CHAPTER XI ROTARY TERMINAL SINGLE-PHASE INDUCTION MOTOR 101. A single-phase induction motor, giving full torque at starting and at any intermediate speed, by means of leading the supply current into the primary motor winding through brushes moving on a segmental commutator connected to the primary Diagram of rotary terminal aingle-plia-w induction motor. winding, was devised and built by II. Eickemeyer in 1891, and further work thereon done later in Germany, but never was brought into commercial use. Let, in Fig. 60, P denote the primary stator winding of a single- phase induction motor, S the revolving squirrel -cage secondary winding. The primary winding is arranged as a ring (or drum) Winding and connected to a stationary commutator, C. The single-phase supply current is led into the primary winding, P, through two brushes bearing on the two (electrically) opposite SINGLE-PHASE INDUCTION MOTOR 173 points of the commutator, C These brushes, B, are arranged so that they can be revolved. With the brushes, B, at standstill on the stationary commutator, C, the rotor, S, has no torque, and the current in the stator, P, is the usual large standstill current of the induction motor. If now the brushes, B, are revolved at synchronous speed, /, in the direc- tion shown by the arrow, the rotor, S, again has no torque, but the stator, P, carries only the small exciting current of the motor, and the electrical conditions in the motor are the same, as would be with stationary brushes, B, at synchronous speed of the rotor, S. If now the brushes, B, are slowed down below synchronism, /, to speed, /i, the rotor, S, begins to turn, in reverse direction, as shown by the arrow, at a speed, /2, and a torque corresponding to the slip, 8 = / — (/i + /2). Thus, if the load on the motor is such as to require the torque given at the slip, s, this load is started and brought up to full speed, / — 8f by speeding the brushes, B, up to or near synchronous speed, and then allowing them gradually to come to rest: at brush speed, /i = / — s, the rotor starts, and at decreasing, fh accelr- ates with the speed /2 = / — s — /i, until, when the brushes come to rest: f\ = 0, the rotor speed is /2 = / — s. As seen," the brushes revolve on the commutator only in start- ing and at intermediate speeds, but are stationary at full speed. If the brushes, B, are rotated at oversynchronous speed: /i>/, the motor torque is reversed, and the rotor turns in the same direction as the brushes. In general, it is: /i+/2 + s=/, where /i = brush speed, /2 = motor speed, s = slip required to give the desired torque, / = supply frequency. 102. An application of this type of motor for starting larger motors under power, by means of a small auxiliary motor, is shown diagrammatically, in section, in Fig. 61. Po is the stationary primary or stator, So the revolving squirrel- cage secondary of the power motor. The stator coils of P0 connect to the segments of the stationary commutator, Co, which receives the single-phase power current through the brushes, B0. 171 ELECTRICAL APPARATUS These brushes, Bv, are carried by the rotating squirrel -cage secondary, Si, of a small auxiliary motor. The primary of this. Pi, is mounted on thr power shaft, A, of the main motor, and carries the commutator, Cj, which receives current from the brushes, B,. These brushes are speeder! up t<> or near synchronism by some means, as hand wheel, H, and gears, G, and then allowed in slow down. Assuming the brushes were rotating in couMici-il.uk- wise direction, Then, while they are slowing down, the (ex- ternal) squirrel-cage rotor. .Si, of the auxiliary motor start* tad . (il, — Rotary terminal an nip-phase inriiu-tiuii motor with i trolling s[ da up, in clockwise direction, and while the brushes, B,, come to rest, .S', comes up to full speed, and thereby brings the brushes, Bv, of the power motor up to speed in clockwise rotation. As soon as Bo has reached sufficient speed, the power motor gets torque and its rotor, So, starts, in counter-clockwise rotation. As So carries Pi, with increasing speed of So and P,t Bj and with il I lie brushes, B„, slow down, until full speed of the power motor, So, is reached, the brushes. BB, stand still, anil the brushes, Bu by their friction on the commutator, <",, revolve together with f„ /*, and 8+ In whichever direction the brushes, B,. are Btarted, in the same direction starts Ihe main motor, So. SINGLE-PHASE INDUCTION MOTOR 175 If by overload the main motor, So, drops out of step and slows down, the slowing down of Pi starts Si, and with it the brushes, Bo, at the proper differential speed, and so carries full torque down to standstill, that is, there is no actual dropping out of the motor, but merely a slowing down by overload. The disadvantage of this motor type is the sparking at the commutator, by the short-circuiting of primary coils during the passage of the brush from segment to segment. This would require the use of methods of controlling the sparking, such as used in the single-phase commutator motors of the series type, etc. It was the difficulty of controlling the sparking, which side-tracked this type of motor in the early days, and later, with the extensive introduction of polyphase supply, the single-phase motor problem had become less important.