Introduction: Danfoss soft starters are widely used in many industrial settings. Recently, we troubleshooted several soft starter malfunctions for our customers and found that the cause was the failure to install a quick-connect fuse at the front end of the soft starter as required by the Danfoss manual. This resulted in damage to the soft starter's SCR (Silicon Controlled Rectifier) when a phase-to-phase or phase-to-ground short circuit occurred in the motor or motor cable. Therefore, this article discusses the short-circuit protection of soft starters. I. Working principle of soft starter:
The main circuit power device of a soft starter is an SCR (Silicon Controlled Rectifier). The working principle of a soft starter is to control the conduction angle of the SCR's trigger signal to control the output three-phase AC voltage. The most common fault of a soft starter is a short-circuit failure of the main circuit SCR, with the thermal effect of short-circuit current being the main cause of SCR damage. The thermal effect of short-circuit current is represented by the value I²t , and the current value during a short circuit and the duration of the short circuit are important reference parameters for short-circuit protection. After the SCR is triggered and turned on, it does not have self-turn-off capability. If the SCR is relied upon for natural commutation turn-off, the longest short-circuit turn-off delay will reach half an AC cycle, which is 10ms for a 50Hz power grid. The short-circuit overcurrent withstand capability of the SCR is generally not very strong, so soft starters require external devices to provide short-circuit overcurrent protection. II. Short-circuit protection for soft starters:
The following discussion uses the Danfoss 220kW soft starter MCD3220 as an example to illustrate the short-circuit protection measures for soft starters. The rated output capacity of the MCD3220 is shown in the table below: When the starting current multiple is 3, the rated full-load current of the motor is 430A. Therefore, its rated output current capacity is calculated as: I<sub> e</sub> = 430 × 3.0 = 1290 A. The Danfoss soft starter MCD3220's main circuit thyristor module is the EUPEC T588N16TOF, with the following technical parameters: Key Data 1: On-state current RMS value 1250A, approximately the same as the rated output current capability of the soft starter 1290A; Key Data 2: Maximum peak effective current in 10ms is 8000A, meaning the maximum short-circuit current should not exceed 8000A if the thyristor is used for natural commutation turn-off; Key Data 3: I²t thermal withstand capability at the highest junction temperature (125℃) is 320000A²s , which is the short-circuit thermal withstand capability of semiconductor devices and a key reference for short-circuit protection. The magnitude of the short-circuit current depends on the type of short circuit and the grid resistance. Short circuit types include simultaneous short circuits of three-phase live wires, short circuits between two-phase live wires, and single-phase live wire-to-ground short circuits, with simultaneous short circuits of three-phase live wires being the most severe. We will analyze the short-circuit current based on this type of short circuit. Figure 2. Short-circuit current under different grid capacities and grid internal resistances. The short-circuit current when all three live wires are short-circuited simultaneously is Isca: where S is the grid (transformer) capacity; Z is the grid (transformer plus cable) internal resistance. Figure 2 calculates the value of Isca using grid internal resistances of 5% and 2% as examples. When the MCD3220 is used in a potentially poor 500kVA grid with 5% internal resistance, the short-circuit current reaches 13222A, which is larger than the maximum peak effective current of 8000A that the T588N16TOF thyristor module can withstand in a single 10ms operation. Therefore, relying on the natural commutation turn-off of the thyristor cannot provide reliable short-circuit overcurrent protection. Therefore, the Danfoss soft starter body only provides overload and instantaneous overload protection. The following figure shows the setting parameters for the instantaneous overload protection of the Danfoss soft starter. Electrical devices commonly used for short-circuit protection are circuit breakers and fuses. Their applications are discussed below. Discussion 1: Using Circuit Breakers for Short-Circuit Protection. When selecting a circuit breaker for short-circuit protection, choose one based on the motor's rated current of 430A. If using Schneider Electric products, the NS630 circuit breaker should be selected. This circuit breaker can be configured with two types of trip units: one is the electromagnetic MA (MAE500) type trip unit, which only provides short-circuit current protection. Since the MCD3000 has built-in overload protection, this model can be used, with a fixed fastest operating time of 20ms. The NS630 can also be configured with another type of motor-specific electronic trip unit, the STR43ME (500A), which provides both short-circuit and overload protection, with a maximum short-circuit protection operating time of 10ms. However, since a circuit breaker is a mechanical device, and mechanical devices have a significant operating lag, Schneider Electric specifies a total breaking time of approximately 60ms. The total breaking time is independent of the type of trip unit selected. Other literature estimates that the circuit breaker's breaking time is delayed by at least one power frequency cycle, i.e., 20ms. If a domestically produced circuit breaker is chosen, taking Shanghai Huatong's products as an example, the fastest short-circuit protection time is a fixed 20ms when using the motor-specific SM30 630A or SM40 630A circuit breaker. The total breaking time is not readily available, but since most domestic products are imitations of imported products, it can be estimated that the actual total breaking time will not be better than Schneider's products. The protection action delay of the aforementioned traditional circuit breakers is much longer than the natural commutation and turn-off time of SCR thyristors, therefore they are unsuitable for short-circuit overcurrent protection of soft starters. Let's look at the advanced function of Schneider NS circuit breakers—the reflex energy trip function. It is said that during a short-circuit fault, it generates a powerful gas pressure that directly acts on a piston mechanism, enabling the circuit breaker to break quickly and reliably. Because the action is very rapid, <10ms, it has the function of naturally suppressing short-circuit current, just like a fast-acting fuse. Its current-limiting characteristic curve is shown in the figure below: Figure 3. Schneider Electric's reflex function for suppressing short-circuit current. As shown in Figure 3, the NS630 has a significant ability to limit short-circuit current when the expected short-circuit current is >30kA. However, 30kA is too large for a thyristor, and the operating delay time is also half a power frequency cycle (10ms). Therefore, the reflex function is not suitable for short-circuit overcurrent protection of soft starters. In fact, Schneider Electric introduced this function only to improve the protection of power cables or busbars. Discussion 2. Using a dedicated fast-acting fuse for semiconductor protection for short-circuit protection (required in the Danfoss manual). Danfoss recommends installing an 800A fast-acting fuse before the MCD3220 soft starter. The fusing characteristics of the fast-acting fuse are shown in Figure 3: The fuse operating time is divided into two segments. The first and fourth segments are the pre-arc time, i.e., the time it takes for the fuse to melt, during which the current is the same as the expected short-circuit current. The specific value of the pre-arc time can be found in Figure 5. A fast-acting fuse with a rated current of 800A can be found. When the short-circuit current is 13222A, the arc initiation time is <2ms; when the short-circuit current is 99165A, the arc initiation time is <0.1ms. The second segment is the arc breaking time, which is the time required for the arc to be generated and extinguished, during which the current continuously decreases.| Figure 5. Arc pre-arc time of fast-acting fuses |
Figure 4. Operating Characteristics of Fast-Blow Fuses Figure 5. Arc Pre-Arc Time of Fast-Blow Fuses In general, the total operating time of a fast-blow fuse during a fault is related to the thermal effect of the short-circuit current. Therefore, the key parameter for a fast-blow fuse is not the operating time, but the cumulative current heat generated during the operating process, I²t . This is consistent with the data indicators required for SCR short-circuit protection. As long as the total I²t value of the fast-blow fuse matches the I²t value of the protected semiconductor device, it can provide effective short-circuit protection. The table below is the selection table for fast-blow fuses required for the pre-stage short-circuit protection of Danfoss soft starters: The Danfoss manual requires a Bussmann 170M6012 semiconductor fuse to be installed before the MCD3220, specifying a short-circuit protection I²t value of 320000A²S for the thyristor, which is the same as the short-circuit protection I²t value required for the T588N16TOF thyristor used in the MCD3220 soft starter. Data for the 170M6012 semiconductor fuse is shown in the table below: Since the arc-breaking I²t value given in the BUSSMANN manual is for a 660V mains voltage, a correction factor for a 400V mains voltage needs to be calculated from a table. From the right-hand graph, the correction factor is approximately 0.61. Therefore, the arc-breaking I²t value for the 170M6012 in a 400V mains voltage is: Arc-breaking I²t = 465000 × 0.61 = 283650. The total I²t value for the 170M6012 = Pre-arc I²t + Arc-breaking I²t = 69500 + 283650 = 353150. This value is close to the I²t value of 320000 required for SCR protection. Conclusion: Dedicated fast-acting fuses for semiconductor device protection can provide short-circuit protection for soft starters. For typical customers, it is essential to require the use of semiconductor fuses recommended by Danfoss for soft starter short-circuit protection. The circuit breaker cannot meet the short-circuit protection requirements of the soft starter. I. Selection of Domestic Fast-Blow Fuses:
Domestically produced fast-acting fuses are also available for semiconductor device protection. To reduce costs, domestically produced semiconductor fuses can be considered for short-circuit protection of soft starters. When selecting a fuse, the relevant parameters of imported products should be referenced, meeting the following requirements: 1. Voltage: ≥ 380VAC 2. Current: Refer to the rated current of the BUSSMANN fuse to be replaced 3. I²t : Due to the lack of domestic standards, there is no relevant product data. However, based on experience, domestically produced fast-acting fuses completely imitate imported products in terms of fuse material and shape, so this data should be similar to imported products. When selecting domestically produced semiconductor fuses, the real concern is the quality of the domestic product, such as whether the fast-acting fuse body will overheat at rated current, whether it will age and fail, and whether it will fail to break circuits. … The table below, using the Shanghai Electrical Ceramics Factory's semiconductor device protection fuses as an example, lists the selection of domestically produced fast-acting fuses suitable for short-circuit protection of Danfoss soft starters: | soft starter model | Fast-acting fuse models | Fuse holder model |
| MCD3007 | NGT00C-380V/125 (63A) | sist101 |
| MCD3015 | NGT00C-380V/125 (125A) |
| MCD3018 | NGTC1-380V/250 (160A) | sist201 |
| MCD3022 | NGTC1-380V/250 (160A) |
| MCD3030 | NGTC1-380V/250 (160A) |
| MCD3037 | NGTC2-380V/400 (280A) | sist401 |
| MCD3045 | NGTC2-380V/400 (280A) |
| MCD3055 | NGTC2-380V/400 (280A) |
| MCD3075 | NGTC2-380V/400 (400A) |
| MCD3090 | NGTC3-380V/630 (500A) | sist601 |
| MCD3110 | NGTC3-380V/630 (560A) |
| MCD3132 | NGTC3-380V/630 (630A) |
| MCD3185 | RST3-500V/800 (700A) | No seat |
| MCD3220 | RST3-500V/800 (800A) |
| MCD3300 | RST3-500V/1200 (1000A) |
| MCD3315 | RST3-500V/1200 (1200A) |
| MCD3400 | RST1-1000V/2000 (1600A) |
| MCD3500 | RST1-1000V/2000 (2000A) |
| MCD3600 | RST1-1000V/2000 (2000A) |
| MCD3700 | RST1-1000V/2000 (2000A) |
| MCD3800 | RST1-1000V/2000 (2000A) |
References [1] MCD3000 Operating Instructions. MG.15.A.41. Danfoss. [2] Compact NS Molded Case Switch Application Guide. SC DOC 797-LV. Merlin Gerin. 2005 [3] Marketing Information T588N. Eupec. 1996 [4] High Speed Fuse Application Guide. Bussmann. [5] High Speed Fuse Catalog. Bussmann. About the Author
Xia Ming: Technical Department Manager, Shanghai Jinxin Electric Co., Ltd., primarily engaged in the application and research of Danfoss drive products. Contact information: No. 5, Lane 248, Xinshi Road, Shanghai, 200083, China; Tel: 021-65618877 statement:
Shanghai Jinxin Electric Co., Ltd. owns all rights to this technical document. Anyone who wishes to reproduce it must indicate the source.
