Introduction
This document serves to explain the fundamentals of motor protection for 3 phase low voltage motors. The evolution of such protection products with reference to local manufacture {NewElec} and the South African economy in the years 1970 to 2021. We remind readers about the South African apartheid era whose influence on local manufacturing companies was substantial. An attempt is made to detail how the marketing of such products was affected.
About The Author
Motor Protection History
Three phase electrical motors orginated in the 1880s. The frames that held them were made of steel or cast-iron whose mass often exceeded the motor requirements. As such the motors tended to be over-designed for their normal usage and could withstand overloading conditions without serious consequences as the generated heat from overloading was absorbed by the motor frame. However, modern computer assisted designs are not as forgiving. The competition is fierce as motors are now imported from Brazil, China, Germany and Japan. All motors are very much on a par with one another. Prices are almost on an equal footing! End user preferences are dictated by stock holding and client service. From a protection viewpoint older generation motors used thermal bi-metal relays and thermistors embeded in the windings. In our modern world thermal bimetals are still widely used because they are least expensive BUT from approximately 11kW upwards better protection is often sought. NewElec , a local manufacturer and designers of electronic motor protection devices, enjoyed a competitive advantage in the late 1970s because they launched a very efficient relay 'The 320 Series' that was extremely successful. European and other manufacturers were, at the time slow to develop such products. One of the distinct marketeable features in the apartheid era was that local manufacturers were heavily supported. Later, a version designed for protection of cranes evolved. The '330 Series'. In the early 1980s this advantage was slowly lost as european manufacturers began developing their own similar products.
Essential Protection
When looking at modern designed motors the following is considered essential protection:
1. Thermal Overload protection.
2. Phase Unbalance protection.
3. Rapid trip on phase loss.
NOTE: Upon a complete phase loss the tripping time should not be delayed by more than 4 seconds.
Physical Size Considerations
In my sales and marketing experience I discovered that the physical size of a protection relay is always an important consideration. The big switchgear manufacturers design their thermal relays to fit directly on their own unique contactors so that a Cutler Hammer (now Eaton) relay would not fit on a Siemens contactor. See illustration here. One thing that must be considered when using such devices is that they bring with them 3 possible "hot-points", that could lead to increased maintenance cost. NewElec was at a dissadvantage in this area as they had no similar products. Certainly they did have more accurate products, see 'LA series' here, but these , although illimanating possible "hot points" were substantially much larger in size. On the otherhand their basic thermal protection relays catered for a wider amperage range. I found this a marketing problem because of two main considerations:
1. Motor control centres for the most part are equipped with small cubicles as a plant's motors are for the most part smallish kW motors.
2. Cubicle (bucket) space is always at a premium and costly.
In order to overcome this problem NewElec launched their 'K' series relays that occupied a much smaller footprint. However, there were other considerations to keep in mind. Typically there existed a cubicle door mounted reset button together with a trip indication light on the MCC cubicle door. It was legally not permitted for an electrician to simply press the reset button without first ascertaining the reason for the trip AND in later years it became incumbent that NO cubicle door on an MCC could be opened unless the technician wore protective clothing. Enter the NewElec 'K' series with an external cubicle door mounted trip indication display!. The 'K series' included in-line current transformers over the entire range! Even more important was that the 'K' series had percentage calibrated relays so that increasingly larger kW motors could make use of interposing CTs to cover larger kW sizes. NewElec could also use this same relay to protect single phase motors!
Motor Construction
The three phase induction motor is the most widely used electrical motor. Almost 80% of the mechanical power used by industries is provided by three phase induction motors because of its simple and rugged construction, low cost, good operating characteristics, the absence of commutator and good speed regulation. In three phase induction motors, the power is transferred from stator to rotor winding through induction. The induction motor is also called a synchronous motor as it runs at a speed other than the synchronous speed.
WATCH THIS Motor Construction Video
A 3 phase induction motor is constructed from two main parts, namely the rotor and stator:
1. Stator: As its name indicates stator is a stationary part of induction motor. A stator winding is placed in the stator of induction motor and the three
phase supply is given to it.
2. Rotor: The rotor is a rotating part of induction motor. The rotor is connected to the mechanical load
through the shaft.
The rotor of the three phase induction motor are further classified as:
1. Squirrel cage rotor.
2. Slip ring rotor or wound rotor or phase wound rotor.
Insulation Materials
When a motor 'burns out', we are referring to the stator windings. The windings are insulated by means of various type of materials as indicated in the NEMA table below. However 80% of standard induction motors are class 'F' insulated. So, what causes the windings to burn? When a motor runs at a load that is above it's rated load (remember that the motor will always try to meet the load or work required from it), heat is generated in the stator. The class 'F' insulation material begins to degrade at 155 degrees Celsius. So, on very occasion that the motor reaches this temperature threshold, the insulation material degrades. Every subsequent similar event degrades the insulation further until a point is reached when the insulation fails. Every overload cummulatively degrades the insulation by about 20%!
See more about INSULATION.
Other insulating material classes are used for specialized applications. For example, a motor that we know will be placed in a high ambient temperature or subject to poor ventilation. I have personally seen employees that are designated the full time job of blowing air onto the motor ribbed cooling frame to remove flour deposits in a bakery industry.
Other insulating material classes are used for specialized applications. For example, a motor that we know will be placed in a high ambient temperature or subject to poor ventilation. I have seen employees that are designated the full time job of blowing air onto the motor ribbed cooling frame to remove flour deposits in a bakery industry.
Non standard insulating class materials are expensive and have prolonged delivery times from manufacturers.
Thermistors
Thermistors are resistances that dramatically change their values upon reaching a specified temperature threshold. In my experience I have seen that industry often specify that all large motors above 75 kW must be equipped with thermistors in their windings. The thinking being that "If all else fails, thermistors will save the motor!" Certainly there is merit in the thinking particulary in safe guarding front and end shield bearings. I have come accross situations where a bearing failure contributed catastrophically to motor loss. Liberal use of thermistors in windings definitely have their place. There are various types of thermistors that we will look at. Generallly these must be sited in the motor windings (one or more per phase) and also in the bearings at the time of manufacture.
When a plant or manufacturing environment is fully automated, readings from thermistors (RTDs) often provide alarms concerning critical motors. There are two main types:
1. Negative Temperature Coefficient. {When temperature increases resistance decreases}
2. Positive Temperature Coefficient. {When temperature increases resistance increases}
Thermally Speaking
It is good practice for motor manufacturers to define a motor's thermal limits by defining the Safe Cold and Safe Hot starting times for their range of motors. It is important first to understand that although most motors are class F insulated, their respective safe cold and safe hot stall times wiil be different because of motor frame size. Although direct on line starting of a motor is deminishing in popularity many are still started that way. When this happens, and because the motor is at rest, the inrush starting current can be in the order of 6 times motor rated load!
Let us take a practical example. Suppose a certain motor has a safe cold starting time of 16 seconds and that this motor is driving a conveyor belt that takes 22 seconds to accelerate to normal speed. In this scenarion, the motor will be experiencing 600% normal full load current for at least two thirds of the required acceleration time. After this period the motor's remaining thermal capacity will be zero. The generated heat will eventually subside to a normal level BUT if that conveyor should be stopped and immediately re-started (Now a hot motor), the thermal capacity will be exhausted long before the conveyor belt is back in operation. The rule of thumb is that the safe hot stall time is about one third of the safe cold staarting time.
1. Negative Temperature Coefficient. {When temperature increases resistance decreases}
2. Positive Temperature Coefficient. {When temperature increases resistance increases}
As can be understood, knowing the thermal limitations of the motor, the actual starting time and knowledge of the application is important information that should always be considered. Newelec incorporated these concepts and designed their very successful 320 series relay which incorporated settings for motor acceleration time, safe number of start per hour, phase loss, unbalanced loads and the maximum motor load. The thermal model thus was taylor made for a given application and motor size. It also incorporated thermal memory.
As electronic motor protection relays do not actually have a direct input relating to winding temperature the heat generated for motor load is
calculated by the below formula:
i 2.t
where:
i = measured load, and t is the time in seconds that the load is applied.
Wish List
Users of motor protection relays were always imposing new chalenges for such products. These were as follows:
1. The relays should be tamper proof.
2. They wanted features relating to short circuit protection, earth fault protection and earth leakage protection.
3. They neede to have door mounted fault indications.
4. They wanted physically smaller products.
5. They wished for the protection relays to memorise events, reasons for trip, motor kilowwat usage and number of hours in service.
6. Pumps required load loss and dry-run protection.
7. There was increased demand for 'intelligent' relays that could communicate and interact computers.
Standardization
The tendency amongst all larger industries was to standardize on a choice of swithcgear. The reason for this was that electrical staff became comfortable and more knowedgeable on a specific type. The spare parts kept in inventory was also deminished. Because NewElec was an independent South African manufacturer of motor protection products BUT did NOT manufacture switchgear they were somewhat at a dissadvantage. Prospects that were using CHI or other brands of switchgear prefered to use that brand's motor protection products. The fact that the larger well established brands had a global footprint appeared to solidify universal acceptance. In order to maintain a fair share of the motor protection market NewElec evolved products that were needed by specialist niche industries such as Joy Mining that needed protection products fitted in explosion proof enclosures on underground machinery, the pump protection market and very intelligent miniturized relays which incorporated digital inputs and several output relays specifically designed for the automation market. As market research indicated that mines were spending vast amounts on pumps as well as pump repairs they developed a product suited to address these challenges. Curiously it was discovered that pump supliers and manufacturers had no interest in pump protection. They were only too happy to sell new pumps or charge for repairing them.
Benefits and features of the NewElec pump protection relays:
- 1. Complete protection of the pump motor including earth leakage and earth fault protection.
- 2. Dry run protection
- 3. Programmable automatic restart of the pump motor after a user progammable time period.
- 4. Limit impellor wear on pumps
Competition
The abundent markets in South Africa flourishing due to the mining , copper, chemical, water, paper, sugar, packaging, synthetic fuels, and power station industries, to name a few was and will continue to be an attraction for competitors in the motor protection field. Competitors were from Germany, Spain, Japan, China, Australia and Canada which all became increasingly well established in South Africa. Often pushing prices downwards. Siemens in particular began gaining a lot of ground with their Simocode range which became an industry standard at Sasol. In order to compete with this product NewElec released their Newcode series that was technically superior but suffered from being over priced for the market. It is safe to say that Siemens took about 50% of the market share by the end of 1995. It was sad to see that the benefits of product life-cycle longevity and local manufacture began to lose the appeal and support it once had. By 1994, those benefits were no longer regarded as important despite the fact that the larger more successful products had a shorter life-cycle whose manufacturers forever encouraged existing users of their products to migrate to newer versions of the same products and so called 'older' models becoming rapidly obsolete. NewElec attempted to get the support of large industries, notably Sasol, who continued to prefer supporting German products. In 2022 the Siemens Simode relays are obsolete, leaving many of their customers in the learch. In contrast, the entire range of NewElec relays are still supported and serviceable. Even those dating back to mid 1970!
Automation
In an effort to deminish the labour components (effects of strike actions) as well as improve efficiencies modern industry are implementing automated systems in their planning and expansions. As such motor protection products of the future need to meet this requirement. Profibus and Modbus communications protocols are widely supported in many indutries. Modern relays must incorporate every possible motor protection requirements, have at least 6 digital inputs and a similar quantity of potential free output contacts for control purposes. These new generation motor protection products should also memorise All types of events that should be downloadable and accessible to a controlling PLC. They must cater for door mounted trip indications and preferably include cable theft detection. Such relays should be programmable by means of user-friendly software by means of a laptop computer or by the PLC. I predict that variable speed drives will evolve similarly in the foreseeable future.