The motor in use is a 3-phase 440V/37kW induction motor. I have received data regarding winding heaters from a motor manufacturer. If the rated voltage is 440V, a single-phase voltage of 24Vac must be applied, and the VA value is said to be 750VA. Since I'm using an inverter and I'm not going to decorate the extra circuit, I can only set the DC current value. I asked the motor maker if it was okay to pass DC current, and I got a reply saying that AC/DC is irrelevant. However, if a motor maker uses DC, they do not check the DC current value to be applied. Is it possible to find an appropriate amount of DC current with only the data of 750VA at 24Vac?

  • \$\begingroup\$ "can you temporarily apply 24 VAC and measure the real power delivered to the winding?" Is this intended to measure W to calculate DC since the given value is VA? I can't do it now because there is no separate circuit configured other than the inverter. If you can do an accurate calculation and it doesn't cost a lot, you can configure it, but I'd like to find another way if possible. "Do you know the winding inductance and resistance?" I don't know. I've asked but they don't tell me. I will ask the manufacturer again. If you get a value, how is it calculated? \$\endgroup\$ May 30, 2022 at 1:54
  • \$\begingroup\$ Yes, it is intended to get true watts instead of VA. Some motor manufacturers publish data for their motors. If you have the manufacturer name and model number for the motor, maybe we can try to look it up online. Are you able to measure the motor temperature? \$\endgroup\$
    – user57037
    May 30, 2022 at 1:57
  • \$\begingroup\$ We are going to install PTC. By composing a circuit, it is not an exact temperature, but it is possible to measure it to some extent. \$\endgroup\$ May 30, 2022 at 2:06
  • \$\begingroup\$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. \$\endgroup\$
    – Community Bot
    May 30, 2022 at 6:10

2 Answers 2


If the manufacturer indicates ac/dc is irrelevant, you can supply 24VDC at \$\frac{750VA}{24V}=31.25A\$ and it should be fine.

  • 1
    \$\begingroup\$ You are neglecting power factor and the winding inductance. Maybe that is correct, but I am not so sure. \$\endgroup\$
    – user57037
    May 30, 2022 at 1:37
  • \$\begingroup\$ @mkeith I had considered that, but the manufacturer indicates that ac/dc is irrelevant, so power factor is irrelevant. Now I don't believe that without checking for myself, but I work with the information I've been given. \$\endgroup\$
    – Bryan
    May 30, 2022 at 1:39
  • \$\begingroup\$ Well, I am reading the question slightly differently. To me the manufacturer told the OP "you can use DC if you want but you will have to figure out the proper current yourself." \$\endgroup\$
    – user57037
    May 30, 2022 at 1:41
  • 1
    \$\begingroup\$ Thank you for answer. What you're saying is that you don't have to think about AC/DC differently. Shouldn't it be taken as an rms value when calculating the current? 24Vac / 1.414 = 16.9Vrms, 750 / 16.9 = 44.3A I'm not sure what's right. In Google, I've seen an article saying that the current value should be lower when using DC current than when using AC current, so I'm not sure if I can simply think and apply it. \$\endgroup\$ May 30, 2022 at 1:45
  • \$\begingroup\$ mkeith is right. "You can use DC if you want but you will have to figure out the proper current yourself." This is the position of motor manufacturers. \$\endgroup\$ May 30, 2022 at 1:48

Based on all the discussion both on the original question and the answer provided by Bryan, it seems that the best option is to find a DC voltage which will produce about 31 amps in the winding. This should be based on whatever you consider to be the normal winding temperature. For example, maybe 25 °C.

The heat produced in the winding will be a function of winding resistance multiplied by the square of the current.

For a 24 VAC source at 750 VA, the RMS current will be about 31 A.

So the correct procedure would be to find a DC voltage which will produce the same current of 31 A at nominal winding temperature.

Copper wire (and aluminum wire also) has a positive temperature coefficient of about 4% for every 10 degrees. For this reason, it is better to heat the motor with a voltage source rather than a current source. The current source may eventually converge on a stable temperature but the voltage source will converge more rapidly and reliably and predictably.

My answer is based on a mixture of theory and experience. I do have experience with copper resistors, but I don not have experience with motor winding heaters. So proceed with caution and use your own judgement. I cannot promise you that the steps I have outlined will produce a good outcome for you.


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