Ac and dc machines

[PG1995]

Hi

It says here, "Off-shore, military, textile industry, marine, computer mainframe, aircraft, and spacecraft applications sometimes use 400 Hz, for benefits of reduced weight of apparatus or higher motor speeds".

Q1: It looks like induction motors are used in airplanes because they are more reliable, require less maintenance, and don't pose ignition/sparking problems because they don't use commutators. What are other significant advantages in using induction motors in airplanes etc.?

Q2: By the way, don't you think many of the problems associated with DC motors can be avoided by using brushless DC motor? Can't brushless DC motors compete with induction motors? Perhaps, they aren't suited for heavy duty work required in industries.

Q3: What type of motors, induction or DC, are used in automobiles?

Q4: It is said that DC motors offer a very high starting torque. Why is it so? Why can't AC or induction motors offer high starting torque?

Regards
PG

Helpful links:
The following links talk about the advantages and disadvantages of AC and DC motors.
1: **broken link removed**
2: https://electricalquestionsguide.blogspot.com/2011/05/dc-motors-advantages-disadvantages-ac.html
3: https://answers.yahoo.com/question/index?qid=20070329091024AAn3OA3

 

[jpanhalt]

Hi

It says here, "Off-shore, military, textile industry, marine, computer mainframe, aircraft, and spacecraft applications sometimes use 400 Hz, for benefits of reduced weight of apparatus or higher motor speeds".

I don't think there is a sharp line between AC and DC motor use. You have to look at the history and available technology of the times. Then, there is reluctance to change what works to something that is new.

For example, 400 Hz was used in aircraft because of the savings in weight for components, namely transformers. However, that 400 Hz was generated on board from batteries by a DC inverter. Many gyros for attitude control still used DC brushed motors in civilian airplanes -- at least until recently (my experience with aircraft instruments is relegated to the 20th century). Solid state sensors may be replacing those motors.

With the advent of solid state devices and microcontrollers, brushless DC motors became much more accessible. Their speed is quite easily controlled, and radio/HF interference is far less than with brushed motors. Thus, they find increasing use.

In early automobiles, where weight was not such a concern, DC motors prevailed as they were easy to use from batteries. Windshield wipers, as an example that today runs from an electric motor, historically ran from manifold vacuum, which could also control speed easily.

While DC motors have high starting torque, so do multiphase AC and brushless DC motors. Model aircraft almost exclusively use brushless DC today. However, on my lathe, I use a brushed DC motor with PWM control. It has very high, low-speed torque and no hesitation at the start. Hesitation at start-up is one thing you may see with brushless DC motors.

John

 

[JimB]

Q1: It looks like induction motors are used in airplanes because they are more reliable, require less maintenance, and don't pose ignition/sparking problems because they don't use commutators. What are other significant advantages in using induction motors in airplanes etc.? I dont know, are there any other advantages?

By the way, don't you think many of the problems associated with DC motors can be avoided by using brushless DC motor?
Yes possibly.
But just replacing one type of motor with another because it is a newer design is not necessarily the way to go.
When an aircraft is designed from scratch, the choice of motor is not just AC, DC or ?? things such as what type of power is available will influence the choice of motor.
In an aircraft many of the motors are hydraulic rather than electric.

Q2: What type of motors, induction or DC, are used in automobiles?
Mostly cheap simple DC motors with a commutator.


Q3: It is said that DC motors offer a very high starting torque. Why is so? Why can't AC or induction motors offer high starting torque?
They can if increase the rotor resistance. But the downside is that he normal running speed torque is poor. But then you could add slip rings and have an adjustable starting resistor. But that adds cost and complication.

JimB

 

[steveB]

I don't feel I have enough experience with motors to answer these questions well, but here is a general observation.

It is interesting how changing technology, not only in the motor design, but also in other areas, can affect how and where motors types are used. Consider the induction motor. It is one of the oldest and simplest designs. It is rugged and reliable also. Historically, it has been used in many applications, but one issue was always that it was hard to control precisely. You could plug it in to the AC power and it would work just fine, but if you wanted to precisely control speed and torque, it was necessary to use other motors that were easier to control.

However, in the last few decades we've seen the development of improved power electronics making high power inverters a reality and we've seen the development of high speed computing devices, such as microcontrollers with real number math capabilities (even double precision math). These technologies make it possible to use induction motors in applications where speed and torque must be precisely and accurately controlled.

What this means is that the induction motor was originally the main workhorse, and was not competitive in some of the emerging applications as time went by, and other motor designs were used there. But, now induction motors are finding their way into some of those applications. Also, they are being considered for new applications, whereas before they would not (and could not) have been an option.