Schneider Electric 58 TRX manual Calculating Resistor Size

Altivar® 58 TRX AC Drives

Dynamic Braking Resistor Kits

Calculating Resistor Size

The standard dynamic braking (DB) resistor assemblies are suitable for a wide variety of drive system stopping applications. However, when the driven machinery may present an overhauling load or large inertia to the drive system, the suitability of the DB resistor assembly should be checked.

The suitability of a DB resistor assembly is determined by analyzing the mechanical system of the driven machinery. From the analysis, the following key parameters are computed:

•The peak braking power required during stopping or speed changes (Pi). The value of Pi determines the maximum allowable ohmic value of the DB resistor.

•The amount of power that must be absorbed (Pd) for a given time (td) by the DB resistors during stopping or speed changes of the drive. The value of Pd and td determine the required time-current characteristic of the DB resistor.

•The calculation of dynamic braking power requires Vdb.

•Vdb = 850 V for 460 V drives Vdb = 375 V for 230 V drives

•The average power that must be dissipated by the DB resistor during an entire cycle of the machine (Pa). The value of Pa determines the required continuous current rating of the DB resistor.

The following example illustrates the process:

Given

The application consists of a 5 hp, 460 Vac, 1740 rpm motor (Nbase = base speed) with a rotor inertia of 0.28 lb-ft2. The motor is being controlled by an ATV58HU72N4 operating in the constant

torque mode. The motor is driving a machine with an inertia 10 times that of the motor with no interposing gear box. The machine resistive (friction) torque is one-tenth of the rated motor torque at full speed. The requirement is to stop in 5 seconds from rated speed at a rate of 2 cycles/minute.

Mechanical System Parameters:

Rated motor torque: Tn = (hp x 5250)/Nbase = (5 x 5250)/1740 = 15.1 lb-ft

Machine cycle time: tc = (60 seconds)/(two operations per minute) = 30 seconds

Machine speed change during deceleration: Nd = 1740 rpm - 0 rpm = 1740 rpm

Machine deceleration time: td = 5 seconds

Mechanical system resistive (friction) torque: Tr = (15.1 lb-ft)/10 = 1.51 lb-ft

Mechanical system overhauling torque: To = 0.00 lb-ft

Mechanical system combined inertia: Jc = 0.28 lb-ft2+ (10) x 0.28 lb-ft2= 3.08 lb-ft2

Mechanical system inertial torque for a 5 second deceleration rate (as set by controller deceleration ramp):

Tj = Jc x (Nd)/(308 x (td)) = 3.08 x 1740/(308 x 5) = 3.48 lb-ft

Required braking torque from motor: Tb = Tj + To - Tr = 3.48 + 0.00 - 1.51 = 1.97 lb-ft

NOTE: The required braking torque must not exceed the motor’s ability to produce torque. For inertial loads, including those depicted in the above examples, the required braking torque must not exceed the torque producing ability of the dynamic braking unit with the recommended braking resistor (approximately 1.5 times the motor rated torque for constant torque applications).

For machines that can continuously overhaul the motor, the value of overhauling torque (To) minus the resistive torque (Tr) must not exceed the motor continuous torque rating at any speed.

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