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Some machines are not satisfied with a single fixed speed; they need to run at two different speeds, low and high. Think of a fan turning in two stages summer and winter, an elevator slowing down to a stop, or a conveyor separating its loading and transport speeds. The classic and economical way to meet this need directly from the grid, without a frequency drive, is the dual-speed (two-speed) asynchronous motor. There are two basic architectures: the single-winding Dahlander connection (which halves or doubles the pole count; 2/4 or 4/8 pole) and two separate, independent windings. In this article we cover dual-speed motor selection from the angle of the Dahlander vs separate-winding difference, the power-torque change between speeds, the terminal connection (high/low speed bridging) and fan-elevator-conveyor applications. Our dual-speed Dahlander asynchronous motors article also focuses on the application side.
What Is a Dual-Speed Motor? The Difference Between Two Architectures
A dual-speed motor lets you obtain two different speeds from a single motor. There are two ways to do this, and the choice depends on the ratio of the two desired speeds.
Single-Winding Dahlander (2/4, 4/8 Pole)
The Dahlander connection bridges a single winding differently to double or halve the pole count, so the speed changes in a 1:2 ratio (for example 2/4 pole = ≈3000/1500 rpm, 4/8 pole = ≈1500/750 rpm). Because it uses one winding it is economical and compact, but only a 1:2 speed ratio is possible. For the basis of the pole-speed relationship see our 2, 4, 6 pole selection guide.
Separate-Winding Two-Speed
When two independent windings are used, the speed ratio is not limited to 1:2; combinations like 4/6 pole (≈1500/1000 rpm) become possible. While more flexible, the motor is larger, heavier and usually more costly. If your speed ratio need differs from 1:2, a separate winding is required.
Power and Torque Change Between Speeds
The most critical point in a dual-speed motor is that the power and torque the motor produces differ at the two speeds, and this difference must be assessed against the load character. The Dahlander connection has two typical arrangements: constant torque and constant/variable power. Knowing whether the load wants constant torque or variable torque (as in fans) is decisive in selection; we explain this distinction in our constant or variable torque article. Since fan power varies with the cube of speed, power demand rises sharply at high speed; that is why variable-torque (quadratic) character usually suits dual-speed fan motors. The real speed is slightly below synchronous due to slip; see our slip and actual speed. For the right power margin our load ratio and correct sizing is useful.
Terminal Connection: High and Low Speed Bridging
The terminal box of a dual-speed motor differs from a standard three-phase motor; in Dahlander there is usually a six-terminal arrangement with different bridging for low and high speed. At low speed (high pole count) the winding terminals are connected one way, at high speed (low pole count) another way; this transition is managed by a contactor group (usually a Dahlander contactor set). Wrong bridging burns the motor, so the nameplate and connection diagram must always be followed. You can find terminal and voltage bridging logic in our terminal connection, star and delta bridging, classic starting in our star-delta wiring diagram, and rotation direction in our rotation direction and phase sequence. For IP protection in the terminal box and cabling see terminal box and cable connection.
Applications
Fan and Ventilation
For two-stage ventilation (e.g. day/night or summer/winter), a dual-speed fan motor provides two speeds without drive cost. For fan power-speed selection see centrifugal and axial fan motor selection and for HVAC projects fan motor supply in HVAC projects.
Elevator and Escalator
In classic (drive-less) elevators, high speed is used for travel and low speed for floor-approach/stop accuracy; a dual-speed motor provides these two speeds. For elevator and escalator motor supply see our elevator and escalator motor supply.
Conveyor and Machine Drive
Conveyors that separate loading and transport speeds, some benches and mixers benefit from two speeds. For conveyor motors and replacement our conveyor belt motor emergency replacement is a guide. If continuously variable speed is needed, instead of dual speed a frequency drive (VFD) with asynchronous motor may be more suitable.
Dahlander Connection Types: Constant Torque, Constant Power, Variable Torque
In Dahlander motors, depending on how the winding is connected at high speed, the motor offers three basic characters. In the constant-torque arrangement, roughly the same torque is produced at both speeds; this suits constant-torque loads like conveyors. In the constant-power arrangement, power stays roughly constant at both speeds while torque varies inversely; this is preferred in some bench applications. In the variable-torque (quadratic) arrangement, power rises markedly at high speed; this is ideal for loads where power rises quickly with speed, like fans and pumps. Choosing the right arrangement requires knowing the load character; our constant or variable torque explains this matching. We also stress that fan power rises with the cube of speed in our fan motor selection.
Separate-Winding vs Dahlander: Cost and Size Comparison
Because Dahlander uses a single winding, it is more compact and usually more economical at the same power; but it gives only a 1:2 speed ratio. A separate-winding two-speed motor allows ratios other than 1:2 (e.g. 4/6 pole) but, housing two independent windings, it grows a size or two, gets heavier and costs more. The first question to ask when deciding is: "Is the ratio of the two speeds I need exactly 1:2?" If so, Dahlander makes sense; if not, a separate winding is required. For the effect of frame size and power matching on stock see our frame size and power matching. Since most two-speed applications concentrate in fans, elevators and conveyors, you can find these sectors' supply needs in our HVAC fan motor supply and elevator motor supply.
Contactor Group and Motor Protection
In a dual-speed motor there are separate contactors for the two speeds with electrical/mechanical interlocking between them; energising both speeds at once means a short circuit. The Dahlander arrangement typically uses three contactors (low speed, high speed and high-speed star bridge). Separate thermal protection values must be set for each speed, because the current drawn differs at the two speeds. For thermal and relay selection see thermal, relay and fuse selection and for protection devices protection devices purchasing. To monitor winding temperature, PT100/thermistor protection is useful.
Efficiency and the Effect of Pole Count
In dual-speed motors, efficiency is usually somewhat lower than an equivalent single-speed motor, because the winding design is optimised for two speeds rather than one point. Efficiency and power factor change between the high stage (low pole count) and low stage (high pole count). We explain the effect of pole count on efficiency in our efficiency and pole count comparison. If the application runs mostly at one speed and energy saving is a priority, a single-speed high-efficiency motor plus a VFD when needed may be more efficient; see our IE3 vs IE4 investment.
Mounting, Protection and Shaft Fit
Dual-speed motors are also offered in standard IEC frames, with B3/B5/B14/B35 mounting options and IP55 protection. The fan application may involve direct blades, the conveyor a gearbox input; for shaft diameter and coupling fit see our shaft diameter and key dimensions. For correct frame-flange matching when used with a gearbox, our reducer motor matching (IEC) and worm gear reducers page are useful. For protection class selection review our IP protection class selection.
Common Mistakes
The most common mistake is burning the motor with wrong bridging; the nameplate and connection diagram must always be followed. The second is setting a single thermal value for both speeds; since each speed draws different current, protection must also be set for two stages. The third is choosing a dual-speed motor when the need is continuously variable speed; in that case a VFD is more suitable. For general purchase mistakes see mistakes made when buying an electric motor and for commissioning commissioning and first startup.
Dual Speed or Frequency Drive?
A dual-speed motor offers only two fixed speeds with no drive cost; if the need is truly two stages, it is the most economical solution. If continuous stepless speed control, energy saving or soft starting is required, a single-speed standard motor with a frequency drive (VFD) is more suitable. Our motor selection for variable-speed applications and when a VFD is needed together clarify this decision. For single-speed low-speed alternatives see 6 and 8 pole low-speed motor and low speed high pole direct drive. For supply and quote preparation our information to provide when requesting a quote and our HEM Motor homepage and efficient electric motors pages are useful.
Frequently Asked Questions
What is the difference between Dahlander and separate-winding two-speed?
Dahlander gives only a 1:2 speed ratio (2/4 or 4/8 pole) with one winding and is economical; a separate winding allows ratios other than 1:2 (e.g. 4/6 pole) but is larger and costlier. For the pole-speed basis see our pole selection guide.
How do you switch between the two speeds?
Different bridging is made in the terminal box for low and high speed, and a contactor group manages the transition. If the nameplate and connection diagram are not followed, the motor is damaged; see our terminal bridging.
Is the motor power the same at both speeds?
No; in a dual-speed motor, power and torque differ at the two speeds and must be selected according to the load character (constant/variable torque). Our constant or variable torque article explains this.
Get a Quote
To quickly confirm the right power rating, pole/speed combination and lead time for your application, talk to our engineering team. Call us on +90 (532) 345 49 86 or open a request through our contact page. When you share the nameplate data, frame type and mounting arrangement, we return a clear quote and delivery schedule the same day.
Dual-Speed (Dahlander) Motor Selection Checklist
- Is the need truly two fixed speeds (or a VFD for continuous control)?
- Is the speed ratio 1:2 (Dahlander) or different (separate winding)?
- Pole arrangement: 2/4 or 4/8?
- Load character: constant torque or variable torque (fan)?
- Are the required power and torque met at both speeds?
- Is the terminal connection and contactor group (bridging) planned?
- Application: fan, elevator, conveyor, machine drive?
- Mounting type, shaft diameter, protection class and delivery plan?
