6.3.2. Direct current motors
If the equation U = U0 + I Ri is adapted in line with current magnitude, one derives an equation with
for working out the current value in the rotor circuit of the motor. If one compares current intensity for switching on and actual operation, we can determine the following:
During switching on current is calculated according to
There is thus more current because
1. the acceleration torque must be forthcoming and
As rotational movement continues a back voltage is induced according to U0 = c • Φ • n whereby current intensity declines. Current intensity decreases more and more as speed increases. Then, as rated speed is attained, operational current is brought into play. The very considerable inrush current leads to
1. a greater heating up of the winding
Consequently, only motors with low rated power may be connected to full mains voltage during switching on. Thus, motors operating on a mains voltage of 220 V between both external conductors may not have a greater power than 0.7 kW.
Direct switching on is only possible for low powered motors.
Starters with series resistor
In the case of higher powered direct current motors the starting currents are limited through a series resistor, the starter. The starter must, moreover, be so dimensioned that peak starter current Isp does not exceed 1.5 times the rotor nominal current (operating current intensity at rated speed). Thus, the following equation applies:
Isp = 1.5 • In
This comprises several series connected resistors which can be switched off as speed increases. The connecting terminals R, L and M should be switched thus:
R to the rotor, L to the mains (lead) and M to the shunt winding (magnetic field).
Starters are manufactured for the operating mode S2.
A direct current motor with a rated power of P = 10 kW and a rated voltage of U = 220 V has an internal resistance Ri = 0.4. How great are:
Starting peak current Isp
U = 220 V
Solution P = U • I
In≈ 45.5 A
Isp = 1.5 • In
Isp≈ 68 A
Ia max = 550 A
Ia max≈ 12 In
Rrepl≈ 3.24 Ω
Ra = Rrepl - R;
Ra = 2.84
Where a starter of at least 2.84 Ω is connected in series, the inrush peak current is restricted to max 68 A. In the absence of a starter the inrush current would be 12 times greater than the rotor rated current.
In practise prestipulated speeds are required for various drives.
In production certain speeds must also be adhered to, moreover such speeds shall also remain constant given loading variations.
Such drive problems can be solved by means of direct current motors.
The equation for calculating the speed of a motor is derived from U0 = C • Φ • n and U0 = U - 1 (Ri + Rv) through equalisation and subsequent solution according to the speed.
Rv is a series resistance which is series switched to Ri.
Subsequently the speed can be set
All these methods are used in practice.
Changing mains voltage.
Changing mains voltage is advantageous where a motor has an own voltage source of differing values. Where direct current conductor mains are available the voltage can be stepwise changed by means of a selector switch. The influence of the mains voltage on the speed can be seen in Figure 97.
Favourable and economical speed setting results from changing the voltage by means of controlled rectifiers (thyratrons or thyristors). There are virtually no losses with these rectifiers. Power dependency becomes irrelevant as rotor resistance does not change during this procedure.
Changing the series resistance of the rotor circuit
Where rotor circuit resistance is increased through a series resistance, speed subsequently decreases.
However, due to the considerable rotor current, this speed control leads to marked power losses. Where this procedure shall serve for speed control, the servo unit is dimensioned for permanent S1 operation. Such a unit is called a speed control starter if it is simultaneously suitable for starting. This control leads to a power drop.
Changing the magnetic flow
The magnetic flow decreases when a speed field controller is switched on to the field winding. The speed increases in the diminished exciter field. In practise speed field starters are constructed permitting a speed increase of up to 200 per cent of the rated speed. The arising losses are relatively low, consequently this control unit is quite economical. Figure 99 depicts the dependence of speed on exciter flow.
Rotational direction control
The rotation direction depends on the current direction in the rotor and the direction of the exciter field. This is determined by the left-hand rule.
A rotational change of direction can therefore be attained
1. by current directional change in the rotor and
In practice the current directional change in the rotor is mainly used. However, the exciter field is repoled in more powerful machines (Leonard converters) as, otherwise, the switching contacts to handle the extremely great rotor currents become too big.
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