AC Motor Types

How do you calculate full torque load for a motor?

Remember formula for horsepower: HP = (Torque (lb-ft) * Speed (rpm) ) / 5252 Solve for torque: Torque (lb-ft) = (HP * 5252) / Speed (rpm)

How do you reverse the rotation of an AC induction motor?

Reverse any two leads: T1, T2, T3

How do you reverse the rotation on a single phase AC induction motor?

Reverse the connections to the starting winding

What are some common voltages for single-voltage 3 phase motors?

230V, 460V, 575V

What is slip of an AC induction motor? How is slip determined?

For a voltage to be induced into the conductor bars of the rotor, the rotor must be rotating at speed slightly below synchronous speed. Slip is calculated as a percentage as follows: Slip % = (( S - R) / S ) * 100 where S = Synchronous speed (rpm) R = Rotor speed (rpm)

What is full-load torque?

In md2.12 as speed increases beyond breakdown torque (point C) torque rapidly decreases to point D. This is full-load torque. Full-load torque is reached at a speed slightly less than synchronous speed.

How does a capacitor motor work? What is the function of the capacitor?

Like a split-phase motor a capacitor motor has a starting and running winding. In addition to this a capacitor is added in series to the starting winding. The capacitor causes a greater phase difference between the running and starting winding, which gives the motor more staring torque.

What does a NEMA A torque curve look like? What are NEMA A motors used for?

NEMA A torque curve looks like a NEMA B, except that it has higher starting current. Must make sure NEMA A motor overcurrent protective devices are adequate to handle increased current. NEMA A motors are used in the same applications as NEMA B motors.

What is starting / locked rotor torque? What is the starting torque of a NEMA B motor? (NOTE TO SELF: This and following 3 cards describe a NEMA B torque curve)

Torque motor develops when rated voltage and frequency is first applied. Starting torque for a NEMA B motor is 150% of full load torque. See md2.12

What are the voltages for a dual-voltage 3 phase motor? Which voltage is preferred? Why?

230V and 460V. 460V is preferred because using higher voltage because the current is cut in half w/o loss of horsepower. Lower current means smaller diameter wiring can be used, which is more cost effective.

What are the 4 types of NEMA motors? What is the most common type? Least common?

4 types of NEMA motors are A, B, C, D. Most common type is B. Least common type is A.

How many leads are there for 3 phase motors? How are they labeled? Which leads connect to the power source?

9 Leads labeled T1-T9. T1 connects to L1, T2 connects to T2, T3 connects to L3.

What phase angle difference in the starting and running windings cause the largest amount of torque to be produced in split-phase or capacitor motors?

90 deg.

How does a shaded-pole motor work?

A shaded-pole AC motor uses a single turn of copper wire around the portion of the main pole. This offsets the magnetic field by 90 deg. The offset causes the rotor to move from the main pole to the shaded pole, which determines the starting direction of the motor. See md2.1.

How does a split-phase motor work?

A split-phase motor contains a rotor and a stationary part that contains a running winding and a starting winding. When first starting, both the starting and running windings are connected in parallel. The running winding has higher reactance (opposition to flow of AC current in a circuit due to inductance) than the starting winding. The running winding current lags behind the starting winding current, which causes a phase difference between the starting and running windings. This sets up a rotating magnetic field producing rotation in the rotor. When the motor reaches 60-80% of full speed a centrifugal switch opens and disconnects the starting winding from the running winding, so the motor runs on the running winding only. If the motor slows to 40% of full speed the centrifugal switch closes and the motor runs on both starting and running windings again. See md2.2

What is pull up torque?

After the motor speed increases torque drops below starting torque at point B on md2.12. This is pull-up torque, which is less than staring torque, but still greater than full-load torque.

What are NEMA B motors used for?

Are used for applications with normal starting and running torque such as light conveyors, fans, pumps and machine tools.

What is breakdown torque?

As speed continues to increase from point B to point C on md2.12 torque reaches 200% of full-load torque. This called breakdown torque.

How are 3 phase motors wired for low voltage? High voltage?

Both wye and delta 3 phase motors are connected in parallel for low voltage. They are connected in series for high voltage. See md2.6-2.9

Compare and contrast capacitor-start, capacitor-run, and capacitor-start-and-run motors (Think in terms of design, start and running torque).

Capacitor-start motors run much the same way split-phase motors function. They have a centrifugal switch, which opens to disconnect both the starting winding and the capacitor (see md2.3) and around 60% to 80% of full rated speed. It has high starting torque. Capacitor-run motors do not have a centrifugal switch and the starting winding and capacitor is connected in series at all times. This motor has medium starting torque and somewhat higher running torque than a capacitor-start motor. The capacitor used in this motor has a lower-value than the capacitor used in capacitor-start motors. Capacitor-start-and-run motor has two capacitors: a high value one connected in series with the starting winding, and a low value one also connected in series with the starting winding. When the motor reaches a certain speed a centrifugal switch opens, which disconnects the high value capacitor from the circuit. The low value capacitor is always connected to the circuit. These motors have same starting torque as a capacitor-start motor and has higher running torque than either the capacitor-start or capacitor-run motors (see md2.5).

Torque curves can be useful in understanding motor performance under load. Which curves in md2.10 show appropriately sized motors? Which do not?

Constant load 1 and variable load 1 are appropriately sized motors because they are able to accelerate to rated speed. Constant load 2 does not have sufficient starting torque to turn the rotor. Variable load 2 cannot reach rated speed. Constant load 2 and variable load 2 will most likely overheat until the overload relay trips.

Single phase induction motor produces a pulsating torque, why?

Each time the stator's magnetic field reverses direction, there is an instant when the magnetic flux is zero and no torque is produced. As a result, the torque is pulsating which gives single-phase motors a tendency to vibrate.

What does a NEMA C torque curve look like? What kind of applications are they used for?

See md2.11. NEMA C motors are designed for applications, that require high starting torque for hard to start loads such as heavily loaded conveyors, mixers, and crushers. Starting torque is typically higher than the starting torque found in NEMA B motors (150% of full-load torque), although they have low starting current. Slip and full-load torque in NEMA C motors are the same as NEMA B motors.

What does the NEMA D torque curve look like? What applications are they used for?

See md2.13. Starting torque for a NEMA D motor is approximately 280% of full-load torque. It is well suited for hard to start applications such as punch presses and oil well pumps. It has no breakdown torque, as torque decreases until full-load torque is achieved. Slip is between 5-13%.

In the motor nameplate in md2.14, what does the service factor represent? What is the class of insulation for this motor? What are the classes of insulation from lowest to highest?

Service factor is multiplied by horsepower to determine the horsepower the motor can be operated at. In this example the horsepower is 30. So 30 * 1.15 = 34.5. The class of insulation is F. The classes of insulation from lowest to highest (in terms of their ability to handle heat) is: A, B, F, H.

Name 3 types of single phase AC motors.

Shaded-pole, split-phase, capacitor motors.

What is starting / locked rotor current? What is starting current for a NEMA B motor? What is full-load current?

Starting / locked rotor current is the current supplied to the motor at rated voltage initially when the motor is at rest. For a NEMA B motor this is 600-650% of full-load current. Full-load current is the current supplied to the motor with the rated voltage, frequency, and load and the rotor is up to speed.

How do you determine the max temperature the motor can operate in md2.14?

The insulation class identifies the allowable temperature rise from an ambient temperature of 40 C (104 F). The insulation class in this example is F, which has a 105 C temp rise. So max temperature would be 40 C + 105 C = 145 C for a motor with a service factor of 1.0. However, this motor has a service factor of 1.15. Motors with a service factor of 1.15 can add an additional 10 C to their temperature rise. So the max temp of this motor would be 155 C.

What are the main differences in the starting and running winding in a split-phase motor? (In terms of number of coils, thickness of wire).

The running winding has larger diameter wire and more turns than the starting winding.

Describe what happens to the slip, voltage, current, and torque of a rotor when a load is applied to an AC induction motor.

When a load is applied to an AC induction motor, rotor speed slows, slip increases. The increased slip causes more conduction bars of the rotor to cut the flux lines of the stator coils increasing rotor voltage and current. Torque also increases as a result of increased voltage and current.

Describe what happens to the slip, voltage, current, and torque of a rotor when a load is removed from an AC induction motor.

When a load is removed from an AC induction motor, rotor speed increases, slip decreases. The reduction in slip causes fewer conduction bars of the rotor to cut the stator flux lines, which causes a decrease in voltage and current. The reduction of voltage and current causes a decrease in torque.

Why is starting current so high?

When the rotor is a rest and the stator's rotating magnetic field begins to rotate, the slip is at 100%. This causes a large voltage to be induced in the rotor, which in turn causes a large influx of current.