5. The motor field current and flux are usually held constant and the W-L method thus represents in essence a constant-torque, variable-horsepower system.
6. Opening the field for reversal purposes may cause dangerous runaway , instability , and exceedingly high armature currents; and the field is more highly inductive than the armature-opening a highly inductive circuit will produce more severe arcing and voltage breakdown than an interruption of the armature circuit.
7. The total applied voltage across the unprotected armature then is practically twice the voltage that may occur at the moment of starting without any protective resistance in series with the armature.
8. A dc dynamo acts as a motor when its armature is connected to a supply and when the dynamo develops a counter emf that is less than the terminal voltage .
9. Magnetic brakes are effectively and extensively used in conjunction with a type of speed control called jogging .
10. For a number of practical situations associated with the operation of electrical machinery , the change of an output quantity with time is governed by the following 1st order differential equation .
Example
A 500V,100hp,2500r/min,separately excited dc motor has the following parameters:
Rf = 109 Ω , Vf = 300V,
Ra = 0.084 Ω ,Kf = 0.694V/(A·rad/sec)
Assuming the field voltage to be held constant at 300V, use MATLAB to plot the motor speed as a function of armature voltage with the motor operating under no-load and also under rated full-load torque as the armature voltage is varied from 250V to 500V.
clc
clear
% Motor parameters
Rf=109;
Ra=0.084;
Kf=0.694;
% Constant field voltage
Vf=300;
% Resulting field current
If=Vf/Rf;
%Rated speed in rad/sec
omegrated=2500*(Pi/30);
%Rated Power in watts
Prated=100*746;
%Rated Torgue in N-m
Trated=Prated/omegrated;
%Vary the armature voltage from 250 to 500V and calculated speed
for n=1:101
Va(n)=250*(1+(n-1)/100;
% Zero torgue
T=0;
omega=(Va(n)-T*Ra/(Kf*If))/(Kf*If);
Noloadrpm(n)=omega*30/pi;
% Full-load torgue
T=Trated;
omega=(Va(n)-T*Ra/(Kf*If))/(Kf*If);
FullLoadrpm(n)=omega*30/pi;
end
plot(Va,NoLoadrpm)
hold
plot(Va(20),NoLoadrpm(20),'+')
plot(Va(50),NoLoadrpm(50),'+')
plot(Va(80),NoLoadrpm(80),'+')
plot(Va,FullLoadrpm)
plot(Va(20),FullLoadrpm(20),'o')
plot(Va(50),FullLoadrpm(50),'o')
plot(Va(80),FullLoadrpm(80),'o')
hold
xlabel(‘Armature Voltage [V]')
ylabel(‘Speed [r/min]')
text(270,2300,'+ = Zero torgue')
text(270,2100,'o = Full-Load torgue')
本文关键字：电动机 变频器基础，变频技术 - 变频器基础
