Factors to keep in mind when choosing discharge resistors

Factors to keep in mind when choosing discharge resistors

The discharge resistor is measured in Ohm values ​​(Ohm) with the main purpose is to drain excess electrical energy generated by the engine during braking or inertia. These excess energies, if not dissipated, will cause great losses such as fire and damage to equipment. So, what is the most appropriate choice of discharge resistor?

The working principle of the discharge resistor

During the motor stop, the engine becomes a generator that generates an alternating current.

At this time, the protection circuit of IGBT with Diode against reverse current becomes a rectifier circuit that converts the alternating voltage (AC) into direct current (DC).

Then, proceed to bring DC current to DC bus causing DC voltage to increase. When the DC voltage at the DC bus rises above the device protection level, there are 2 cases:

– Case 1: The device will report the overvoltage error.

– Case 2: A sudden increase in DC bus will cause an explosion of IGBT + capacitors.

=> To avoid the above errors should install an additional discharge resistor.

==> Discharge resistance will consume excess energy during the engine stop, reverse rotation with large inertia.

Which factor should be used to select the discharge resistor?

To choose a discharge resistance suitable for the inverter must be based on many factors such as the capacity of the motor, the load of the motor, the inverter manufacturer … In addition, when selecting discharge resistance, it is necessary to follow the instructions. of inverter manufacturers.

In fact, the choice of discharge resistance is usually based on two main factors:

  • Discharge power capacity (W)
  • Resistance value (Ohm)

How to choose the discharge resistor capacity?

The discharge capacity selected must ensure two factors: technical features and economic efficiency. Accordingly, the way to choose the most standard discharge resistance capacity is to discharge a lot of electricity, then choose a large capacity and vice versa to ensure economic requirements.

However, the inverter discharges more or less depending on a load of each application.

For example:

– Load lifting crane, hoist … (potential load), the engine will work as a generator. When downloading and braking, it takes a long time to download, so it is necessary to choose a type of discharge resistor with a large capacity (discharge resistance capacity is 1/2 or 2/3 of the engine power).

– Loads with a long braking time, large inertia such as centrifuges, extractors, flywheels, etc. It is important to choose a large capacity resistor.

How to choose the value of the discharge resistor?

The discharge resistor value is determined by the braking current design of each manufacturer and each type of inverter. Accordingly, the inverter manufacturer will give the MIN value of the resistor (ie related to the braking MAX current of the inverter).

=> Therefore, when selecting the resistor value, you must choose a value greater than or equal to the MIN value of the lookup table provided by the supplier.

Currently, most inverter manufacturers control discharge according to the PWM principle (pulse modulation method) so the resistance value can be selected to be 1.5 to 2 times the MIN value of the inverter.

-> You may be interested in Brake unit for Fuji Electric inverters

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