Regenerative power for BLDC, stepper, and servo motors

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om_tech_support_JT
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Regenerative power for BLDC, stepper, and servo motors

Postby om_tech_support_JT » Wed Nov 27, 2013 6:53 pm

Q: I am using your BX series brushless DC electromagnetic brake motors for a lifter application. In the manual, a regeneration unit is recommended for a large inertia load or gravitational load type application due to the fact that back EMF generated from a back-driven motor may damage the driver circuit. I am trying to find out if there’s any way for me to calculate the regeneration power so I can also include some other components into my design to protect my circuits?

A: For our brushless DC, stepper, and servo motors, it is important to connect the recommended regeneration unit if the motor is driving a large inertial load or gravitation load. Our regeneration unit is basically a large resistor circuit which dissipates this excessive regenerative energy as heat to prevent damage to the driver's internal components. The main reason for the regenerative energy is because the rotor is made of permanent magnets, and the stators are made of copper coils (omitting laminations). We know that current is induced onto a conductor when it passes magnetlc flux. Therefore, when the motor rotates, essentially it is a rotating magnetic field around copper conductors, thus it will generate current in the stator coils, which will pass through the driver circuits. This induced current surge may damage the driver's internal electrical components if a regeneration unit is not used. For DC systems, the power supply may be damaged over time.

Motor regeneration power can be estimated by using the formula below:

Regeneration power = 0.1047 x T x N,

Where T is the load torque, and N is the RPM of the motor (not the gearhead output speed). When you apply this formula, please note that the T and N in the formula are the load torque of the motor and actual motor speed, instead of the gearhead output torque and speed.

For example, if you have a 20:1 gear ratio for the application, and the load creates a torque of 1 N.m, and rotating at 100 RPM. Then the load torque will be 1 N.m divided by 20 divided by 90% (gearhead efficiency ) = 0.0555 N.m. The total RPM is 100 x 20 = 2000 RPM on the motor. Therefore the total regeneration power is 0.0555 x 2000 x 0.1047 = 11.62W.

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