Operation Modes of Electric Drive

Consider single-mass reduced computing scheme of mechanical part of electric drive (heavy-mass is possible, it doesn’t have significant value) for which the equation of mechanical motion will have the view

(2.6)

- For reciprocal motion and

(2.7)

- For rotational motion

In equation (2.6) and (2.7) all parameters are reduced to motor shaft:

F – force generated by electric motor, [N];

M – moment of electric motor, [Ng];

F_s – static resistance force, [N];

M_s – static resistance moment, [Nm];

F_d =dynamic force, [N];

m – mass, [kg];

ν – linear speed, [m/s];

M_d=dynamic moment, [Nm];

J – inertia moment, [kg m²];

ω- angular speed.

Physical units of forces and moments are the following:

;

Equations (2.6) and (2.7) allow to define the character of mechanical motion of electric drive.

So if dynamic moment is absent M_d=0, than

(2.8)

If moment of drive is equal to static resistance moment then motion is attained (or rest). So electric drive moves with even speed (or motionless).

With non-attained duty the moments mentioned are not equal, dynamic moment is present and motion will be accelerated of fallen.

So for rotation of electric drive equation (2.8) is called the equation of attained motion, and equation (2.7) is called the equation of non-attained motion.

The value (module) of components in left part of equations (2.6) and (2.7) and the character of its activity (sign) determine the peculiarities of performance duty for every definite electric drive.

The value (module) and sign of components in the right part of equations (2.6) and (2.7) do not have intrinsic independent value and they are defined only by the ratio of component in the left part of equations.

Consider the effect of module and sign of component of left part of equation (2.7) on example of load lifting mechanism for non-attained duty of mechanism for six cases.

1 Motor starting Motor slowing down		Starting on load lifting
	Slowing down when lifting
2 Motor starting	—	Starting when lowering load
3 Braking Lifting	—	Electrical bracing when lifting
4 Braking, slowing Braking, acceleration		Electrical braking when lowering down
	Acceleration of load when braking lowering down
5 Motor is de-energized		Self-braking of load when lifting
6 Motor is de-energized		Self-starting of load when lowering down

The given above table shows following features of non-attained duties of lifting mechanism:

a) “+” sign before value of moment of motors points on drive mode of its operation;

b) “-“ sign before value of moment M of motor indicates of its braking duty;

c) “+” sign before the value of static moment M_s points to the lowering down of a load irrespective of motional or braking performance of a drive.

d) “-“ sign before the value if motor static moment M_s points to the duty when load lifting irrespective of braking duty of a motor.

e) When motor is de-energized non-attained motion of electric drive is realized due to accumulated kinetic or potential energy.

Basic problem in the field of electric drive mechanics is determination of speed V and power P at the attained duty, and determination of speed V, path S and power P at the time function in non-attained transitional duties

V,S,P = f(t)

Дата добавления: 2014-01-05