Perhitungan Pengaman Motor Listrik
7 Agustus 2011, 11.25
Diposting Oleh: Ari Sulistiono | lokasi Tag: Control System, Ilmu Listrik
Pada dasarnya “Motor protection” dan “motor circuit protection” adalah dua hal yang berbeda dan membutuhkan suatu perhitungan yang berbeda pula.
Untuk mencegah motor terbakar, kita harus menyiapkan sebuah cara untuk melindunginya dari arus lebih (overload, hubung singkat, atau ground-fault). Sebelum kita melangkah ke bahasan lebih lanjut, sebaiknya ada tidak bingung dengan motor protection dan circuit protection. Motor protection adalah sebuah sistem pengaman motor yang tehubung dengan rangkaian tenaga motor. Sedangkan circuit protection adalah sebuah sistem pengaman untuk rangkaian tenaga listrik daripada motor itu sendiri.
Silahkan simak gambar 430,1 pada NEC untuk membantu anda melihat perbedaan dari keduanya dengan jelas. Disana dijelaskan persyaratan untuk perlindungan overload motor pada Bab III dan persyaratan perlindungan hubung-singkat dan ground-fault pada Bab IV dan V.
Tabel atau nameplate? Untuk menentukan KHA (Kemampuan Hantar Arus atau ampacity) minimum dari konduktor power supply motor, anda harus mengetahui secara jelas berapa banyak arus yang akan mengalir saat tarikan pertama motor. Tapi disana juga ada berbagai jenis arus pada aplikasi motor (simak “Motor Current Basics” pada halaman 80). Arus beban penuh / full-load current (FLC) atau amper beban penuh / full-load amperes (FLA) Manakah yang kita gunakan untuk perhitungan motor Anda?
Di dalam standar NEC tidak diijinkan penggunaan nilai Amper Beban Penuh / FLA yang tertera pada nameplate untuk menentukan KHA / ampacity konduktor atau ukuran kabel yang digunakan, percabangan rangkaian hubung-singkat dan ukuran perangkat ground-fault overcurrent, atau pun nilai rating amper disconnecting switches [430,6 (A) (1)]. Tetapi disini kita harus menggunakan nilai FLA motor untuk menentukan ukuran overload-protection motor secara terpisah sesuai dengan Bagian III Alenia 430 [430.6(A)(2)]. Perhatikan pengecualian berikut:
- Jika Anda menghitung motor overload protection secara terpisah untuk torsi motor, gunakan nilai arus locked rotor pada nameplate [430,6 (B)].
- Jika ada variable-frequency drive (inverter) sebagai pengendali motor, gunakan maximum operating current yang ditandai pada nameplate (motor atau kontrol). Jika nilai tersebut tidak terdapat pada nameplate, gunakan 150% dari nilai yang ditemukan di dalam tabel NEC [430,6 (C)].
- Motor dengan Torsi Tinggi (umumnya dibuat dan untuk beroperasi dibawah kecepatan 1,200 rpm) biasanya memiliki nilai FLC yang lebih besar dibandingkan dengan motor multispeed. Untuk motor seperti ini, gunakan rating arus tertera pada nameplate [430.6(A)(1)].
- For a listed motor-operated appliance, use the FLC marked on the nameplate of the appliance (rather than the horsepower rating) to determine the ampacity (or rating) of the disconnecting means, branch-circuit conductors, controller, and branch-circuit short-circuit and ground-fault protection [430.6(A)(1) Ex 3].
Overload protection. Overload protection harus sesuai dengan 430 Bab III. Ukuran perangkat proteksi beban lebih didasarkan pada rating yang tertera pada nameplate motor (hal ini berfungsi untuk melindungi kerusakan gulungan motor akibat arus yang timbul oleh locked-rotor atau rotor macet/seret/nge-jam) [430,31].
Fig. 4. Branch-circuit conductors are protected against overloads by the overload device.
You can use a single overcurrent device, sized per 430.32 requirements, to protect a motor from overload, short circuit, and ground faults.
Branch-circuit conductor size. Branch-circuit conductors to a single motor must have an ampacity of not less than 125% of the FLC as listed in Tables 430.247 through 430.250 [430.6(A)(1), 430.22(A)].
When selecting motor current from one of these tables, note that the last sentence above each table allows you to use the ampacity columns for a range of system voltages without any adjustment. Select the conductor size from Table 310.16 according to the terminal temperature rating (60ºC or 75ºC) of the equipment [110.14(C)].
THHN/THWN is a common conductor insulation type that can be used in a dry location at the THHN 90ºC ampacity, or in a wet location at the 75ºC ampacity for the THWN insulation type. Regardless of the conductor insulation type, size the conductor per 110.14(C).
In 110.14(C)(1)(a), we read that equipment terminals are rated 60ºC for equipment rated 100A or less (unless marked 75ºC). Today, most equipment terminals are rated at 75ºC. Look for that specification, so you can use the 75ºC column if your conductors are also rated for 75ºC. If this is the case, you may save considerable money on your project. If you can’t find that specification, use the rules of 110.14(C).
Test your knowledge by answering this question: What size branch-circuit conductors are required for a 7½-hp, 3-phase, 230V motor (Fig. 1 on page 76)?
The motor FLC from Table 430.248 is 22A. The conductor is sized no less than 125% of motor FLC: 22A 3 1.25 = 27.50A. As per Table 310.16, a 10 AWG conductor is rated 30A at 75ºC.
The minimum size conductor permitted for building wiring is 14 AWG [310.5]; however, some local codes and many industrial facilities require branch-circuit conductors to be 12 AWG or larger.
Feeder conductor size. Perform feeder conductor size calculations the same way as for branch circuits, but use the different ampacity rules provided in 430.24. Conductors that supply several motors must have an ampacity of not less than:
(1) 125% of the highest rated motor FLC [430.17], plus
(2) The sum of the FLCs of the other motors (on the same line). Find the FLC in the NEC Tables [430.6(A)(1)].
The highest rated motor is the motor with the highest FLC [430.17]. Determine the “other motors in the group” value by balancing the motor FLCs on the feeder being sized, then select the line that has the highest rated motor on it (Fig. 2 on page 78).
Branch-circuit short-circuit and ground-fault protection. Each motor branch circuit must be protected against short circuit and ground faults by an overcurrent device sized no greater than the percentages listed in Table 430.52. The motor branch-circuit short-circuit and ground-fault protective device must be capable of carrying the motor’s starting current, and it must comply with 430.52(B) and 430.52(C).
A branch-circuit short-circuit and ground-fault protective device protects the motor, the motor control apparatus, and the conductors against short circuits or ground faults, but not against overload [430.51] (Fig. 3 on page 78).
It bothers many electrical practitioners to see a 14 AWG conductor protected by a 30A circuit breaker, but branch-circuit conductors are protected against overloads by the overload device (Fig. 4). That device is sized between 115% and 125% of the motor nameplate current rating [430.32]. See 240.4(G) for details.
Where the branch-circuit motor short-circuit and ground-fault protective device values derived from Table 430.52 don’t correspond with the standard overcurrent device ratings listed in 240.6(A), you can use the next higher overcurrent device rating. The “next size up protection” rule for branch circuits [430.52(C)(1) Ex 1] doesn’t apply to the motor feeder overcurrent device rating (Part II).
Keeping it straight. Articles 430 and 250 are the largest of the NEC Articles, and arguably the most misapplied. Something else these two Articles have in common but not with the other Articles is a “Figure 1” you can use as a guide.
In the case of Art. 430, this figure is a simple representation of the motor system with the correct Part of Art. 430 noted for each area of application. At the beginning of this article, we said that using Figure 430.1 will help you to not confuse motor protection with circuit protection when in actuality it can do much more. Spend some time working with it, and you’ll see how useful it really is.
If you base each motor project on Figure 430.1, you will reduce — if not eliminate — Art. 430 application errors.
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Tags: Control System, Ilmu Listrik