Universal Motors Theory

Introduction :

We know that the motors run on AC or DC supply known as Synchronous or asynchronous motors and DC motors respectively. Universal motors are kind of hybrid ones from the ac and dc motors combined operational.

Construction of universal Motors:

The principle used in the working of universal motors is electromagnetism. The stator is used as the electromagnet for the creation of the magnetic field. It is the commutated one series wound motor also referred to as AC series motor. The field coils of the stator are connected in a series manner with the windings of the rotor via commutator. The universal motors are constructed very similar to the DC series motor but a slight modification is done to operate it on AC power.

Properties of Universal Motors:

Following are the distinctive properties of the universal motors:

  • They have high starting torque.
  • They run at high speed.
  • They are lightweight.
  • Their size is compact.
  • They are normally used in portable equipment and power tools as well in many house-hold appliances.
  • They are easily controlled using tapped coils which are the electromagnetic process or through the electronic circuits.
  • They are noisy because the commutator has brushes and when the rotor rotates, the brushes get scratchy and the noise get to be produced.

There are some series-wound motors available which only operate on AC power as well. If you connect a normal AC wound motor to an AC power then it would run in a very bad manner resulting in the shutdown of the breaker if there is connected any.


When it comes to the efficiency and you take the AC power reference, then it would be the rotational frequency which will determine the efficiency because it is often greater than the input frequency supplied because motors run on the rotational frequency and greater the rotational frequency, greater the efficiency but in case of universal motors, for smaller motors efficiency is 30% and for larger ones, it is 70-75%.


The simplest way to operate the motor on a single-phase ac power supply is to take a DC motor and run it on AC power. We know that the induced torque is

                                                  Tind = KΦIA

If the supplied voltage polarity is reversed to the series or shunt dc motor, the direction of the field windings flux and armature current reverses respectively and hence, the resulting torque which is the induced one continues to be produced in the same direction as before. Due to this, the achievement of pulsating but the unidirectional induced torque is possible from the DC motor when it is connected to the AC power supply. This design is only practical for the DC series motor because the armature current and field winding current must reverse at exactly the same time. It is not practical for the DC shunt motors because there exists a very high field inductance which tends to delay the field winding current reversal resulting in unacceptable motor’s average induced torque reduction.

For the effective functioning of DC series motor on AC power, the motor’s stator frame and field poles must be completely laminated. If the lamination is not done in the proper way, then the core losses will be very much. When the stator frame and the field poles of the motor are completely laminated, this motor is called universal motor because of its functioning on both AC and DC power.

When the motor is running on an AC power source, the performance of the commutator will be much poor than the running of the motor on the DC power source. The extra sparking at the brushes is caused by the transformer action inducing the voltages in the coils undergoing commutation. This extra sparking shortens the brushes’ life in a significant manner and it can become the source of radio-frequency interference in certain environments.

Torque-Speed Characteristics:

 The torque-speed characteristics of the universal motors differ by the input sources. There are two main reasons for that which are described in the following:

  • The field and armature windings have quite a large reactance at the 50 or 60 Hz. A significant part of the supplied voltage is dropped across these reactances and therefore, the induced voltage across armature EA is smaller during AC operation than it is with the DC operation. We know that

                                                       EA = KΦw

The motor will run slowly for a given armature current and induced torque on alternating current than it would be on direct current.

  • We know that the peak voltage of an AC system is √2 times its RMS value, so due to this property of AC voltages a magnetic saturation could occur near to the value of the peak current. This magnetic saturation could effectively lower the RMS flux of the motor tending to reduce the motor’s induced torque for the given peak current level. We know that decrease in the flux increases the speed of the machine. Due to this effect, the decreased speed offsetting can be caused by the first effect.

Applications of Universal Motors:

The universal motors have the dangling torque-speed characteristics for the ac operation on a series DC motor due to which it is not suitable for the constant-speed applications. Although, the motor size is compact and gives more torque per ampere current than any other available single-phase induction motor. It is therefore used where lightweight and heavy torque are important. Typical applications are vacuum cleaners, drills, similar portable tools, and kitchen appliances. 

Related topics;

  1. Advantages and disadvantages of universal motor
  2. Why induction motor is widely used in industry?
  3. Why slip ring is used in induction motor?
  4. Why induction motor is used in electric vehicle?