Speed control vs torque control?
What Is the Difference Between Speed Control and Torque Control?
Speed control and torque control are two different ways of commanding an electric motor drive.
The simplest distinction is:
Speed control: “Keep the motor at this speed.”
Torque control: “Produce this amount of turning force.”
If a conveyor has to run continuously at 30 metres per minute, the main process target is speed. If a winding machine must maintain a defined web tension as roll diameter changes, torque and tension become much more important.
Speed and torque are not independent, however. When mechanical load increases, a motor operating in speed mode must normally produce more torque in order to maintain the commanded speed.
For the wider background, see Understanding Variable Frequency Drives and What Is Torque Control?.
The difference in one sentence
Speed mode: the drive regulates motor speed to a speed reference and varies motor torque as required by the load.
Torque mode: the drive regulates motor torque to a torque reference; motor speed becomes dependent on the mechanical load and is normally constrained by a speed limit.
What actually happens in speed control?
Suppose a PLC gives the drive a reference of 1000 rpm. If mechanical load increases and actual speed begins to fall, the speed controller increases the torque-producing motor current in order to bring the shaft back toward the reference.
Speed control therefore does not mean simply holding output frequency at one fixed value.
In higher-performance vector-control systems, the drive actively regulates the current components associated with motor flux and torque.
Basic V/F control can provide useful speed control for many general applications, while sensorless vector or closed-loop control can provide tighter performance where required.
What actually happens in torque control?
In torque mode, the primary reference is not shaft speed but the required motor torque.
A winder, for example, may receive a torque command corresponding to the force needed to maintain material tension.
It is too simplistic to say that torque control merely “holds motor current constant.” Motor current contains components associated with magnetic flux and torque production. Vector-control methods use a motor model and measured electrical quantities to regulate the torque-producing component much more accurately.
Why does torque mode need a speed limit?
This is one of the most important practical differences.
If a motor is commanded to produce torque and the opposing mechanical load suddenly disappears, the motor can accelerate rapidly because very little load remains to oppose the commanded torque.
Real drive systems therefore commonly combine a torque reference with a maximum speed limit.
For example:
Torque reference: 15 N·m
Maximum speed: 1200 rpm
The drive attempts to produce the commanded torque but does not allow motor speed to exceed the configured limit.
A winder makes the difference easy to understand
A winding process is one of the clearest examples of the relationship between speed and torque.
If line speed and web tension remain constant while the roll diameter increases, roll speed must decrease because each revolution winds a greater length of material.
At the same time, the required shaft torque increases because the same tension force acts at a larger radius.
The basic relationships are:
Rotational speed = Line speed / (π × Diameter)
Torque = Web tension × Radius
and:
P = T × ω
or in common motor units:
P(kW) = T(N·m) × n(rpm) / 9550
Interactive winder example
Change the line speed and web-tension values below. As the roll grows, watch rpm decrease while required torque increases.
Training note: Real winding systems must also consider motor and gearbox efficiency, acceleration torque, roll inertia, load-cell or dancer feedback and the selected control algorithm.
What is actually happening in the animation?
The nip roll on the left maintains line speed. This is fundamentally a speed-control problem.
The winder on the right must pull the material with a defined tension. As roll diameter increases from 120 mm toward 340 mm, the required rpm falls.
At the same time, the same web-tension force acts through a larger radius, so the required winding torque increases.
This shows why speed and torque control are not competing technologies. They are often two different control requirements inside the same machine.
Typical speed-control applications
Centrifugal fans and pumps are classic speed-control applications. The process usually regulates flow, pressure or airflow by changing motor speed.
Conveyors are also commonly operated in speed mode. If conveyor load increases, the drive increases motor torque in order to keep belt speed near the requested value.
Using speed mode therefore does not mean torque is unimportant. The drive still needs enough torque and current capability to maintain speed under load.
Typical torque-control applications
Torque control is particularly useful in winding and unwinding, web-tension systems, dynamometers, load sharing and certain extrusion processes.
Where several motors share one mechanical load, torque reference, droop, master-follower or load-sharing strategies may be used to divide the work between motors.
See What Is Torque Control? for the detailed topic.
Does a crane use speed control or torque control?
For cranes and hoists, saying simply “use torque control” is too simplistic.
Many hoisting systems are commanded primarily by a speed reference. However, strong low-speed torque, torque limits, brake sequencing, encoder feedback and regenerative behaviour are all critical.
A crane therefore needs excellent torque performance, but that does not automatically mean the drive operates in pure torque-reference mode throughout every movement.
A mechanical holding brake and the machine safety architecture remain separate engineering requirements.
Do pumps need torque control?
Most centrifugal pump applications are primarily speed-, pressure- or flow-control problems. They are generally treated as variable-torque loads.
Positive-displacement pumps can behave much more like constant-torque loads.
This distinction is explained further in What Is Constant Torque for a VFD?.
Is constant torque the same as torque control?
No.
Constant-torque load describes the mechanical characteristic of a machine that requires approximately similar torque at different speeds.
Torque-control mode describes a drive operating mode in which torque reference is the primary regulated command.
A conveyor can therefore be a constant-torque load while the VFD is still operating in speed-regulation mode.
Is a torque limit the same as torque control?
No.
A motor can operate in speed mode at 1000 rpm while the drive also applies a maximum torque limit.
The main controlled variable is still speed; torque is merely constrained.
In torque mode, the torque reference becomes the primary command and speed is typically used as a limit.
What happens in speed mode when load increases?
When load increases, the motor must produce more torque. A well-tuned speed regulator detects the resulting speed error and increases torque-producing current to bring the motor back toward its speed reference.
In induction-motor systems without an encoder, slip compensation can also help reduce load-dependent speed drop.
Do you need an encoder for torque control?
Not always.
Modern sensorless-vector drives can estimate motor flux and torque from electrical measurements and a mathematical motor model.
Encoder feedback becomes more valuable where the application requires extremely low-speed operation, near-zero-speed performance, very fast dynamic response or tighter torque and speed accuracy.
See Open-Loop vs Closed-Loop Control for the wider explanation.
The relationship between speed, torque and power
The relationship is:
P = T × ω
and in common motor engineering units:
P(kW) = T(N·m) × n(rpm) / 9550
If torque remains the same while rpm doubles, mechanical power approximately doubles.
Above the motor's rated frequency, however, the available voltage normally reaches its limit and the motor enters the field-weakening region. Available continuous torque then normally decreases as speed continues to rise.
Where does the Traverse function fit?
Winding processes may need more than simple speed and torque control. Material sometimes also needs to be distributed across the roll in a controlled pattern.
The Traverse function changes drive frequency around a centre value according to a defined pattern and is used in certain textile, winding and fibre applications.
Traverse is not the same as torque control, but both functions can appear on the same winding machine.
Should I choose speed control or torque control?
Ask what the machine actually needs to regulate:
If shaft speed or line speed is the critical variable: speed control is normally the starting point.
If force, tension or motor moment is the critical variable: torque control becomes important.
If both matter: the application may need speed/torque limits, master-follower control, speed trim, tension feedback or a manufacturer-specific combined control mode.
For drive selection itself, continue with What to Consider When Choosing a VFD.
Speed control vs torque control FAQ
Does a motor produce torque while operating in speed mode?
Yes. The speed regulator continuously commands the torque required to maintain the requested speed.
Can motor speed be limited while using torque mode?
Yes. In fact, a speed limit is normally an important part of practical torque-mode operation.
Does torque control always require an encoder?
No. Sensorless vector drives can perform torque estimation and regulation without a shaft encoder, although feedback can improve demanding low-speed and dynamic applications.
Should a fan normally use torque mode?
Usually not. Most fans are variable-torque loads and are controlled primarily by speed or process demand.
Must a crane use torque-control mode?
No. Hoist control can combine speed regulation, vector control, torque limits, feedback, brake sequencing and other functions.
Does constant-torque load mean torque-control mode?
No. Constant torque describes the load characteristic; torque control describes the drive's regulator mode.
Why does winder rpm decrease as roll diameter increases?
Each revolution takes up a greater length of material, so fewer revolutions are required to maintain the same line speed.
Why does winding torque increase as the roll grows?
The same web-tension force acts at a larger radius, so the required moment increases according to T = F × r.
Related VFD control topics
What Is Torque Control? · Understanding VFDs · Choosing a VFD · Vector Control · Open / Closed Loop