CR4® - The Engineer's Place for News and Discussion®


Previous in Forum: Voltage Factor for Voltage Transformers   Next in Forum: Changing an AC Package Unit 460 v 3 Phase to 230 v 3 Phase
Close
Close
Close
12 comments
Guru
Engineering Fields - Mechanical Engineering - New Member

Join Date: May 2008
Location: CHENNAI, TAMIL NADU, INDIA.
Posts: 1794
Good Answers: 62

Electric Heater Power Control by Voltage Variation

11/10/2017 10:26 PM

Dear CR4 Members,

We have 4 Electrical Heaters of 4 KW each to heat water to 95 Deg.C and constantly maintain the temp., to heat milk and normal procedure is either by Thermostat or RTD control through on-off system.

Now I have been informed that Thyristor or VFD principles can be thought as alternative to control the voltage to increase or decrease the heat liberated by the heater.

The heaters are designed for 415 volts, phase to phase and 3 will work at a time and 4th one will be stand by.

The power liberated is proportional to ( V^2)/R and naturally voltage variation will alter thee heat liberated by heaters.

I seek the advise of CR4 members to share their viws /advise,

Thanks,

DHAYANANDHAN.S

Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 19994
Good Answers: 751
#1

Re: Power Control in Heaters by Voltage Variation

11/10/2017 11:08 PM

Please quit shouting.

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
Register to Reply Off Topic (Score 4)
Power-User

Join Date: Jan 2010
Location: Hyderabad
Posts: 160
Good Answers: 3
#2

Re: ELEC.HEATER POWER CONTROL IN HEATERS BY VOLTAGE VARIATION

11/11/2017 2:42 AM

Thyristor control is widely used in Oil & Gas as well as power stations for heaters.

You can safely go for it.

Just make sure that the thyristors do not introduce too much harmonics in to your system.

It is generally a combination of some fixed and some variable heaters.

__________________
Raghunath
Register to Reply
Guru

Join Date: Dec 2010
Posts: 1221
Good Answers: 89
#3

Re: ELEC.HEATER POWER CONTROL IN HEATERS BY VOLTAGE VARIATION

11/11/2017 7:31 AM

TRIACS should be fine for the size of resistive load you mention. Firing circuits are simpler than Thyristors (SCRs).

The zero-voltage(voltage zero-crossing) firing, integral cycle method is recommended for heaters. Unlike phase angle firing, it does not cause sudden steps in current or DC currents which cause EMI, supply harmonics problems .

Roughly speaking, ON for a variable number of complete cycles out of 100 varies the heating power in fine steps much better than thermostat enabling closer temperature control with RTDs or thermistors as sensors. As mentioned in other posts, one or more heaters can be on continuously to match the minimum heating demand, so not all require Triac control.

Modern microcontrollers can easily handle the necessary selection of cycles, I suggest breaking into groups so average over 100ms is fairly constant, 1 second may thermal cycle the wire elements in the heaters too much. I suggest you consult the heater makers about what cycling will not affect life - most heaters with thermostats stay on for minutes.

Register to Reply Score 1 for Good Answer
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain, and now disconnecting as Little England and Wales (not too sure about Wales bit, either). Kettle's on.
Posts: 26721
Good Answers: 701
#4

Re: Electric Heater Power Control by Voltage Variation

11/11/2017 9:51 AM

One would need a 3-term controller to do that. The equipment can be discovered and selected using an internet search. The next biggest challenge will be tuning the loop.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
Guru
Hobbies - Fishing - Old Salt Hobbies - CNC - New Member United States - US - Statue of Liberty - New Member

Join Date: Mar 2007
Location: Rosedale, Maryland USA
Posts: 5213
Good Answers: 269
#5

Re: Electric Heater Power Control by Voltage Variation

11/11/2017 10:27 AM

Simple temperature control driving a solid state contactor. Changed a wash tank over to this type system. Has already paid for itself. The old dish washer heater would wear the tubular heater out in 3 to 4 years. The change over has run for 12 years with no problems. Maintains the water at constant temperature of 140 F.

__________________
Life is not a journey to the grave with the intention of arriving in a pretty, pristine body but rather to come sliding in sideways, all used up and exclaiming, "Wow, what a ride!"
Register to Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 19301
Good Answers: 1129
#6

Re: Electric Heater Power Control by Voltage Variation

11/11/2017 5:55 PM
__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Register to Reply
Power-User

Join Date: Sep 2016
Location: SPAIN
Posts: 170
Good Answers: 7
#7

Re: Electric Heater Power Control by Voltage Variation

11/11/2017 7:30 PM

Listen, choppers are used when they are needed.

Your system seems slow enough to be controlled with a contactor for the 4th heater. If that works, it should be fine.

You can do it with a silicon switch as well, but in this case, I doubt it will be cost effective. You can make the numbers yourself.

__________________
Building the future!
Register to Reply
Power-User
New Zealand - Member - Kiwi

Join Date: Oct 2007
Location: Christchurch, New Zealand
Posts: 304
Good Answers: 28
#8

Re: Electric Heater Power Control by Voltage Variation

11/12/2017 12:23 AM

This can best be done with a PID controller and solid-state relay setup.

There are many PID heating controllers available that have SSR drive outputs. Add in a 3-phase SSR with a sufficiently-high current rating and you are mostly there.

__________________
paulusgnome
Register to Reply
Member

Join Date: Jul 2017
Posts: 7
#9

Re: Electric Heater Power Control by Voltage Variation

11/12/2017 8:11 AM

Unlike a drive and motor situation where a VFD could be used to save power there is no power saving benefit on heater systems.

It takes a fixed amount of heater wattage to raise you fluid to the desired temperature so the introduction of an SCR will not give you any power savings unless your current system is greatly overshooting temperature and then coming back down.

A good temperature controller with PID function that is properly auto tuned is very accurate as long as heater kW is grossly over sized and will give you a much longer functional life.

If service life of the contactor is a concern I have not notice any better performance with SCR units over a standard contactor as long as they are sized properly to handle the closing arc.

Register to Reply Score 1 for Good Answer
Power-User
New Zealand - Member - Kiwi

Join Date: Oct 2007
Location: Christchurch, New Zealand
Posts: 304
Good Answers: 28
#10
In reply to #9

Re: Electric Heater Power Control by Voltage Variation

11/13/2017 3:04 PM

If it is just a fluid that you are heating, and if the fluid is not damaged by higher temperatures, then running the heater at 100% is fine.

Here, I design heating for big (1.5 ton) lumps of butterfat, lard, palm oil and the like, and it is often necessary to back the heating off some to avoid overheating the product. Phase changes of the product introduce a whole new set of problems.

That said, it is not usually necessary to drop the heater power, it usually runs from a PID controller that does a PWM on the heater power.

__________________
paulusgnome
Register to Reply
Guru
United States - Member - New Member Engineering Fields - Power Engineering - New Member

Join Date: Sep 2006
Location: California, USA, where the Godless live next door to God.
Posts: 4408
Good Answers: 732
#11

Re: Electric Heater Power Control by Voltage Variation

11/13/2017 4:06 PM

You can get a PID temperature controller and have it bang the contactors on and off, that's not the problem. The only issue is that when you do that, the heaters are either all the way on or all the way off, so you end up with "overshoot" of the temperature within the cycles. Solid State Relay (SSR) control gives you more ACCURATE control of the heating process (meaning less overshoot) and no wear and tear on contactors, but if accuracy of the control is unimportant and contactors last you a long time anyway, I agree with the others, it's pointless. The down side of SSR control is that the SSRs produce heat; about 1-1/2W per running load amp. That heat must be dissipated properly that that often limits how and where you can mount the controller.

__________________
** All I every really wanted to be, was... A LUMBERJACK!.**
Register to Reply
Guru
Safety - Hazmat - New Member United States - US - Statue of Liberty - New Member Engineering Fields - Chemical Engineering - Old Hand

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 13569
Good Answers: 155
#12

Re: Electric Heater Power Control by Voltage Variation

11/14/2017 12:41 PM

Switching power to control heaters is typically the way it is done. It comes down to having the appropriate digital "word" for the number of heaters available on-line.

Thus, not all the heaters will be "on" at the same time, necessarily, and the milk will not be scorched.

__________________
If it ain't broke, don't fix it. Just build a better one.
Register to Reply
Register to Reply 12 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

67model (1); Ivanov327950 (1); James Stewart (1); JRaef (1); khelectrical (1); ozzb (1); Paulusgnome (2); PWSlack (1); raghun (1); SolarEagle (1); Tornado (1)

Previous in Forum: Voltage Factor for Voltage Transformers   Next in Forum: Changing an AC Package Unit 460 v 3 Phase to 230 v 3 Phase

Advertisement