Controlling heat15 September 2004
Michael Urbassik explains how to calculate and manage the heat generated by drives
The integration of new IGBT (insulated gate bipolar transistor) and other solid-state technology to control hoist and travel motors on cranes has presented new challenges for the control packaging design. Much care is taken in controlling and managing the heat in the design of the core drive units but little attention is given to this issue when integrating these core drives into a complete crane motion control package with all of its associated equipment required by various world standards and guidelines. This paper presents a methodology on calculating thermal energy produced based upon the RMS duty cycle of the motor. It also briefly introduces various thermal management solutions. While this paper is based on North American standards for DC motors, the concepts presented can be adapted to other world standards.
Some people may be asking why this topic is being addressed. The new technology for cranes provides advance performance in controlling hoist and travel motors never achievable before. But with this improved performance comes the penalty of heat production and heat limitations of the components used in the core drive technology become the limiting factor in designing complete enclosed control solutions. Primary heat sources are the power diodes, SCRs (silicon controlled rectifiers) and IGBTs, which carry the full motor current when conducting. If the enclosure heat rise is ignored, it could lead to numerous drive faults and shut down of the motion as all drives have heat sink thermal protection and some components may have thermal shut down protection. Therefore, this topic is important because estimating and managing the heat rise is necessary for trouble free and reliable operation.
The steps to a trouble free installation can be broken down to a two-phase process. Phase 1 involves estimating the heat load in an enclosure. This process begins by calculating the RMS current through the power devices. Next, a determination of generated watts needs to be done. Then the actual heat rise inside the enclosure can be estimated. Phase 2 is the thermal management phase. This phase involves analysis of the ambient conditions and the decision on the proper cooling method convection, or forced cooling.
RMS current calculation
To estimate the enclosure heat rise we first need to know the average current flowing through the power semiconductor devices.
Using the RMS method accomplishes this and takes into account cycle time, accelerating, running, and decelerating currents that the motor and power devices experience. The general equation for calculating RMS current is presented in Figure 1.
To determine the RMS current we need to look at the actual motor duty cycle of the application. For constant potential North American cranes, the DC series motor is most widely used for the hoist. This motor is generally based on the 30-minute motor rating. Table 1 of the AISE DC Mill Motor standard No. 1 defines the 30-minute motor duty cycle as a 30% time on duty cycle.1 This cycle is defined as a continuously repeated duty cycle of 5 minutes duration with 100% current on for 1.5 minutes and 0% current for 3.5 minutes until the winding temperature stabilizes at 75°C measured by a thermometer. Generally, temperature stabilization is reached by 60 minutes (1 hour).
Appling this information to the equation in Figure 1 permits the simplification of the RMS current. Furthermore, changing the time on for the current flow allows us to produce a family of simplified equations for the hoist motor's full load amp (FLA) as shown in Figure 2.
Hoist, 30% time on
Hoist, 50% time on
Hoist, 100% time on
This process can be applied to any motor rating thus simplifying the task of calculating RMS current for any standard motor application. Figure 3 shows the simplified equations for a DC travel application.
Travel, 30% time on
Travel, 50% time on
Travel, 100% time on
These travel equations are based upon a defined duty cycle of: 0.1 minute (6 seconds) 100% accelerating current, 1.4 minute 30% current (free run), 0.1 minute 100% deceleration current, and 3.4 minute 0% current. Total cycle time 5 minutes; repeated until temperature stabilizes (1 hour).
Estimating Generated Watts
After defining the RMS current our next step is to determine the watts generated or the BTU/Hr generated by each drive or motion panel. We also need to account for the heat rise of other current carrying parts such as cable, contactors, transformers, etc. This generally adds approximately 15% more watts or BTU/Hr. Therefore; a multiplication factor of 1.15 should be applied to the total watts or BTU/Hr calculated.
Drive manufactures provide tables or graphs of Power Losses, which are used to calculate the generated heat loads. Tables are generally expressed as watts per amp as illustrated in Figure 4.2
Graphs however, are generally expressed as watts per switching frequency at continuous current. These graphs need to be converted into W/A format to be useful for calculations. Figure 5 illustrates this process for a 500V, 87 continuous amp variable frequency drive switching at 3 KHz.3
The simplified general equations for calculating generated watts and BTU/Hr is given in Figure 6. The 1.15 factor accounts for the additional watts generated by the power wire, contactors, relays, etc that are on the motion panel. Additional heat loads such as isolation transformers, line or load reactors, or heat generated from sunlight exposure need to be added to these equations.
Figure 7 provides the results of calculated watts and BTU/Hr for a digital DC hoist application using a 10HP frame 602 motor having a 30-minute current rating of 44 amps. From the table in Figure 4 the W/A for a NEMA Size 2 controller is five. Combining this value with the equations from Figures 2 and 6 yields the results in Figure 7.
Hoist, 30% time on:
Hoist, 50% time on:
Hoist, 100% time on:
After estimating the generated Watts or BTU/Hr the enclosure heat rise must be determined. This requires the total surface area of the enclosure available for convection heat flow in square feet. Generally, this includes the two sides plus the front plus the back (minimum of six inch clearance required to use) plus the top of the enclosure. Figure 8 provides a general formula derived from a graph for convection cooling of an enclosure.4
Continuing in our example the enclosure dimensions selected for the NEMA size 2 Respondor DDC hoist control was 60 inches high by 32 inches wide by 19.3 inches deep. The total surface area available for convection cooling is 47 square feet. The temperature rise inside this enclosure for the duty cycle examples we have been considering are shown in Figure 9.
Hoist, 30% time on:
Hoist, 50% time on:
Hoist, 100% time on:
A practical total temperature limit inside of an enclosure with industrial rated (85°C) components is 65°C. Based on our hoist example this means we can operate in an ambient of 56°C for 30% time on, 53°C for 50% time on, and 52°C for 100% time on. A higher ambient would require supplemental thermal management such as forced cooling or air conditioning.
Thermal analysis can be a difficult and tedious process. The effort, however, is worth the pain for a trouble free installation. The analysis using the equations provided can be mechanized for easier use using Microsoft Excel or similar programs. There are also some free software solutions available for downloading from the Internet. Two useful sites personally discovered are www.thermoelectric.com/2002/fs/index5 and www.hoffmanonline.com/PDFCatalog/SpecifiersGuide/indexThermal.htm4
Applied Thermal Management Solutions
Many thermal solutions can be used to manage the heat rise inside an enclosure. Solutions applied by the author include: direct enclosure air vents, cooling fans, circulating fans, air conditioners, and insulated enclosures. Figure 10 is an example of an application using air vents located directly over the drive heat sink. This design channels the major heat flow to outside of the enclosure. The disadvantage of this design is the possibility of dust contamination when the heat flow is not present (drive is powered down).
Air conditioners must be applied to meet the ambient temperature specified. The typical enclosure mounted air conditioner unit has an ambient operating temperature limit of 55°C and is not always suitable for severe vibration applications such as moving cranes without modification. Special AC units with high temperature refrigerants and designed for severe vibration service are available. Thermoelectric AC units are a good choice if the heat loads can be kept low. The advantage of these units is no refrigerant and its compact design. Figure 11 shows an application designed for a 74°C ambient using thermoelectric units and an insulated enclosure. Special designed AC units and thermoelectric units are available for either AC or DC power bus systems.
Other alternative cooling technologies such as heat exchangers, vortex coolers, and heat pipe liquid cooling have been considered for use but not applied on crane applications. These technologies each have advantages and disadvantages that must be reviewed before selecting. Each installation must be analyzed to select the best cooling method based on customers specifications and preferences.
Incorporating solid-state drives into crane control systems requires careful thermal analysis. Estimating or knowing the duty cycle of the crane motion allows for a cost efficient thermal management design solution. The methodology and process presented in this paper can vary widely. The end result however, is estimating and managing the enclosure heat rise which, provides for a trouble free operation, protects the temperature sensitive components, and achieves the maximum production benefits from the new technology.