TRANSFORMER vs MOTOR
- Thread starter lquadros
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Re: TRANSFORMER vs MOTOR
Even though the NEC went Metric for the 2002 edition, it uses hP whereas the metric desgination would be kW. Perhaps Charlie B can help out, he is very good at explaining this type of information.
Even though the NEC went Metric for the 2002 edition, it uses hP whereas the metric desgination would be kW. Perhaps Charlie B can help out, he is very good at explaining this type of information.
Ed MacLaren
Senior Member
Re: TRANSFORMER vs MOTOR
The power factor of the system supplied by a transformer or generator is determined by the type of load, whether it is resistive (heating, incandescent lighting) or inductive (motors).
Loads are calculated in kva when the power factor is not used in the calculation, or is not known.
When the load power factor is used in the calculation, the power is expressed in kw.
There is no way that the manufacturer of a transformer or generator can predict what type of load the purchaser will have, so they normally rate their equipment in kva.
Ed
The power factor of the system supplied by a transformer or generator is determined by the type of load, whether it is resistive (heating, incandescent lighting) or inductive (motors).
Loads are calculated in kva when the power factor is not used in the calculation, or is not known.
When the load power factor is used in the calculation, the power is expressed in kw.
There is no way that the manufacturer of a transformer or generator can predict what type of load the purchaser will have, so they normally rate their equipment in kva.
Ed
- Location
- Lockport, IL
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- Retired Electrical Engineer
Re: TRANSFORMER vs MOTOR
The KVA rating of a transformer (or a generator, for that matter) is based on the device?s ability to dissipate heat. For example, consider one set of loads that consists of 12 KW of real power and 5 KVAR of reactive power. Next, consider another set of loads that consists of 5 KW of real power and 12 KVAR of reactive power. In both cases, the total load adds up to 13 KVA. A transformer supplying the first set of loads would have to dissipate the same amount of heat as a transformer supplying the second set of loads. The first set has a power factor of 92%, and the second set has a power factor of 38%. But as Ed pointed out, the transformer is not going to care. All the transformer sees is the 13 KVA.
I believe that a motor is rated in HP or in order to allow the electrical designer to select motors to match the intended mechanical equipment. The pumps, fans, and compressors of the world are rated in HP or KW (or sometimes in BTU, but that can be converted to either HP or KW). In order to make the mechanical designers happy, we need to give them motors that have ratings at least as high as their mechanical equipment. But then we need to take one more step. In order to select the branch circuit conductors (and other stuff), we need to convert the motor HP to the actual current that the motor will draw. That requires us to take into account the motor?s electrical efficiency and its power factor. Fortunately, we don?t need to do the actual math. The NEC has tables that give us the amps, based on HP and voltage. The tables already take the efficiency and HP into account.
The KVA rating of a transformer (or a generator, for that matter) is based on the device?s ability to dissipate heat. For example, consider one set of loads that consists of 12 KW of real power and 5 KVAR of reactive power. Next, consider another set of loads that consists of 5 KW of real power and 12 KVAR of reactive power. In both cases, the total load adds up to 13 KVA. A transformer supplying the first set of loads would have to dissipate the same amount of heat as a transformer supplying the second set of loads. The first set has a power factor of 92%, and the second set has a power factor of 38%. But as Ed pointed out, the transformer is not going to care. All the transformer sees is the 13 KVA.
I believe that a motor is rated in HP or in order to allow the electrical designer to select motors to match the intended mechanical equipment. The pumps, fans, and compressors of the world are rated in HP or KW (or sometimes in BTU, but that can be converted to either HP or KW). In order to make the mechanical designers happy, we need to give them motors that have ratings at least as high as their mechanical equipment. But then we need to take one more step. In order to select the branch circuit conductors (and other stuff), we need to convert the motor HP to the actual current that the motor will draw. That requires us to take into account the motor?s electrical efficiency and its power factor. Fortunately, we don?t need to do the actual math. The NEC has tables that give us the amps, based on HP and voltage. The tables already take the efficiency and HP into account.
sc57ford
Member
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- South Carolina
Re: TRANSFORMER vs MOTOR
And the NEC motor tables are on standard effiency motors, not the premium high effiency types that are commonly specified for industrial and commercial uses. Having said that, sometimes the premiums are replaced with standards when that time comes around. Maybe one of these days standard effiency motors will be a thing of the past.
[ February 13, 2004, 02:50 PM: Message edited by: sc57ford ]
And the NEC motor tables are on standard effiency motors, not the premium high effiency types that are commonly specified for industrial and commercial uses. Having said that, sometimes the premiums are replaced with standards when that time comes around. Maybe one of these days standard effiency motors will be a thing of the past.
[ February 13, 2004, 02:50 PM: Message edited by: sc57ford ]
Re: TRANSFORMER vs MOTOR
--------"ALWAYS WONDERED ABOUT WHY TRANSFORMERS ARE RATED IN KVA AND MOTORS IN KW OR HP AND WHAT ABOUT GENERATORS"--------
The quick answer is transformers convert electrical energy to electrical energy. whereas an induction motor works very similar to a transformer by inducing voltages and currents in the rotor of the machine, and for that reason it has sometimes been called a 'rotating transformer', there are other similarities the Magnetization curves are similar the motors is less pronounced than the transformer, but the reason for the motor being rated in Horsepower is the motor converts electrical energy into Mechanical energy ergo the HP rating.
Although the Air-gap power is listed in watts the final result is calculated into Horspower.
The efficiency of the motor is determined by all the inherant loses associated with the motor itself and are similar to the transformer.
When modeling a motor or a transformer the circuits are very similar if not identical.
They both have copper losses(I^2R losses, hysteresis and eddy currentlosses) and core loses(magnetization losses) but then the motor has Rotor copper losses, Friction and windage losses and Miscellanious losses and this is the biggest difference between the transformer and the Induction motor and the efficiency of a transformer is always much greater than the induction motor.
Generators are rated KVA for the same reason as transformers they convert but the reverse of motors they convert Mechanical energy to electrical energy. but again have the same losses as the motor. And instead of a horsepower output the power input is listed as Applied Torque X the mechanical speed of the prime mover, albeit the Niagara Falls, turbine type or a diesel drive, the bottom line is power in must equal power out plus the losses inherant to the system.
WOC
--------"ALWAYS WONDERED ABOUT WHY TRANSFORMERS ARE RATED IN KVA AND MOTORS IN KW OR HP AND WHAT ABOUT GENERATORS"--------
The quick answer is transformers convert electrical energy to electrical energy. whereas an induction motor works very similar to a transformer by inducing voltages and currents in the rotor of the machine, and for that reason it has sometimes been called a 'rotating transformer', there are other similarities the Magnetization curves are similar the motors is less pronounced than the transformer, but the reason for the motor being rated in Horsepower is the motor converts electrical energy into Mechanical energy ergo the HP rating.
Although the Air-gap power is listed in watts the final result is calculated into Horspower.
The efficiency of the motor is determined by all the inherant loses associated with the motor itself and are similar to the transformer.
When modeling a motor or a transformer the circuits are very similar if not identical.
They both have copper losses(I^2R losses, hysteresis and eddy currentlosses) and core loses(magnetization losses) but then the motor has Rotor copper losses, Friction and windage losses and Miscellanious losses and this is the biggest difference between the transformer and the Induction motor and the efficiency of a transformer is always much greater than the induction motor.
Generators are rated KVA for the same reason as transformers they convert but the reverse of motors they convert Mechanical energy to electrical energy. but again have the same losses as the motor. And instead of a horsepower output the power input is listed as Applied Torque X the mechanical speed of the prime mover, albeit the Niagara Falls, turbine type or a diesel drive, the bottom line is power in must equal power out plus the losses inherant to the system.
WOC
Re: TRANSFORMER vs MOTOR
Normally rating for any equipment is the power output of that equipment.
Transformer is source and hence can not decide KW or KVAR demand of load. Hence while designing transformer, we specify KVA rating that is KW rating at unity PF - which is ultimate value of PF.
Motors are sink. They consume power both active & reactive and hence decide PF. Motors are required to give mech o/p and hence KW rating of motor is mech o/p rather than elect i/p. This is to suit mech HP/KW rating.
Generator ratings on other hand are always expressed in KVA but with specified PF.(normally 0.8 lag)
This is because while designing generator, to calculate HP/KW rating of prime mover you need to assume power requirement of load. 100 KVA generator with 0.8 PF lag can feed 80 KW & 60 KVAR. Generator will perform nicely as far as it's power traingle is intact.
[ March 04, 2004, 06:27 AM: Message edited by: Neelesh ]
Normally rating for any equipment is the power output of that equipment.
Transformer is source and hence can not decide KW or KVAR demand of load. Hence while designing transformer, we specify KVA rating that is KW rating at unity PF - which is ultimate value of PF.
Motors are sink. They consume power both active & reactive and hence decide PF. Motors are required to give mech o/p and hence KW rating of motor is mech o/p rather than elect i/p. This is to suit mech HP/KW rating.
Generator ratings on other hand are always expressed in KVA but with specified PF.(normally 0.8 lag)
This is because while designing generator, to calculate HP/KW rating of prime mover you need to assume power requirement of load. 100 KVA generator with 0.8 PF lag can feed 80 KW & 60 KVAR. Generator will perform nicely as far as it's power traingle is intact.
[ March 04, 2004, 06:27 AM: Message edited by: Neelesh ]
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