Electric motors run at a speed determined by two factors: the mains frequency (50 Hz in Europe) and the number of pole pairs in the windings. The formula is straightforward: n = (60 × f) / p, where n is the synchronous speed (rpm), f is the mains frequency (Hz) and p is the number of pole pairs. More pairs = lower speed. This is the basis of all IEC standard motors and the principle behind pole-changing motors.
Pole pairs and corresponding speeds
| Number of poles | Pole pairs (p) | Synchronous speed (50 Hz) | Rated speed (±slip) |
|---|---|---|---|
| 2-pole | 1 | 3000 rpm | ~2900 rpm |
| 4-pole | 2 | 1500 rpm | ~1450 rpm |
| 6-pole | 3 | 1000 rpm | ~960 rpm |
| 8-pole | 4 | 750 rpm | ~720 rpm |
| 10-pole | 5 | 600 rpm | ~575 rpm |
| 12-pole | 6 | 500 rpm | ~480 rpm |
What are pole-changing motors?
A pole-changing motor has windings that can be reconnected to change the effective number of pole pairs. By switching the wiring configuration in the control cabinet, the winding arrangement converts, for example from 2-pole (3000 rpm) to 4-pole (1500 rpm), or from 4-pole to 8-pole. This makes multiple fixed speeds available without a frequency drive.
A variable frequency drive (VFD) delivers continuously adjustable speed over a wide range, but is more expensive, requires more space and introduces harmonics into the network. A pole-changing motor provides two or three fixed speeds, is more compact and simpler to maintain. Choose a VFD when you need continuous speed control; choose a pole-changing motor when two or three fixed speeds are sufficient.
2-speed motors
The most common variant: one motor, two fixed speeds in a 1:2 ratio (e.g. 4/8-pole: 1450/720 rpm). Available in two winding topologies:
- Dahlander winding (PAM): simple reconnection of a single winding, always 1:2 ratio. Lower-cost construction, but power at low speed is reduced (approximately 30-40% of full speed power).
- Separate windings: each winding has its own pole count and power rating. More flexible in power and speed ratio, but a larger and more expensive motor due to two complete windings.
Typical applications: fans with low and high power modes, compressors, mixers, centrifuges and hoists.
3-speed motors
Three independent speeds in one housing, always built with three separate windings (a Dahlander cannot achieve this). More expensive and heavier than a 2-speed, but provides a third intermediate speed regime. Popular combinations: 4/6/8-pole (1450/960/720 rpm) for heavy ventilation systems, or 4/8/12-pole (1450/720/480 rpm) for process fans.
When to choose a multi-pole or pole-changing motor?
- Soft start: start at low speed, then switch to high speed. Reduces starting current and mechanical shock.
- Energy saving at partial load: fans and pumps consume approximately 12.5% of full-speed power at half speed (cubic law). Two speeds save significantly.
- Process phasing: a hoist or mixer runs slowly at first, then at full speed. Improves precision and productivity.
- No VFD space or budget: a simple two-position switch in the control cabinet is cheaper than a frequency drive for straightforward 2-mode applications.
- ATEX zone: pole-changing motors are available in Ex versions (Ex db, Ex ec) for explosive environments where VFDs add additional complexity.
Specifications and standard versions
| Parameter | Standard offering |
|---|---|
| Winding type | Dahlander (1:2 ratio) · Separate windings (free ratio) |
| Pole combinations | 2/4 · 4/8 · 6/12 · 4/6/8 · 4/8/12 pole |
| Power range | 0.18 kW, 315 kW |
| IEC frame | 63 to 315 |
| IP class | IP55 standard · IP65/IP67 on request |
| Wiring | 6 terminals (Dahlander) · 9 or 12 terminals (separate windings) |
| Switching types | Star/star · delta/star · high/low power |
| Certification | CE · Ex db / Ex ec on request · Class Society on request |
Multi-pole motors in marine applications
In the maritime sector, pole-changing motors are widely used for anchor windlasses (slow haul-in, fast pay-out), winches, cranes and pump systems. Class Society certification (BV, LR, DNV, ABS, RINA, ClassNK) is available. See also our article on marine class certifications.
Delivery and stock at Van Bodegraven
Van Bodegraven stocks 2-speed and 3-speed motors in all common pole combinations. Standard configurations are often available from stock; specific power combinations or ATEX versions are supplied to order (3-8 weeks depending on configuration). Contact us for a tailored quotation or browse our product list of 6,800+ motors.
No rights can be derived from the information in this article. Contact us for a concrete quote and current delivery status.
Need a pole-changing motor?
Send us your required speeds, power and application. We will advise on the best pole combination and winding type.