Starting of an electric drive involves a change in its state from rest to a steady-state speed of rotation. The process of starting is the most important phenomenon in the entire operation of the drive. An electric motor is started by switching on the supply.
There are, however, three possibilities which have to be considered:
1. Interference with the supply in the form of an excessive voltage drop which is more than that can be tolerated by other equipment or other consumers connected to the same power supply circuit.
2. The starting currents will add to the motor heating by an amount that depends upon their rms values and the frequency of starting. The excessive currents may cause damage to the motor itself.
In a dc motor the limitation may be good commutation rather than heating, as dc machine have a certain maximum limit for the current dictated by the commutation process.
3. Damage to the connected load through too rapid acceleration. This may also be taken to include unacceptable discomfort to passengers in lifts and trains.
In some cases none of the above is applicable and full- voltage or direct-on-line (DOL) starting is permissible, but in many cases one or more have to be guarded against, usually at the expense of additional equipment.
Necessity of Starting Equipment:
If a motor is switched directly on to the supply it will, except in the small sizes, draw a current much above the permissible limit. The main purpose of the starting equipment is, therefore, to limit the starting current to a safe value without, at the same time, reducing the starting torque to a value less than that required.
This limitation of current may be accomplished by reducing the voltage applied to the motor by means of a resistance in series with the motor or by some other suitable means as briefed below:
1. Direct-on-Line Starting or Full-Voltage Starting:
This involves the application of full line voltage to the motor terminals.
Whether or not this method of starting is used, depend, upon the following factors:
(i) Size and design of the motor,
(ii) Kind of application,
(iii) Location of the motor in the distribution system and
(iv) Capacity of the power supply circuit and the rules governing such installations as established by power supply companies.
As a general rule, dc motors up to 2 kW and squirrel- cage induction motors as well as certain small synchronous motors up to 4 or 5 kW are usually started by this method.
2. Reduced-Voltage Starting:
The starting of a dc motor is usually accomplished by inserting a suitable external resistance in the armature circuit and as the motor speeds up the starting resistance is cut out in steps.
Reduced voltage for starting of 3-phase induction motors is achieved by:
(i) Stator resistance starting
(ii) Stator reactor starting
(iii) Star-delta starting and
(iv) Auto-transformer starting.
The above methods are applicable equally to the synchronous motors. With reduced voltage starting the transition to full voltage may be done either before or after synchronisation although the former method is usually preferred.
Slip-ring induction motors though can be started by using the starting methods that are used in case of squirrel-cage induction motors, are usually started with full-line voltage across the stator terminals and by introducing variable resistance in each phase of the rotor circuit.
The external resistance introduced in each phase of the rotor circuit not only reduces the current at the starting instant but increases the starting torque also. As the motor accelerates, the external resistance is cut out in steps so that the available electromagnetic torque remains maximum during the accelerating period. Ultimately when the machine attains the normal speed, the rotor winding is short circuited automatically.
3. Starting by Means of Smooth Variation of Voltage or Frequency:
With ac motor-dc generator sets, dc motors can be started by smooth variation of applied voltage and with variable frequency sources both induction and synchronous motors can be started by smooth variation of supply frequency, simultaneously varying proportionally the voltage applied to the motors.
4. Starting of AC Commutator Motors:
AC commutator motors may be started by applying a reduced voltage or by shifting the brushes. Both of these methods are also employed for speed control, and in most cases starting can be satisfactorily affected by adjusting the control equipment to the low-speed position and then switching on full-line voltage. In some cases it may be necessary to provide in addition special starting tapings on an auto-transformer or a series resistance.
Special Features of Starting Equipment (Starters):
There are a number of features common to starters of motor of different types. Firstly, it is desirable to provide some protection against the flow of excessive currents for a prolonged time duration, even though currents of the same magnitude are permissible for a short period during the initial acceleration. Such ‘overcurrent’ protection can be had by providing means to return the starter to the ‘off ‘ position or by disconnecting the supply by other means.
The operation of the device may be achieved electromagnetically by energization of solenoid connected in series with the motor, or thermally by a bimetallic strip heated by the motor current. In the former case a “dashpot” is required to prevent operation during acceleration period; in the latter case, the inherent delay caused by the thermal capacity of the device may be sufficient.
Secondly, it would be pointless to provide starting equipment which remained in the “full-on” position in the event of failure of power supply, since the restoration of supply would cause the very result for the prevention of which the starting equipment was provided. Furthermore, unexpected restarting of the motor and connected load could endanger equipment and personnel. A “no-volt release” is, therefore, provided in all starters, to return them to the “off’ position in the event of failure of supply.
A third common feature is a matter of definition. Most starters operate in a number of discrete steps and not by continuous smooth variation. It is necessary to define the number of steps, n, as the number of accelerating positions, including “full-on”. Since the starter has an “off’ position, there are (n + 1) positions in all.
The study of behaviour of an electric drive during transient period (i.e., the duration of starting, braking, and transition from one period to another) is of great practical importance, as it helps in determining the requisite control equipment for starting, braking etc. so as to have minimum energy loss during such operation and also to ensure that the machine operates stably after the transients die out.
Determination of the time during which a transient will continue to exist is based on integration of the equation of motion for the drive, i.e., Eq. (1.11). Solving the equation for time, we have-
The time required to change the speed of drive from ω1, to ω2 will amount to-
In the above Eq. (1.45) the inertia J is assumed to be constant. In case J varies with time or speed, the equation may be rewritten suitably.
For the case, where ω1 is zero at t equal to zero and ω2 is equal to a steady-state speed ωn attained by the drive, accelerating time is given as
The above Eq. (1.46) shows that the acceleration time duration depends upon the area under the curve relating speed ω and 1/TM – TL. Also, as the steady-state speed of the motor is reached, the term (TM – TL) tends to be zero and hence 1/TM – TL tends to be infinite and this would lead to an infinite accelerating time. This difficulty is overcome, in practice, by computing the time required to reach say 95 to 98 per cent of the final steady-state speed. The desired value of accelerating time for any motor-load combination can be achieved by suitably modifying the speed-torque characteristic of the driving motor.
Acceleration Time for Specific Nature of Motor and Load Torques:
For determination of accelerating time it is necessary to know the motor torque TM, load torque TL, and inertia constant J. We now will derive expressions for the acceleration time for different motor and load torque conditions.
(i) Constant Motor and Load Torques:
Such a situation may arise, for example, during starting of a hoisting mechanism with a constant load torque. If the motor also develops a constant torque during starting, which is possible with shunted armature connections of a dc series motor or rotor resistance starting of a wound rotor induction motor or a double cage induction motor, the net torque available for acceleration (TM – TL) will also be constant, and acceleration time, will therefore, be given as-
Also, if the motor is started on no load, i.e., TL = 0, the time required to bring the motor from rest to no-load speed is given as-
The time t1 given by the above Eq. (1.48) is called the mechanical time constant of the motor and denoted by tm. For dc motors and induction motors rated at 1.000 rpm, tm varies from 0.4 to 0.6 second.
(ii) Linearly Varying Motor Torque and Constant Load Torque:
For a particular motor (such as for a slip-ring induction motor with high resistance rotor circuit) let the motor starting torque be Tst (i.e., when ω = 0, TM = Tst) and be zero when speed is synchronous, ωs (i.e., when ω = ωs; TM = 0). The relationship between motor torque and slip s is expressed as-
Substituting dω= – ωsds and TM = sTst in Eq. (1.46), we have-
Similarly the acceleration time duration for different motor torque and load torque conditions can be determined using Eq. (1.46).
Energy Consumption during Starting:
An expression for the energy lost in the armature circuit of a dc shunt motor can be derived as follows-
The voltage equations of a dc shunt motor is-
The above equation is also valid for a separately excited dc motor-
The equation of motion on no load will be-
Multiplying Eqs. (1.52) and (1.53), we have-
Substituting V/K = ωn in Eqs (1.52) and (1.54), we have-
IaRa = K (ωn – ω) ……. (1.55)
Now energy consumption during starting is given by the expression-
Thus, energy lost in the armature circuit of a dc shunt motor or a separately excited motor during starting on no load will be equal to the kinetic energy stored by the armature and is thus independent of the armature circuit resistance.
If the motor were started with a constant load torque TL, the energy lost in the armature circuit during starting could be determined as follows-
The equation of motion will, now, be of the form-
From Eqs. (1.55) and (1.58), we have-
Now energy consumption during starting is given by the expression-
DC Series Motor:
Since a dc series motor cannot be started on no load, let us assume that it is started with a constant load torque TL. The equation of motion will be
Therefore, energy consumed in the armature circuit of a dc series motor is given by the expression-
From above Eq. (1.62) it is obvious that unlike in a dc shunt motor, the energy dissipated in the armature circuit of a dc series motor depends upon the armature circuit resistance.
Three-Phase Induction Motor:
The mechanical torque developed in a 3-phase induction motor is given as-
And the equation of motor under no-load condition, neglecting friction torque becomes-
The total energy loss in the rotor circuit of an induction motor during starting period is given as-
From above Eq. (1.65) it is revealed that the energy lost in the rotor circuit during starting is equal to the kinetic energy stored by the rotor and is independent of rotor circuit resistance. The total energy loss including the loss in the stator circuit, however, depends upon the rotor and stator resistances. An expression for the total energy lost in the motor during a change in speed can be given as-
During starting, the energy lost in the motor will, therefore, be given as-
Where, J is the moment of inertia of rotor.
The energy loss during starting of a motor can be reduced by adopting the following methods:
(i) Reduction in moment of inertia of rotor.
(ii) Smooth variation of applied voltage in case of dc shunt motors.
(iii) Smooth variation of supply frequency (V/f control) in case of induction motors.