DC Motors KL3073

Direct Current (DC) Machines Fundamentals Dc motors are dc machines used as motors the same physical machine can operate as either a motor or a generator-it is simply a question of the direction of the power flow through it. Dc motors are compared by their speed regulations.

Types of dc motors The separately excited dc motor The shunt dc motor. The permanent-magnet dc motor. The series dc motor. The compounded dc motor

The Equivalent Circuit of a DC Motor its equivalent circuit is exactly the same as that of a generator except for the direction of current flow. Figure: (a) The equivalent circuit of a dc motor. (b) A simplified equivalent circuit

The Equivalent Circuit of a DC Motor The internal generated voltage in this machine is given by the equation. induced torque developed by the machine is given by These two equations, the KVL equation of the armature circuit and magnetization curve, are necessary to analyze the behavior and performance of a dc motor

Separately excited and shunt dc motors A separately excited dc motor is a motor whose field circuit is supplied from a separate constant- voltage power supply a shunt dc motor is a motor whose field circuit gets its power directly across the armature terminals of the motor When the supply voltage to a motor is assumed constant, there is no practical difference in behaviour between these two machines

The Separately excited and shunt dc motors (a) The equivalent circuit of a separately excited de motor. (b) The equivalent circuit of a shunt dc motor. The KVL equation for the armature circuit is:

The Terminal Characteristic of a Shunt DC Motor The terminal characteristic of a motor is a plot of its output torque versus speed. How does a shunt dc motor respond to a load? When load ↑ τ load > τind so ω↓ EA ↓ = KФω↓ IA ↑= (VT – EA ↓)/RA τind ↑ = KФIA↑) τ load = τind

Induced Torque in the Rotating Loop Output characteristic of a shunt dc motor (a) Torque-speed characteristic of a shunt or separately excited dc motor with compensating windings to eliminate armature reaction. (b) Torque-speed characteristic of the motor with armature reaction present.

Speed Control of Shunt DC Motors In general, there are 3 methods to control the speed of dc shunt motor: Adjusting the field resistance RF (and thus the field flux) Adjusting the terminal voltage applied to the armature. Less common method: Inserting a resistor in series with the armature circuit

Speed Control of Shunt DC Motors Changing the Field Resistance

Speed Control of Shunt DC Motors Changing the Field Resistance The effect of field resistance speed control on a shunt motor's torque speed characteristic

Speed Control of Shunt DC Motors Changing the Armature Voltage The effect of armature voltage speed control on a shunt motor's torque-speed characteristic.

DC Machine Construction Inserting a Resistor in Series with the Armature Circuit. The effect of armature resistance speed control on a shunt motor's torque-speed characteristic.

Safe Ranges of Operation for the 2 common methods Field Resistance Control If a motor is operating at its rated terminal voltage, power and field current, then it will be running at rated speed, also known as base speed. Field resistance control can control the speed of the motor for speeds above base speed but not for speeds below base speed. To achieve a speed slower than base speed by field circuit control would require excessive field current, possibly burning up the field windings.

Safe Ranges of Operation for the 2 common methods Armature Voltage Control If a motor is operating at its rated terminal voltage, power and field current, then it will be running at rated speed, also known as base speed. Armature voltage control can control the speed of the motor for speeds below base speed but not for speeds above base speed. To achieve a speed faster than base speed by armature voltage control would require excessive armature voltage, possibly damaging the armature circuit.

The Effect of an Open Field Circuit What would happen if the field circuit were actually opened while the motor is running? The flux in the machine will drop, and EA will drop as well. This would cause a really large increase in the armature current, and the resulting induced torque would be quite a bit higher than the load torque of the motor. Therefore, the motor’s speed starts to rise and just keeps going up. This condition is called runaway.

THE PERMANENT-MAGNET DC MOTOR A permanent magnet dc motor (PMDC) is a dc motor whose poles are made of permanent magnets. Advantage: Since the motors do not require an external field circuit. No field circuit copper losses. Smaller than corresponding shunt dc motors. Disadvantages: cannot produce as high flux density >lower induced torque PMDC motors run the risk of demagnetization. not possible to control the speed of the PMDC motor by varying the field current or flux.

THE SERIES DC MOTOR A series dc motor is a dc motor whose field windings consist of a relatively few turns connected in series with the armature circuit. The Kirchhoff's voltage law equation for this motor is The equivalent circuit of a series dc motor.

Induced Torque in a Series DC Motor the flux is directly proportional to the armature current, at least until saturation is reached. the load on the motor increases, its flux increases too. so the speed deceases The induced torque is τind =KφIA φ =c IA τind =Kc(IA)2

The Terminal Characteristic of a Series DC Motor Using equation φ= cIA the torque-speed characteristic curve for the series motor can be derived as ideal torque-speed characteristic is

Speed Control of Series DC Motors Unlike with the shunt dc motor, there is only one efficient way to change the speed of a series dc motor. That method is to change the terminal voltage of the motor. If terminal voltage is increased, the speed will increase for any given torque.

THE COMPOUNDED DC MOTOR A compounded dc motor is a motor with both a shunt and a series field. Such a motor is shown in Figure below. The equivalent circuit of compounded dc motors: (a) long-shunt connection; (b) short-shunt connection.

THE COMPOUNDED DC MOTOR The KVL for a compounded dc motor is: VT = EA + IA (RA + RS) and the currents are: IA = IL - IF IF = VT/RF   The net mmf and the effective shunt field current are:  Fnet = FF ± FSE - FAR  IF* = IF ± (NSE/NF) IA – FAR/NF

The Torque-Speed Characteristic of a Cumulatively Compounded DC Motor (CC) There is a component of flux which is constant and another component which is proportional to its armature current (and thus to its load). CC motor has a higher starting torque than a shunt motor (whose flux is constant) but a lower starting torque than a series motor (whose entire flux is proportional to armature current). The CC motor combines the best features of both the shunt and series motors. Like a series motor, it has extra torque for starting; like a shunt, it does not overspeed at no load.

The Torque-Speed Characteristic of a Cumulatively Compounded DC Motor (CC) A comparison of the torque-speed characteristics of each of these types of machines is shown below: (a) The torque-speed characteristic of a cumulatively compounded dc motor compared to series and shunt motors with the same full-load rating. (b) The torque-speed characteristic of a cumulatively compounded dc motor compared to a shunt motor with the same no-load speed.