32 Electric Motors Example: 400 V mains, U Back EMF @ 1000 RPM = 100 V, U DC critical = 1000 V n critical = u 1000 RPM = u 1000 RPM = 5656 RPM U DC critical U BackEMF@1000 RPM u — 2 1000 V 100V u — 2 A brake resistor can be used to overcome such critical situations. Unfortunately there is no standard used by motor manufacturers to provide information about the back EMF. Some manufacturers state back EMF related to 1000 RPM while others use nominal speed of the motor. Sometimes the value of factor ke is given in radians and must be converted to RPM. U EMF = ke = = 2 / 1000 60 Where peak values are provided the voltage must be divided by square root of two in order to get the RMS value.
Also advanced motor data like motor resistance and inductances are stated in diff ering ways. Sometimes they are given as phase/phase values, and sometimes as phase/star values. U RMS = U Peak 3 2 1.4.1.2 Torque and Speed Range The torque of a PM motor is proportional to the motor current, and its speed is proportional to the feeding frequency. At nominal torque and speed, a certain voltage is required. If the FC can deliver a higher voltage, the speed can be increased further. This results in a higher power at constant torque. When the voltage has reached an upper limit, the motor enters the fi eld weakening area.
Operation in fi eld weakening is only possible with suitable frequency converters. Motor mechanics and insulation must support the higher speed and withstand the higher voltage.