VFD Speed Control vs Torque Control
Short answer
- In speed control a VFD holds the commanded speed and lets torque vary; in torque control it holds the commanded torque and lets speed vary.
- Torque control suits winders and unwinders, force limiting, holding with a set torque and load-sharing followers.
- Sensorless vector drives can control torque for many jobs, while closed-loop vector with an encoder gives full torque at or near zero speed.
- A torque-controlled drive that loses its load accelerates towards maximum speed, so a speed limit must always be set.
In speed control, a VFD holds the motor at the commanded speed and delivers whatever torque the load needs to stay there. In torque control, the drive delivers the commanded torque and the motor runs at whatever speed that torque produces. Most applications use speed control. Torque control is for jobs where force or tension is the thing that matters, and it must always be paired with a speed limit.
What is the difference between speed and torque control?
Inside a vector drive the control loops are nested. In speed mode, the speed regulator compares the speed reference with actual speed and produces a torque demand, which the inner torque (current) loop then delivers. In torque mode, the speed loop is bypassed for the reference: the external command goes straight to the torque loop.
| Speed control | Torque control | |
|---|---|---|
| Drive holds | Speed | Torque |
| Allowed to vary | Torque, up to the torque limit | Speed, up to the speed limit |
| If the load lightens | Speed stays put, torque falls | Motor accelerates until something limits it |
| Typical uses | Pumps, fans, most conveyors, mixers, machine axes | Winders and unwinders, tension control, force limiting, load sharing followers |
Which applications need torque control?
- Winding and unwinding. Paper, film, textile, wire and metal strip must be kept at a steady tension while the roll diameter changes. Holding torque in proportion to diameter holds tension.
- Force limiting. Material handling or pressing equipment that must never exceed a set force, whatever the load does.
- Holding a position with a set torque, such as pressing against a stop with no motion.
- Load sharing. When two or more motors are rigidly coupled to the same load, a common pattern is one drive in speed control as the master and the others in torque control as followers, taking their torque reference from the master so the motors share the work. Drive makers document this as master/follower or master/slave operation. Motors that are not rigidly coupled should not be torque followers.
If the job is "run at this speed," speed control is the right answer even when the load varies a lot. Speed mode already adjusts torque to suit.
Which control modes can regulate torque accurately?
Torque control needs a drive that controls the motor's torque-producing current separately from its magnetising current, which is what vector control does.
- V/Hz (scalar). Controls voltage and frequency, not torque. Fine for speed control of pumps and fans; not a torque control method.
- Sensorless vector. The drive tunes itself to the motor and estimates speed and torque from the motor current and voltage. Many sensorless drives offer a torque mode that is accurate enough for a lot of applications, but performance near zero speed is weaker.
- Closed-loop (flux) vector. Uses an encoder for actual speed and position. This gives the most accurate torque control, including full torque at or near zero speed.
The exact accuracy figures vary by drive model and depend on correct motor data and tuning. Read the manual for the drive's stated torque accuracy in each mode, and run the auto-tune as the manual specifies (some modes need it with the motor coupled to the load).
What happens when a torque-controlled load runs away?
If the load disappears, for example a web breaks or a coupling lets go, the drive keeps delivering the commanded torque into nothing. With no load to absorb it, the motor accelerates towards its maximum speed. That is a runaway, and it can damage the machine and injure people.
So every torque-controlled drive needs a speed limit set, and the drive's own speed-limit parameters are there for exactly this. Winder drives often add web-break detection as well. The speed limit should be set just above the highest speed the process needs, not left at the motor's maximum.
The drive's internal limits are not a safety function on their own. If a runaway or loss of holding torque could hurt someone, the machine needs a risk assessment and properly designed safety measures, and hoisting or vertical loads need a mechanical brake designed for the job. That design work is for qualified people.
How do speed and torque limits interact?
One variable is in control and the other acts as a limit. In speed mode, the torque limit caps how hard the drive will push to hold speed; if the load needs more, speed sags. In torque mode, the speed limit caps how fast the motor may go; if the load lightens, the drive holds at the limit instead of running away.
Many applications use this deliberately. A common winder approach is to run in speed mode with the reference set slightly above the line speed, and use the torque limit to set the tension. While the web is intact, the torque limit governs and the motor runs at line speed; if the web breaks, the motor only rises to the slightly higher speed reference. Drives also offer modes that switch between speed and torque control, or that use the minimum or maximum of the two references. Check what your drive calls them and how it switches.
Do you need an encoder for torque control?
Not always. Sensorless vector torque control works for many winders and load-sharing jobs that run well above zero speed. An encoder becomes necessary when you need full, accurate torque at or near zero speed, such as holding tension on a stopped web, starting a heavy roll, or very low-speed operation, and some drives require closed-loop mode for particular functions such as certain unwinder setups.
The encoder must suit the drive's encoder interface (type, voltage, resolution) and be mounted and wired properly, with shielded cable. Encoder faults in a torque-controlled drive cause their own problems, so most drives have encoder-loss detection; make sure it is enabled.
What should you check when commissioning a torque-mode drive?
- Motor data and tuning. Enter the motor nameplate data exactly and run the auto-tune the manual specifies. Torque accuracy depends on it.
- Speed limits in both directions. Set forward and reverse limits just beyond what the process needs, and confirm the drive actually holds at the limit with the load removed, under controlled conditions.
- Torque limits. Set motoring and regenerating torque limits separately if the drive allows it.
- Reference scaling. Confirm what 100 percent of the torque reference means in that drive (motor rated torque is common, but check), and that the analogue or network reference is scaled to match.
- Braking. An unwinder holding back tension, or a follower being driven by the load, returns energy to the drive. The drive needs a braking resistor, a regenerative unit or a common DC bus sized for it, or it will trip on overvoltage.
- Encoder loss and web break detection, where used, enabled and tested.
Document the final parameters. A torque-mode drive replaced with factory defaults will not behave like the one it replaced.
VFD wiring, parameter changes and commissioning involve hazardous voltages and moving machinery, and should be done by qualified people following the drive manual and site lockout rules.
Further reading on this site: VFD control modes; how a VFD works; incremental vs absolute encoders; dynamic braking and braking resistors; motion control vs PLC logic.
If you are replacing a drive, an encoder or a braking resistor on a torque-controlled application, send us the model numbers and motor nameplate data, and we will source matching parts for you to check against the drive manual.
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What is the difference between VFD speed control and torque control?
In speed mode the drive's speed regulator compares reference and actual speed and produces a torque demand, so the drive holds speed and torque varies. In torque mode the external reference goes straight to the torque loop, so the drive holds torque and the motor runs at whatever speed results. Most applications use speed control.
Which applications need VFD torque control?
Winding and unwinding paper, film, textile, wire or strip at steady tension, equipment that must never exceed a set force, holding against a stop with a set torque, and load sharing, where a master drive in speed control passes a torque reference to followers on the same rigidly coupled load.
Do you need an encoder for VFD torque control?
Not always. Sensorless vector torque control works for many winders and load-sharing jobs running well above zero speed. An encoder and closed-loop vector control are needed for full, accurate torque at or near zero speed, such as holding tension on a stopped web, and some drives require closed-loop mode for certain unwinder functions.
What stops a torque-controlled motor from running away?
The drive's speed limit. If the load disappears, the drive keeps delivering commanded torque and the motor accelerates towards maximum speed. Set the limit just above the highest speed the process needs. The drive's limits are not a safety function on their own, and vertical loads need a properly designed mechanical brake.