmax  <   Fbf    1.5 sin -                  (17)

Dynamic Brake

During normal hoist operation, the regenerative braking capacity of the drive motor is utilized to slow the hoist down to creep speed. Unlike the mechanical drum brake, the regenerative brake force is not constant. The retarding effort is adjusted by the speed regulator to achieve a constant deceleration, independent of the conveyance weight.

The regenerative deceleration and motor acceleration is tested concurrently with the drum brake, by raising and lowering the conveyance when the hoist is loaded to its maximum rated capacity. The acceleration and deceleration rates are analyzed from the speed curve to determine if the hoist is starting or stopping abruptly. Safe limits for acceleration and deceleration have been established by the metal, nonmetal, and sand and gravel federal regulations. The maximum normal acceleration and deceleration should not exceed 6 ft/s2.

Occassionally, the armature current limit is reached when the maximum load test is conducted. This condition occurs when the armature current recording is "clipped" at the current limit value. This condition will provide a slower than normal deceleration rate which may cause the hoist to run into an overtravel or overspeed condition, or the hoist may exceed the allowed time in current limit and emergency stop. When the maximum load test causes the hoist to operate in current limit, either the current limit must be increased or the maximum rated load capacity must be decreased.

A dynamic braking resistor provides emergency backup protection in the event that the mechanical braking system fails. As previously mentioned, the hoist motor can act as a generator to slow the hoist down to creep speed during a normal stop. However, during emergency stopping the motors retarding effort is not utilized unless a dynamic braking resistor is properly installed.

There are many philosophies and methodologies that can be incorporated when selecting and installing a dynamic braking resistor in a hoist control system. A comprehensive discussion on dynamic braking design would be prohibitively lengthy, and thus, can not be addressed in this paper. Instead, an actual system that was tested by the Mine Electrical Systems Division will be discussed.

This particular dynamic braking system provided protection in the event that both the control system and holding brake failed simultaneously. A one line diagram of the system is shown in figure 3.

Fig. 3. One Line Diagram of a DB Control System

The loss of the control system power to the field is detected by the field loss relay. This transfers the drive system into the dynamic braking mode by deenergizing the field loss and the dynamic braking relay. At the same time the holding brake sets, thus the two braking systems complement each other. If during a control system shutdown, the holding brake should fail, the overhauling load will cause the motor to be driven as a generator supplying braking current into the dynamic braking resistor and field current to the shunt field. The dynamic braking resistor would dissipate the energy generated by the overhauling load, and the load would be lowered at a safe slow speed. Thus a "runaway" condition would be prevented. It is important to remember that dynamic braking does not stop the load like the holding brake, but rather is designed to limit the speed to a small percentage of the base motor speed.

The holding brake and the dynamic brake operate simultaneously when control power is lost. Therefore, the following two test methods were developed which measure only the dynamic braking contribution to the hoist braking capacity. Precautions must be taken when conducting these tests so that the holding brake can be quickly applied if the hoist begins to overspeed.

The first method of testing the dynamic braking resistor is by manually deactivating the holding brake while running the hoist up to rated speed, and then tripping the control power circuit breaker. The hoist should slow down to a predesigned dynamic braking speed and continue at this speed until the holding brake is reactivated, thereby stopping the hoist. This is the most severe test of the dynamic braking resistor because the motor is running at full speed when dynamic braking is initiated. Consequently, the initial value of armature current being dissipated by the dynamic braking resistor is very large. However, as the hoist slows down to the final dynamic braking speed, the armature current assumes a steady state value.

Another less severe test can be performed which also demonstrates the dynamic braking capacity and measures the final dynamic braking speed. For this method, the holding brake is deactivated and the control power is removed while the conveyance is at rest. The overhauling load will then begin traveling down the shaft and accelerate the hoist. The data obtained from this test procedure is shown in figure 4.

Fig. 4. Dynamic Brake Test Data

As the hoist approached base motor speed, 300 fpm, the armature current began to develop due to the residual field magnetism. The armature current, in turn, supplied current to the motor field through the dynamic braking resistor. The field quickly built up to 90 percent of its normal operating level. Once the field was established the hoist rapidly decelerated to its final dynamic brake speed with a small under-damped response. The designed dynamic brake speed of this system is 40 percent of base motor speed. This dynamic braking system effectively provided additional backup protection to the hoist which would activate in the event of multiple failures and prevent a high speed runaway condition on the hoist.

CONCLUSIONS

It is due to the complexity of the hoisting system and the potential for serious injuries or fatalities that has caused an increased awareness and concern about hoisting systems. This concern not only comes from MSHA, but also from the industry and is due to the many accidents that have occurred during the past several years. According to studies made by E. D. Seals of MSHA's Health and Safety Analysis Center in Denver, Colorado, defective equipment and maintenance errors accounted for 69 percent of the total hoisting accidents at Metal/Nonmetal Mines and 79 percent at coal mines. These studies indicate that "the major fault associated with defective conveyances was related to an electrical malfunction". Mr. Seals also concludes that the potential for hoist accidents could involve from one to over a hundred miners. In addition to the concern over personnel safety, industry also is concerned about the economics and productivity of hoist operations. The loss of a hoisting system can completely shut down a mining operation. Depending upon the extent of the damage, this shut down can exist for several months. It is because of the potential hazard to personnel that MSHA's Mine Electrical Systems Division has developed the capability to check out and commission hoists. In the past, this commissioning was exclusively requested by MSHA's enforcement personnel but recently requests have come directly from mining companies. These tests provide an opportunity for the mines' personnel to become familiar with the hoist's operation and capabilities, and also provides documentation and reference for future tests. This testing of the hoisting system should become a periodic occurrence. By analyzing these tests and comparing them to previous tests, the integrity of the hoisting system can be determined. These tests should also be performed after any major mechanical or electrical work has been performed on the hoisting system.

At the present time testing, data collection, and analysis work for a particular hoisting system involves approximately 3 to 10 days. Approximately one to three days are spent actually doing the tests. Methods are presently being investigated to shorten the testing, data collection, and analysis periods. One method that appears promising, is the use of a personal computer with interfacing equipment. By using a computer and its associated peripheral equipment it is estimated that it would only take a day or less to completely check out a hoist and generate a report.

Other methods are currently being employed to monitor hoist operations to insure reliability. One method that is extensively used and required in other countries such as the United Kingdom is a hoist monitoring system. This is a complete system with its own sensors that independently monitors the hoist. It is not part of the hoist control but can initiate a shutdown of the hoist. The unit can store the operating parameters of the hoist when it was commissioned and then compares the hoist operation to this stored data. If something should exceed the stored data the monitor would initiate a shutdown. This system can become very elaborate depending upon the user's needs.

As technology advances and new system designs for hoists and developed, these systems will have to be evaluated from a safety standpoint. This safety analysis process must start not before a major accident occurs.

BIBLIOGRAPHY
  1. William J. Helfrich, "Mine Hoist Control System," Mine Safety and Health Administration, Mine Electrical Systems Division, Proceedings of the Sixth WVU Conference on Coal Mine Electrotechnology, July 28-30, 1982.

  2. E.D. Seals, Hoisting Accidents in Metal/Nonmetal Mines, 1978, 1979, and1980, Mine Safety and Health Administration, Health and Safety Analysis Center.

  3. E. D. Seals, Hoisting Accidents in Coal Mines 1979, 1980, and 1981, MineSafety and Health Administration, Health and Safety Analysis Center.