DC Motors: Characteristics

EMI noise suppression

Brushed DC motors generate EMI (Electro Magnetic Interference) noise, whenever the brushes pass between two commutator segments.

The higher the supply voltage, speed and current - the higher the noise emission.

The emission also depends on number of segments (voltage drop level between neighbouring segments).

EMI is emitted by two ways:

  • Conduction along power supply wires
  • Radiation through the air


The noise has to be limited according to relevant Directives and Standards.

For that, JE can make tests and implement noise suppression means. Preferably, the whole customer appliance should be tested.

Additional to means such as twisting and shielding of wires, there is a choice of several suppression components:

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Losses and Efficiency

No-load losses PO:
Friction losses at bearings and at brushes, Hysteresis and Eddy current losses.
For these reasons a no-load current IO must flow, which in turn causes some winding losses too.

Load losses:
At rising load torque, current increases and thus winding losses increase.
The current reaches its maximum value Istall when rotor is stalled.
The power loss is Ploss max = U2 / R

At point of max. efficiency there is best ratio Pmech / Pel = Pmech /(U • I).
Efficiency at this point is about η = (1 - √ IO/Istall)2

At point of max. mechanical output power, the speed is half of no-load speed.
Output power Pmech max is little less than U2 / 4 R.


DC motor lifetime is limited by wear of brushes. It depends on

  • Speed (commutator surface speed)
  • Current load (average current in and peaks, start/stop frequency)
  • Brush configuration, brush material and commutator design

Frequent start/stop operation, with high current to accelerate the rotor each time, is more critical for lifetime than continuous operation.