Page 102 - Industrial Power Engineering and Applications Handbook

Starting and control of slip-ring induction motors 5/83
The use of wound motors is on the wane, for reasons of to speed variation, would appear as slip loss in the
inherent advantages of a squirrel cage motor over a wound rotor circuit. For instance, at 25% slip, the power
motor and the availability of static drives (Chapter 6) output will be 75% minus the cooling effect and this
which can make a squirrel cage motor perform the same 25% will appear as a slip loss in the rotor circuit.
duties as a wound motor and even better. Nevertheless, 4 Restriction in starting current and a requirement for
the use of wound motors exists and will continue world- high starting torque to accelerate heavy rotating masses
wide for years. Where slip-ring motors are employed sometimes limit the use of a squirrel cage motor. For
whose only purpose is starting or a limited speed control such applications a slip-ring motor provides a better
the resistance or electrolyte method of starting has been alternative.
most commonly adopted, for reasons of cost. Static drives 5 As discussed in Section 2.7.1. during start-up, the
generate harmonics and distort the supply voltage, and rotor is more vulnerable to damage due to excessive
call for larger sizes of cables. It is also cumbersome and heat in the rotor compared to the stator. But in slip-
cost-inhibiting to provide filter circuits, particularly when ring motors a major portion of this heat is shared by
the installation is small. the external resistance, in proportion to its resistive
But where accurate speed control is the process value. Therefore. a slip-ring motor can be switched
requirement, static controllers, termed 'slip recovery ON and OFF more frequently, compared to a squirrel
systems' (Section 6.16.3) are recommended. which in cage motor. It can also withstand a prolonged starting
addition to exercising extremely accurate speed control, time, while accelerating heavy loads. Now the external
also conserve slip losses. Static drives are discussed in resistance will have to be suitable for such duty/load
Chapter 6. Below we will describe a procedure to deter- requirements.
mine the value of resistance, its steps and switching and
control schemes for these steps for a rotor resistance starter. Slip loss
An electrolyte starter is almost a standard product like
a motor and the manufacturer, depending upon the number From equation (1.9), slip loss = S . P,. If the full-load
of starts and the speed control requirement, can adjust slip is Sand the speed varied to slip S, the additional slip
the quantity of electrolyte, depth of electrodes etc. loss due to the increased slip
= P,(S, - S)
5.1 Important features of a slip-ring = kW (S1 - S) (ignoring rotor losses).
motor Example 5.1
If the speed of a 125 kW, 1480 r.p.m. motor is varied at
These motors are switched through their rotor circuit by constant torque to 750 r.p.rn., then the additional slip loss
inserting suitable resistances and then removing them E 125 x (0.50 - 0.01 33)
gradually. In view of their varying characteristics through = 125 x 0.4867
their rotor circuit, they can provide the following features:
or 61 kW
1 The external resistance adds up to the total impedance where S = "O0 - 1480 = 0.0133 and SI = 0.5
1500
of the motor windings and limits the starting current.
It also improves the starting power factor. Disadvantages
2 Since the performance of an induction motor can be
varied by altering the rotor parameters, a slip-ring A slip-ring motor is expensive, as are its controls,
motor, through its rotor circuit, can be made to suit compared to a squirrel cage motor. It also requires
any specific torque and speed requirement. meticulous and periodic maintenance of the brushes, brush
3 The speed of a slip-ring motor can be vaned through gear, slip-rings, external rotor resistances etc. A squirrel
an external resistance. Therefore the torque can be cage motor is thus prefemed to a slip-ring motor. A slip-
maintained at any value up to the pull-out torque in ring motor also requires a larger space for the motor and
the entire speed range by suitably varying the external its controls.
resistance. (See circle diagram in Figure 1.16 and
Section 1.10.2). At lower speeds, however, the 5.2 Starting of slip-ring motors
efficiency of the motor will be poor, as the output is
proportional to the speed. The efficiency would be These can be started by adopting either a 'current limiting'
roughly in the ratio of the two speeds, i.e. method or a 'definite time control' method. In a current
limiting method the closing of contactors at each step is
governed by the current limiting relays which permit the
accelerating contactor of each step to close when the
In fact, it would be even worse as a result of the motor current has fallen from its first peak value to the
equally reduced cooling effect of the fan at lower second pre-set lower value. The relays determine the
speeds. Since kW = Nr . T, kW would vary with closing time by sensing the motor data between each
speed, the torque remaining almost the same throughout step and close only when the current has fallen to a pre-
the speed range. The motor would draw the same determined value of the current relays. The closing
power from the supply as before, which, proportional sequence is automatic and adjusts against varying loads.