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Old April 25th, 2005, 04:47 PM   #1
leitmotif
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VFD to control heaters ??

CONCEPT is to use off the shelf VFD as low cost method to do proportional control on heaters. I know you can use relays and turn on/off more heaters - which may work fine
BUT this is "step" control not "variable / proportional control"

Basically turn up the speed output of the VFD and get more power to the heaters.

Heaters would have to operate in "threes" and they would have to closely match each other unless recified.

VFD output could be rectified to reduce noise problems. In this case recifiers would have to closely match each other.

Lessee
REASONS WONT WORK
1. Motors are not heaters. VFD controls motors.
2. VFD would not get back (or be able to calculate them) speed signals from motor. Therefore would trip.
3. VFD may not go low enough on voltage to give low range control of heaters. Has to have enough volts to get motor to roll at low frequency (RPM).
4. Rectifiers cannot handle the hi freq carrier signal ??
5. VFD requires other signals or calculations from a motor that a heater setup cannot replicate??

REASONS COULD WORK
1. VFD has variable output both frequency of pulses and magnitude of pulse. Combination of two is power.
2. Heaters don't care about frequency. Besides they would be fed off rectified VFD output.
3. Pulse control is used on heater applications is it not? VFD basically produces a pulse.
4. In limited research have not found anything saying it cannot be done.

Thoughts ??
Dan Bentler
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Old April 25th, 2005, 04:58 PM   #2
Peter Nachtwey
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This is a silly idea.

A drive cost much more than a heater controller and is suitable for controlling systems with time constants in the millisecond range. Heater systems have time constants in the many seconds or minutes range.

It would be a waste of a drive. I will stop there.
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Old April 25th, 2005, 08:08 PM   #3
rdrast
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Or just use something like this:
http://www.watlow.com/products/contr...r/dinamite.cfm
?

Watlow, Eurotherm, and many other manufacturers make phase-angle controlled heater controllers.
A good deal simpler, cheaper, smaller, and more suited to purpose than a VFD.
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Old April 25th, 2005, 08:43 PM   #4
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Keep it simple!

Quote:
VFD output could be rectified to reduce noise problems. In this case rectifiers would have to closely match each other.
The PWM waveform output from today's VFDs would not lend itself to rectification. The waveform only approximates a sine wave when used on an inductive load. Using resistive heater material would not yield the same response. Just use the simpler methods, that is always the best and least expensive.

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Old April 25th, 2005, 09:22 PM   #5
Mtn_Bkng_Dave
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Actually a modified DC drive is more suitable for this type of application.

Wire drawing machines used this method for the heat control for the annealing section of the machine.

The trick is to modify the SCR ouput bridge so that the gated 3 phase power is output out of the drive instead of DC.

We fed this power into a 50kva tranformer that stepped the voltage from 480 down to about 50volts for greater current output across the annealer sheaves.

The drive reference was a follower reference from the main line tachometer. Faster speed........more annealer voltage ouput and viceversa.

Hope this helps,

Dave
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Old April 25th, 2005, 09:35 PM   #6
leitmotif
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Thanks guys. I wondered if temperature controllers were cheaper. Was thinking of a project involving heaters at school - have no heater controllers but a fair amount of VFD. Also have some "old" DC drives.

Dan
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Old April 25th, 2005, 10:05 PM   #7
Mtn_Bkng_Dave
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If it is a small project system the heater controller is definitely the way to go.....The drive based system I was referring to would max out at almost 1000 amps of output to the heaters. Don't really think ya wanna unleash that on the lab bench!


Dave
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Old April 25th, 2005, 10:59 PM   #8
leitmotif
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Hmmm 1000 amps would probably dim the building lights. I am sure someone would question this. I am sure Facilities would get real ticked off at overloading the panel.

I guess in this case we are just not gonna use what we got.

Dan
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Old April 26th, 2005, 06:38 AM   #9
Steve Bailey
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For a lab project, instead of a dedicated controller, use a micro PLC and a solid state relay.

It can teach techniques involved in PWM and PID.
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Old April 26th, 2005, 08:10 AM   #10
leitmotif
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Steve

That is what is installed now. Thermocouples were used to monitor temp.
Two problems:
1. Thermocouples picked EMI and screwed up the readings.
2. Solid state relays do not provide proportional control. Instructor wants PID control.

Solution to 1 was fairly simple turn off heaters read TC into PLC (SLC 5/05) then if not warm enough turn heater bck on.
They could not find a way to do PID with the relays, was thinking a VFD would give them proportional control.

Budget problems so use what you got.
I am donating the 3 phase pump.
They do have some DC drives that may be adaptable.

That is another project and I have my own to finish up yet. It is also to learn PLC. Making progress. Described in other posts. I think you have seen them.


Dan
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Old April 26th, 2005, 08:36 AM   #11
Steve Bailey
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Dan,

You can do PID control with a solid-state relay. Use PWM techniques. Set up a duty cycle for the solid state relay. If the PLC output to the solid state relay is a contact output, make it a relatively long duty cycle. If the PLC has transistor or triac outputs the duty cycle can be short. Let the CV of the PID function control the percentage of the duty cycle that the solid state relay is on.
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Old April 26th, 2005, 08:41 AM   #12
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Further food for thought!

If you use SSRs, you can achieve PID given that you have a controller/PLC that is capable of doing time proportioning output.
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Old April 26th, 2005, 08:43 AM   #13
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Keep it simple.

If your heater is resistive, you can achieve PID control by two ways: use a controller (stand alone PID controller, or a PLC), with a time proportional output, connected to a standard zero-crossing solid state contactor. Just google on: "solid state contactors" for a variety of suppliers.

OR use a PLC with a PID analog output, or even just a 0-5k potentiometer, to generate a 4-20mA control signal to a: http://www.power-io.com/products/dm2540.htm

Both answers above are low EMI, zero-crossing solutions that are common in the industry.
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Old April 26th, 2005, 05:56 PM   #14
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If this is a school project that doesn't have to run 24/7 for a long time you can get by with regular relays using the PWM or time percentage methods described above. Just make sure your cycle time is several seconds so you aren't cycling your relays several times a second.

Both the AutomtionDirect.com 205 instruction manuals and the Allen Bradley Progam Reference manual have examples of timed output from a PID loop.

Note that if you are picking up noise on your thermocouple (T/C) this may not eliminate it, as any switching of AC voltage is going to produce EMI and RFI. Check the wiring on your T/Cs and make sure you are grounding the shield on the T/C wires at one end.
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Old April 27th, 2005, 01:00 AM   #15
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Dan,

I don't think that anyone said that the Variable Frequency Drives would not work, only that it would not be an EFFICIENT use of the equipment for normal industrial applications. I once hooked an audio amplifier to a DC motor. It wasn't a very efficient speaker, but you could listen to a news report from the motor...

I think that you could control a 3-phase heater with a VFD. A VFD will send out a voltage with NOTHING connected to the leads. I have checked the outputs many times with a voltmeter and no motor connected. I don't see why it will not work with some heaters connected.

If you don't have a 3-phase heater, then find 3 that are close to the same resistance (measure with ohmmeter) and wire in a delta or star configuration, depending on the size (kW) of VFD and the size (kW) of heaters. MAKE SURE THAT YOU HAVE A PROPER FUSE OR BREAKER ON YOUR POWER SUPPLY. Add your thermocouple for a feed-back signal to the PLC, add a potentiometer for setpoint input, write your PID block to control the VFD, and you will be in business.

Now will come the nay-sayers with all the reasons why you shouldn't do this. Some of them are teachers, and some of us are doers. Besides, you have got to let the smoke out of this equipment evey now and then, or it starts to clog up the chimney.

Last edited by Lancie1; April 27th, 2005 at 01:09 AM.
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