Field Instrumentation

What is the purpose of the Universal Motor?

characteristics of universal motor

What is Universal Motor?

A unique kind of motor called a universal motor is made to operate on either a single phase of AC power or a DC supply. These motors typically have series wound armatures and field windings, which results in strong starting torque. Because of this, universal motors frequently arrive pre-installed in the machinery they are designed to power. The majority of universal motors are built to function at speeds more than 3500 RPM. Due to the reactance voltage loss that exists in AC but not in DC, they operate at a slower pace under an AC source than they do under a DC supply of the same voltage.

Construction of Universal Motor :

A DC machine’s architecture is remarkably similar to that of a universal motor. Field poles are installed atop a stator, which is what it comprises of. On the field poles are woven field coils.

However, the entire magnetic route, including the armature and the stator field circuit, is laminated. To reduce the eddy currents that operate on AC, lamination is important.

The commutator has brushes sitting on it, and the rotary armature is of the wound kind with straight or skewed slots. Due to the current induced in the armature coils, the commutation on AC is worse than that for DC. Because of this, the brushes that are used have high resistance.

Universal motor’s primary components are as follow,

  1. Field Core (Rolled Steel or Aluminium)
  2. Armature
  3. Frame
  4. End plates

Typically, through bolts are used to secure the field poles to the frame. The frame is frequently built to be an essential component of the machine it supports.

Laminations that are strongly packed together make up the field core. The laminations are made to hold a two-pole motor’s two field poles. The tiny DC motor’s armature is comparable. Essentially, it is a laminated core with either straight slots or slots that are not straight and a commutator to which the armature winding’s leads are attached. The commutator and core are both pressed on the shaft. The ends of the frame are where the end plates are situated. Bearings are housed within plates. A cast end plate is a common feature of universal motor frames. From this kind of engine, just one plate may be removed.

Working of Universal Motor :

1. Working in AC Supply :

  • The armature and field winding of a universal motor both experience the same ac current flow when powered by an AC supply. Because the armature winding and the field winding are connected in series, the same current is carried by both while the current-carrying armature conductors are in the magnetic field.
  • In order to maintain the same force direction experienced by armature conductors, the direction of the magnetic field and the direction of the armature current reverse.
  • It still generates a single-direction torque when supplied with AC power. Armature winding and field winding are in phase because they are coupled in series. When a result, the direction of current in the armature and field winding simultaneously reverses as the polarity of AC periodically changes.

2. Working in DC Supply :

  • The universal motor behaves like a DC series motor when it is supplied with a DC supply.
  • The field winding and armature conductors both experience the same DC current.
  • In the field winding, it creates an electromagnetic field.
  • A mechanical force and torque are applied to the current-carrying armature when a current-carrying conductor is put in a magnetic field, which causes the rotor to begin rotating.
  • The conductors of the armature likewise carry the same current. A current-carrying conductor encounters a mechanical force when it is exposed to an electromagnetic field.
  • The rotor begins to revolve as a result of this mechanical force, or torque. The Fleming left hand rule determines the direction of this force.

Therefore, universal motors operate on the same principles as DC series motors, whether they are powered by an AC or DC supply.

Speed and Load Characteristics of Universal Motor :

A universal motor’s speed/load characteristics are comparable to those of a DC series motor. A universal motor’s speed is low under full load and extremely high under no load. Gear trains are typically utilised to achieve the desired speed on the demanded load. The properties of the speed and load apply to both AC and DC supplies. The series motor produces less torque when powered by an AC supply. The direction of rotation can be changed by switching the connections between the fields and the armature.

Types of Universal Motors :

They are classified into two types:

  1. Compensated type universal motor and
  2. Uncompensated type universal motor

Advantages of Universal Motor :

  • The universal motor may be used with both DC and AC power sources, unlike other motors, as its name suggests. However, DC motors can only function effectively when a DC source is used as the input source. Similar to this, AC motors function well when an AC source is used as the input source.
  • When compared to other motors, the universal motor is more affordable. Due to its straightforward design and lack of a requirement for a permanent magnet stator to be wound around the rotating rotor, it is potential to be cost-effective.
  • They may rotate at a very high speed due to a greater current ranging from 3.6K rpm to 25K rpm since the field winding and armature winding are constructed in series, just like series DC motors. This makes it possible for them to be used in applications that call for very fast rotations.
  • The use of the universal motor is preferred in many portable instruments needing compact size due to its tiny size and lighter weight.
  • The usage of universal motors in numerous everyday handheld equipment that are powered directly by the AC main supply is made possible by their ability to deliver strong torque at low speeds.
  • The power ratings for universal motors range from 5W to 500W. The smaller size and higher rating of the universal motor make it ideal for use in portable drill machines and blenders.

Disadvantages of Universal Motor :

  • Eddy current losses and hysteresis cause the efficiency to decrease.
  • Due to the significant reactance of the field and the armature windings, the power factor is poor.
  • At the brushes, there is excessive sparking.
  • Comparatively to other AC and DC motors like induction motors, universal motors produce more noise. With an increase in the universal motor’s speed, noise levels likewise rise.
  • Even a small unbalance might result in strong vibrations that could harm a motor or the surrounding area. As a result, careful balancing of the universal motor is necessary for its proper performance.
  • The universal motor is challenging to reverse so that it runs in the opposite direction, unlike DC motors.

Applications of Universal Motor :

  • Many household gadgets, including vacuum cleaners, drink and food mixers, domestic sewing machines, etc. employ universal motors.
  • Portable drills, blenders, and other appliances employ universal motors with greater ratings.
  • Appliances for the kitchen and polishers both employ universal motors.
  • Electric shavers and Grinders employ these motors.

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