Constant Torque: What,When,How,Use And 9 Facts You Should Know

The previous post explained the exertion of zero torque and its consequences in detail. This post concerned giving information on constant torque will be discussed.

Torque is not a function of speed, but its characteristics depend on the speed of a rotating body. This dependency on the speed of rotation gives an account for constant torque. The torque exerted on the load does not change at every rotation aspect; then, we said the torque is “constant.”

We know that the torque exerted on any object makes them rotate at a certain speed. Since some external force is also required to be exerted, that body can rotate continuously with angular acceleration. A constant torque can relate to angular acceleration.

What is constant torque?

Constant torque can be defined as “the amount of torque required to maintain the constant running rate of the object at all speeds, under rotational motion.”

Simply, we can say that the torque acting on the rotational body must be constant for high speed and low speed. This means that, even if the speed of the rotating body increases or decreases, the application of torque must be the same.

Constant torque can also be defined as an output torque that does not change for any input rotation. So any rotating system is a candidate for constant torque by maintaining the applied torque at the rotating phase.

Can torque be constant?

Yes, torque can be constant. If a body is rotating freely with a constant radius and the application of external force is perpendicular to the rotational axis, then torque can be constant.

Since we know that torque is just a cross product of the radius vector of the rotational path and applied external force, the radius must be the same at every point with respect to the center of mass of the rotational axis and the application of an external force must be in perpendicular direction.

τconst=|r|const|F|const sin(θ)

If the rotational path does not have a fixed center, the radius will change, so the torque varies with the radius by increasing the linear velocity. In a rotational path with a fixed center, the radius will be constant with constant torque.

When can torque be constant?

Torque is associated with the slipping rate of the object. If the slipping rate is kept constant for every speed and varying frequency, then torque can be constant. Even when the angular velocity increases linearly at each interval, one can get constant torque.

The linear increase of angular velocity gives constant angular acceleration; thus, we can say constant torque is referred to as constant angular acceleration in a rotating frame.

If we talk about AC and DC motors, the torque is proportional to the power generated on them. The shaft fitted-in motor works on the torque principles. So in electric motors, constant torque can be achieved when the conjunction controller and proper load condition are employed.

Where can torque be constant?

A constant torque is usually employed in induction motors. But it cannot create constant torque on its own to operate at all velocities.

A device called variable-frequency drive is used where torque can be constant at every speed of the rotational shaft. So the induction motors are fitted with VFD.

The constant torque in VFD describes the variation of horsepower with the rotation speed. There will be constant torque if horsepower increases linearly with the rotational speed. Equally, it is also important to maintain the frequency below 60Hz. If it exceeds 60Hz, then horsepower becomes constant.

Block diagram of variable frequency drive
Image credits: Wikimedia commons

How can torque be constant?

The torque acting on the rotational system defines the rotational inertia developed in that system. If rotational inertia is kept constant, then angular acceleration will be equal to torque leading to a constant amount of torque to exert on the rotational system.

Rotational inertia defines the resistance of an object to change its rotation. The formula gives it as; [latex]I=\frac{L}{\omega}[/latex]; Where L is the angular momentum and ω is angular velocity.

So, if angular velocity increases linearly at every equal interval of time, angular acceleration will be constant, which tends to exert constant torque for their rotation.

Constant torque use

Constant torque is widely used to high current at low-speed rotation. Other than that, there is a wide range of applications in various fields; some of them are listed below:

Centrifugal pumps

An important application of constant torque is centrifugal pumps. The impellers of the centrifugal pumps are fitted with a series of curved vanes sandwiched between two discs. These impellers rely on the constant torque with high-speed rotation.

Motorized sewing machine

Modernized sewing machines use VFD technology to control the torque exerted on the driving wheel of the machine. If the speed varies, the torque will be constant.

Wound rotor-motor

The wound-rotor induction motor, also known as the wound ring motor, consists of two rings where external resistance connects rotor windings to the spring ring. The external resistance controls the speed and torque developed in the motor.

DC excited flux switching machines

DC excited flux switching machines consist of DC windings at the rotors, which provide good control over the torque produced at every rotor speed.

Cooling fans

The rotor and stators of the cooling fans induce constant torque by reducing the heat within the rotor. A type of motor called PCS motor is employed in cooling fans to execute constant torque on rotors.

Conveyor belts

Conveyor belts offer high speed and constant torque irrespective of the amount of load applied to the belt shaft.


An electric elevator is a constant torque machine in which the stator of the induction motor drives the elevator action for high and low load applications.

Constant torque springs in elastic actuators

In the human-robot interaction era, a series of parallel springs are used in elastic actuators, enhancing good control over the torque and overcoming the torque loss at a different speed.

Permanent magnet synchronous motor

An AC synchronous motor in which a permanent magnet excites the fields to generate sinusoidal EMF is called a permanent magnet synchronous motor. It consists of a rotor and stators that creates a magnetic field at each rotation.

This motor is in sync with the speed and frequency of the rotation and the rotational period is equal to the integral value of the AC cycle. Thus the speed of the motor is in synchronous with the rotating magnetic field intended to constant torque.

Constant torque walking

In robotics, constant torque techniques are employed to support the leg base to enhance stable walking.

Constant torque loads

Constant torque loads refer to applying the same amount of torque at low and high speeds. It is also associated with horsepower, which directly depends on the speed. The constant torque load drives positive displacement and reciprocating action in pumps, conveyors, and compressors.

A good example of a constant torque load is a drum hoist. Let us consider a drum of diameter d, which can hold a maximum load weighed up to 1000kg. There will be some tension T acting on the string of the drum, so the torque exerted on them is given as [latex]\tau=T\frac{d}{2}[/latex]

If the speed is kept constant, the load does not accelerate torque is equal and opposite to exerted force at the drum side. If the hoist speed is increased to a certain value, then the motor attains the maximum speed. Now hoist speed will be in correspondence with the drum speed.

But, load torque at the motor is reduced due to friction. However, the full load torque is equivalent to the motor torque, thus balancing the required amount of force to hoist at every maximum speed. So steady state of torque is achieved on the load and the drum hoist.

constant torque
Constant torque on drum hoist

Constant torque acting on a uniform circular wheel

A constant torque acting on the circular wheel often changes the angular momentum of the wheel constant. This means that the change in the angular momentum is equal to the applied torque on the rotating wheel.

Since we know that angular momentum is a product of moment of inertia and angular velocity, i.e., L=Iω, the angular velocity defines the change in the angle of rotation per unit of time.

When the torque is applied, the angular velocity changes, leading to a change in angular momentum. It is given as

[latex]\tau=\frac{\Delta L}{\Delta t}[/latex]

Suppose applied torque is constant; that means there will be a constant change in the angular momentum per unit time interval.

Constant torque vs variable torque

Constant torque drives relatively high current even at the low speed of the rotor where variable torque cannot; it is the main difference between constant and variable torque. Other than this, some other comparisons are listed in the below table:

Constant torqueVariable torque
Constant torque gives the same amount of torque for all values of speed.Variable torque means there will be a change in torque as the speed changes.
If the speed increases by double, the horsepower also doubles, maintaining the torque steady.In variable torque, if speed reduces, the torque gained by square times and horsepower is reduced by cubical value.
The horsepower changes linear with speed, and thus, torque will be constant.The horsepower remains constant throughout the action.
The efficiency of a rotor is a little less as it has to provide the same torque for all speeds.The efficiency of the rotor is more as it does not affect the motor shaft.
Energy loss is often in constant torque as it provides the same amount of torque at high and low speeds.Saves a lot of energy as the application of torque decreases or increases, when speed decrease or increases.
The lifetime of the constant torque drive is less.The lifetime of variable torque drive is comparatively more.
The huge cost of installation and more difficult to maintain.Installation of variable torque drive is affordable and easy to maintain.
Table of Constant torque vs Variable torque

How can constant torque be controlled in the motor?

The torque can be controlled by maintaining the pressure exerted on the load in the circuit.

Generally, torque depends on the angular force acting on the mechanical load. If exertion of angular force is maintained at a constant rate, then we can easily control the constant torque.

How does speed affect the torque in the motor?

The torque and the speed possess an inverse relation; as the speed increases, torque tends to decrease.

In a motor, the gear systems are provided to control the speed so that the input gear has less number of teeth than the output gear. So when the output gear rotates, the input speed decreases, and torque consequently increases.

Do voltage and torque proportional to each other?

Yes, voltage and torque are proportional to each other as both of them depends on the power.

Power is a factor that is closely related to voltage. Torque is also produced due to the power exerted on the rotor drive. Hence, torque and voltage are related; the torque at the rotor shaft and the voltage developed in the motor circuit also increase.

What is meant by horsepower?

A horsepower is just a unit of power that defines the rate of work done per unit time on the system.

Horsepower generally defines the output power from an engine or motor. The horsepower has a different value for a different system. On this basis; they are classified as

  • Mechanical horsepower
  • Metric horsepower
  • Electrical horsepower
  • Boiler horsepower
  • Hydraulic horsepower
  • Air horsepower


By understanding the concept of constant torque, we can conclude this post by stating constant torque is a quantity, meaning a change in speed does not alter the value of torque. It depends on the angular momentum and constant output power associated with the system.

Keerthi Murthi

I am Keerthi K Murthy, I have completed post graduation in Physics, with the specialization in the field of solid state physics. I have always consider physics as a fundamental subject which is connected to our daily life. Being a science student I enjoy exploring new things in physics. As a writer my goal is to reach the readers with the simplified manner through my articles. Reach me –

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