Q & A INDEX

SECTION 3 - BRUSH THEORY AND CHARACTERISTICS

Question 3:6 How Is Current Density Calculated?  

For DC machines use the formulae:  

                   Current                        

Total Brush Circuit Contact Area  

Where Current = The actual operating current of the machine.

This is usually less than the rated full load Amps on the name plate.

Total Brush Circuit Contact Area = 

Contact area of 1 brush (width x thickness) x number of brushes in the machine.                                                                                                          

                                                 2*

* Divide by 2 because to complete a circuit half the brushes conduct current to the armature, the other half conduct it out and complete the circuit.

For AC slipring motors and other current transfer rings the current density is calculated as: 

                               Actual Amps                                          

Brush Thickness x Width x Number of Brushes Per Ring  

With slipring motors remember the actual amps may not be the same as the rated rotor or secondary amps on the name plate as this is a full load rating.

Determining the actual rotor amps of an underloaded machine is not possible by a standard AC clamp-on ammeter as the frequency is very low. If you don't have any instruments that can measure around 1-2 Hz then, as a guide, take the actual primary line current of the motor and apply the percentage of full load current to the rotor full load amps to arrive at an approximate value.

Remember that for brushes with a bottom angle the contact area is not exactly the Width X Thickness since the angled face has a wider dimension than the actual thickness of the brush and this should be taken into account in the calculation of the contact area.

When calculating contact area of brushes with saw cuts in the contact face remember the contact area is reduced by the area of the cuts.

This is also a consideration when brushes are running on sliprings with helical grooves in the rings. The effective contact area has to be reduced by the area of the grooves under the brush face. 

Example

Current Density Calculation:

575KW DC Motor    Rated 1400A     440V

Actual Running Amps 800

Brush Size: 25mm Thick x 32mm Wide 

A    Cross Section= 2.5 x 3.2 = 8cm²

B    Brushes per Arm 8 

C    Number of Arms 4  

Total Cross Section for Current Density

TCM² =  A x B x C 

                        2 

             = 8 x 8 x 4      = 128cm²  

                        2  

At full load A/cm² =1400 = 10.94 A/cm²

                                  128  

At actual load A/cm² = 800 = 6.25A/cm²

                                        128

*This would be considered to be too low under normal circumstances. 

Effect of reducing brushes to 6 per arm 

TCM² = A x B x C     = 6 x 8 x 4   =    96cm² 

                    2                      2             

At actual load A/cm²  = 800   = 8.33 A/cm² 

                                          96  

This would be the recommendation which allows for intermittent short term overload and starting currents.

Question 3:7 What Can Be Done About Low Current Density?

Q & A INDEX

The information has been drawn from experience of Morganite Taiwan Limited Application Specialists combined with a wide range of published information from major carbon supplier and motor manufacture. However, the material can not be downloaded, copied or used without written permission.

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