are dc electric motors cheaper to operate than ac motors and why?
DC motors, if you are talking brushed, vs AC motors or Brushless? OK, it depends on the situation, but: Brushless motors have less friction, all other things being equal, and can be somewhat more efficient, wasting less electricity. Brushless motor controllers, if you are operating from DC, can be a bit more complicated, expensive, and a bit less reliable. However if you are operating from AC and the motor is matched to the incoming AC, i.e., 110vac 60 cycle, this is negated. Same for DC brushed motors. If they are operating from the correct voltage DC supply, then the controller is minimal, but still a bit of resistance from the brushes. However, to answer your question as asked, negating the start-up costs, a correctly designed AC motor operating from matched AC is the cheapest to operate- minimal controller, maximum efficiency. After that, brushed or brushless motor on DC , or brushed motor on AC, just depends on lots of different parameters, cost of failures, etc.
No... a dc electric motor would require the power be transformed into ac before the motor could work.
It is difficult to determine what is a fair comparison. Most AC motors are operated by direct connection to an AC supply with only a nearly 100% control device such as an on/off switch. Similarly, most DC motors are operated from a DC supply such as a battery. AC motors are used where power ratings are required. DC motors that are battery operated are usually quite low powered compared to AC motors. Larger machines are generally more efficient than small machines. An AC and a DC motor of equal power rating each operating from a matched power supply will be operate at comparable efficiencies. A large DC motor would likely be somewhat less efficient than a large 3-phase AC motor because of the commutator and field excitation losses. A small single phase motor would likely be somewhat less efficient than a small DC motor. When a DC motor is used on an AC supply it requires an AC/DC converter or rectifier. When an AC motor is used on a DC supply, it requires a DC/AC converter or inverter. If an AC and DC motor of equal power ratings are compared and one requires a converter while the other one doesn’t, the converter – motor combination will be less efficient. If an AC and a DC motor of equal power ratings are each equipped with a converter for speed control purposes, the converter – AC motor combination is likely to be more efficient for a large power rating and the DC combination is likely to be more efficient for a small power rating. A brushless permanent magnet DC motor can operate only with an electronic converter unit. It is essentially the same thing as a permanent magnet synchronous AC motor. This type of converter – motor combination is more efficient than the equivalent system with a commutator type DC motor or an induction motor.
The power company charges you for the energy (kw-hr). If you are going to move a constant load, let's say 50 KW. I doesn't matter what type of motor you use, you are going to need 50 Kw of power to move the load, and the company is charging you for that value, so speaking of the price you pay for the energy, it would be the same. But, there is an additional cost because the electrical company doesn't supply you with DC, you'd have to buy an inversor to supply it to the motor.
