Difference Between AC and DC Motors

Main Difference Between AC & DC Motor

An electrical machine is a device that converts mechanical energy into electrical energy & vice versa. A motor is a type of electrical machine that converts electrical energy into mechanical energy. These motor can either run on AC electrical energy or DC electrical energy. Therefore, the motors are classified into two main types; AC motor & DC motor.

Both types of motor generate mechanical energy used for moving any mechanical load etc. but their construction, control, efficiency & applications are quite different. You may know more about the basic information about AC and DC Current and Voltage in the previous post.

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Before going into the list of differences between AC & DC motor, let’s discuss how a motor works & the basics of AC & DC motor.

What is AC Motor?

It is a type of electrical machine that converts AC electrical energy into mechanical energy.

There are two types of AC motors; Asynchronous (induction) motor & synchronous motor

In asynchronous or induction motor, the stator is made of multiple windings, while the rotor (squirrel cage or wound type) is made of closed loops of conductors. The AC is supplied to the stator which generates an alternating flux called a rotational magnetic field.

This magnetic flux generates induced current in the rotor according to faraday’s law of induction. The induced current opposes the magnetic field & starts revolving in its direction. Thus the induction motor either single-phase or 3-phase works on the principle of electromagnetic induction between the stator & rotor.

In a synchronous motor, a separate DC is supplied to the rotor through slip rings to generate its own magnetic field or a permanent magnet is being used. The AC input is supplied to the stator to generate a rotational magnetic field.

The rotor magnetic field gets locked with the stator rotational magnetic field & starts rotating at the exact same speed. Since the rotational magnetic field depends on the supply AC frequency, the speed of the motor only depends on the input frequency. Therefore, it is called a synchronous motor.

Since the input is always applied to the stator, there is no use of carbon brushes. Thus they are more reliable & needs less maintenance. They are more efficient than DC motor in generating greater mechanical power.

But controlling the speed of an AC motor is a little difficult then DC motor. Since it mainly depends on the input frequency, we use a device called VFD (variable frequency drive) to vary the input supply frequency to the motor. They are also called AC motor drives. Their direction is controlled by changing the polarity of only starter winding.

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What is DC Motor?

It is a type of electrical machine that converts DC electrical energy into mechanical energy.

The basic principle on which DC motor operates is “when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force mutually perpendicular to the magnetic field & current direction”.  The direction of the force is determined by Fleming’s left-hand rule.

The armature of the DC motor is made of multiple conductor windings. It is placed inside a housing made for a permanent magnet that generates the magnetic field. The armature carries the direct current supplied by any DC source such as batteries. The magnetic field interacts with the current-carrying conductors of the armature. Thus the armature experiences a mechanical force.

Since the input is applied to a moving part (rotor) of DC motor, we use carbon brushes & commutator to supply alternating current to the armature. Therefore, they are called Brushed DC motor. The brushes & commutator wear out with time, so they need frequent maintenance. The sparks between the commutator also decrease its efficiency & generate noise.

In BLDC (Brushless DC) motor, the stator is made of multiple coils that surround the rotor armature made of permanent magnets. The DC is changed into 3 phase AC using thyristors & supplied to the stator coils to generate a rotating magnetic field. In such motors, the input is applied to the stationary part thus it does not require brushes or commutator. It helps in increasing the performance of the motor as well as its efficiency. You may also know more about stepper motors and servo motors in the previous details posts.

Therefore in DC motor, the input can be applied to the rotor (in brushed DC motor) & as well as the stator (in BLDC motor).

The speed of the DC motor can be easily controlled by varying the input voltage. One simple method of controlling the input voltage is PWM (pulse width modulation). The direction of the DC motor reverses by changing the polarity of the input DC supply. You may also read about the Cable Size Calculation for LT & HT Motors.

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Key Differences Between AC and DC Motor

The conclusion of this article is that the AC motors are used for their best output power performance, their reliability & the requirement of less maintenance. While the DC motor used because of their easier speed control & direction. But their frequent maintenance is very costly. Overall, using a VFD with an AC motor can provide a less expensive solution to the problem.

Related Posts:

• Difference Between Single Phase & Three Phase Induction Motor
• DC Machine – Construction, Working, Types and Applications
• Speed Control of DC Motor – Voltage, Rheostatic & Flux Control Methods
• Single-Phase Induction Motor – Construction, Working, Types & Applications
• Three-Phase Induction Motor – Construction, Working, Types & Applications
• What is Motor Starter? Types of Motor Starters and Motor Starting Methods
• Direct Online Starter – DOL Starter Wiring Diagram for Motors
• Difference between AC Drives and DC Drives