FR 2949038 A1 20110211 FR 1055780 A 20100716 POWER ELECTRONICS COOLING SYSTEM WITH FLOW ORIFICE CONTROL BACKGROUND OF THE INVENTION This application relates to a power electronics cooling system wherein a plurality of motor controllers are provided with a single cooling system, and wherein the flow leading to the motor controllers is adjusted dependent on conditions at cold start-up of the aircraft. Power electronics are becoming widely utilized in many applications. One particular application is the use of motor controllers on an aircraft. As aircraft become more and more complex, more sophisticated high power motors provide various functions. In one system, motor controllers are provided with a single cooling system which circulates a cooling fluid such that the motor controllers are maintained at an acceptable temperature during use. The motor controllers may provide any number of functions. Some of the functions are required to be operational upon initial start-up of the aircraft, while others can be left off for a period of time until the cooling system can warm up. Some of the larger motor controllers have a flow orifice that provides a high volume of cooling fluid, while other of the motor controllers may have a flow orifice that provides a limited amount of cooling fluid. The amount of cooling fluid is typically determined by the amount of cooling required at operational conditions. Thus, a large motor controller would typically require greater cooling fluid volume. On the other hand, some of the smaller motor controllers must be operational upon initial start-up of the aircraft, and thus demand cooling fluid initially. A problem can occur, particularly at initial start-up in very cold conditions, for example, below -30 C. At initial start-up in very cold conditions, the viscosity of the cooling fluid can be very high, and thus there may be a tendency for the cooling fluid to flow in undesirable volumes to the larger motor controllers having a larger orifice, resulting in an insufficient amount of cooling fluid being delivered to the smaller motor controllers. In the prior systems, it has been known to increase the system pressure for the cooling fluid. This may somewhat overcome the orifice pressure drop 1 mentioned above during cold conditions. However, this also requires cooling system components such as cold plates, cooling lines, pumps and valves, which are all necessarily designed for the higher system pressure. These components thus become larger, heavier, and more expensive, which is all undesirable. SUMMARY OF THE INVENTION A system includes a plurality of motor controllers. A cooling system for the plurality of motor controller has a common inlet line, a common outlet line, and a plurality of parallel branch lines leading to the plurality of motor controllers. A control selectively increases the flow to certain of the plurality of motor controllers and limits the flow to others of the plurality of motor controllers. In addition, a method of operating a cooling system for motor controllers is disclosed and claimed. These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a motor controller rack according to the present invention. Figure 2 is a schematic of a cooling system for the rack of Figure 1. Figure 3 shows another cooling system according to this invention. Figure 4A shows a first flow orifice. Figure 4B shows a resistor which may be incorporated into the Figure 4A embodiment. Figure 5A shows another portion of the system of Figure 3. Figure 5B shows another feature of the Figure 5A system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An aircraft 18 may be provided with a motor controller rack 20. The rack 20 incorporates a frame 22 mounting a plurality of large motor controllers 24, and smaller motor controllers 26. The motor controllers can provide any number of functions. Some example functions for the large motor controllers would include common motor starter controllers, cabin air compressor system motors, starter 2 controllers for both an auxiliary power unit and a main engine, etc. Some of the smaller motor controllers could include a ram fan motor control, and a power electronics cooling system motor control. As can be appreciated from Figure 2, a single cooling system is provided for cooling each of the several motor controllers 24 and 26. As shown, an inlet 32 provides a cooling fluid, which circulates through the controllers 24/26, into an outlet line 34. An orifice 36 is provided on a line leading to the small motor controllers 26, and another type orifice 136 may be mounted on the larger motor controller 24. While this distinction between the orifice 36 and the orifice 136 has been tied directly to the size of the motor controllers, other reasons may drive the selection of the different type of orifice, as will be detailed below. Figure 3 shows a portion of the Figure 2 system. In Figure 3, the motor controllers 24 are motor controllers which will not need to be started initially. The motor controller 26 will be of the sort which must be started immediately upon start- up of the overall system. When the system is started at very low temperatures, the cooling fluid may have such a high viscosity that insufficient cooling fluid will flow through the orifice 36 to the motor controller 26, which at the time is the only motor controller requiring cooling fluid. The orifice leading to the larger motor controller 24 will be larger due to the high volume which will be required at normal operational conditions. Thus, that orifice will allow greater flow. At start-up, this can result in insufficient fluid delivered to cool controller 26, while excess flow is delivered to the controllers 24, which may not yet be operational. Thus, a controller 200 is provided which controls operation of the orifices 36 and 136, particularly at start-up. As shown in Figure 4A, the orifice 36 can be provided with a heating element 56. An inlet line 50 leads to a restriction 52, and then to an outlet 54, with outlet 54 connecting to a motor controller 26. The heating element 56 may be as shown in Figure 4B, and may be a negative temperature co-efficient ceramic resistor. A voltage is applied across the resistor, and the resistor creates heat to direct into the orifice 36. The negative temperature co-efficient resistor has its resistance increase with lower temperatures. At system start-up, the resistor dissipates heat, especially at lower temperatures. 3 The heat from this resistor will increase the temperature of the coolant in the orifice 36, and reduce its viscosity. Reduced viscosity will result in reduced pressure drop and increased flow leading to the motor controller 26. The amount of flow can be controlled by controlling the voltage drop across the resistor. While the ceramic resistor 56 is utilized in one embodiment, other ways of providing heat to the cooling fluid in the restriction 52 may be utilized. At the same time, the orifice 136 can be provided with a cooling function. Thus, as shown in Figure 5A, a cooling element 60 may be associated with the restriction 52 between the inlet 50 and outlet 54 of the orifice 136. As shown in Figure 5B, the cooling element 60 may be provided with a body 64 and a plurality of thermoelectric coolers 62 spaced radially around the restriction 52. Thermoelectric coolers are known, and receive a voltage to provide cooling. This cooling will increase the viscosity of the coolant flowing through the restriction 52, and thus reduce the volume flow leading to the motor controller 24 which does not require cooling as it is not initially operational at start-up. With the present invention, the control 200 is thus able to control the fluid flowing through the orifices 36 and 136 and ensure that adequate coolant fluid flows to those controllers that are being utilized initially at start-up. Notably, soon alter start-up, the coolant will heat up sufficiently such that its viscosity is low enough that it will operate properly even without the control at the orifices 36 and 136. At that point, the control may cease. While both cooling and heating are disclosed, systems that do only one of the two would come within the scope of this invention, and the use of only one of the two will still increase flow to the desired controllers, and decrease flow to the others. One coolant that may be used is available under the trade name Dow-FrostTM. Essentially, this is a propylene glycol mixture with water, and may be a 60/40 mixture. Of course, other coolant fluid would come within the scope of this invention. Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following daims should be studied to determine the true scope and content of this invention.