Differences Between Shunt and Series DC Motors on Drilling Rigs
A typical drilling rig will be equipped with either shunt or series wound motors, rarely a mixture a of both. The type of motor chosen is made at the system design stage and is unlikely to change through the lifetime of the rig. This article describes the parameters which inform the decision to use either.
Theory of Operation
DC Motors consist of an armature and field. It is the interaction of the magnetic flux from each of these which generates the motor torque. For high power DC motors the armature and the field fluxes will be produced by electric windings, but on (very) small motors the field flux is more liklely to be produced by a permanent magnet. The type of DC motors used on drilling rigs originated from the traction industry, i.e. trains. In the early days the motor speed and torque would have been controlled by adjusting the field current as this operates at a much lower current and voltage than the armature, and so controlling equipment would have been cheaper. With the advent of high power thyristors it became possible to control the armature current more efficiently which then led to the SCR control systems that are commonly found on drilling rigs.
Fig 1: Shunt Motor Connection
Shunt motors have a field coil which is independent from the armature winding. It is usually fixed at around 50.5A (for a standard GE752 motor, 57A for the Hi-Torque version) and requires a DC voltage of about 110VDC to achieve this. The armature current and voltage is regulated between 0-800A (1050A intermittently for Drawworks, 1400A for a Hi-Torque motor on Top Drive), 0-750VDC and is controlled by an SCR (thyristor bridge). The resistance of the armature is very low, and it would appear that Ohm's Law would indicate that, with a fully phased-up SCR bridge at 750VDC connected across it, the current flow would be very high, but this is not the case. How can this be? The reason is that the motor rotates, and this generates a back-EMF in the armature which opposes the forward drive of the SCR which makes the apparent resistance much higher. If, however, the motor is loaded the speed drops and the back EMF reduces and the current can increase for the same voltage. If the motor is stalled completely then there is no back EMF and only the armature resistance (plus cable resistance) is in circuit to limit the current unless the SCR regulator limits it by reducing the voltage (which is what happens in real life).
If the field flux is weakened, less back EMF is generated at a given speed and so less SCR voltage is required to maintain a particular speed for a given load, but there is a trade-off in that less torque is generated. Conversely if the field is strengthened I need more SCR voltage to maintain a particular speed, but I get more torque. The amount torque and speed achieved for a given set of armature and field conditions is a characteristic of the particular model of motor.
For a fixed field, the SCR voltage is very nearly linearly proportional to the speed of the motor, so the designer can use this in his regulator design as his speed feedback. An encoder, tacho or other feedback device is not required. If I want to connect the motors in parallel (as a Mud Pump drive, for example) I have a problem because if the motors are not perfectly and exactly matched in every respect (which is impossible to achieve) one motor will begin to absorb more of the load than the other, which will tend to reduce the back EMF, meaning that it will absorb even more current until the current limit of the SCR acts to limit it. The current limit for parallel motors might be 1600A (800A per motor), so if one motor is absorbing the lot it's not going to last very long. For this reason parallel motor systems with shunt motors have a current balancing system which adjusts the field current of one (or both) of the motors to compensate (see the article on Troubleshooting Mud Pump Field Supplies for more details).
Fig 2: Series Motor Connection
Series motors differ from shunt motors in that instead of having a separate, fixed, field supply, the field winding is connected in series with the armature. This has a big cost advantage to the designer in that no separate field excitation is required. The downside is that the SCR armature voltage is now not linearly proportional to speed because the field flux varies with the armature current and load. This can be overcome by using an independant speed feedback device (tacho or encoder) or by utilising a speed calculator circuit which factors in the changing field flux dependant on armature current.
The huge advantage of series motors is that when connected in parallel they will naturally load share. This is because if the load increases in one motor so does the field flux which increases the back EMF making the armature draw less current, and vice-versa. They are naturally self-regulating. The danger with this system is that if one of the motors becomes mechanically disconnected from the load it will be subjected to the full armature voltage and will overspeed. For this reason a protective circuit such as Mud Pump Sprocket Slip Detection is used to ensure that both motors are loaded. (The description 'Sprocket Slip' refers to the drive chain and sprockets found on dual motor Mud Pumps where at high speed the drive chain can rise off the sprockets if too slack).
So there you have it, not much to choose between either, except that with series motors you save on field supplies and field-regulated load sharing, but you do need sprocket slip protection. For the SCR system designer there is another factor for consideration where reversing drives (e.g. Rotary Table, Drawworks) are used. To reverse a DC motor either the field or armature voltage must be reversed, not both. With shunt motors this is fairly easy to achieve by field or armature reversing but with series motors the switching required is a little more involved. With shunt motors the design engineer can implement field weakening or strengthening to suit a particular application, e.g. winch controls, which is not practical with series motors, and will not be a consideration if the field supply has fixed voltage and/or current.
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