Electric Motor Heater 7

[gurse]

I have a 7.5 kW motor with 110V, 40W anti condensation heater. But i have only 240 V supply for the heater, can I put a resistor in series with the heater to reduce the voltage please.

 

[edison123]

Better to use a 220/110 V step down transformer.

Muthu

http://www.edison.co.in

 

[itsmoked]

Better to use a 240V heater.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zlatkodo]

Hi, Gurse,
Maybe you can try with one diode connected in series with heater.
ACW

 

[gurse]

Thank you for your replies but I would like to use an appropriate power rated resistor, any comments on use of resistor please.

 

[edison123]

130 V, 47.2727 W resistor, if you are that particular.

Keith - Normally, the space heaters are run at half their rated voltage (typically 2 of 220 V rated heaters connected in series for a 220 V supply) to ensure a lower failure rate since changing a failed space heater is a pita.

Muthu

http://www.edison.co.in

 

[electricpete]

I get 366 ohms, 50 watt external resistor...

Heater parameters:
Prated = 40W
Vrated = 110V
Irated = Prated/Vrated = 40 / 110 = 0.36A
Rheater= Vrated/Irated = 110V/0.36A = 300 ohms

External resistance to keep current through space heater at Irated = 0.36A:
Rext = V / I – Rint = 240 / 0.36A – 300 = 366 ohms

Minimum power rating of external resistor would be 0.36^2 * 366 ~ 50 watts.
Of course you have to see what's resistance ratings are available, select one considering it's effect on motor space heater, and adjust the resistor minimum power rating according to the actual external resistance selected.

Someone double check me on the calculations.



=====================================
(2B)+(2B)' ?

 

[edison123]

pete

Space heater current = 40W/110V = 0.3636 A.
This current x voltage across external resistor (=240-110=130)V = 47.2727 W

Muthu

http://www.edison.co.in

 

[electricpete]

Hi Muthu

We both got roughly the same answer. Yours was quicker.
You were working toward specification in terms of a space heater (watts, voltage).
I was working toward specification in terms of a power resistor (resistance, watts).
Both good answers imo. I didn't look closely at yours before I posted.




=====================================
(2B)+(2B)' ?

 

[itsmoked]

Understood Muthu. You're right, a transformer gets you that.

You should NOT use an external dropping resistor. It's an insult to the planet. It will consume an additional waste of 438kWhrs a year. That's 1.13 x 438 = 500 pounds of completely pointless carbon dioxide. Do the right thing, as edison suggests, and put in a transformer. Run that 50W heater at something less than 120Vac since you sure don't need 40W worth of heat to do the job on a measly 7.5HP motor.

Get this transformer

wire it for 380V primary and feed the secondary to your heater. That will be 76Vac.

That will put the heater at 16W.

That's also about a $124/yr operating cost savings.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[che12345]

Dear Mr gurse

1. I am in full agreement with learned Mr itsmoked " You should NOT use an external
dropping resistor... It will consume an additional waste of 438kWhrs a year..."
2. I consider learned Mr itsmoked subsequent proposal (27 Jun 19 20:41)
" wire it for 380V primary and feed the secondary to your heater. That will be
76Vac . That will put the heater at 16W ... " is the BEST solution!
i.e. connect the 240V heater source to any "standard transformer" available in
your location with [primary voltage] rated close to 380V or 400V (exact value is not
critical).
3. Conclusion:
a) the heater would be operating "under voltage"; no issue with [reduce wattage],
but would increase its life considerably,
b) transformer loss would be lower than the external dropper resistor,
c) the size and the price of the transformer would not be very much more than the
external dropper resistor.

Che Kuan Yau (Singapore)

 

[gurse]

Considering all the options suggested, I have decided to use a diode in series with the heater.

 

[crshears]

You're welcome.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[itsmoked]

Not so fast cshears! You don't want any credit for this 'solution'.

--------------------------------------------------------------------------------

Poor choice grasshopper...

You're going to follow that decision with:

You said it's a 110V 40W heater. Using a diode results in it dissipating 80 Watts. I give it a week and maybe scorched motor windings.

Then:

Inevitably:

Keith Cress
kcress -

http://www.flaminsystems.com

 

[crshears]

Hey Keith, I wasn't taking credit; I was merely trying to make a subtle statement about gratitude...& FWIW going diode wouldn't have been my choice, either; if anything I would add my concurrence to che12345's ringing endorsement of your recommendation.

The only minor tweak I might have added is that if a transformer with a choice of both input and output taps was used, following commissioning I'd monitor for a week to ten days and see what sort of more-or-less standing delta T was achieved, which, if inadequate, would suggest selecting other taps to give just a smidgen more [or maybe less] of anti-condensation heat.

OP says 240VAC is what's available; if wired to the 380V hi-side terminals, would the upset in V/Hz ratio be of any consequence? Perhaps not in a transformer of such low rating?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

What Keith said.
40 Watts @ 110 Volts = 160+ Watts @ 240 Volts. The diode will cut that in half to 80+ Watts.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr gurse

1. " Considering all the options suggested, I have decided to use a diode in
series with the heater " . NO! NO! NO!!! You will regret if you insist on doing so.
2. With due respect, Mr zlatkodo proposal is [NOT] going to work. Technically [wrong].
3. Follow learned Mr itsmoked (27 Jun 19 20:41) proposal.
4. I have 50+ years of working experience, trust me. Learned Mr itsmoked'S proposal
is the [BEST] and after all, free-of-charge.

Che Kuan Yau (Singapore)

 

[waross]

Hi CR. Don't go over the V/Hz, but going under the V/Hz limit is not a problem. The flux curve of a transformer is pretty linear up to the saturation Knee point. The V/Hz limit keeps you below the knee point.
Under voltage on a motor is an issue with reduced torque, not saturation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr Dear Mr gurse

1. My apology for the typographical error:
" 2. With due respect, Mr zlatkodo proposal is [NOT] going....
should read " 2. With due respect, [Mr crshears] proposal is [NOT] going....

2. There is [NO] V/Hz problem at all to connect a 240V on a 380V or even 400V
tapping. See advice by learned Mr waross .

Che Kuan Yau (Singapore)

 

[crshears]

Thanks, Bill; I'm going to study on this just to wrap my head around the concept and do my best to cement it as firmly as I can in my mathematically challenged mind.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]