Universal Motor (Complete Guide)

What is a universal motor?

 A universal motor is a special type of electrical motors which is designed to run on DC or single phase AC source, and uses an electromagnet as its stator to create its magnetic field.

The stator’s field coils are connected in series with the rotor windings through a commutator, It also known as a commutated series-wound motor.

The universal motor is a DC series motor, but is modified slightly to allow the motor to operate properly on AC power source.

Universal motor is called so because, it can run on both AC and DC source.

Universal motor
Marrrci, CC BY-SA 3.0, via Wikimedia Commons

Universal Motor Working Principle

Universal motors work on the same principle as DC series motors, regardless of whether AC or DC powers them.

Depending on the supply, a universal motor can be either a DC motor or a single-phase AC motor.

With the application of a DC power source to the universal motor, it works just like a DC series motor. In the field winding of the motor, the flow of current produces an electromagnetic field.

In addition, the armature conductors also carry a current of the same magnitude. Conductors encased in electromagnetic fields undergo a mechanical force. As a result of this mechanical force, or torque, the rotor begins to rotate. Fleming’s left-hand rule determines the direction of this force.

When an AC supply powers a universal motor, it will produce the same unidirectional torque it can produce. The armature winding is connected in series with the field winding, so both of them are in the same phase.

Since the polarity of AC constantly changes, the current flowing in the field winding and the armature also reverses simultaneously as the polarity changes. As a result, the magnetic field and armature current are reversed so that the direction of force experienced by armature conductors does not change.


There are various applications where universal motor is used, such as:

  • portable power tools and equipment
  • washing machines
  • Vacuum cleaner
  • small fans
  • kitchen appliances
  • Table fans
  • Sewing machines

Why are Universal Motors So Loud?

Universal motors much less often used for equipment that is in continuous use as the commutator has brushes. Besides, because of the commutator, universal motors are typically very noisy.


Even when used with AC power source, this motor can rotate at a rotation speed above the frequency of the supply, and because most electric motor properties improve with speed, however, universal motors are inefficient: up to 70–75% for larger motors and around 30% for smaller motors.


A universal motor has the following advantages:

  • High starting torque.
  • Can run at high speed
  • Lightweight, compact and portability
  • Relatively easy to control
  • Low cost and price


Carbon brushes
Stan Zurek, CC BY-SA 3.0, via Wikimedia Commons

  • Maintenance and short life problems caused by the commutator and brushes.
  • Electromagnetic interference (EMI) issues due to any sparking
  • Totally-enclosed fan cooled (TEFC) universal motors would be impractical due to the increased risk of overheating, so this kind of electrical motors may only be used where clean air is present.
  • This motor type is not suitable for working in hazardous and explosive atmospheres, because it produces spark during its normal working.

Why Universal Motor Has High Speed?

Generally, universal motors have a high speed, between 4000 to 16000 RPM, and can be over 20,000 RPM because, when the field windings are in line with the armature winding, they automatically reduce the voltage and the current through them as the speed increases, giving a reduction in field strength at high speeds.

Accordingly, no theory has been developed in order to predict to what extent a motor will be capable of racing at its theoretical maximum speed.

Speed Control

Speed control of universal motor is easy compared to induction motor because, induction motor is a constant speed motor.

Two types of controlling speed of universal motor:

  • Using of a thyristor circuit to obtain continuous speed control.
  • Stepped speed control using multiple taps on the field coil.

Universal Motor vs Induction Motor

Feature Universal Motor Induction Motor
Brushes Uses brushes No brushes
Efficiency Low efficiency (30 – 75%) High efficiency
Speed control Easy speed control Poor speed control
Speed Higher speed Lower speed
Power Source Runs on AC and DC power source Runs on AC source only
Work at Hazardous atmosphere Not suitable for hazardous explosive atmosphere Suitable for hazardous explosive atmosphere
Applications Light hand tools and home appliances Used for heavy industrial applications
Maintenance Needs more maintenance due to brushes issues Lower in maintenance
Noise Noisy working Not so loud

I have written a detailed article about three phase induction motor, you can find it here.

Universal Motor is not an Induction Motor. A universal motor is a series-excited motor that uses a single energy phase.They are pretty different from each other.

Induction motors are incredibly efficient, using less current per horsepower. Universal motors require more amperage per horsepower than induction motors.

Why Can A Universal Motor Work on AC and DC?

Because the armature and field windings are used in series, a universal motor can operate with a DC or AC supply.

There is no difference in the construction of a universal motor and that of a series-wound DC motor.

In order to make a universal motor, some modifications have been applied that allows it to work with either AC or DC supplies.

In a universal motor, the field poles are mounted on a stator. On the field poles, coils are wound around which they oscillate. A universal motor has both a stator field circuit as well as an armature circuit laminated together. It is necessary to use laminates when operating on AC power to reduce the eddy currents during the process.

The commutator and brushes are positioned on the axes of the universal motor’s rotary armature, consisting of straight or skewed slots. Due to the induced current in the armature coils, AC commutation is worse than that for DC. Having high-resistance brushes is therefore necessary.

Read my detailed article about AC vs DC power. You can find it here.

How to use a universal motor as a dC generator?

There is no doubt that Universal Motors can also be used as DC generators. For this purpose, the rotor will need to be separated from the stator windings, and one of the coil sets will need to be powered by DC current.

However, they are DC series motors, and general-purpose generators are not what they are made for. Getting it started is difficult since the magnetic field is insufficient. It will take a significant portion of your generated energy to support that magnetic field with a brushed rotor.

To produce electrical energy, magnetic flux lines need to be cut with coils. As a result, you have several different types to choose from, but a universal motor makes a poor generator.

Why is the universal motor used in home appliances and power tools?

Among the many benefits of universal motors is that they have high starting torque, can run at high speeds, are lightweight, and are compact. Typically, they are found in portable power tools and equipment, alongside many other everyday products, like household appliances.

Using tapped coils or electronic means, they are easy to control because, they can be managed electromechanically.

It must be noted, however, that the commutator has brushes that wear down over time, and this makes it less likely to be used for machinery that is constantly being used. Of course home appliances are not continuous duty and universal motors are suitable for them.

Can We Run Universal Motor Without A Load?

No, you can’t run a universal motor at no load. Due to the very small flux on very light loads or when there is no load, the motor runs at a dangerously high speed, which may damage it mechanically.

As I have mentioned earlier, the motor armature current is determined by the load. When there is no load or very little load, the motor’s armature current is very small.

When a dc series motor is used Φ α Ia, Due to the small Ia, the flux produced under no load is also very small. As per the speed equation, N α 1/Φ as Eb remains constant almost.

The speed-armature current and speed-torque characteristics reveal that the motor will frequently rotate at dangerously high speed under low armature current and low torque conditions.

A series motor should never be started under mild or no-load circumstances. As a result, it is not used for belt drives because, breaking or slipping of the belt causes the whole weight on the motor to be thrown off, causing the motor to operate with no load, which is unsafe.

Why is a Universal Motor Has No Capacitor?

In universal motors, both windings are energized in order to create a rotating magnetic field, there is no need to energize an auxiliary start winding or to use a starting capacitor to generate phase shift like single phase motors.

The field and armature of a universal motor, like a DC motor, are both driven by windings on the same path.

Each winding has its own alternating current, so external induction isn’t necessary. Universal motors are self excited motors.

Why does Universal Motor Have a Spark?

Universal motors have series-wound windings. The field and armature are connected in series, and commutation is achieved mechanically with brushes and a commutator. It is normal to occur spark sometimes in the universal motor due to brushes and the commutator.

Because Wearing brushes reduce spring pressure, and the roughness of the commutator segments or carbon dust between the commutator segments can cause excessive sparking.

How Much DC Voltage Do I Need to Run A Universal Motor?

Universal motors can function at a wide variety of voltages, but their speed is highly dependent on the voltage. In general, if an AC voltage is provided, the motor will run on the same DC voltage at the same speed.

If your system is rated at 220 volts, that is most likely the RMS figure, which is the corresponding Dc voltage.

Assuming it’s a 220-volt universal motor, it’ll require around 220 volts DC to run regularly. It will work with as little as 200 DC volts or as much as 240 volts, but it will run sluggish and hot or fast and hot, using more current than you may desire.

Is Universal Motor Self Starting?

Because universal motors are designed for single-phase operation, they are unable to self-start without an auxiliary stator winding driven by an out of phase current of near 90 degrees. Universal motors are self excited but not self starting.

The excitation winding are internally connected and no starting capacitor needed.

You can temporarily transform it into a two-phase motor before starting it so that it is self-starting.

We can introduce an additional auxiliary winding called a starting winding to achieve this. As a result, the stator of a single-phase motor has two windings: (1) the main winding and (2) the starting winding.

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