Resistors

Current Limiting Resistors

     Areas Of Usage

     ♦ High Power Test Laboratories

     ♦ Motor Starting

     ♦ Electric Arc Furnaces

     ♦ Induction Furnaces

     Features

     ♦ Stainless steel grid elements

     ♦ Typically 2 mm hot dip galvanized steel enclosure on demand

     ♦ High thermal capacity to absorb high currents

     ♦ Rugged-shock resistant construction

     ♦ High Altitude ratings

     ♦ Corrosion resistant nameplate

     ♦ Specially designed units for hazardous and extreme locations

     ♦ Designed and tested to applicable IEC and IEEE standards

High Resistance Neutral Grounding Device

     Areas Of Usage

Approximate phase to earth fault current in 240-5000 V networks:


     ♦ Solidly grounded systems: 1000-6000 A
     ♦ Low resistance grounded systems: 100-1000 A
     ♦ High resistance grounded systems: 2-10 A
     ♦ Ungrounded (or delta) cable systems: 0,3-2,8 A/km: 2-10 A
 

High Resistance Grounding (HGR) systems are used in power systems where phase to ground short-circuit current is desired to be limited zero to a few amperes by placing highresistance between neutral point of transformer (or generator) and ground. As it is not possible to locate the fault point in delta connected systems, an artificial neutral point is created and delta connected system can be grounded. This allows a fault current of a few amperes thus locating the fault point gets easy. When the neutral point is grounded thru high resistance, both continuity of operation is provided during fault condition and sufficient current (typically between 2A and 10A) flow is provided for ease of locating the fault point.

     Features

     ♦ 0 – 10A analog ammeter and 0 – 250V analog voltmeter with setting on panel

     ♦ Automatic door switch for power shut down

     ♦ Test button to indicate fault

     ♦ Green light for normal conditions

     ♦ Intermittent alarm and red light during ground fault

     ♦ Pulse/Normal Selector swith to magnify current and intermittent pulse during ground fault.

     ♦ Auxilliary free contacts at pulse and alarm position during fault

     ♦ Dimensions (LxWxH)= 60 x 100 x 200 cm (Other enclosure types avaible on demand.)

     ♦ Suitable for 240….4160V three phase systems

 
     Usage

     ♦ 0 – 10A analog ammeter and 0 – 250V analog voltmeter with setting on panel

     ♦ Automatic door switch for power shut down

     ♦ Test button to indicate fault

     ♦ Green light for normal conditions

     ♦ Intermittent alarm and red light during ground fault

     ♦ Pulse/Normal Selector swith to magnify current and intermittent pulse during ground fault.

     ♦ Auxilliary free contacts at pulse and alarm position during fault

     ♦ Dimensions (LxWxH)= 60 x 100 x 200 cm (Other enclosure types avaible on demand.)

     ♦ Suitable for 240….4160V three phase systems

 

Dynamic Braking Resistors

     Areas Of Usage

Dynamic braking resistors are used to stop ac and dc motors with inverter control. Design of Dynamic Braking Resistors depend on recommended braking periods or specific applications.

     Tips

Electric motors in certain cases run as a generator by the machine that it is operating and feeds the electric network. If the motor is required to stop or slow down to nominal speed in such cases, a resistor with suitable ohmic and watt ratings is connected to inverter braking terminals in order to convert the energy generated by motor to dc current and convert it into heat through dynamic braking resistor. Required power of a braking resistor (dynamic braking) in order to brake a motor that rotates at maximum speed and maximum load must be equal to the power of the motor. However, as the braking time is generally around a few seconds, a resistor power rating that is smaller than motor’s rated power can be over loaded for a short time in order to provide an economical way of braking.

 

     Warning 1

Ohmic value of resistors for braking purposes can not be less than stated dynamic braking resistor manufacturer catalog values. Otherwise, inverter and resistor can be damaged. If the ohmic value chosen is larger than inverter catalog’s recommened value, braking time gets longer. Braking time is determined by the weight and speed of rotating parts.

 

     Warning 2

If braking time is unknown, as a safety tolerance , the resistor watt rating should be chosen the same as the motor’s power rating.

 

     Example 1

Suppose that an electric motor with a power rating of 100 kW, is lifting a load for 60 seconds and descending the load for 60 seconds with dynamic braking. The operation factor (ED%) for this system will be ED% = 60/(60+60) = 50 % . As a result of this fact, this braking resistor will operate for 60 seconds and cool down for 60 seconds within the operation cycle of 120 seconds of the crane. As a result of this fact, a braking resistor that will brake a 100 kW motor for 60 seconds must withstand operating 0,50 x 100 = 50 kW continuously and 100 kW for 60 seconds.

 

     Example 2

If a 100 kW electric motor is running 10 seconds and dynamic braking is being applied for 80 seconds within a time frame of 90 seconds, as the braking time is longer than 60 seconds, it is considered as continuous braking and an dynamic braking resistor which can withstand 100 kW continuously is strongly recommended.

 

     Example 3

Suppose a 100 kW electric motor is rotating a fan and after the frequency of motor becomes zero, the fan stops with a braking resistor at 6 seconds. If the motor will start running after Tr = 54 seconds, a smaller wattage rating for this application can be selected.

 

Overload time (Tbr) = 6 seconds.

 

Braking Resistor Power at Overload (Pbr) = Pmotor x Tbr / (Tbr + Tc) = 100 x 6 / (6+54) = 10 kW . So a resistor with a power rating of 10 kW is enough for his application.

 

Over Load Multiplier of this resistor (OLM) = (6+54)/6 = 10.


So an 10 kW resistor must withstand the power of 10 x 10 = 100 kW for 6 seconds.

 

Relationship between Over Load Multiplier / Cooling time / Over Load Time can be seen in the following graph.

Generator Neutral Grounding and Leads Cubicles

     Features

Up to 13,8 kV 3000 A

     ♦ Test terminals for current and voltage transformers secondaries

     ♦ Circuit diagram included in terminal box
     ♦ 100 W anti-condensation heater included againist condensation
     ♦ Zinc – oxide lightning arresters and surge capacitors on demand
     ♦ Voltage transformers fuses with signal contacts
     ♦ M.V. fuse protection

Harmonic Filter Resistors

     ♦ Designed and tested to applicable IEC and IEEE standards

     ♦ Stainless steel resistor elements

     ♦ Excellent high voltage strength

     ♦ Low inductance resistor elements for easy tuning

     ♦ Large variety of harmonic filters for optimum solution

     ♦ Stainless steel, hot dip galvanized steel or aluminum resistor enclosures on demand

     ♦ Corrosion and heat resistant electrostatic paint for indoor and outdoor applications

     ♦ Insulators with high creepage distance are provided on demand for highly polluted areas and high altitudes

     ♦ Compact design

     ♦ Elevation stands are available

     ♦ Cooling fans are available on demand

Motor Starting and Control Resistors

     Areas Of Usage

     ♦ Overhead cranes

     ♦ Lift trucks

     ♦ Machine tools

     ♦ Conveyors

     ♦ Cement Plants

     ♦ Industrial Controls

     ♦ Steel Mills

     ♦ Ships and Submarines

     Features

     ♦ Designed and tested to applicable IEC and IEEE Standards

     ♦ Designed for all wound motor, squirrel cage motor and induction motors

     ♦ Durable construction

     ♦ Stainless steel grid or resistance alloy elements

     ♦ High performance in heavily polluted areas

     Types

     ♦ AC Squirrel Cage Induction Motor Resistors

     ♦ Crane control resistors

     ♦ Starting Resistors

     ♦ Speed Control Resistors

     Selection Details

     ♦ Application

     ♦ Power of motor

     ♦ Rotor current

     ♦ Rotor voltage

     ♦ Starting Torque

     ♦ Number of speeds

     ♦ Desired starting time (seconds)

     ♦ Duty Cycle

Neutral Grounding Resistors

     Areas Of Usage
     ♦ Reducing single phase fault currents which occur in M.V. electrical networks to prevent damages on transformers and generators
     ♦ Reducing temporary over voltages occurred by braking instantaneous fault current, . Providing long-life for switchgear
     ♦ Reducing step voltages to a harmless level for staff
 

Neutral Grounding Resistors are used for resistance grounding of industrial power system. They are generally connected between ground and neutral of transformers, generators and grounding transformers. Neutral Grounding Resistors are used in order to limit maximum fault current to a value which will not damage the equipment in the power system, yet allow sufficient flow of fault current to operate protective relays to clear the fault. Although it is possible to limit fault currents with high resistance Neutral Grounding Resistors, phase to ground short circuit currents can be extremely reduced. As a result of this fact, protection devices may not sense the fault. Therefore, it is the most common application to limit single phase fault currents with low resistance Neutral Grounding Resistors.

     Features

     ♦ Stainless-steel resistor elements

     ♦ Current transformer included (EN 60044-1)

     ♦ Bolted resistor element connections instead of welded connections in order to be able to assemble spare parts on site immediately

     ♦ Typically, RAL 7032(and others) painted

     ♦ Typically, 2 mm hot dip galvanized steel enclosure

     ♦ High thermal capacity to absorb high currents

     ♦ High altitude ratings

     ♦ Custom made lifting eyes provide secure lifting

     ♦ Rugged shock-resistant construction

RC Filters

     Areas Of Usage

RC Filter components are made of low inductance resistors and capacitors in order to protect M.V. motors, generators and dry type transformers, shunt reactors againist very fast transient and over voltages. R–C snubber filters are connected in parallel between motors, generators etc. and switchgears.

     Advantages

     ♦ Longer insulation lifetime for machinery

     ♦ Protects againist insulation failures between windings

     ♦ Protects againist reignitions between the contacts of vacuum circuit breakers and other switchgear due to high frequency over voltage

     ♦ Protects againist high frequency over voltage specificly caused by arc furnaces

     ♦ Optional earth fault relay by arc furnaces

     Application Areas

Large motors, generators, induction and arc furnaces, shunt reactors, dry type transformers.