DC Motors
What are the three basic speeds called which DC motor operates at?
1.Base speed 2.Below base speed 3.Above base speed
How may Watts equal one Horsepower?
746
What are some examples of Series motors applications?
A automotive starters, cranes, hoists, electric buses, street cars, subway trains, electric locomotives.
Define speed regulation
A comparison of a motors speed between full and no load.
Define Motor.
A device that converts electrical energy into mechanical energy.
Define commutator.
A mechanical rectifier converting AC to DC.
Define shunt motor.
A motor containing only a shunt field.
What will happed to the speed of a shunt DC motor if the voltage is reduced to the shunt field?
Above base speed
Define locked rotor current
Amount of current produced when voltage is applied to a motor and the motor is not turning.
What will happed to the speed of a shunt DC motor if the voltage is reduced to the armature?
Below Base Speed
The armature of a 5 hp, DC motor measures 500 milliohms. The applied voltage is 120 volts and the FLA is 40 amps. What is the CEMF when the motor is delivering it's rated horsepower?
CEMF=Applied EMF - Effective voltage CEMF=120V - 20V CEMF=100V
What are the three main types of DC motors?
Compound, series, and shunt.
Define brushes.
Soft material semi conductor provides connection to rotating machines.
What is the most accepted method for revrsing the direction of a DC motor?
Swap the armaure leads WHY? You could put it in differential if you switch the field leads.
Define Compound DC motor:
Uses series and a shunt field.
Define Cumulative compound motor:
When the series and shunt field aid each other in the production of magnetism.
Define interpoles.
pole pieces used to coret armature reaction connected in series with the armature.
The armature of a 10 hp, DC motor measures 1 ohm. The applied voltage is 240 volts and the FLA is 76 amps. What is the effective voltage when the motor is delivering it's rated horsepower?
E=IR E=76A X 1Ω E=76V
Explain how a series field is constructed?
Few turns of large wire with a low resistance.
What would happed if a Compound DC Motor lost its shunt field?
It will become a series motor
Why do some DC drives leave voltage on the shunt field when the motor is not in operation?
Keeps the motor dry with heat.
How do you determine the direction of rotaion of a DC motor (CW/CCW)?
Look at it from the commutator end.
Explain how a shunt field is constructed.
Many turns of small wire with a high resistance.
What will happen to the direction of rotation of a DC motor if the incoming power leads are reversed?
Nothing
What three factors determine the amount of CEMF in a DC motor?
Number of turns of wire in the armature. Strength of the magnetic field of the pole pieces. SPEED OF THE ARMATURE.
Explain how to operate a shunt DC motor at Base Speed and darw a schematic?
Apply full voltage to the armature and shunt field.
Torque is proportional to _________________ if field excitation remains constant.
Armature current.
The speed regulation for a DC motors is proportional to the _?
Armature resistance
Why do DC motors draw large inrush current (LRA) during starting?
Because the CEMF is low.
The armature of a 10 hp, DC motor measures 1 ohm. The applied voltage is 240 volts and the FLA is 76 amps. What is the CEMF when the motor is delivering it's rated horsepower?
CEMF=Applied EMF - Effective voltage CEMF=240V - 76V CEMF=164V
What are the characteristics of a shunt DC motor?
Constant speed motor within 10%, poor starting torque.
Define series DC motors
Contains only a series field winding.
Define Field loss relay:
Current relay connected in series with the shunt field.
The armature of a 5 hp, DC motor measures 500 milliohms. The applied voltage is 120 volts and the FLA is 40 amps. What is the effective voltage when the motor is delivering it's rated horsepower?
E=IR E=40A X 500mΩ E=20V
Explain how to operate a shunt DC motor Below Base Speed and draw a schematic?
Full voltage on the shunt field and add resistance in series with the armature. Reduced voltage to the armature full voltage to the shunt field.
Eplain how to operate a shunt DC motor Above Base Speed and draw a schematic?
Full voltage to armature and reduced voltage to shunt field.
What are the characteristics of a compound DC motors?
Good speed regulation and good starting torque. Most commonly used motor.
What are the characeristics of a series DC motor?
High starting torque with poor speed regulation.
The armature of a 5 hp, DC motor measures 500 milliohms. The applied voltage is 120 volts and the FLA is 40 amps. What is the locked-rotor current during start up?
I=E/R I=120V/500mΩ I=240A
The armature of a 10 hp, DC motor measures 1 ohm. The applied voltage is 240 volts and the FLA is 76 amps. What is the locked-rotor current during start up?
I=E/R I=240V/1Ω I=240A
Define Differential Compound Motor:
Series and shunt field oppose each other in the production of magnetism.
Define locked-rotor torque.
The amount of torque produced by a motor at the time of starting.
What is the principal of operation of a DC motor?
The attraction and repulsion of magnatism.
Define armature.
The moving part of a motor or generator containing a shaft, insulated windings, slotted iron core, and a commutator
What two factors determine the amount of torque developed by a DC motor?
The strength of the magnetic field in the armature, and the strength of the magnetic field in the poles.
Define torque.
The turning force developed by a motor. It also increases with current.
Define Counter Electromotive Force
The voltage induced in the armature of DC motor that opposes the applied voltage and limits amature current. opposite polarity to applied voltage.
Why is the core material of a pole piece laminated?
To reduce eddy currents.
Define armature reaction.
Twisting or bending of the main magnetic field, and its directly proportional to armature current.
