de5h

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Corporate Engineering StandardDesign Standard: DE5H

Electrical Technology Network

DE5H

Power Control Devices in Electrical Heating and

Other Variable Power Applications

Table of Contents

1. User Guidance ....................................................................................................................................................2

1.1 Scope .........................................................................................................................................................21.2

Applicability ................................................................................................................................................2

1.3

Benefits ......................................................................................................................................................2

1.4 References .................................................................................................................................................2

2. General ................................................................................................................................................................2

3. Magnetic contactors / ElectroMechanical relays ............................................................................................2

4. Mercury Displacement Relays (MDR) ..............................................................................................................3

5. Sil iconcontrolled recti fiers ..............................................................................................................................3

6. Sol id State Relays (SSR) ...................................................................................................................................4

7. Type of control ...................................................................................................................................................5

8. Comparison ........................................................................................................................................................6

List of Figures

Figure 1.

Circuit for inverse parallel connection of silicon-controlled rectifier units as alternating-currentcontactor .................................................................................................................................................7

Figure 2.

Time-proportional control action .............................................................................................................7

Figure 3.

Basic SCR control systems for electric heating applications..................................................................8

Figure 4A. Example SCR control system for a small to medium process gas heater ............................................10

Figure 4B. Example SCR control system for a large process gas heater ..............................................................11

Red text indicates revisions made in the April 2008 issue.

Document revised April 2008 / Entire document reaffirmed April 2008

Contact [email protected] e-mail for more information.This document may be used and reproduced for DuPont business only.

Copyright 2001, 2008 E.I. du Pont de Nemours and Company. All Rights Reserved. (Unpublished)(Engineering) Page 1 of 11

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DE5H Power Control Devices in Electrical Heating and Other Variable Power Applications

1. User Guidance

1.1 Scope

This standard covers the selection of suitable methods and devices for controlling electrical

heating and other similar variable power applications on the basis of performance requirementsand costs.

1.2 Applicability

This document offers recommended guidelines for the application and selection of power devicesused to control electrical heating and other variable power applications. The guidelines containedin this standard are applicable to all sites within DuPont.

1.3 Benefits

The proper selection of power control devices for electrical resistance heating systems is veryimportant to system performance, life cycle, and electrical energy efficiency.

1.4 References

NEC National Electric Code

2. General

2.1 In the simplest and lowest initial cost type of control, the power supply to electrical loads isturned on and off by means of automatically controlled magnetic contactors. Circuits requiringcontrol of power not exceeding approximately 3/4 kW may be switched on and off without the useof magnetic contactors by using suitable thermostat contacts directly.

2.2 The most common and economical power control device in use in industry today is the

silicon controlled rectifier (SCR). These are solidstate (or static) devices in which no activemechanical, thermal, or electronemitting components are used.

2.3 Other control schemes used infrequently and, therefore, not included within the scope of thisstandard are as follows.

2.3.1 Varying automatically the voltage applied to the load by means of inductionvoltageregulators or multitap transformers.

2.3.2 Using power input controllers that are a type of circuit interrupter, in which the lengthof time that power is supplied to the process is automatically varied in reference to thedeparture from a set point.

2.3.3 Adjusting automatically a resistance in series with the load elements.

3. Magnetic contactors / ElectroMechanical relays

Generally, a magnetic contactor is selected only where cyclic operation about a set point issuitable for the processheating application and the minimal installed cost is of primeconsideration. The control condition is aggravated by the necessity of purposely placing a deadband in the system to prevent too frequent operation, with the resultant excessive wear anddamage to the contactor, particularly where the larger sizes are used. A typical minimum cycle

Document revised Apri l 2008 / Entire document reaffirmed April 2008

Copyright 2001, 2008 E.I. du Pont de Nemours and Company. All Rights Reserved. Used under Copyright License. Page2 of 11

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time is 15 seconds, longer cycle times are preferred. In addition, magnetic contactors are noisyand may cause a serious vibration problem that prevents all but the smallest sizes of contactorsfrom being mounted on the same panel (or within enclosures) with control and measuringinstruments. Magnetic contactors usually are unsuitable for use in dirty or contaminated areasbecause of frequent shutdowns, due to the severe service, and the high cost of contactmaintenance.

4. Mercury Displacement Relays (MDR)

4.1 Mercury displacement relays switch power to loads by applying AC or DC control voltage toits solenoid. This causes a plunger to raise and lower the mercury level, thus making andbreaking the power to the load circuit. The contacts do not wear due to the mercury in thecapsule. MDRs must be mounted and remain in the upright position at all times. They come in1, 2 or 3 pole configurations. They can switch considerable amounts of power at a low cost,similar to electromechanical relays, and they offer long life operation, similar to solid-state relays.

4.2 Because MDRs contain mercury, they should not be placed in the normal waste stream.Mercury is a hazardous material and therefore all MDRs are required by law to be recycled and

disposed of properly.

5. Siliconcontrolled rectifiers

5.1 SCR units are the solidstate equivalent of ignitron and thyratron tubes. SCR units aremuch like ordinary silicon rectifiers except for being designed to block in the forward direction untila small signal is applied between the gate and the cathode. When a SCR unit is fired, it willcontinue to conduct, even when the gate signal is removed, until the anode positive potential iseither removed or made negative. If an ac potential is placed on the anode, the SCR unit willconduct every half cycle (provided the gate is fired) similar to an ignitron tube.

5.2 It is necessary to take precautions to ensure that the semiconductor rectifiers are not

subjected to (a) excessive voltage or current overload conditions due to choice of unsuitablecomponent values in the circuit, (b) excessive inrush or transient voltages or currents, (c) highambient temperatures, (d) voltage surges higher than peak inverse voltage rating due principallyto transformer switching transients, (e) inadequate heat sinks or cooling fins, etc. Installation ofan electrostatic shielded type isolation transformer is recommended in the ac supply to the unit toprotect SCRs and prevent voltage spikes from being transmitted back upstream and affectingother devices on the same ac source. Devices with a peak forward voltage (PFV) and peakreverse voltage (PRV) of 2.5 times rms line voltage have generally proven satisfactory whensurge suppression devices are properly applied on the line side of the controller. Since solidstate contactors have a limited shortterm overload rating, it is important to select a unit with acurrent rating equal to, or greater than, the maximum peak load to be carried. Generally, ratingsare based upon a maximum ambient temperature of 120oF (49oC). For operation at highertemperatures, the units should be derated in accordance with manufacturers recommendations.

5.3 SCR units are very reliable and should have very long service life. As is common with manysolidstate devices, they have characteristics that the longer they last, the better are the chancesthat a failure will not occur. If a failure is to occur, the chances are that it will happen during initialoperation rather than later. A failure can cause the SCR unit to either open up completely or toshort out. In most cases, the SCR fails shorted so a satisfactory means of disconnecting theheater from line voltage, such as a contactor or shunt trip breaker, must be used for overtemperature protection. The controller adjusts output voltage so the load must be sized to restrict



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the current rating or a current limit circuit must be added to the controller. Medium to large SCRpower controllers are available with optional shorted SCR alarms and current limiting controls.

5.4 By connecting two SCR units in inverse parallel, as shown on Figure 1, it is possible tocontrol a singlephase ac current. When two rectifiers are used in inverse parallel in an ac circuit,both rectifiers will conduct alternately, permitting both the positive and negative parts of the cycle

to pass. When conduction to the load is desired, the gates of SCRs are allowed to fire. When itis desired to stop conduction, the gates are prevented from firing. Unlike a magnetic contactor,the frequency at which the control circuit can permit the static contactors to operate can be veryfast (theoretically up to once every 16.7 milliseconds) without damage.

5.5 If SCR units should misfire due to electrical noise on the gate signal lead, filtering orshielding of the gate lead may be required.

5.6 True-RMS meters are recommended when making load measurements on systems usingphasecontrolled SCR units, as moving coil or averaging meters can give inaccurate readings atless than 100% demand. When measuring the output of burstfired SCR units, a filter circuitmust be applied to the meter to smooth the response or the meter indication will cycle.

5.7 SCR control systems are normally purchased as a packaged system from specialtysuppliers. The type of SCR or other type of control is based on the type of heater and how itneeds to be controlled. Local versus remote heater temperature control, need for safety controls,monitoring features and local or remote alarms should be based on process/personnel hazardrisk, and business loss and how the heater controls fit into the overall process control strategy.SeeFigure 4Afor an example of an SCR based control system for a large process gas heater.See Figure 4Bfor an example of an SCR based heater control system for small to medium sizeprocess gas heater.

6. Solid State Relays (SSR)

6.1 Solid-state relays (SSRs) control load currents through solid-state switches such as triacs,SCRs, or power transistors. These elements are controlled by input signals coupled to theswitched devices through isolation mechanisms such as transformers, reed relays, oroptoisolators. Optical isolation allows the output circuitry to be energized when infrared lightstrikes a photosensitive device, which results in the virtual absence of electrically generatednoise, and allows for electrical isolation of the input to the output. Applications are where rapidon/off cycling would quickly wear out conventional electromechanical relays. General-purposeSSRs have on/off cycle lifetimes as high as 100,000 actuations.

6.2 SSR failure modes are primarily determined by the triac or SSR switching characteristics.Most failures take the form of SSR false turn on with no turn-on signal. For example, turn on mayoccur if operating temperatures exceed the thyristor rating. Also, transients from the switched loador from an ac line can momentarily exceed the thyristor breakover voltage, or steeply rising load

voltages can couple into the thyristor input through stray capacitances in the thyristor and causeturn on.

6.3 The primary failure mechanism of an SSR is mechanical fatigue in the power semiconductorstructure, caused by thermal cycling. Since semiconductor switches can dissipate significantamounts of power, solid-state relays must generally be heat sinked to minimize operatingtemperature.Proper heat sinks for most conditions are available or are an integral part of theSSR. SSRs generate heat because of the voltage drop present in all semiconductor devices. A40-A relay, for example, typically drops 1.2 V during conduction and, thus, dissipates 50 W of



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heat. However, SSR heat generation generally does not require special system design. Thesedevices usually mount on circuit boards or control panels containing other semiconductor devices.Cooling and heat-sinking methods used for these devices are likely to be adequate for the SSR.One of the most common causes of SSR failures in industrial applications is not derating the SSRfor the installed ambient temperature. Higher than expected enclosure or localized spottemperatures (limited or blocked ventilation/cooling) temperatures can cause early and repeatedfailures of SSRs. Consult the manufacturers derating charts for use with free-air and heat sinkapplications.

6.4 Some SSRs designed for controlling ac loads incorporate a zero-voltage turn-on circuit thatswitches the load on or off only when the power-line sine wave passes through zero. Some solid-state relays also incorporate snubber circuits or zero-crossing detectors to reduce spikes andtransients generated by interrupting load current. Highly capacitive loads such as lamps andheaters that produce high inrush currents at turn on generate little electromagnetic interference ifactuated when line voltage is zero.

6.5 SSRs are available with zero-voltage switching and proportional firing (example: converts a4-20ma signal to time proportional output), or direct connection to a heating controller with a time

proportional (pulsed DC) output.

7. Type of control

7.1 Static contactors can be applied to either single or threephase loads as an onoff, timeproportional, or a modulatedproportional type control. Even with onoff control, better operationcan be expected than with magnetic contactor control since the hysteresis and dead time iseliminated.

7.2 Timeproportional control (burst firing)ideally is suitable for static contactors. Timeproportional control is a form of onoff control where the control instrument is set to a basicrepetitive unit of time instead of waiting until the process temperature reaches the control point

before applying or dropping the power. The time over which power is applied to the process in agiven unit of time is dependent upon the deviation of the process temperature from the processset point. In processes where the thermal time constant or thermal inertia of the system is longcompared with the selected repetitive interval, the temperature of the process will be smoothed oraveraged to a constant level. The shorter the interval, the smoother will be the resultant processtemperature. Time-proportional control is not suitable for switching transformer primaries; phase-angle control is required.

Since static contactors are not limited by the frequency of operation, they can be used with avery high repetitive rate control. When the process is first fired, the temperature will be wellbelow the set point and the process will call for full power over a given period, as illustratedonFigure 2. As the process temperature nears the set point, the process will require lessheat and will begin to cut back on the applied power by providing certain off intervals. As

the process temperature levels off at the set point, the offtime intervals are increased sothat the average power applied will be that required to provide for the process losses.Burstfired units thermally shock heating loads with low thermal inertia, causing rapidtemperature changes and fatigue at connections. Full onfull off characteristics also causemechanical forces on system components. This type of controller cannot be used for (a)variable resistance loads, (b) inductive loads such as transformers, or (c) where soft start orcurrent limit is required.



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7.3 Modulatedproportional control (phase-angle fired) can be used in most applications. Itcosts slightly more than burstfired controllers and induces harmonics on the supply systemwhich may be objectionable, especially when phasecontrolled heaters are a major portion of theload on the system. Providing a supply transformer dedicated to the heating system can reduceproblems associated with transients coupled back into the site power system.

7.4 Where two or more heating elements are employed, especially if the heating loads are large,there are two common methods of control.

7.4.1 The heating elements may be switched on and off in a stepping manner to satisfy therequired load (heating) demand.

7.4.2 One or more base load heating elements may be switched on in conjunction with oneor more swing load heating elements, which are controlled proportionally on and off to suitthe required load (heating) demand.

7.5 For accurate temperature control where the heating load is at a fixed temperature and thesize of the heating load does not change except over a relatively narrow range, a minimum costinstallation can be designed with a manually switched heat input adjusted slightly below minimum

(or standby) requirements, supplemented by an additional heat input controlled by temperaturesensing instruments operating magnetic contactors to switch the additional heat, which shouldprovide a total heat input just slightly above the maximum heat required. Where conditions aresuch that the difference between maximum and minimum energy required is small relative to themaximum energy required, such a system can provide very accurate control in a very lowcostinstallation. Timeproportional control can be used with this arrangement for control of thenarrowband heating, if desired.

8. Comparison

8.1 Solid state devices (SCR & SSR) have the following advantages over magnetic contactors:

a. Low maintenancethere are no contact tips to dress or replace and no moving parts towear. Dust and dirt particles cannot affect the operation of SCR/SSR devices; however,excessive quantities will reduce heattransfer capabilities that may result in prematurefailure of the devices.

b. Quiet operationSCR/SSR devices are silent in operation. Magnetic contactors areinherently noisy because of the necessity to quickly pull in and drop out.

c. Vibration freesince there are no moving parts, SCR/SSR devices can be mounted on thesame panel.

d. No exposed arcSCR/SSR devices are sealed, there is no electric arc in the air (animportant consideration in ambient conditions where a hazardous atmosphere could

possibly exist due to gases or dusts). These devices permit their safe use in Class I,Division 2; Class II, Division 2; and Class III, Divisions 1 and 2 hazardous locations providingsurface temperatures do not exceed 80 percent of the ignition temperature (in degrees C) ofthe hazardous material (National Electrical Code requirement).

e. Long lifeyears of servicefree life of SCR/SSR compared to a relatively lower servicefreelife of magnetic contactors.

8.2 The main advantage of the magnetic contactor over SCR/SSR devices is lower initial cost.



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8.3 Basic SCR control systems for electric furnaces and heating applications, such aselectrically heated steam or gas heaters, autoclaves, and special boiler pressure controls areshown on Figure 3.

Figure 1. Circuit for inverse parallel connection of silicon-controlled rectifier units as alternating-currentcontactor

Figure 2. Time-proport ional control action



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Figure 3. Basic SCR control systems for electric heating applications

* Maximum load volts 99% of line volts

** Maximum load volts 93% to 97% of line volts

3A

Figure 3BFigure 3A

.**

Figure 3C

3D

Figure 3D Figure 3E



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Figure 3. Basic SCR control systems for electric heating applications (continued)

* Maximum load volts 99% of line volt

** Maximum load volts 93% to 97% of line volts

3F

Figure 3F Figure 3G

3E

Figure 3H

3H.

Figure 3J



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Figure 4A. Example SCR control system for a small to medium process gas heater

SCR

W/

Current

limit

OTC

I / I

FM

E / I

3

Remote Pot

Local/Remote Sw

4-20 ma DC from DCS

SCR Hi

Temp

Power On

Off / Enable Sw

Process (permissive)

Interlocks from DCS

ResetOvertemperature

Controller

Process Temp

to DCS

Process Flow

to DCS

Heater Currentto DCS

Heater Voltage

to DCS

Shorted SCR

to DCS

Shorted SCR

I2T HighSpeed Fuses

Contactor

Disconnect Sw

480 Vac - 3 phase

Electrostatically

Shielded Transformer

480 - 480Y208

Figure 4 A

Example - Large Process Gas Heater (see DE5H Section 5)

Circulation heater 50 Kw - 480 Vac

3 phase - 4 wire (Wye)

1. SCR power-pack: Six SCR (Three pair back-to-back) Voltage proportional power (phase-angle fired)

SCR, with current limit, shorted SCR & SCR high temperature alarms (see DE5H Figure 3G).

2. Basic temperature control, monitoring, and alarms in DCS (Process Control Controls System)

3. Safety layer: Agency approved overtemperature controller, sensing TC welded to heater sheath,

magnetic contactor to interrput 480 volt power to SCR.

1

Manual adjustment

potentiometer for test/

maintenance

Contactor

Enabled

to DCS

SCR Hi Temp

to DCS

Shutdown from OTC



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Figure 4B. Example SCR contro l system for a large process gas heater

SCR

W/

Current

limit

TC

3

SCR Hi Temp

Power On

Off / Enable Sw

Process (permissive)

Interlocks from

process control

system

Reset

OvertemperatureController

Shorted SCR

I2T High

Speed Fuses

Contactor

Disconnect Switch

480 Vac - 3 phase

Electrostatically

Shielded Transformer

480 - 480Y208

Figure 4 BExample - Small to Medium Process Gas Heater (see DE5H Section 5)

Circulation heater 15 Kw - 480 Vac

3 phase - 3 wire (Wye)

1. SCR power-pack: 3-phase w/ two controlled legs (four back-to-back SCR), open heater / shorted SCR

detection & SCR high temperature alarms (see DE5H Figure 3F).

2. Basic temperature control integral to control cabinet (local) , local dedicated alarms and a dry contact for

remote general alarm.

3. Safety layer: Agency approved overtemperature controller, sensing TC welded to heater sheath,

magnetic contactor to interrput 480 volt power to SCR.

Shutdown from OTC

OTC

A

Remote

General Alarm

Contactor

enabled

Temperature

Controller

Ammeter & Switch

Process Gas



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