THICK FILM on STEEL Resistor Technology

For use in Dynamic Braking and AC/DC Motion Control Applications

Author: Wilson Hayworth, Applications Engineer
wilson.hayworth@irctt.com

International Resistive Company, Inc.

ABSTRACT:

Thick Film on Steel Resistor Technology is uniquely suited for wide-scaled usage in resistor applications where high power density, surge handling capability, and low inductance are required and space is limited. Thermal management is optimized utilizing stainless steel substrates with superior strength to shock and vibration.

This paper is a general guideline aimed at familiarizing the users with the characteristics of TFS for dynamic braking and motion control of AC and DC motor drives.

The IRC Thick Film on Steel, (hereafter "TFS"), products represent high power handling technology that offer a reduction of system inductance and size with a cost effective component. Heat dissipation and operating temperature ranges exhibit excellent performance characteristics. Applications requiring speed control can maximize dynamic braking and surge handling by using this monolithic, multi-layer ceramic device constructed on a stainless steel substrate.

A flameproof package and integrated fusing offer failsafe circuit protection that has been proven to meet the UL requirements for various applications. With current and temperature sense options and a wide range of resistance values available, IRC will custom design the circuit and substrate to meet the requirements of your application in a discrete package.
 
 

PRODUCT DESCRIPTION:

TFS is a ceramic encapsulated resistor constructed on a non-corrosive 300 or 400 series stainless steel substrate. Ceramic dielectric layers insolate the substrate with a matching thermal expansion coefficient to the stainless steel. A layer of a conductor and resistor are added and finally coated with an insulating layer of glass. Figure 1 (below) shows the typical construction of the resistor on the stainless steel substrate.

TFS can handle up to 600W/in² (100W/cm²) and 400 degrees C continuous operation. Under surge conditions, peak temperatures of 500 degrees C are nondestructive. Standard construction exceeds withstanding voltages of 3500 VDC for 1 second and this product is robust to humidity, arcing, shock and vibration. The performance is stable over the operating temperature range with a typical TCR of 1200 ppm/ degrees C, (TCR values of 150 to 3500 ppm/Degrees C are also available).

IRC offers flexibility with the TFS geometry and can accommodate various flat surfaces as well as contoured shapes such as stainless steel tubing. Custom features including mounting holes and lead attachment options are available to meet the design needs.

TFS DYNAMIC BRAKING DEVICES:

Dynamic Braking is critical to motion control acting as a high voltage clamp to dissipate regenerative energy induced from electronic switching. Without the resistive brake, the drive could slip off line or not stop in the desired time. Whether the braking is used to hold a constant speed or if the system is used as a shutdown for safety, the coupled inertial load acts to drive the motor as a generator and the energy built up in system must be dissipated to instantaneously change the rotational velocity. For a three-phase induction motor, defining the instantaneous power to be dissipated and the current rating of the switching device will determine the minimum braking resistance value. The three parameters that define the required resistor performance are the switch current limit, the voltage levels during the braking action, and the system inertia. Where the generated current, voltage, and instantaneous power are dependent on the motor drive design, the average and continuous power is dependent on the system’s load inertia and the braking frequency.

The dynamic braking data shown in Figure 2 (below) comes from a power tool with a low inertial load. The coast down time without a braking resistor is over 5 seconds. Figure 3 shows the temperature profile generated during the braking action with the TFS Resistor mounted to a heat sink in still air. The generated heat can be dissipated at a faster rate with airflow or a water-cooled system, which will reduce the steady state temperature of the resistor. Most applications can be defined by a maximum brake time and either a maximum generated current or resistor temperature. The maximum resistance value is determined by the maximum braking time. The maximum temperature of the resistor or the current limit of the switch defines the minimum resistance value.
 
 

SIZING THE BRAKING RESISTOR

In order to determine the relationship of the instantaneous power, P_i, generated during the braking cycle, the formula derived from Ohms Law is used to compare the elevated DC link voltage, V_e, and the braking resistance, R_b,

This is important when defining the surge characteristics required for the resistor rating.

A factor in determining the minimum resistance value, R_bmin, is the current limit of the switching mechanism, I_s. By design, the resistor should be sized so that the generated current of the motor does not exceed the rating of the electronic switch.

For synchronous speed, w_s = 2pf/N_p [rad/sec], where f is the power frequency and N_p is the number of induction pole pairs. Using the actual rotor speed, w_r [rad/sec], the motor slip, S, is calculated by:

The rated power of the drive, P_r, is used to calculate the rated torque, T_r.

Taking into account the torque overload factor, T_o, usually between 150% and 200%, the effective torque, T_e, for braking calculations is

By assuming that the angular deceleration, a, is constant from the synchronous speed, w_s, to zero, the braking time, t_b [sec], can be calculated using the effective torque, T_e. The inertial load, J_l [kgm²], of the drive system determines the brake time:

Other data can be obtained from the following equations in understanding the requirements of your application.

To calculate the continuous power rating, P_c, the duty cycle of the braking interval is considered. Where the duty cycle is calculated by the braking and cycle time d_c = t_b / t_cy.

And with the thermal resistance of the heat sink, R_th, the temperature rise of the resistor can be estimated by:

EXAMPLE

Calculate the braking resistor needed for a 3 KW drive system. This drive consists of a 4 pole induction motor, a 10A rated switching mechanism, the rotational speed is 1750 rpm at 60 Hz, the coupled inertial load is 1 kgm², and the elevated DC link voltage, Ve, is 780 V.

Assuming a 175% overload factor, or T_o= 1.75, and calculating the number of pole pairs, 4/2, or 2, the synchronous and rotational speed are:

The rated torque and maximum available torque are then calculated as

Assuming constant angular deceleration, the braking time is then calculated by:

If the desired nominal braking time is 5 seconds, then:

And the instantaneous resistor power, P_i, is determined to be:

With the upper rail voltage of 780 V, the resistor is calculated to be

From the Kinetic Energy equation:

To determine the continuous rating for the resistor, apply the duty cycle. For this example, assume one braking cycle per minute, or 5 seconds per 60 second interval:

In summary, the resistor needs to meet the following parameters in order to handle the dissipated energy for a 5 second braking cycle:
 

• Instantaneous Power = 6900 W
• Average Power = 3549 W
• Continuous Power = 296 W
• Resistance value = 88.2 W

To verify that the generated current is within the current limit of the switching device, which is 10A, the lowest resistance value is considered. The 88 W resistor has a 10% tolerance and therefore the lowest possible resistance is 79.2 W. The peak generated current during the braking cycle is then:

This is less than the switch rating of 10A and the assumption that the switch is on continuously is worst case. However, further safety for the switch may lead the designer to change the device to a higher rating such as 15A. By upgrading the switch to a higher rating, the system can be modified for faster braking if the application requirements change.
 

APPLICATION GUIDELINES:

TFS offers excellent thermal transfer that allows high power densities for surge handling as well as continuous operation. With proper heat sinking, this range can be greatly increased. After the resistor has been sized for value, the instantaneous and average power dictates the physical size of the resistor and the heat sink. Various air and water cooled aluminum heat sinks are readily available. The thermal conductivity is also improved when thermal greases or pads are used to interface the heat sink and resistor. The heat sink is typically mounted to the resistor with screws as the fastener; however, rivets, staking, or clamp assemblies are viable alternatives. Lead wires and terminals are constructed to the requirements of the application.

When specifying the braking resistor, the power requirements should be identified as the instantaneous power or surge, the average power and braking cycle, and the continuous power required for the application. The environmental temperature and the maximum resistor operating temperature should also be designated if your application has constraints.

Because the inertial loads will vary for each drive application, resistor sizes will vary for the same motor coupled to the different systems. In order to limit the number of resistance values required, the designer should consider a few values that can be linked in series for a higher total resistance, or in parallel to lower the total resistance. This allows for less resistance values to be inventoried and greater flexibility to properly size the brake for each application.

Flameproof products with encapsulated electrical contacts meet most standards for environment and proximity. Fusing elements offer surge protection to the controlling device with a failsafe open mode, while thermistors or thermocouples provide data for closed loop temperature control.

Thermal shock and vibration testing has shown TFS Technology to be superior to other resistor technologies available on the market. Thermal shock test results of 20,000 cycles of a dry boil and quench test show no performance degradation. The accelerated testing consists of heating the resistor up to 140 Degrees C and then quenching the resistor in water at 26 Degrees C repeatedly. The stainless steel substrate is resilient to vibration testing where typical ceramic substrates are brittle and fail.

CONCLUSION:

TFS technology offers significant advantages for motion control. With exceptional thermal transfer characteristics, power density, and size qualities, this product is an overall improvement to existing resistor configurations. The robust nature of stainless steel offers improved reliability over other substrate systems such and alumina or FR4 for shock, vibration, and heat dissipation. TFS is a cost effective solution for motion control applications with superior performance characteristics.