usa.siemens.com/ids Integrated Drive Systems Siemens Integrated Drive Systems optimize system cost when VFD current derating is required due to operational requirements How do I derate a VFD when I need to operate in unusual service conditions such as high ambient or altitude, high or low frequency or current overload? Problem description When sizing a VFD to operate a motor, one of the important parameters to consider is the operating current that the motor demands from the VFD. In a SINAMICS PERFECT HARMONY GH180 VFD, the power cells are arranged in series per phase, see Figure 1. Figure 1 Arrangement of Cells in a Nine Cell VFD *Variable Frequency Drive (VFD) may be used interchangeably with adjustable frequency drive (AFD). For usual service environments, the maximum VFD output current is determined by the maximum cell output current and transformer kVA rating. With properly sized transformer, the VFD can achieve output current ratings equal to the cell maximum continuous rating. In fact, the three different product lines of GH180 PERFECT HARMONY VFDs are determined by their cell output current ranges, as seen in Table 1. Answers for industry. Table 1 Power cell rated maximum continuous output currents Types of SINAMICS GH180 VFD Product Lines Cell Amps 40 A 70 A 6SR4 100 A 140 A 200 A Air-cooled VFD 260 A 315 A 375 A 6SR3 Derating current due to high ambient temperature (air-cooled VFD only) When the VFD is operating in an ambient temperature greater than 40 °C but less than 50 °C, the cell output current must be derated to prevent overheating of the cell IGBTs. The SINAMICS PERFECT HARMONY GH180 VFD may not be able to operate in ambient conditions over 50 °C ambient because the control boards may be damaged or malfunction at high temperature. Please consult the factory for any operating conditions above 50 °C ambient temperature. The required motor current is denoted as IMOTOR. If the ambient temperature is within usual service conditions, then IREQUIRED 1 = IMOTOR , otherwise: 500 A 660 A IREQUIRED 1 = 720 A The usual service environment is the normal environment in which the VFD is operating. This environment is defined as having ALL of the following conditions in addition to other conditions1: 1) The ambient temperature in which the VFD is operating does not exceed 40 °C (104 °F). 2) The altitude (measured in feet above sea level, or FASL) at which the VFD is operating does not exceed 3300 ft (1000m). 3) The VFD operates with continuous output current with frequency between 10 Hz and 167 Hz. 4) The overload requirement of the application is 110% of the rated cell current for 1 min / 10 min (1 minute of overload current for every 10 minutes of nominal operation). If the VFD is operating in a usual service condition, then the current values in Table 1 define the maximum continuous current output for the different cell types in each of the GH180 product configurations. However, if the VFD is operating in an unusual service environment in which one of the above conditions is not satisfied, then the maximum output current of the VFD may need to be derated in order to keep the temperature of the junction connections inside the IGBTs of the cells below the manufacturer’s maximum rating. The derating of the current may result in using a cell with a higher continuous current cell, which could change the required VFD product line (see Table 1). The derating of the cell current can be due to the following four reasons: 1) High ambient temperature 2) High altitude 3) Frequency considerations 4) High overload requirements. Note that if a VFD is operating inside a control air-conditioned house, the ambient temperature is controlled due to the HVAC system, and temperature derating of the cell current is normally not necessary. 2 ambient 880 A 1250 A [(60 – Temperature 20 ] (°C) 0.67 This derating method only applies to the air-cooled VFD. Note that 720A cells cannot operate above 40 °C ambient temperature. Therefore, if a VFD is operating at an ambient temperature greater than 40 °C , and the application requires current greater than 660A, a water-cooled VFD may need to be used. For a water-cooled VFD, standard design allows for the cooling limits are defined by a combination of maximum ambient air temperature and the coolant inlet temperature. Above this limit, the cells may not be able to operate because the control boards may be damaged in this condition, and the factory should be consulted. As the maximum VFD ambient temperature increases from 40 °C to 50 °C, the maximum VFD coolant inlet temperature decreases from 47 °C to 42 °C (See Figure 2). This relationship between maximum air temperature and maximum inlet coolant temperature is due to residual heat removal from the cell cabinet through air-to-liquid heat exchangers installed on top of the cell cabinet. Please consult the factory if the operating temperatures are in the conditions when control boards may fail. VFD Ambient Temperature (°C) Water-cooled VFD IMOTOR VFD Coolant Inlet Temperature (°C) Figure 2 Temperature Conditions for Power Cells in a Water-cooled VFD Derating current due to high altitude When the VFD is operating in an altitude higher than 3300 ft (1000 m), the lower density causes a reduction in heat transfer capabilities from the surrounding air. This reduction causes the connections in the cell IGBTs to be more easily overheated, and therefore the output current must be derated. Furthermore, voltage strike and creep distances are reduced at higher altitudes, which may impose an upper limit on the operational altitude of the power cells. Continuous Cell Current with Overload Requirements Some applications may require intermittent overload conditions. Depending on the magnitude of the overload, cell currents may need to be derated to stay within the temperature tolerance of the manufacturer. Usually, the overload condition is defined as the percentage overload for 1 minute of overload duration per 10 minutes of normal operation (1 min/10 min). Table 2 shows the cell continuous current derates when different overload conditions are required by the application. If the altitude is within usual service conditions less than or equal to 3300 feet (1000 meters), then IREQUIRED 2 = IREQUIRED 1. Note that these derates assume ambient temperature < 40 °C and altitude of below 3300 ft. Therefore, the calculated current IREQUIRED 3 should be used when selecting the appropriate cell. If the altitude is above 3300 feet (1000 meters) but below 13,123 feet (4000 meters) for air-cooled 6SR3 VFD or below 10,000 feet (3048 meters) for air-cooled 6SR4 VFD and watercooled VFD, then Table 2 Cell current derates for different overload conditions and classes (CL) Cell Continuous Current No Overload Required 110% of nameplate 1min/10 min (CL-1) 150% of nameplate 1min/10min (CL-2) 40 A 40 A 40 A 70 A 70 A 70 A 100 A 100 A 100 A 140 A 140 A 140 A If the operating conditions are beyond the altitude current derating limits, then the factory must be consulted. 200 A 200 A 200 A 260 A 260 A 260 A Derating current due to operating frequency To adequately operate the motor, the operational output frequency (FOUT) of the VFD must match the operating frequency of the motor. Note that for applications above 330 Hz, the factory should be consulted. 315 A 315 A 300 A 375 A 375 A 300 A 500 A 500 A 400 A 660 A 660 A 450 A 720 A* N/A N/A 880 A 880 A 660 A 1250 A 1250 A 950 A IREQUIRED 1 (Altitude (ft) -3300)×0.5) 116700 IREQUIRED 2 = [ ] IREQUIRED 1 (Altitude (m) -1006)×0.5) 15090 IREQUIRED 2 = [ ] If the required operating frequency is within usual service conditions between 10 Hz and 167 Hz, then IREQUIRED 3 = IREQUIRED 2, but if the continuous (i.e. > 1 minute) output frequency FOUT is between 0.5 Hz and 10 Hz, the new required cell output current IREQUIRED 3 can be calculated as follows: IREQUIRED 3 = IREQUIRED 2 [ 0.5 + (F20 )] Air-cooled 6SR4 VFD Air-cooled 6SR3 VFD Water-cooled VFD *720 A cells does not have overload capability For all other overload conditions not defined here, the overload current is used as the continuous current (No Overload Required column in Table 2) to select the appropriate power cell. OUT The relationship between the carrier frequency and the VFD output frequency is typically defined as FC ≥ 3.6 × FOUT, where FC is the carrier frequency of a single power cell. If output frequency FOUT is between 167 Hz and 330 Hz, then the carrier frequency is used to calculate the current derate due to high switching frequency. Note that for the 720A cell, the maximum carrier frequency is 400 Hz, which may impact performance above approximately 111 Hz. The new required VFD cell output current is calculated as follows: IREQUIRED 3 = IREQUIRED 2 600) x 0.2 [1- (F - 600 ] c Determining Cell Currents After Derates The method to determine the cell size and current capacity required for a particular motor current (IMOTOR) after derating due to the different service conditions are as follows: 1) Calculate the current required by the motor after derating due to the following conditions if applicable: a. High Ambient Temperature b. High Altitude c. Unusual Operating Frequency 2) Using the calculated derated current, select the appropriate power cell after considering overload requirements from Table 2. 3 Siemens Integrated Drive Systems can mitigate this increase in system cost when the VFD output current must be derated due to environmental conditions such as ambient temperature, altitude, frequency, and/or overload requirements. Because the system is integrated within Siemens products, the motor can be designed around the VFD. The motor can be designed to lower the required motor current and raising the required voltage to satisfy the power requirement from the customer. This design change may not have a significant effect on the cost of the motor, since this change typically does not increase the shaft height, which is a primary cost lever for the motor. In our previous case study, if the customer were to purchase an integrated drive system, the motor can be designed to require 140A and 6.1kV because 258A ×3300V ≈ 6.1V 140A Case study In this case study, a customer wanted a SINAMICS GH180 VFD that can deliver an output current of 258 A to the motor with 3.3 kV. Normally, with this current requirement, we would have offered the customer an air-cooled 6SR4 nine cell VFD. However, an unusual service condition was requested where the VFD must be rated for 170% Overload for 10 seconds in 10 minutes and 150% Overload for 30 seconds in 10 minutes. Because of the unusual overload conditions, the cell current must be derated. In this case, the cell is derated to the worst case scenario, which was the 170% Overload. At 170% Overload, the VFD must output about 439A. With this derating, the smallest power cell that can provide the required current is a 500A cell. The proposal was revised to offer the customer an air-cooled 6SR3 nine cell VFD with 500A cells. Integrated Drive System Optimized Solution General speaking, an air-cooled 6SR3 VFD can be about 20-40% more costly than an air-cooled 6SR4 VFD, while a water-cooled VFD can be about 5 times as costly as an air-cooled 6SR4 VFD. Therefore, from a system cost perspective, it is a significant system cost reduction if a smaller size VFD can be used for an application. For a system that is not integrated and is not optimized for cost, derating the output current may result in significant system cost increase due to the change in VFD sizes and product lines. Siemens Industry, Inc. 3333 Old Milton Parkway Alpharetta, GA 30005 1-800-241-4453 info.us@siemens.com usa.siemens.com/ids 140A ×1.7 = 238A Because a 260A cell is available in the 6SR4 product line, we can then offer the customer an air-cooled 6SR4 nine cell VFD along with this motor compared to the original 6SR3 nine cell VFD with 500A cells. By providing an integrated drive system, Siemens provides several benefits for this specific customer: • The customer receives the same power output at lower cost compared to if they had only bought the air-cooled 6SR3 nine cell VFD. • The customer is receiving a brand new motor to replace their older existing motor, which will greatly extend the life of their system. • The air-cooled 6SR4 nine cell VFD has a 51% smaller footprint than the air-cooled 6SR3 nine cell VFD. This makes installation considerably easier, faster, and less expensive. For Siemens, new business is generated for our factory, and Siemens motors have gained a foothold with this specific customer. Siemens Integrated Drive Systems is truly a win-win situation for both the customer and Siemens. To learn more about Integrated Drive Systems, visit www.usa.siemens.com/ids. Subject to change without prior notice Order No.: DTAN-00031-1014 Printed in USA © 2014 Siemens Industry, Inc. The information provided in this flyer contains merely general descriptions or characteristics of performance which in case of actual use do not always apply as described or which may change as a result of further development of the products. An obligation to provide the respective characteristics shall only exist if expressly agreed in the terms of contract. All product designations may be trademarks or product names of Siemens AG or supplier companies whose use by third parties for their own purposes could violate the rights of the owners.