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For high-power motors around 1000 kW, the price of a DC motor is much higher than that of an AC motor of the same power. I know there is a commutator and brushes, but I don't think that is the only reason behind the higher cost. What else accounts for the price difference?

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    \$\begingroup\$ 1000kW as in 1MW ? Or 1000W for electric bikes for example \$\endgroup\$
    – Solar Mike
    Commented Dec 25, 2017 at 17:40
  • \$\begingroup\$ Example pricing? \$\endgroup\$ Commented Dec 25, 2017 at 20:03
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    \$\begingroup\$ 'kW', not 'Kw', for kilowatt if that's what you mean. \$\endgroup\$
    – Transistor
    Commented Mar 25, 2021 at 12:54

5 Answers 5

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Motor physical size and manufacturing cost is more closely proportional to torque capacity than to power capacity. If you compare two motors with the same power capacity, a higher speed, lower torque motor will be smaller and cost less than a lower speed, higher torque motor.

A motor with a higher operating voltage and lower current requirement will have a lower cost than a motor with lower operating voltage and higher current.

A 1000 kW AC motor will most likely be a three-phase, squirrel-cage, induction motor. Since the current is carried by three conductors rather than two, that represents a slight advantage over the DC motor.

The wound rotor and commutator of a DC motor is quite a bit more complicated then the rotor of an induction motor. It is made out of copper and iron rather than aluminum and iron. The heavier rotor of the DC motor may require more expensive bearings. The rotor design is probably the most important factor in making the DC motor more expensive.

If the DC motor stator has permanent magnets, that could make it more expensive or not depending on the magnet material.

There are many more manufacturers making AC motors and many more AC motors used at 1000 kW. At 1000 kW, DC motors have rarely been used except in applications requiring adjustable speed. That gives the AC motors a big advantage in manufacturing cost because of economies of scale. More competitors making AC motors would tend to drive the selling price down.

Since DC motor buyers are buying motors for adjustable speed use, they are buying an electronic speed control along with the motor. They will be comparing the motor - controller package prices, not the motor prices. They may also be more concerned about quality than a lot of AC motor buyers. The least expensive AC motor on the market may be of lower quality than available DC motors rated 1000 kW.

Regarding Motor Power and Type and VFD Use

In the motor manufacturing industry, a large motor generally refers to a motor that is physically larger than the frame sizes with dimensions specified by IEC and NEMA. In terms of power, large motors are generally those rated about 400 kW and above. Medium motors are rated about 1 to 400 kW. Small motors are 1 kW and less.

Induction motors are available with power ratings up to about 22 MW, but there are more manufacturers at lower power ratings. Induction motors rated a 1 MW are not at all uncommon. Large induction motors are manufactured by Siemens, TECO-Westinghouse, ABB, General Electric, Toshiba, WEG, Marathon Electric and others.

Large synchronous motors are manufactured by Siemens, TECO-Westinghouse, ABB, General Electric and Toshiba and probably others with power ratings between 1 and 100 MW.

A synchronous motor might be selected for a 1 MW application, but an induction motor seems more likely. In either case, the operation voltage is not likely to be less than 2000 volts and more likely to be about 4000 volts.

It is really not likely that a DC motor would be considered for any application at the 1 MW power level. If it is considered it would not make sense to consider only the price to the motors. A reasonable comparison should include the motor, controller (or starter) and projected future maintenance costs. Even including a VFD with the AC motor, the DC system is likely to be more expensive, particularly if maintenance and reliability is considered.

VFDs are available for both induction and synchronous motors at all power levels at which motors are available. Most large AC motors are not used with VFDs, but many are and the very largest motors may require VFDs for starting.

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    \$\begingroup\$ A few notes: A 1MW AC motor is quite likely to be synchronous (not induction), and AC motors with VFDs are increasingly being used for variable-speed operation, even up to those large capacities. \$\endgroup\$ Commented Dec 26, 2017 at 7:27
  • \$\begingroup\$ @Someone Somewhere see addition to my answer. \$\endgroup\$
    – user80875
    Commented Dec 26, 2017 at 18:05
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If you are speaking about dc brushless motors, one reason is better design. Combined with lower quantities you have to pay for that. In addition there are magnets, which may be very expensive so special matterials will allow stronger fields in smaller volumes.

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    \$\begingroup\$ Thanks for ur answer. But, I just wanted answer about normal DC motor, not about BLDC \$\endgroup\$ Commented Dec 25, 2017 at 16:52
  • \$\begingroup\$ Then i can only guess its because of the brushes and magnets. \$\endgroup\$
    – user76844
    Commented Dec 25, 2017 at 16:53
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    \$\begingroup\$ There is no 'normal' anymore for ac or dc motors. Thanks to permanent magnets with immensely powerful magnetic fields of 10,000 Gauss or more, BLDC motors and huge ac motors with powerful torque are much more common, hence the proliferation of hybrid cars and buses. \$\endgroup\$
    – user105652
    Commented Dec 25, 2017 at 23:17
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PMDC motors contain permanent magnets which are not cheap. The amount of magnet required is approximately proportional to the power. Also the magnetic flux density available from magnets is lower than electromagnets which means the motor may need to be bigger. Induction motors and wound-rotor motors contain only copper windings and iron pole pieces.

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It's actually much much simpler than that: the answer is sales volume. In most applications using motors that size, AC is going to be selected 20:1 over DC, maybe even more. I work for a motor manufacturer, in the last 6 years in my small office service the Western US we have not sold one single DC motor over 50HP; not even one INQUIRY to sell one. But during that time we have probably sold 10-20 AC motors over 600HP. If someone wanted to buy a 1000kW DC motor right now we could do it, but it would be a special order and likely cost 2x the price of an AC motor that we would ship off the shelf.

In order to build it, we would have to shut down a production line and re-tool it to make the parts, meaning lost production of the motors we DO sell every day. We will make you pay for that...

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The rotor of an AC motor has permanent magnetisation, the rotor of a DC motor has its magnetisation wander around all the time through the communicator. That means that the rotor of a DC motor has to support all directions of magnetisation (and must not lose significant power when changing magnetisation, so it needs to be laminated), while the rotor of an AC motor just needs to support a single orientation of magnetisation (in case of a synchronous motor) or is intended to lose a lot of power when being remagnetised (in case of an asynchronous motor) while not needing copper wire packages or solder points, so it is cheaper and more resilient against heat.

However, at 1000kW anything but a synchronous machine seems kind of unusual.

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