SCR Alert
In response to WASIP Alert #22-80-08 and the NSAA Special Bulletin dated December 10, 1980, the Rocky Mountain Lift Association held meetings on January 13, 1981, and January 27, 1981, to discuss the issues pertaining to D.C. drives proposed in these letters.
During this discussion, five specific failure modes were outlined:
- Failure of lift to slow on command
- Failure of lift to stop on command
- Lift starts without command
- Lift accelerates at an uncontrolled rate
- Lift over-speeds
These failure modes do not exclusively pertain to any particular drive system. The five can be reduced to the single problem statement, that the operator has lost control of the lift. This can happen in all drive systems, electric, “A.C.” or “D.C.”, or internal combustion.
A practical solution proposed was an independent shutdown device, isolated from the normal stop circuit, which will disconnect the power to the prime mover and stop the lift. This device should interrupt the armature circuit or activate an auxillary trip circuit breaker or cut the fuel supply to an internal combustion prime mover. If required, additional braking devices should be activated to regain control and stop the lift.
This emergency shut-down circuit should be made operational by a manually operated device at each loading and unloading station. The device should also be designed in such a way as to be fail safe.
As a minimum requirement all new and existing equipment should be equipped with over-speed and over-current protection. New installations should also include field loss protection, armature contactors and loss of speed feedback protection. Even though over-speed and over-current protection can cover these failures, it is recommended that these additional protective devices be phased into older installations within a reasonable time frame.
In closing, it is the concensus of the RMLA that after losing control of the drive system, the objective should be to regain control and stop the lift. The operation of this emergency shut-down circuit at any time during normal operation must stop the lift with no possibility of damage or endangerment to either the public or the lift.
Answers to any questions or additional information can be obtained by contacting me.
Signed for the Rocky Mountain Lift Association by Carl Stewart, Winter Park Recreation Assoc.; Lee Burton, Beaver Creek; Dave Larson, Vail Assoc., Inc.; James K. Bunch, Steamboat Ski Corp.; Doug Allen, Copper Mountain Corp.; Keil Thompson, Copper Mountain Corp.
Fincor
Sir:
In response to the article by Mr. Philip H. Berger published in the January issue of SAM and the NSAA Special Bulletin concerning SCR drives that accompanied that article, permit me to make the following comments on how we at Fincor view the situation.
Some years ago a number of drive manufacturers promoted a concept that solid-state regulator electronics and power conversion devices (SCR’s) could be used to start and stop the adjustable speed drive motor and thus eliminate the need for electromechanical relays and motor contactors. This concept did reduce the cost of the product, but it also meant that these manufacturers were relying entirely upon the internal regulator electronics to start and stop the drive motor. In addition, it meant that the means to positively disconnect the motor from the rectified power source had been eliminated.
Unfortunately, solid-state electronics can malfunction when subjected to excessive voltage spikes or electrical noise problems. Frequently these problems can become intensified where utility lines are quite long and/or are subject to electrical storms. A failure or malfunction of the electronics in drives relying on the internal regulator electronics to start or stop the drive motor can result in the motor starting unexpectedly or result in a condition where the operator can not stop it by using the normal stop circuit. Our standard approach for ski-lifts has always been to provide a dc armature contactor as a positive means to start and stop the drive motor.
We have always believed the drive units should be designed and manufactured with fail/safe features such as motor field-loss protection, armature contactors to handle full-load motor current, overspeed protection, and automatic shutdown of tachometer feedback equipped units in the event the tachometer signal is interrupted.
Programmable controllers and solid-state logic control are proven solid-state electronic products. They can be cost effective and can certainly reduce maintenance problems. However, like SCR motor controllers, they too can be sensitive to ac power line problems. We consider it good practice, when these devices are used in conjunction with SCR drives, to design the stop circuit with an electro-mechanical relay separate from the solid-state logic to ensure a positive means to open the motor contactor.
We believe the best assurance for safe and reliable operation of any type ski-lift drive equipment starts with a sound basic design and includes installation by qualified electricians using practices recommended by the National Electrical Code. Frequent inspections by qualified personnel will help ensure that the equipment is kept in proper operating condition and no potentially hazardous conditions exist.
Richard G. Wilson
Mgr. Mktg. Communications
Fincor, Incom International
B77 Subcommittee on SCR
The Electrical Subcommittee of the B77 Committee has developed the following guidelines as a basis for a proposal on SCR Drives that will be submitted for inclusion in the Standard Revision. Prior to that it will be submitted to the B77 Executive Committee at its March meeting in Denver. The approach has been to come up with a proposal that is simple and straightforward, clarifying the potential problems and not overly complicating the control circuits of our modern tramways.
Define the controls and circuits so there are only two levels of stop controls for tramways. Level One controls being the basic general stop circuits within the drive and control electronics which would include normal manually initiated stops and automatic stops initiated by the various stopping devices such as counterweight and carriage limit switches, safety gates, derailment switches, etc.
Level Two controls being a simple independent stop circuit that could be manually activated at each loading and unloading station and automicly for an over-speed or roll back of a tramway.
It is felt that the integrity and reliability of a Level Two stop circuit could be improved and simplified with a limit set on the number and type of stop devices allowed in the circuit. The Level Two stop control would be the emergency stop circuit as we now know it and automatically set the emergency brake.
The proposal would require that the control circuits for tramways guard against the following five conditions:
Condition 1.—Tramway will not slow down when given the command to do so.
Condition 2.—Tramway will not stop when given the command to do so.
Condition 3.—Tramway overspeeds beyond control settings and/or maximum design speed.
Condition 4.—Tramway accelerates faster than normal design acceleration.
Condition 5.—Tramway self starts or self accelerates without given the command to do so.
The Level Two stop circuit would stop the lift automatically for conditions three and five and manually for all five conditions. Further, the Level Two stop circuit would disconnect the power source to the prime mover being A.C. power to the SCR converter, fuel to the diesel engine, etc. and set the emergency brake when activated.
Also being considered is that an electrical or electronics engineer be required to test and certify that the drives and controls for a particular tramway meet the intention of the electrical standards for tramways. This would be similar to the testing certification required for wire rope used on tramways.
Additional requirements will be considered for D.C. electric powered tramways including:
- Motor field voltage loss protection shall be required.
- Tachometer loss protection required for those D.C. Drives using some for speed reference.
- Controls be designed, manufactured and tested for minimum sensitivity to electrical noise and other electrical emissions, such as noise spikes from power lines and lightning, radio transmitter, thyristor (SCR), solenoid or relay noise at levels and frequencies that could initiate any of the five conditions outlined above.
LeRoy W. Schultz, P.E., Chairman,
B.77 Electrical Subcommittee
Consumer ski shows
Sir:
With pleasure I commend Paul Robbins for the depth of his personal investigation into the condition of consumer ski shows. And you, too, for your willingness to publish his full story!
The article received considerable comment at a meeting in Denver of the major U.S. ski show producers, during which they formed a new association . . . “Ski Shows of America.” Their emphatic and unanimous reaction was: there is nothing ailing consumer ski shows which at the same time is not ailing the whole ski industry that the shows attempt to represent, and that is financial instability.
With adequate financial support the shows will once again create lively, colorful, informative environments that attract enormous crowds and satisfy everybody. To this end exactly the new association was formed, and a lot of sensible, new measures will be taken to guarantee this adequate financial support. And the ski industry will be one of the primary beneficiaries.
Harry A. Leonard
Canadian development
Sir:
I cannot help but comment on your editorial regarding the investment climate in Canada (Issues, November ’80 issue).
One of the major reasons for our development attitude and the present investment climate is the Ski Development office. The existence of this office provides a central contact and reference point for the many government agencies involved in ski development and the ski industry. The Ski Coordinator acts as a coordinating agent, and assists the various government agencies in their dealings with the ski industry, and vice versa.
We, in the Ski Development office, would be overstating our role if we were to say that the bureaucratic maze has been totally eliminated, but you can be assured that the links of communication between government and industry are open and working.
I am very pleased with our track record to date, and believe that we are meeting the expectations of both the government and the ski industry.
Dan Matthews,
Coordinator, Ski Development
Ministry of Lands, Parks
and Housing, B.C.
In calling attention to the favorable investment climate in Canada we should have mentioned the role of Dan Matthews, who is putting into practice the principles he taught as Director of the ski area management program at Humber College—Ed.
