All motors and generators have two main parts: the stator, which is a stationary coil of wire wound around a core of laminated steel, and the rotor, the rotating portion which is made up steel lamination and aluminum bars with end rings. Magnetic flux crosses these bars and induces a voltage between them. The rotor shaft sticks out beyond the housing. For a generator, a prime mover (i.e. crank, water wheel, steam turbine, diesel engine, etc.) is attached to the shaft. This applies the torque necessary to turn the rotor. For a motor, the shaft is connected to a mechanical load.

GENERATORS
The input to a generator is mechanical power. If its output is direct current, it is a DC generator, if its output is alternating current, it is an alternator.

TORQUE
Torque is defined as turning force. In a generator, the prime mover applies torque to make the rotor turn. Torque is directly proportional to force and distance (i.e. more force equals more torque and a longer crank equals more torque).

Torque (T) = Force (f) x Distance (d). The unit of force is the Newton (N) while distance is measured in meters (m) and torque is measured in Newton meters (N.m), or as is commonly used in the US, Ft-Lbs.

MECHANICAL TO ELECTRICAL POWER
Power is a rate of energy and is proportional to the product of speed and torque In order to have mechanical power, you must have motion as well as torque. For example, it takes more power to move a rotor fast than it does to move it slow.

PM = (S x T) / 9.55, where PM is mechanical power S is speed T is torque 9.55 is a constant for metric measurement.

The losses involved in this process are critical to the performance of the generator. Some mechanical power is required by the prime mover to cause the rotor to turn. This amount of power is not converted to electrical power and is considered rotational loss. When a load is connected to the generator, it draws current from the generator. This, in turn, requires more mechanical power from the prime mover. Therefore, rotational losses increase under load.

In the conversion from mechanical to electrical power, we encounter what is known as counter torque. This is due to Lenz's Law which states that induction tends to oppose whatever causes it. Counter torque is the repulsion force between two magnetic fields. There are some other losses that must be taken into account. Although small, some of the electrical power is lost in the winding resistance. They are called copper losses. Also, an additional amount is lost in what is called stray power loss. The power that remains after overcoming these losses is delivered to the load.

MOTORS
The input to a motor is electrical power. The output of the motor is mechanical power, which is transferred by the rotor shaft as torque to the load. To drive a load at a particular speed, you need a certain amount of torque. As the motor drives the load, mechanical power is drawn from the motor. This causes the motor to draw electrical power from its power source.

TORQUE
Defined as turning force and should be viewed as mechanical energy. The torque produced by a motor depends on the strength of the stator and rotor, and the design of motor, such as the number of poles, number of windings on the armature coil, and the way the armature coil is wound and connected.

There are a number of different types of torque.

Locked Rotor Torque (Breakaway Torque). This is the torque developed when the rotor is not turning.

Full Load Torque. The torque needed to produce rated power at full load speed.

Accelerating Torque. This is the difference between the torque delivered by the rotor and the torque needed by the load. It does not last very long as this is the torque that speeds up the motor. When rotor and load reach correct speed, there is no longer accelerating torque.

Breakdown Torque. Maximum torque a motor can develop without an abrupt drop in speed.

Starting Torque. This is usually less than twice full load torque, even though starting current can be 5 or 6 times Full Load current. This is due to the low power factor of the rotor.

ELECTRICAL TO MECHANICAL POWER
Magnetic forces produce torque on the armature conductors, which apply a torque to the rotor. The torque drives the load. The motor draws electrical power from its source at the same time and rate the mechanical power is being used by the load. As the load increases, the motor current increases. The source knows how much mechanical power is being used by the amount of Counter Electromotive Force (CEMF) being generated in the armature. EMF is another name for voltage. While CEMF is a real voltage which opposes the voltage applied to the armature, it cannot be measured separately. Prior to start, the rotor is not stationary. CEMF is zero and current is at a maximum. The net voltage is also at a maximum. The combination of these parameters creates a high torque. At the instant of startup, the rotor begins to turn. As rotor speed increases, more and more CEMF will be introduced into the armature. The net voltage decreases causing armature current to drop. This results in a decrease of output torque.

IMPORTANT DEFINITIONS

SPEED
As soon as you turn on a 3 phase motor, current flows in the stator coils and the stator magnetic field begins to revolve. The field's rate of travel is known as the synchronous speed, which depends on the number of poles in the motor and the frequency of the applied voltage. Induction motors always run slower than synchronous speed, while synchronous motors run at exactly synchronous speed.

SLIP
This is the ratio of slip speed to synchronous speed. Slip speed is equal to the difference between rotor speed and synchronous speed. Percent slip is slip multiplied by 100. When the rotor is not turning, the percent slip is 100%. Slip relates to other motor parameters as follows:

Voltage. At start, when slip is I00%, the voltage is maximum. As the rotor begins to turn, slip reduces and so does voltage.

Frequency. As slip decreases, so does frequency.

Inductive Reactance. A motor has resistance and inductance built in, and they are constant. Inductive Reactance is dependent on frequency and slip. When the rotor is not turning, the frequency and slip are maximum and so is the inductive reactance. When the rotor is turning, the inductive reactance is low and power factor approaches unit.

Rotor Impedance. Rotor impedance is the phasor sum of resistance and inductive reactance. While resistance is constant, inductive reactance changes with slip. At start, inductive reactance is high and impedance is mostly inductive. The rotor has low, lagging power factor. As the rotor picks up speed, the inductive reactance goes down, eventually equaling resistance.

EFFICIENCY
This is the ratio of the output power to input power. Only part of the electric power going into a motor is transferred to the load as mechanical power. Some is lost in the resistance of the stator windings and is known as copper losses, which are the only variable losses. They vary with the load in proportion to the current squared. A little is lost in the stator core and is known as core losses. The rest is transmitted across the air gap to the rotor. Some of the rotor power is lost in rotor resistance. Finally the power needed to overcome winding and friction losses reduces the mechanical output still further.

There are two ways to determine motor efficiency, first determine the electrical power into the motor and the mechanical power out (Efficiency = P_in / P_out x 100), or second, determine the electrical input and losses, (Efficiency = Input losses / Input x 100).

POWER FACTOR
Induction motors always run with a lagging power factor, because they draw magnetizing current from the AC Source. Synchronous motors, however, may run at either leading, unity, or lagging power factor depending on how much magnetizing current is drawn from the excitation supply. The magnetizing power may come from either the DC or AC supply. If only part of the magnetizing is done by the AC, the motor is under excited. Under excited synchronous motors run at a lagging power factor. In normal excited motors, current in the DC field coil supplies just the right amount of magnetization. A normal excited motor runs at unity power factor. A motor where the DC field supplies too much magnetization is known as over excited. An over excited motor runs at leading power factor.

MOTOR & GENERATOR TESTING
There are two classes of tests that are typically run on motors, serial tests and type tests. Type tests are normally done in conjunction with serial tests as a part of the design and development cycle to fully verify the electrical specifications and the performance of the motor. Serial tests only are probably most commonly part of production testing. During the serial test, a locked rotor test (also known as a short circuit test) and a no load test are usually performed. Type testing is more detailed and adds load testing and heat run tests. From the results of these tests, the efficiency of the motor can be calculated.

TYPICAL MOTOR/GENERATOR TESTS

Locked Rotor Test
Tests made with a locked rotor can determine current and torque at start as well as copper loss at full load. First the rotor is locked into position. This eliminates rotational losses. Then reduced voltage is applied. Readings are taken of torque produced and current at this reduced voltage to determine the starting values. The voltage is increased slowly until rated current is drawn by the motor. At this point, all of the power going to the motor is lost in the windings. This is the copper loss at full load. It is then possible to compute equivalent resistance of the motor as seen by the power source. From this, the equivalent resistance of the motor as seen by the power supply may be calculated using the equation R = P/i2 .

No Load Test
To make a no load test, the motor is operated at full voltage with no load connected to it. Copper losses can be computed by I2R, where I is equal to the current at no load and R is the equivalent resistance of the motor (determined from the locked rotor test). The input power is dissipated as power lost due to rotational and core losses (fixed losses) and power lost due to copper losses at no load current. By subtracting copper loss from input power, it is possible to determine the fixed losses. Once fixed losses are known, it is possible to compute the efficiency at full load. At no load, very little torque is produced and rotor current is low. Power factor under no load conditions is very low (usually less than 0.5). As load increases, the power factor tends to improve (typically 0.8 or better).

Type Test
During the Type Test, a load test and a heat run may be performed. During a load test, a load is coupled to the motor and a complete load run is made under actual conditions in which the input power, stator current, and slip are determined for each load step. The Heat Run Test determines whether the motor will remain within allowable temperature limits during operation. Using the results of these tests and those of the serial tests, the loss separation method is used.

Therefore, Power in - losses = Power out, where losses are:

• P_core = Core Losses
• P_Cu = Copper Losses
• P_rot = Rotational Losses
• P_friction = Friction Losses
• P_add = Additional Losses
• Or, P_in (P_core + P_Cu + P_add + P_rot+ P_friction) = P_out