Discuss What kind of motor is this? in the UK Electrical Forum area at ElectriciansForums.net
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Well Marconi is close enough that I have to give it to him. Yes, made by BTH, yes brushes are shorted together and run on a comm to create rotor repulsion, yes power is applied to stator only. However, repulsion is only used for starting - it's a Repulsion-Start, Induction-Run (RSIR or R/I) machine.
There are two ways to do this. Century and others in the USA, used a centrifugal mechanism like the starting switch of a split-phase induction, but instead of operating a switch, as the motor approaches full speed it does two things. It triggers a spring-loaded actuator that lifts the brushes off the comm (to save wear) and puts a shorting ring around all the commutator segments to convert the armature into the equivalent of a squirrel cage.
This BT-H (then part of AEI) avoids the centrifugal mechanism by having two windings on the rotor. The repulsion winding brought out to the commutator, and a separate squirrel cage. At a standstill, the high slip frequency puts the cage current out of phase due to its inductance (which is why inductions typically have poor starting torque.) But the repulsion winding takes over and accelerates the motor, at a speed and direction controlled as Marconi mentions, by the setting of the brush rocker. As the speed increases and the slip frequency reduces, the cage torque dominates and the relative placing of the windings in the slots causes the magnetic flux to bypass the repulsion winding, so the motor runs as an induction. Unlike the Century there is still brush wear, but it is low because the brushes are passing minimal current.
Why go to all this complication to start an induction motor? Simply because repulsion motors have excellent starting characteristics. Smooth and torquey, without the excessive line current required by other starting methods, despite being single-phase (which were always inferior to 3-phase at starting). RSIRs handle high inertia loads and starting against full torque, but once running they are not as efficient as inductions and lack the near-constant speed. Hence, to deal with difficult loads with only single-phase power available, start as induction for best torque and lowest overcurrent, then run as induction for best efficiency and stability.
I've no idea why Tel sees it heading to the skip. It's worked fine for 80 years and will probably be good for another 80. I'm sure skips will have been banned by then.
The round thing bottom centre
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