The power (kW) rating of a motor has to do with how much torque it will deliver at its rated speed. So even though the speed is the same, a 4kW motor will deliver 25% more torque than a 3kW motor. If your load only needs the torque of a 3kW motor, certainly a 4kW motor will work. But if you were using a 4kW motor, a 3kW will most likely NOT work for the obverse reason; the 3kW motor will only deliver 75% of the torque of the 4kW, so as a result the motor will slow down under load, which increases the slip, cause it to draw more current and overload. The 4kW on a 3kW load however will cost more and it will consume slightly more energy because the parasitic losses will be slightly higher. Also, as motor ratings increase, so do the physical szes. There may not be a physical difference between a 3kW and a 4kW, but there is between a 1.1kW and a 5.5kW.

The torque required from the motor is determined by the load. The only issue with fitting a larger motor is that of starting it:

• Are the incoming cables up to the job?
• Are the starter fuses up to the job?
• Is the starter wiring up to the job?
• Is the starter overload device up to the job?

Any method to check kilowatts rating for electrical motor if the nameplate missing?

Fluke makes a clip on meter that will measure volts, amps, kw and power factor. It is just what you need in your plant.

If you use 3KW starter, it may trip on no-load current on 4KW motor, but that again depends also on the load type. Better use the correct starter/ set the starter to the correct parameters.

The motor running not on full load of course will not create a failure hazard - (4KW in place of 3KW) but the underloading will have effect on efficiency and powerfactor.

Your first question is pretty much well answered.

For the second question, you can check it with the 'no load current' with similar motor. In addition, you can compare the frame size and no load power of similar motor of the same manufacturer. The no load current of the small size motors (in 3 kw range) is generally more than 50% of FLA (full load current). See the previous thread http://cr4.globalspec.com/thread/43648 and it may help.

- MS

Several fairly good answers, but none exactly right.

Since the motor can only draw current required to meet load, AND since most motors are installed with excess capacity, you should have no problem using same starter and same cabling and same fusing to support larger motor connected to 3 kW load. DO NOT change fusing (it protects the cabling) and DO NOT change overload heaters (they are matched to the "3kW" load).

Using bigger motors than required (by even 20% oversize) will definitely lower motor efficiency (a lot) and will dramatically lower power factor (increasing heating of cabling between starter and motor) so only use larger size motor in crisis until you can replace the smaller motor with right-sized motor.

Because of last paragraph- check actual on-load amperage of all your motors and compare with motor nameplate. If amperage is less than 75% of nameplate, consider changing out motor (and starter heaters) when convenient. The efficiency gain and improved power factor will pay for new motor in a few years, especially if motor is 24/7 run-time.

That is also why you should not use biggest motor everywhere (plus the added costs of larger starters, cabling, fusing, etc.). Electric is only utility where bigger is NOT lower unit cost- larger motors cost proportionally more per kW, starters are the same, and even cabling is proportionally more expensive as it gets bigger. Example- a 400 amp starter will cost about 6-7 times cost of 100 amp starter.

Best of luck.

hi...

Motor KW doesnot depends not only the rpm, the main thing to be considered is Torque. Thats why only for the same rpm also depends upon the requirement of torque there are different type of ranges.

The problem of stocking spares for multiple sized motors, overloads, wiring, energy efficiency, cost of down time etc all have an impact on what you decide is most cost effective.

In many plants they will deliberately oversize the motors by at least 15% to ensure they do not prematurely trip overloads, have a bit of head room for operation etc.

The spares issue is also a real consideration. With multiple motors of all different frame sizes, metric and US etc you can suddenly end up with a huge inventory of spares. By standardizing on a fewer number and make of motors you not only reduce your motor inventory, but reduce the number of starters, OL's, cabling, couplings, gears, motor bases etc. It also reduces the problems and choices in a breakdown situation.

Some machine I work on cost in excess of $1000 per minute of down time. Down time is charged to the department that can be blamed, so if a motor fails, the electrical department is docked the time. When your bonus or performance is based on the cost of down time attributed to your department you will do everything in your power to minimize that time. Standardization is one key factor.

though both the motor has same RPM the power delivered will be different. thats the torque of 4 kw motor will be more than 3 kw

Pls. do not worry about it.

We did the same experiment many times.

Since mechanical dimensions (eg. coupling, shoes brackets) are identical, not any electrical problem will occur.

You need 3 Kw. Your 4 Kw rated motor will draw only 3 kw's from the mains supply. And your motor also will be capable to give an extra 1 kw if needed.

Theoretically you do not need to change any setpoint in your starter system, because your mechanic mass, and therefore the load is the same. However first you must check it for a few motor starts. There might be some unseen parameters, in connection with the construction of the new motor, should be taken into consideration. Then you can make some minor changes for thermal or magnetic overload.

Coming to non labelled motors, there is no practical way to determine the power rating for them. However you can obtain some estimations, by electrically comparing them. Very similar to the short circuit test, achieved for the transformers, of which the secondary windings are shorted.

As well as you know the induction motors and the transformers are working with the same principle. Motor windings can be considered as the primary circuit of a transformer. The only difference between then is the secondary windings. Secondary windings in motors are one loop shorted as squirrel cage.

Now what you will do is obtain a powerful transformer such as 1 KVA. 220/42 Volts. (Of course single phase)

Connect the 220 side to the mains supply. 42 side to any coil of the unknown motor. Such as U-X or V-Y or W-Z. It is obvious the motor will not turn with this 42 Volt single phase, but only will hum. (makes Hummm sound) . At that point please measure the current (Amperage) at the 42 Volt circuit, by using a clamp tester. Read it and write somewhere. (In fact you have measured the inductance of the coil)

Now make the same experiments with the known motors and make a list.

Since the RPM speeds are the same, The similar currents are pointing the similar rated motors.

Kindest Regards

the diffrent motor size simply to match the tourque of the equipment fitted with the motor .

for the 4 kw motor inpleace of 3kw you have to check the rate of the circuit(contactor,overload,cable size) if suitable no problem but it will consume more power.

the way you explain to check the motor kw is workable.

The easier method will be to guess based on frame size, which can be decided by measuring and then comparing with the standard frames.

Many good even better answer, thanks guys for all this useful info. i'll try it and share it together.