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Coreless DC Motors

Coreless DC motors from MICROMO differ from conventional, iron rotor DC motors in that they are equipped with an ironless, self-supporting, skew-wound copper coil. Because of this construction, the rotor is very light yielding a low moment of inertia. The rotor also rotates smoothly without cogging.

MICROMO offers the following coreless DC motors:

Features Coreless DC Motors Flat Coreless DC Motors High Power PMDC Motors
Commutation Precious Metal or Graphite Precious Metal Graphite
Motor Diameter 6 ... 38 mm 15 ... 26 mm 34 to 100 mm
Motor Length 15 ... 63 mm 5.2 ... 8.2 mm 94-202 mm
Shaft Diameter 1.5 ... 6.0 mm 1.5 mm
Nominal Voltage 1,5 ... 48 Volt 2 ... 12 Volt 24 ... 180 Volt
Speed 5000 ... 13000 rpm 5500 ... 10000 rpm 3000 ... 6000 rpm
Continuous Torque 0.11 ... 110 mNm 0.05 ... 3.0 mNm
Links to Data Sheets [Coreless DC Motors Data Sheets] [Flat Coreless DC Motors Data Sheets] [High Power PMDC Motors Data Sheets]

Benefits of Coreless DC Motors:

  • No magnetic cogging
  • High acceleration and deceleration due to the low moment of inertia
  • Low electromagnetic interference
  • Low inductance
  • High efficiency
  • Linearity between voltage and speed
  • Linearity between load and speed
  • Linearity between load and current
  • Small torque ripple due to the multi-segment commutator
  • Compact design

Precious Metal and Copper/Copper Graphite Commutation Systems

For low power small DC motors, commutation systems using precious metals are chosen because of their low contact resistance. For higher power applications a small DC motor with a copper commutator and copper-graphite brushes will achieve longer life. The copper graphite brush commutation system is also more forgiving under overload conditions.

Performance Lifetime

The performance lifetime of coreless DC motors depends on:

  • Operating point - torque, load, and speed
  • Operating conditions - duty cycle
  • Ambient conditions - environment, shock, and vibration
  • Method of integration into other systems - ie. how is the coreless DC motor mounted into the system

Since any combination of the above mentioned parameters may occur, it is impossible to give guaranteed lifetime figures.

The lifetime depending on the application type may exceed 10,000 hours. Higher speeds cause accelerated mechanical wear, resulting in reduced lifetime. Excessive high current and temperature shortens the lifetime. On the average, lifetime of up to 1,000 hours for precious metal brushes, and more than 3,000 hours for graphite metal brushes can be expected when the coreless motors are operated within recommended values indicated on the data sheets.

It is advisable that the current under load in continuous operation should not be higher than one-third of the stall current. In small DC motors with graphite brushes the relationship between stall current and current under load depends on the delivered power and frame size. The motors should not be operated at the stall torque MH, otherwise after a short period of time, the commutation of the windings could be damaged. The motor develops its maximum power P2MAX, at exactly half the stall toruqe MH which also corresponds to half the speed. For reasons of life performance, this working point should only be selected for intermittent periods. For exceptional long life performance, brushless DC motors are available.

Operating Information

Temperature - Humidity

The proven design of the bonded multi-layered skew wound rotor retains a high degree of mechanical rigidity even at elevated temperature.  The majority of the rotors have an epoxy-coated coil for a temperature range up to +125°C.  Galvanic zinc plated and passivated housings give protection against corrosion housings give protection against corrosion in environments with a relative humidity up to 80%.  Nickel plated housing in environments with a relative humidity of up to 98%.  For applications under extreme environmental conditions, special executions can be offered upon request.

Commutator - Brushes

Multi-segmented precious metal or copper commutators and multi-fingered precious metal or copper impregnated graphite brushes are used.  The life performance depends mainly on the speed (mechanical wear) and current (electro-erosion) applied to the motor.  For extended lifetime, reinforced brushes, commutators, and special commutator lubrication are available. The commutation signals of precious metal brushes and graphite brushes do not directly compare.  Precious metal commutation generally has a cleaner signal than graphite commutation.  Current consumption, commutation signals, and noise level of individual samples are not necessarily representative of production quantities.

Bearings - Lubrication

The sintered metal bearings (lubricated under vacuum) and the ball bearings are permanently lubricated with special oil.  Relubrication is not necessary and not recommended.  The use of non-approved lubricants on the coreless DC motors can negatively influence their function and lifetime.  Ball bearings are recommended for continous operation at low and high speeds and for elevated radial shaft load.

Position of operation - Mechanical load - Vibration

The coreless DC motors can be operated in any position.  Ball bearings are recommended for high radial load requirements.  Vibrations of 5-10 g at frequencies up to 1,000 Hz will not limit the function of the motors.  However, as these small DC motors are precision engineered products, they should be handled with care to ensure the desired performance.

Solder terminals - Lead wires

The motors are supplied standard with solder terminals and optionally with lead wires.  The plastic cover with the solder terminal withstands instantaneous temperatures of soldering or welding, provided the equipment used has the adequate power.

Mounting of the coreless DC motors

If the motors are mounted on the front-face holes, the screws must not exceed the indicated depth, otherwise the magnet will be displaced resulting in damage to the motor.

Flat, Coreless DC Motors

Flat, coreless DC motors uses an ironless rotor technology with three flat, oval coils provides the basis for leading-edge performance in applications with minimal space.

The flat, coreless rotor rotates in an axially directed magnetic field with a minimum moment of inertia – without cogging or iron losses. The precious metal commutation is designed to allow deployment at ultra-low starting voltages, which is made possible by the reduced brush contact resistance. Therefore, flat coreless DC motors are particularly suitable for battery operated portable devices.

High Power Permanent Magnet DC Motors

Permanent Magnet DC Motors offer good efficiencies and a high peak load through the use of premium ferrite materials and optimized magnet systems. The steel tube housing with externally accessible brushes provides for a rugged, high-quality design. The Permanent Magnet field wound DC motors are rated in power from 10 W to 750 W with mounting options for worm, planetary, and spur gears with torques up to 130 Nm as well as brakes, incremental encoders, and tachometers.