How to Reverse the Direction of Rotation of a Universal Motor
Introduction
A universal motor is a high-speed electric motor that can operate on either an alternating-current or direct-current supply when designed for the applicable voltage and frequency. It is widely used in portable electric drills, grinders, vacuum cleaners, mixers, sewing machines, small machine tools, and other equipment requiring high starting torque and compact construction.
The direction of rotation of a universal motor can be changed by reversing the direction of current through either the armature winding or the field winding relative to the other. The current must be reversed in only one of these windings. If the connections of both windings are reversed simultaneously, the relative direction of the field flux and armature current remains unchanged, and the motor continues rotating in the same direction.
Before studying the reversing methods, readers may review the
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Universal motor direction reversal using a DPDT selector switch to change the armature-current direction relative to the series field winding. |
Working Principle of a Universal Motor
A universal motor is essentially a series-wound motor. Its field winding and armature winding are connected in series, which means that the same current flows through both windings.
The electromagnetic torque developed by the motor depends on the interaction between:
- The magnetic flux produced by the series field winding
- The current flowing through the armature conductors
- The direction of the magnetic field
- The physical position of the armature conductors
When a universal motor operates on an AC supply, the polarity of the supply changes during every half-cycle. Consequently, the direction of current through both the armature and field windings changes simultaneously.
Although both currents reverse, the direction of the developed torque remains the same because the field flux and armature current retain the same relationship with each other. This is one reason why a universal motor can produce unidirectional torque on an AC supply.
It also explains why merely swapping the incoming AC supply conductors does not reverse the motor. The direction of current through both series-connected windings would still change together.
To reverse the motor, the direction of current through one winding must be changed with respect to the other.
Methods of Reversing a Universal Motor
The direction of rotation of a universal motor can be reversed using either of the following methods:
- Reversing the field or armature connections
- Using two oppositely wound field windings
Both methods change the relationship between the armature current and the main magnetic field. However, their switching arrangements and practical applications are different.
Method 1: Reversing the Armature or Field Connections
The first and most common method is to reverse the armature connections while keeping the field winding connections unchanged.
Alternatively, the field winding connections can be reversed while the armature connections remain unchanged. Either arrangement reverses the direction of the developed electromagnetic torque.
However, the armature and field connections must not both be reversed at the same time. Reversing both windings would leave their relative electromagnetic relationship unchanged.
Armature-Reversing Method
In the armature-reversing method, the two armature leads are interchanged using a properly rated universal motor reversing switch.
During normal operation:
- Current flows through the series field winding in a particular direction.
- Current flows through the armature in its original direction.
- The interaction between the field flux and armature current produces torque in the forward direction.
During reverse operation:
- The field current remains unchanged.
- The armature current reverses.
- The relationship between the field flux and armature current changes.
- The motor develops torque in the opposite direction.
This method is commonly used in portable electric drills and other manually controlled equipment.
Reversing a Universal Motor with a DPDT Switch
A Double Pole Double Throw switch, commonly called a DPDT switch, can be used to reverse a single-field universal motor.
The DPDT switch simultaneously changes two electrical connections. When it is wired in a crossed arrangement, it interchanges the two armature leads without changing the field winding connections.
Operation in Position 1
When the DPDT switch is in Position 1, the switch blades connect terminals A to A₁ and B to B₁.
The current flows through the series field winding and the armature in the directions shown in the circuit diagram. Their electromagnetic interaction causes the motor to rotate in one direction.
Operation in Position 2
To reverse the universal motor, the switch is transferred to Position 2.
In this position:
- Terminal A is connected to A₂.
- Terminal B is connected to B₂.
- The direction of armature current reverses.
- The direction of field current remains unchanged.
- The motor develops reverse torque.
A DPDT switch for motor reversing must be selected according to the motor’s voltage, running current, starting current, switching duty, and applicable utilization category. Selection based only on the normal operating current may be inadequate because a universal motor can draw a much higher current during starting.
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| Figure 1: Reversing the direction of a universal motor: (a) armature-reversing method using a DPDT reversing switch and (b) two-field method using a three-position selector switch. |
Method 2: Reversing a Universal Motor Using Two Field Windings
The second method uses two separate field windings installed on the same magnetic core. These field windings are wound in opposite directions.
One winding is used for forward rotation, while the other winding is used for reverse rotation.
The two windings may be identified as:
- FORW field: Produces forward motor rotation
- REV field: Produces reverse motor rotation
Only one field winding should be connected in series with the armature at any given time.
When the forward field winding is energized, its magnetic polarity causes the motor to rotate in one direction. When the reverse field winding is energized, it produces the opposite magnetic polarity, causing the motor to rotate in the opposite direction.
Three-Position Selector Switch
A three-position selector switch can be used to control a two-field universal motor. Its positions are normally arranged as:
- Forward
- OFF
- Reverse
In Position 1, the FORW field winding is connected in series with the armature, and the motor runs in the forward direction.
In Position 2, the REV field winding is connected in series with the armature, and the motor runs in the reverse direction.
The center OFF position separates the forward and reverse selections. This provides a transition point between the two operating directions.
The two-field arrangement can provide simpler switching connections because there is no need to interchange the armature leads. Instead, the control system selects the appropriate field winding.
Applications of the Two-Field Reversing Method
The two-field reversing method is commonly used in applications requiring controlled bidirectional motion, including:
- Motor-operated rheostats
- Electric actuators
- Servo systems
- Positioning mechanisms
- Automatic control systems
- Small machine tools
- Indexing equipment
- Remote-control mechanisms
The arrangement is particularly suitable when the motor direction must be selected electrically through push buttons, relays, contactors, or another control system.
For further study of bidirectional positioning applications, readers can review
Power and Control Circuit of a Two-Field Universal Motor
A two-field universal motor can be controlled using forward and reverse contactors.
The circuit consists of two main sections:
Power Circuit
The power circuit carries the motor operating current. It contains:
- The motor armature
- FORW field winding
- REV field winding
- Forward contactor main contacts
- Reverse contactor main contacts
- Suitable motor protection
Control Circuit
The control circuit determines which contactor is energized. It may include:
- Forward push button
- Reverse push button
- Stop push button
- Forward contactor coil
- Reverse contactor coil
- Holding contacts
- Electrical interlocking contacts
- Overload relay contact
This type of arrangement is known as a forward-reverse motor control circuit.
Readers unfamiliar with the general function of starting circuits can refer to the article on
Forward Operation of the Motor
When the FOR push button is pressed, forward contactor F becomes energized.
The main contacts F₁ and F₂ close, connecting the armature and FORW field winding to the electrical supply.
The operating sequence is:
- The operator presses the FOR push button.
- Forward contactor coil F receives control voltage.
- The main contacts F₁ and F₂ close.
- The FORW field winding is connected in series with the armature.
- Current flows through the motor.
- The motor develops torque in the forward direction.
A normally open auxiliary contact of contactor F may be connected as a holding contact. It keeps the forward contactor energized after the operator releases the momentary FOR push button.
Reverse Operation of the Motor
When reverse operation is required, reverse contactor R becomes energized.
The main contacts R₁ and R₂ close, connecting the armature and REV field winding to the supply.
The sequence is:
- The operator presses the reverse push button.
- Reverse contactor coil R receives control voltage.
- Main contacts R₁ and R₂ close.
- The REV field winding is connected in series with the armature.
- The reverse field winding produces opposite magnetic polarity.
- The motor develops torque in the reverse direction.
The armature current does not necessarily need to be interchanged in this method. The opposite direction of the selected field winding provides the required reversal.
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| Figure 2: Power and control circuit for reversing a two-field universal motor using forward and reverse contactors. |
Interlocking in a Forward-Reverse Control Circuit
Interlocking is an essential feature of a motor reversing contactor circuit. It prevents the forward and reverse contactors from being energized simultaneously.
If both contactors close at the same time, the two oppositely wound field circuits may be incorrectly energized. This can cause excessive current, incorrect motor operation, contactor damage, or mechanical stress.
The primary interlocking methods are push-button interlocking, auxiliary-contact interlocking, and mechanical interlocking.
Push-Button Interlocking
Push-button interlocking uses the contact arrangement of the forward and reverse push buttons to prevent both directional commands from being applied simultaneously.
Operating one direction interrupts or blocks the control path associated with the opposite direction.
Auxiliary-Contact Interlocking
Auxiliary-contact interlocking is commonly used in industrial motor control circuits.
A normally closed auxiliary contact of forward contactor F is connected in series with reverse contactor coil R. Similarly, a normally closed auxiliary contact of reverse contactor R is connected in series with forward contactor coil F.
When forward contactor F is energized, its normally closed interlocking contact opens the circuit supplying reverse coil R. The reverse contactor therefore cannot operate.
When reverse contactor R is energized, its normally closed contact opens the forward contactor control circuit. This prevents forward contactor F from operating.
Mechanical Interlocking
A mechanical interlock physically prevents both contactors from closing simultaneously.
Electrical and mechanical interlocking may be used together when additional protection against simultaneous contactor operation is required.
For related explanations of control devices, see
Comparison of the Two Reversing Methods
Armature-Reversing Method
- Uses a single field winding
- Reverses the armature connections relative to the field
- Commonly uses a DPDT switch
- Suitable for portable tools and manual control
- Requires switching of the armature conductors
- The field connections remain unchanged
Two-Field Method
- Uses two oppositely wound field windings
- Selects either the FORW or REV field
- Can use a three-position selector switch
- Can be controlled using forward and reverse contactors
- Suitable for automatic and remotely controlled systems
- Requires dependable interlocking
Important Points to Remember
The following points are important when studying universal motor direction reversal:
- Swapping the two incoming AC supply wires does not normally reverse a universal motor.
- The armature or field connections must be reversed relative to the other winding.
- Reversing both the armature and field connections simultaneously does not change the direction of rotation.
- A DPDT switch can interchange armature connections in suitable low-power applications.
- A two-field motor uses separate forward and reverse field windings.
- Only one field winding should be energized at a time.
- Forward and reverse contactors require electrical or mechanical interlocking.
- The reversing device must be rated for the motor voltage, current, starting duty, and switching frequency.
- The motor and driven equipment should be suitable for bidirectional operation.
- Rapid reversal can produce high current and mechanical stress.
Universal Motor Speed and Direction Control
Some applications require both speed control and direction control.
Direction control is achieved by changing the relationship between the armature current and field flux. Speed control is achieved using a suitable voltage-control or electronic-control method.
These two functions should be coordinated carefully. The reversing switch, speed controller, motor, and driven equipment must be electrically and mechanically compatible.
Additional information is available in the guide to
Frequently Asked Questions
How can the direction of a universal motor be reversed?
The direction can be reversed by interchanging the armature leads while keeping the field winding unchanged. It can also be reversed by keeping the armature connections unchanged and reversing the field connections.
A two-field universal motor can be reversed by selecting one of two oppositely wound field windings.
Why does changing the AC supply polarity not reverse a universal motor?
Changing the supply polarity reverses the current through both the field and armature windings simultaneously. Their relative magnetic relationship remains unchanged, so the direction of the developed torque does not change.
Which switch is used to reverse a universal motor?
A correctly rated DPDT switch can be used to reverse a single-field universal motor by interchanging its armature leads.
A three-position selector switch can be used for a two-field universal motor. Forward and reverse contactors may be used in automatic control systems.
What is a DPDT switch?
DPDT stands for Double Pole Double Throw. This switch can simultaneously change two separate electrical connections between two operating positions.
When wired with crossed conductors, it can interchange the armature leads of a universal motor.
What is the purpose of the two field windings?
The field windings are wound in opposite directions. One produces the magnetic field required for forward rotation, while the other produces the field required for reverse rotation.
Only one field winding should be energized at a time.
Why is interlocking required in a motor reversing circuit?
Interlocking prevents the forward and reverse contactors from closing simultaneously. This protects the motor windings, contactors, control circuit, and driven equipment from an incorrect operating condition.
Can a universal motor be reversed while it is running?
Immediate reversal of a rotating motor can produce high current and significant mechanical stress. In normal applications, the motor should be stopped before the opposite direction is selected unless the complete drive system is specifically designed for rapid or plug reversing.
What happens if both the armature and field leads are reversed?
The direction of rotation does not change because the relative direction between the armature current and magnetic field remains the same. Only one set of connections should be reversed to change the direction of torque.
Conclusion
The direction of rotation of a universal motor is reversed by changing the direction of current in either the armature winding or the field winding relative to the other. The most common method is to interchange the armature leads using a properly rated DPDT reversing switch while keeping the field connections unchanged.
A second method uses two field windings wound in opposite directions. The FORW field winding operates the motor in one direction, while the REV field winding produces rotation in the opposite direction.
For automatic motor direction control, forward and reverse contactors can be used to select the required field winding. Push-button, auxiliary-contact, and mechanical interlocking prevent the two contactors from operating simultaneously.
A correct universal motor reversing circuit must consider the motor’s voltage, starting current, operating duty, switching frequency, mechanical load, and protective requirements. The motor manufacturer’s terminal markings and wiring diagram should always be followed when selecting or evaluating a direction-control arrangement.
Readers who want to continue studying rotating electrical machines can explore
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