Why do Motors Need Protection?
Generally, many different types of protection are required in motor drive applications, including protecting power transistors, motors, or any part of the system. The current protection of the inverter is one of the most important items. It not only prevents any potential damage to the power transistor, but also prevents demagnetization of the motor in the event of a malfunction or when the control becomes unstable. Overcurrent protection (OCP) and short circuit protection (SCP) are two common protection methods, but there are still differences between the two. This article introduces the difference between OCP and SCP and various protection schemes.
The Difference between OCP and SCP
Short-circuit protection is part of over-current protection. Figure 1 shows the relationship between different current protections. The short circuit protection includes ground fault protection, arm short protection and phase short protection.
Figure 1. Comparison of Over-current Protection and Short-circuit Protection
The different short-circuit modes and current paths are shown in Figure 2. As shown in Figure 2(a), when the motor winding is short-circuited to the motor housing (usually grounded), or when the motor cable is short-circuited to the ground, a ground fault occurs. Figure 2(b) shows the short circuit of the bridge arm, which means that the high-side IGBT and the low-side IGBT are turned on unexpectedly at the same time and generate extremely high current. Figure 2(c) shows the phase-to-phase short circuit, which occurs when the motor windings of different phases are short-circuited. In the three examples, the current amplitude is limited by the impedance of the current path.
Figure 2. Short-circuit Current Path
How to Set OCP Points?
No matter what type of overcurrent situation is faced, how to set the protection current is very important. To solve this problem, we should first identify the vulnerable components in the system. In most cases, the IGBT will be damaged earlier than the freewheeling diode. When the inverter transfers power to the motor, overcurrent conditions are more likely to occur. Whether the OCP trip value should be set as the current that the IGBT can handle depends on the control method adopted by the system. In Volt/Hertz control, the current value at startup cannot be predicted.
For example, a permanent magnet motor used in a refrigerator with FOC has a rated current of 1A rms, but it can withstand a 120% overload for 10 minutes during the initial start-up cooling period. In this case, the peak current is 1 A rms * 120% * Sqrt(2) = 1.7 A peak. Considering the current control overshoot margin, +/- 2 A can be selected as the current feedback range in the control. Then 2.5 A can be set as the OCP trip point. Even if a 5 A IGBT with a saturation current of 30 A is used in the inverter circuit, the current trip point does not need to be set to 10 A or higher. The high current trip point may cause the motor to demagnetize.
Figure 3. The Relationship between the OCP Point and the Magnitude of Other Currents
Summarize
This article introduces the difference between OCP and SCP and various protection schemes. In order to meet application requirements, appropriate protection methods should be carefully selected.
One of the disadvantages of these methods is the inability to detect ground fault current, as shown in Figure 2(a). As long as the sum is not zero, the software can calculate three-phase currents and identify ground faults. Its response speed may not be enough to prevent IGBT damage. A magnetic core, a current detection resistor on a positive DC bus rail and desaturation protection are also optional grounding protection methods.
