BLDC MOTOR BRAKING TECHNIQUES (Part-1)

Whenever an electric motor coupled system requires braking, it is a wide practice to employ motor braking technique, unless the system requires braking at very low or zero speeds.

All types of electric motors can be braked using mainly 3 types of braking techniqueswhich are;

• Dynamic braking
• Plug braking (or Plugging)
• Regenerative braking

Although, each braking can be employed for any type of motor, the performance of different techniques and system level requirements vary with respect to motor type and motor parameters.

This article will investigate different braking techniques along with case studies.

Dynamic Braking

Dynamic braking employs a braking resistor to dissipate motor energy. The basic diagram of a motor system with dynamic braking is shown in figure 1.

Figure 1 – Motor Drive Circuit with Dynamic Braking Circuit

To brake the motor; Q7 is to be turned on so that the motor energy on the bus is dissipated the resistor. However, while brake resistor is active; Q8 must be turned off, so that the power source is separated from the motor bus and the resistor only dissipates the energy of the motor. The BEMF generated by the motor is rectified through the body diodes of Q1 – Q6 and current flows to the brake resistor.

The dynamic braking depends on purely Kirchhoff Laws wherein the braking current is linearly dependent on motor BEMF. Therefore, the designer should decide the value of braking resistor by evaluating required maximum braking torque and minimum speed that the motor braking will be active.

Case Study

A brushless DC Servo motor with the specifications given in Table 1 is used with dynamic braking circuit;

Table 1 – Brushless Servo Motor Specifications

The system requires dynamic braking at full power down to 1000 rpm. The generated BEMF at 1000 rpm with no load applied will be around 15 Volts. If the braking will be at full rated torque, 9 A should be drawn from the bus. The body diodes voltage drop will be 0.7 volts per diode. Therefore, total circuit resistance should be no more than 1.5 ohms. The motor has 0.33 ohms line to line resistance. A typical MOSFET used along with the brake resistor will have on resistance around 0.05 ohms. Therefore, the brake resistor should be selected around 1 ohm to provide required braking. The instantaneous power dissipation of resistor will be 81 Watts at the minimum braking speed.

So, what will be the braking characteristics when the motor is operating at full speed? The BEMF of the motor will be around 45 VDC and when the brake MOSFET is turned on, the instantaneous current of the resistor will be around 30 A. In order to limit the average braking power, the braking MOSFET should be operated using PWM at a duty cycle around 30%. So, the braking circuit should be able to handle at least 30A instantaneous current. Also braking MOSFET will have an additional switching dissipation. Graph 1 shows the maximum braking torque as a percent of rated torque with respect to motor speed.

Graph 1 – Braking Torque (%) vs Motor Speed (rpm)

Of course, such a braking power will probably yield a big deceleration and the BEMF and peak currents will decrease in a short time. However, a single failure at peak operating point may result in component failure and the system will completely fail in that case. The designer should consider peak stresses to avoid failures.

If the required braking resistor is calculated to be less than the internal resistance of the motor, the dynamic braking can be applied by turning on all low side MOSFETs of the controller so that the motor terminals are shorted together. The current flows within the terminals of the motor and motor resistance acts as a braking resistor. This method eases the dynamic braking however, the braking power becomes less controllable and all braking power is dissipated as heat over the motor.

Another system level consideration for dynamic braking is the disconnect switch, which is shown as Q8 in figure 1. Q8 should be turned off before the brake is enabled and as soon as the brake is off, the Q8 should be turned on to restore bus voltage for next motoring operation. Since braking resistor is connected to the bus, with decreasing motor speed, the bus voltage is reduced. Therefore, Q8 should turn on with a high voltage difference between its drain and source pins. Since the bus has a very small impedance due to its low resistance nature and bulk bus capacitance, the inrush current will create a huge stress over Q8 at each turn on.

Figure 2 shows a simulation result for a typical 500-Watt motor controller, where Q8 is turned on as soon as bus voltage drops to 15 VDC.

Figure 2 – Bus Connect Switch Simulation Result

At the turn on instant, a huge inrush current flows through Q8 resulting in over 5 kW instantaneous power dissipation over the MOSFET and total switch on dissipates around 1mJ energy. If the brake circuit is frequently used, Q8 is thermally and current-wise stressed continuously. The stress can be reduced by employing additional circuitry such as pre-charge circuits, inrush current limiters etc. However, these additional components will affect system efficiency and dynamic performance for motoring operations.

Dynamic braking is a simple braking technique whenever the system has a 1 quadrant (source only) power source and the braking energy should be dissipated externally. However as discussed through, the braking circuit has several limitations at the system level, which should be considered at the system level design.

In the next article, we will discuss regenerative braking.