The Theory of Variable Speed Drive

Methods of speed control.
The speed of a driven load often needs to run at a speed that varies according to the operation it is performing. The speed in some cases such as pumping may need to change dynamically to suit the conditions, and in other cases may only change with a change in process. Electric motors and coupling combinations used for altering the speed will behave as either a "Speed Source" or a "Torque Source". The "Speed Source" is one where the driven load is driven at a constant speed independent of load torque. A "Torque Source" is one where the driven load is driven by a constant torque, and the speed alters to the point where the torque of the driven load equals the torque delivered by the motor. Closed loop controllers employ a feedback loop to convert a "Torque Source" into a "Speed Source" controller.

Mechanical.
There are a number of methods of mechanically varying the speed of the driven load when the driving motor is operating at a constant speed. These are typically:

Belt Drive

Chain Drive

Gear Box

Idler wheel drive

All of these methods exhibit similar characteristics whereby the motor operates at a constant speed and the coupling ratio alters the speed of the driven load. Increasing the torque load on the output of the coupling device, will increase the torque load on the motor. As the motor is operating at full voltage and rated frequency, it is capable of delivering rated output power.
There is some power loss in the coupling device resulting in a reduction of overall efficiency. The maximum achievable efficiency is dependant on the design of the coupling device and sometimes the way it is set up. (e.g. belt tension, no of belts, type of belts etc.)
Most mechanical coupling devices are constant ratio devices and consequently the load can only be run at one or more predetermined speeds. There are some mechanical methods that do allow for a dynamic speed variation but these are less common and more expensive.
Mechanical speed change methods obey the 'Constant Power Law' where the total power input is equal to the total power output. As the motor is capable of delivering rated power output, the output power capacity of the combination of motor and coupling device (provided the coupling device is appropriately rated) is the rated motor output power minus the loss power of the coupling device.
Torque 'T' is a Constant 'K' times the Power 'P' divided by the speed 'N'.

T = K x P / N

Therefore for an ideal lossless system, the torque at the output of the coupling device is increased by the coupling ration for a reduced speed, or reduced by the coupling ratio for an increased speed.

Magnetic.
 There are two main methods of magnetically varying the speed of the driven load when the driving motor is operating at a constant speed. These are:

Eddy Current Drive

Magnetic Coupling

These methods use a coupling method between the motor and the driven load which operates on induced magnetic forces. The eddy current coupling is quite commonly employed, and is easily controlled by varying the bias on one of the windings. In operation, it is not unlike an induction motor, with one set of poles driven by the driving motor, hence operating at the speed of the driving motor. The second set of poles are coupled to the driven load, and rotate at the same speed as the driven load. One set of poles comprises a shorted winding in the same manner as the rotor of an induction motor, while the other set of poles is connected to a controlled D.C. current source. When the machine is in operation, there is a difference in speed between the two sets of poles, and consequently there is a current induced in the shorted winding. This current establishes a rotating field and torque is developed in the same way as an induction motor. The coupling torque is controlled by the D.C. excitation current. This method of coupling is essentially a torque coupling with slip power losses in the coupling.

Hydraulic.
There are two main methods of hydraulically varying the speed of the driven load when the driving motor is operating at a constant speed. These are:

Hydraulic pump and motor

Fluid Coupling

The fluid coupling is a torque coupling whereby the input torque is equal to the output torque. This type of coupling suffers from very high slip losses, and is used primarily as a torque limited coupling during start with a typical slip during run of 5%. The constant power law still applies, but the power in the driven load reduces with speed. The difference between the input power and the output power is loss power dissipated in the coupling.
In an extreme case, if the load is locked (stationary) and the motor is delivering full torque to the load via a fluid coupling, the load will be doing no work and hence absorbing no power, with the motor operating at full speed and full torque, the full output power of the motor is dissipated in the coupling. In most applications, the torque requirement of the load at reduced speed is much reduced, so the power dissipation is much less than the motor rating.
In the case of a hydraulic pump and motor, the induction motor operates at a fixed speed, and drives a hydraulic pump which in turn drives a hydraulic motor. In many respects, this behaves in a manner similar to a gear box in that the hydraulic system transfers power to the load. The torque will be higher at the load than at the motor for a load running slower than the motor.

Electrical.
There are a number of methods of electrically varying the speed of the driven load and driving motor.
 These are:

D.C. Motor

Universal Motor

Schrage motor

High Slip Motor (Fan Motor)

Slip Ring Motor

Variable Frequency Drive and Induction Motor

The D.C. motor
The DC Motor was traditionally a very common means of controlling process speed. It is essentially a "Torque Source" controller and is usually used with a tachogenerator feedback to control the speed of the driven load. The D.C. motor consists of a field winding and an armature. The armature is fed via brushes on a commutator. The D.C. motor is available in two main formats, Series wound and shunt wound. Small D.C. Motors are often series wound giving the advantage of improved starting torque. With a series wound D.C. motor, speed control is achieved by regulating the voltage applied to the motor. All the motor current passes through the voltage regulator.
A shunt wound motor has separated field and armature windings. The torque output of the motor is varied by controlling the excitation on the armature winding while maintaining full voltage D.C. on the field. The voltage regulator only passes the current to the field winding, dissipating much less power than in the case of the shunt wound motor.
D.C. motors are a torque source, and so are able to operate well under high transient load conditions. At low speed, the D.C. motor is able to deliver a high torque.

The universal motor
The Universal Motor is a motor with a wound armature and a wound stator. The armature is fed via brushes on a commutator, and is essentially the same as a D.C. motor. The universal motor will operate off a single phase A.C. supply and accelerates until the load torque equals the output torque. Domestic appliances, such as vacuum cleaners, and small hand tools such as electric drills use this technology. The speed is changed by reducing the voltage applied to the motor. This is often a triac based voltage controller similar to a domestic light dimmer.

A Schrage motor
The Schrage Motor is a very special motor with a brush/commutator fed rotor and a slip ring fed rotor and a wound stator, and due to the way it is constructed is able to be speed controlled by variation of the position of the brushes relative to the field windings. The rotor has two windings, one of which is driven by the commutator/brush assembly and the other is driven by means of slip rings. These motors are usually of European origin and found of some of the older machines imported for specialised applications such as carpet making.

High Slip Induction Motor
An induction motor with a high rotor resistance is a high slip motor and is often referred to as a fan motor or a type F motor. The torque capacity of this motor is high at low speeds and low at synchronous speed. By reducing the voltage applied to the Type F motor, the available torque is reduced and consequently, when coupled to a fan load, the speed reduces. A type F motor has a high power dissipation in the rotor and is only useful for smaller single phase and three phase machines. The actual speed is dependant on the stator voltage, motor characteristics and load torque. Voltage controllers are either transformers, variacs or SCR based solid state controllers.

Slip ring motors
Slip Ring Motors are induction motors with a wound rotor with the rotor winding accessible via slip rings. Changing the value of external resistance connected in series with the rotor windings, will vary the torque curve of the motor. With a high value of resistance in the rotor circuit, the slip ring motor will behave like a type F motor. With the slip ring motor, the stator voltage is held constant at line voltage, and the rotor resistance is varied to alter the torque capacity of the motor and hence the speed. This type of speed control is used on large machines because the rotor power dissipated is external to the motor. Typical applications are in hoisting and dragline type machines associated with dredging machines.

Variable frequency drives. (VFDs)
The speed of standard induction motors can be controlled by variation of the frequency of the voltage applied to the motor. Due to flux saturation problems with induction motors, the voltage applied to the motor must alter with the frequency. The induction motor is a pseudo synchronous machine and so behaves as a speed source. The running speed is set by the frequency applied to it and is independent of load torque provided the motor is not over loaded. This is achieved by the use of VFDs.