Sunday, 15 October 2017

Interview questions and answers on MOTORS...

By Ask Biomedical
An AC motor is an electric motor driven by an alternating current (AC). It commonly consists of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field.

A DC motor is a mechanically commutated electric motor powered from direct current (DC). The stator is stationary in space by definition and therefore so is its current. The current in the rotor is switched by the commutator to also be stationary in space. This is how the relative angle between the stator and rotor magnetic flux is maintained near 90 degrees, which generates the maximum torque.

1. Low cost,
2. long life,
3. high efficiency,
4. large ratings available (to 1 MW or more),
5. large number of standardized types.
Disadvantages of AC Motors:
1. Starting inrush current can be high,
2. speed control requires variable frequency source.
Advantages of DC Motors:
1. Precision positioning,
2. High holding torque,
3. Long lifespan,
4. Low maintenance,
5. High efficiency
Disadvantages of DC Motors:
1. Some can be costly,
2. Require a controller,
3. Higher initial cost,
4. Requires a controller.

A series-wound motor is known as a universal motor when it has been designed to operate on either AC or DC power. It operate well on AC because the current in both the field and the armature (and hence the resultant magnetic fields) will alternate in synchronism and hence, the resulting mechanical force will occur in a constant direction of rotation.

The following are major designs and manufacturing standards covering electric motors:
International Electro-technical Commission: IEC 60034 Rotating Electrical Machines
National Electrical Manufacturers Association (USA): NEMA MG 1 Motors and Generators
Underwriters Laboratories (USA): UL 1004 – Standard for Electric Motors

6)What is the back emf ?*
ans. When the motor armature continues to rotate due to motot action the armature conductor cuts te magentic flux, and therefore emf are induced in them the direction of this induced emf is such that it oppose the applied voltage therefore this is called Back emf. .

7)What is a stepper motor for?*
Stepper motors are DC motors that move in discrete steps. They have multiple coils that are organized in groups called "phases". By energizing each phase in sequence, the motorwill rotate, one step at a time. With a computer controlled stepping you can achieve very precise positioning and/or speed control.

8)What are the types of stepper motors?*
Permanent magnet stepper.
Hybrid synchronous stepper.
Variable reluctance stepper.

9)How do you control a stepper motor?*
Stepper motors can move an exact amount of degrees (or steps) when told to do so. This gives you total control over the motor, allowing you to move it to an exact location and hold that position. It does so by powering coils inside the motor for very short periods of time.

10)What is the use of stepper motor driver?
The EasyDriver is a simple to use stepper motor driver, compatible with anything that can output a digital 0 to 5V pulse (or 0 to 3.3V pulse if you solder SJ2 closed on the EasyDriver). The EasyDriver requires a 6V to 30V supply to power the motor and can power any voltage of stepper motor 

11)What are the advantages of a stepper motor?*
Advantages of step motors are low cost, high reliability, high torque at low speeds and a simple, rugged construction that operates in almost any environment. The main disadvantages in using a stepper motor is the resonance effect often exhibited at low speeds and decreasing torque with increasing speed.

12)What is the difference between a stepper motor and a servo motor?*
Stepper vs Servo. The basic difference between a traditional stepper and a servo-based system is the type of motor and how it is controlled. ... Steppers don't require encoders since they can accurately move between their many poles whereas servos, with few poles, require an encoder to keep track of their position.

13)How does a stepper motor controller work?*
First is the pulse generator, also known as a stepper motor controller or indexer. ... A step motor has two primary parts; the rotor, the moving piece, and the stator, the stationary piece. The stator contains coils of wire called windings. The rotor spins on bearings or bushings inside the stator.

14)Where is a stepper motor used?*
Today, stepper motor applications are all around us: they are used in printers (paper feed, print wheel), disk drives, clocks and watches, as well as used in factory automation and machinery. A stepper motor is most often found in motion systems requiring position control.

15)How does it work stepper motor?*
Stepper motors consist of a permanent magnetic rotating shaft, called the rotor, and electromagnets on the stationary portion that surrounds the motor, called the stator. Figure 1 illustrates one complete rotation of a stepper motor. ... You may double the resolution of some motors by a process known as "half-stepping".


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