Which of the following motors is preferred when quick speed reversal is the main consideration?
| 1) | Squirrel cage induction motor |
|---|
| 2) | Wound rotor induction motor |
| 3) | Synchronous motor |
| 4) | D. C. motor |
| 5) | NULL |
Induction motor speed control from stator side
- By changing the applied voltage: From the torque equation of induction motor,
- By changing the applied frequency. Synchronous speed of the rotating magnetic field of an induction motor is given by,
A Dahlander motor (also known as a pole changing motor, dual- or two speed-motor) is a type of multispeed induction motor, in which the speed of the motor is varied by altering the number of poles; this is achieved by altering the wiring connections inside the motor.
What will happen if DC motor is used without starter? Explanation: It would cause intolerably heavy sparking at the brushes which may destroy the commutator and brush-gear. Sudden development of large torque will cause mechanical shock to the shaft, reducing its life.
Synchronous motors are made in speeds from 1800 (two-pole) 150 rpm (48-pole). They operate at the constant speed without slip an important characteristic in some applications. Their efficiency is 1-2 to 5% higher than that of induction motors, the higher value at the lower speeds.
If the air gap of an induction motor is increased, the following will happen: The permeability of the magnetic circuit rotor-to-stator will decrease. The magnetic flux in the air gap will decrease and leakage fluxes will increase. This will cause a reduction in the maximum available torque.
⇒ When the load is above____ a synchronous motor is found to be more economical.
The three phase induction motors are self-starting due to rotating magnetic field. But the motors show tendency to draw very high current at the time of starting. Hence there should be a device which can limit such high starting current. Such a device which limits high starting current is called a starter.
Efficiency:IE3 average value is 91.5%, IE2 average value is 87%. Note: The above all are calculated value of electromagnetism. From the above data, we can see that: one 7.5KW motor which has Grade 1 energy efficiency can save 892 degree than Grade 2.
When energized, a direct on line (DOL) starter immediately connects the motor terminals directly to the power supply. In smaller sizes a motor starter is a manually operated switch; larger motors, or those requiring remote or automatic control, use magnetic contactors.
Slip-ring motors are widely used when high starting torque is required. Rheostatic and liquid immersed resistance starters are used for starting the slip ring motors.
- Methods of Starting Three Phase Induction Motors.
- Direct-on-Line Starter (DOL)
- Star Delta starter.
- Auto-Transformer Starting.
- Rotor Resistance Starter.
- Review.
When 3 phase voltage applied across the stator winding for starting of induction motor, high inrush currents magnetize the air gap between the stator and rotor. An induces emf is generated in the rotor windings of the induction motor because of the rotating magnetic field.
The two main types of starters are Manual Starters and AC Magnetic Motor Starters, commonly known as Motor Starters. The manual starter is operated manually.
: a definite speed for an alternating-current machine that is dependent on the frequency of the supply circuit because the rotating member passes one pair of poles for each alternation of the alternating current.
With a 4-pole machine, the rotor will move from 1 pole pair to the next in 1 AC supply cycle, so will do half a rev in one supply cycle - the synchronous speed is therefore 50/2 revs per second, or 1500 rpm. A 4-pole induction motor will have a nominal speed of 1450 rpm or thereabouts.
6 poles = 1,000 RPM (~970 RPM) 8 poles = 750 RPM (~720 RPM) For a 60 Hz three phase supply: 2 poles = 3,600 RPM (nameplate would list around 3,500 RPM) 4 poles = 1,800 RPM (~1,7.
Poles = number of poles of your motor. So, for example, if you have a 4 pole motor on 60 Hz then RPM=60*(2/4)*60=1800 rpm.
For a 3-phase motor, if you count a total of 12 coil groups, it has 4 magnetic poles. For a 12-pole 3-phase machine, there will be 36 coils. The number of magnetic poles in the rotor is equal to the number of magnetic poles in the stator.
The stator windings also connect to the ac power as in an induction motor. The stator magnetic field rotates in sync with the line frequency. The rotor winding in a synchronous motor may receive current in a variety of ways, but usually not by induction (except in some designs, only to provide start-up torque).
Above a certain size, synchronous motors are not self-starting motors. This property is due to the inertia of the rotor; it cannot instantly follow the rotation of the magnetic field of the stator. Once the rotor nears the synchronous speed, the field winding is excited, and the motor pulls into synchronization.
First of all think of a DC Motor. If it is an 8-pole motor is will have 8 field coils evenly spaced all the way around it. Now, in an AC Motor with 3-phases you will have the 8-poles 3 times. That means you would have 24-groups of coils.
A 4 pole motor has more torque so it will lose LESS RPM under load than the 2 pole, which in the end will make it faster than a 2 pole motor.
The equation for calculating synchronous speed is: S = 120 f/P speed = constant (120) times frequency of power source (60 Hz) divided by number of poles used in the motor (P).
Applications of the Shaded Pole Induction MotorThey are suitable for small devices like relays and fans because of its low cost and easy starting. Used in exhaust fans, hair dryers and also in table fans.
Shaded-pole motors of the asymmetrical type shown are only reversible by disassembly and flipping over the stator, though some similar looking motors have small, switch-shortable auxiliary windings of thin wire instead of thick copper bars and can reverse electrically.
Definition of shaded-pole motor: a single-phase induction motor in small ratings made self-starting by flux displacement derived by means of a permanently short-circuited, high-resistance winding or loop encircling a fraction of the pole pitch or pole piece.
A Shaded Pole motor is an AC single phase induction motor. The auxiliary winding, which is composed of a copper ring, is called a shading coil. When power is applied to the stator, the flux in the main part of the pole induces a voltage in the shading coil, which acts as a transformer secondary winding.
The number of pole pairs (p) or number of poles for an asynchronous motor determines the motor's synchronous speed: Two-pole and four-pole asynchronous motors are typically used for centrifugal pumps. The rotational speed of these motors can be varied using frequency inverters.
Which of the following motors has two separate windings on the
- A. Repulsion motor.
- Repulsion induction motor.
- Repulsion start induction run motor.
