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FEATURE DRIVES, CONTROLS & MOTORS
considering 4pole motors
When selecting a DC micromotor for applications ranging from aerospace through to
downhole drilling inspections, a 4pole motor could be the answer. Greg Dutfield of maxon UK looks into their benefits and features
D
espite having the same footprint as a 2pole design, 4pole motors are capable of generating more torque and are stronger, meaning they can more closely maintaining speed when a load is applied. As
such, they can be the ideal choice for DC applications requiring high torque in a light and compact envelope. Pole count refers to the number of permanent magnet pairs within a
motor. 2pole motors have a single pair of magnets in opposing north and south position. When current is applied, a magnetic field is created between the magnet pole pair, forcing the rotor to turn. Motor configurations also include 4pole, that includes two pole pairs of magnets, through to multi-pole designs that include up to 12poles. Pole count impacts the motor’s speed and torque characteristics. The
lower the number of poles, the higher the motor’s speed. This is because each mechanical revolution of the rotor depends on the completion of the magnetic field cycle of each pole pair. The more permanent magnet pairs a motor has, the more magnetic field cycles required, meaning that it
takes longer for the rotor to complete a 360˚ revolution. Speed is divided by the number of pole pairs at a fixed frequency, so taking a 10,000rpm 2pole motor as the basis, a 4pole motor would generate 5,000rpm, a six-pole motor would run at 3,300rpm, and so on.
GeneratinG torque Increasing the number of poles enables the motor to generate more torque. Taking a 4pole motor, its torque is greatly increased because its compact design features a thinner magnetic return path, enabling more space for a two-pair pole of permanent magnets and, in the case of maxon motors, a patented thicker knitted winding. While a 4pole motor can fit within the same footprint as a 2pole
design, it should be noted that further extending pole count, from six to 12 poles, means that frame size and mass must increase accordingly to accommodate the extra copper, iron, and magnets required. In addition, a motor’s strength is typically defined by its speed-torque
gradient, which means that a stronger motor can more closely hold its speed when a load is applied. The speed-torque gradient is measured by the reduction in speed for each 1mNm load. A lower figure and shallower gradient means the closer a motor can maintain its speed under load. A stronger motor is achieved thanks to the same design attributes that
also help it reach higher torque, such as increased windings, and by using the optimum materials in their manufacture. For this reason, a 4pole motor design is stronger than a 2pole motor of the equivalent frame size. For example, a 22mm maxon 4pole motor has a speed-torque gradient of
19.4rpm/mNm, meaning that for every 1mNm applied, it loses just 19.4rpm, whereas the same frame size 2pole maxon motor has a speed-torque gradient of 110rpm/mNm. Not all motor manufacturers match maxon’s design and materials specification, so the speed-torque gradient of an alternative brand 2pole motor might be higher, signifying a weaker motor.
When should you use a 4pole motor? The strength and low weight of a 4pole motor makes them beneficial for applications across the aerospace industry as well as in power tools and mobile robots. Wheeled or tracked robots need to negotiate rough terrain, obstacles, and steep inclines, and 4pole motors provide the torque and strength to overcome these loads, while helping achieve a compact, lightweight design. Small size combined with a low speed-torque gradient is also vital for
downhole drilling inspection within the oil & gas sector. For this application, a compact 2pole motor wouldn’t be strong enough, while a multi-pole motor would be too large, so maxon has designed a 32mm 4pole motor to fit.
optimisinG a 4pole motor 4pole motors are often suitable for applications in extreme environments or conditions, requiring the ability to operate under high temperature, pressure and vibration. Specifying additional design features such as sleeves as well as techniques to enhance heat dissipation, compact 4pole motors can handle extreme operating conditions long-term. While motor specification is fundamental, in order to optimise an
application, the design of the complete drive system should be considered, including gearboxes, encoders, drives and controls. This is where maxon’s engineers can help.
maxon UK T: 01189 733337
5 DESIGN SOLUTIONS DECEMBER/JANUARY 2023 4
www.maxongroup.co.uk
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