Induction Motor

Overcurrent protects the wire, overload protects the motor.
Over load and over current protection is simple....and sometimes not. For me it is easy to think of it this way. The overload protects the motor, the overcurrent protects the wire. Because the time/current curves are very far apart from each other, don't ever think that one will adequately protect for the other.

Many electricians size the wire from the starter to the motor based on the ampacity of the overload protec ...

If BOTH bearings are not properly insulated, operating on the variable frequency drive (VFD) will cause bearing damage. This is even more pronounced with bearings that take axial thrust loads, since there's now another contact surface in addition to the "normal" radial loading. Note that the damage MAY appear on the housing, rather than the bearing - it all depends on how the energy dissipates within the bearing region.

True, some of your observed temperature rise may be a result o ...

Induction motor with squirrel cage rotor has two different speeds. The synchronous speed refers to the stator rotating magnetic field, which depends on the number of poles and frequency. The other speed is the rotor's. The rotor speed will be always slower than the stator speed, we call it slip. Without the slip, the squirrel cage induction motor doesn't have torque. Even without load the motor has slip. It increases with the load, according to the current x torque curves you can check it out ...

1. Explain why the efficiency of the induction motor increases when the mechanical load increases.
2. Explain why the input power factor of the induction motor increases when the mechanical load increases.

Explain:
1. The efficiency of the induction increases when the mechanical load increases because as the motors load increases, its slip increases, and the rotor speed falls. Since the rotor speed is slower, there is more relative motion between the rotor and the stator ...

Dealing with the change in power factor. A 10HP (7.5kW), 400v, IE2, B3, Induction motor at full load will have a power factor of 0.86. At 75% power the power factor is 0.81 and at 50% it is 0.71. Assuming full load then your 10HP is 7.5kW at power factor 0.86 has a reactive power of 4.45kVAr. If the load drops to 75% and the capacitors are still in circuit, the machine only requires 4.07kVAr so it is now 0.38kVAr leading or +0.99. at 50%, 3.82kVAr leading or +0.98.

Dealing with the ...

Two phase motor works the same way as the single phase one. The only difference is in the voltage level applied. As with two phase motors voltage lever is higher the design must be adjusted so the capacitor voltage does not exceed its limit.

The question is why would anyone want to have two phase motor if all 3 phases are available and the far more reliable 3 phase motor can be used?

There is only one advantage of two phase (or call it single phase) motor. The direction ...

Q: What size feeder cable and overcurrent device and overload protection and disconnect switch are required for two single phase motors, 10 hp, 230V, if the terminals are rated for 75C?

The cable shall have an ampacity of not be less than 125% of the motor's rated full load current. Then you need to derate the cable for factors like ambient temperature and number of conductors in the tray or conduit, underground installation, and voltage drop.

The overload rating depend ...

Every motor type (synchronous, squirrel cage and wound rotor induction, direct current, permanent magnet, linear, switched reluctance, etc.) all have a "sweet spot" in terms of efficiency. Some are designed to work well at low- to moderate-power ratings and high speed; others are best suited for high-power and slow speeds.

Most low-power motors are designed to operate against fairly simple load torque profiles; this means they can be optimized quite readily because there isn't much ...

Allowable current density is not a random choice. It is directly related to the temperature of the conductor and is dependent on the insulation (on the individual strand, on the turn, and on the completed coil), the method and efficiency of the cooling, the heat transfer properties of the surrounding magnetic materials, and the nature of the current itself (AC or DC).

As a really rough approximation, a current density of 3000 ampere / mm^2 equates to something like a temperature ri ...

In a single phase AC motor (at starting) there is only one pulsating field. For the sake of analysis only it could be represented by two rotating fields of equal magnitudes and rotating in opposite directions. If the rotor of this single phase motor is moved by any means, the stator field is still a pulsating field but the rotor induced currents will produce a rotating field and for the sake of analysis this rotating field is added to the component of the stator field rotating in the same dir ...

When using a standard DOL (direct on line starter), star delta starter or even n liquid starter standard bearings in the motor is not a problem. When using a Soft Starter or VFD it is recommended that bearings are insulated. This is to prevent rotor/bearing racking which could result in a direct short. But in the event where a none insulated motor is available it could be used as a short term option. Unfortuna ...

Motors are sized based on the application requirements.
High starting torque (required for a high inertia load) usually translates into light loading at rated speed and voltage. The driving factor is NOT the efficiency under the loaded condition - it is the ability to start the process in the first place.

Environmental conditions also affect design choices - high ambient temperatures, high altitudes, and presence of hazardous (e.g. combustible or flammable) materials will nece ...

In a star connected motor, if the voltages are equal, and if the impedance of all the phases is equal, the star point will have a zero potential, and it won't matter if you connect it to earth. However, in case of unequal voltages or unequal impendences, there will be some current flowing continuously through the earth conductor connected to the star point, which is not a desirable condition.

There is a world of difference between a 3 Phase motor and a transformer. For a start tge ...

If there is no name plate, you won't be able to know the rated voltage too. In that case, you may have to guess the approx. HP depending upon the size of the motor, select appropriate auto transformer and then run at no load by slowly increasing the voltage till it reaches steady state. I am however still not sure if this will give you the correct rated voltage. There are standard ratings of three-phase 380V, 415V & 440V. I do not know if differentiation between these is possible, because ...

If the rotor of 3 phase induction motor is locked and stator is free to rotate, then would stator rotate? If yes then in which direction?

In normal conditions, it cannot happen. If the load on the shaft of an induction motor would be such that the motor cannot start, and if there would be no protection of the induction motor, then the stator winding would just burn down.

BUT if the induction motor would run at full (rated) speed and if the rotor would be blocked instant ...

There are two very basic ways of determining the correct point at which the control gear should change the winding configuration from 'star' (starting mode) to 'delta' (running mode). Both methods determine the point at which the rotor has achieved it's maximum speed in star, and therefore the point at which the starter should change to delta.

One is by current measurement: Firstly, set the delta timer to it's maximum setting. Put a clip on (tongue tester) ammeter on one of the li ...

The simplest means to adjust the speed of a (squirrel cage) induction motor is to vary the load applied. Since the rotor is not separately powered, adding load will reduce the shaft speed (i.e. slow the motor down). Removing load will increase the shaft speed. The upper limit is the slip speed (which is slightly below synchronous speed) for the applied frequency; the lower limit is zero speed or "stall" condition.

This does not mean the load adjustment is the best answer in terms ...

1) Continuous operation in the 75-100 percent of nameplate power rating will generally deliver optimum efficiency. However, too much "extra' HP will result in a lower power factor (since the machine is running at less than nameplate loading), which will adversely affect power consumption and utility costs in the long run. As David mentioned - having a bit more than the nameplate value can be a good thing when process changes (such as colder-than-estimated gases) occur. Of course, the best thi ...

In an AC machine that undergoes an inductive type of start - like a squirrel cage induction or synchronous machine started "across-the-line" - the deciding factor on how often a machine can be started in succession is (almost always) the temperature reached by the bars and shorting end rings on the motor rotor. More specifically, it is the brazed joint between the two - because the braze material will "flow" at a lower temperature than the bar or ring material will deform or change properties ...

As we know that the laminations are insulated from each other to prevent the eddy current circulations between the adjacent laminations. Once lamination staking done, how we are gonna earth the core of any transformer or motor? For a large 11KV motor, I have seen a guide bar welded to the laminations at the core back side & that is grounded to body but that eventually lead to the shorting of insulations at one point.

For motors there is typically a welded bar on the back or the ...