Electrical characteristics of the motor//
Electrical characteristics like voltage, frequency and phase of power supply should be consistent with the motor nameplate rating. A motor will operate satisfactorily on voltage within 10% of nameplate value, or frequency within 5%, with combined voltage and frequency variation not to exceed 10%.
Common 60 hz voltages for single-phase motors are 115 volt, 230 volt, and 115/230 volt. Common 60 hz voltage for three-phase motors are 230 volt, 460 volt and 230/460 volt. Two hundred volt and 575 volt motors are sometimes encountered.
In prior NEMA standards these voltages were listed as 208 or 220/440 or 550 volts.
Motors with these voltages on the nameplate can safely be replaced by motors having the current standard markings of 200 or 208, 230/460 or 575 volts, respectively.
Motors rated 115/208-230 volt and 208-230/460 volt, in most cases, will operate satisfactorily at 208 volts, but the torque will be 20% – 25% lower. Operating below 208 volts may require a 208 volt (or 200 volt) motor or the use of the next higher horsepower, standard voltage motor.
Industrial motor nameplate (photo credit: INYOPools.com)
Single-phase motors account for up to 80% of the motors used in the United States but are used mostly in homes and in auxiliary low-horse power industrial applications such as fans and on farms.
Three-phase motors are generally used on larger commercial and industrial equipment.
- Current (Amps)
In comparing motor types, the full load amps and/or service factor amps are key parameters for determining the proper loading on the motor. For example, never replace a PSC type motor (Permanent Split Capacitor) with a shaded pole type as the latter’s amps will normally be 50% – 60% higher.
Compare PSC with PSC, capacitor start with capacitor start, and so forth.
- Hertz / Frequency
In North America 60 hz (cycles) is the common power source. However, most of the rest of the world is supplied with 50 hz power.
Exactly 746 watts of electrical power will produce 1 HP if a motor could operate at 100% efficiency, but of course no motor is 100% efficient. A 1 HP motor operating at 84% efficiency will have a total watt consumption of 888 watts. This amounts to 746 watts of usable power and 142 watts loss due to heat, friction, etc. (888 × 0.84 = 746 = 1 HP).
Horsepower can also be calculated if torque is known, using one of these formulas:
The approximate RPM at rated load for small and medium motors operating at 60 hz and 50 hz at rated volts are as follows:
- Insulation Class
Insulation systems are rated by standard NEMA classifications according to maximum allowable operating temperatures. They are as follows:
* Motor temperature rise plus maximum ambient
Generally, replace a motor with one having an equal or higher insulation class. Replacement with one of lower temperature rating could result in premature failure of the motor. Each 10°C rise above these ratings can reduce the motor’s service life by one half.
- Service Factor
The service factor (SF) is a measure of continuous overload capacity at which a motor can operate without overload or damage, provided the other design parameters such as rated voltage, frequency and ambient temperature are within norms.
Example: A 3/4 HP motor with a 1.15 SF can operate at 0.86 HP, (0.75 HP × 1.15 = 0.862 HP) without overheating or otherwise damaging the motor if rated voltage and frequency are supplied at the motor’s leads. Some motors have higher service factors than the NEMA standard.
It is not uncommon for the original equipment manufacturer (OEM) to load the motor to its maximum load capability (service factor). For this reason, do not replace a motor with one of the same nameplate horsepower but with a lower service factor.
Always make certain that the replacement motor has a maximum HP rating (rated HP x SF) equal to or higher than that which it replaces. Multiply the horsepower by the service factor for maximum potential loading.
For easy reference, standard NEMA service factors for various horsepower motors and motor speeds are shown in this table.
The NEMA service factor for totally enclosed motors is 1.0. However, many manufacturers build TEFC with a 1.15 service factor.
Capacitors are used on all fractional HP induction motors except shaded-pole, split-phase and polyphase. Start capacitors are designed to stay in circuit a very short time (3-5 seconds), while run capacitors are permanently in circuit. Capacitors are rated by capacity and voltage.
Never use a capacitor with a voltage less than that recommended with the replacement motor! A higher voltage is acceptable.
A motor’s efficiency is a measurement of useful work produced by the motor versus the energy it consumes (heat and friction). An 84% efficient motor with a total watt draw of 400W produces 336 watts of useful energy (400 × 0.84 = 336W).
The 64 watts lost (400 – 336 = 64W) becomes heat.
Encoders are devices that translate a signal, whether motion into position or velocity feedback for a motion control system. Take a conveyor system as an application. You want to run the conveyor at 100 feet per minute. The motor that powers this conveyor has an encoder mounted to its shaft.
Output from the encoder goes into the controller and as long as the output signal is telling the controller that everything is fine – the motor is running at the correct speed – it continues running at the current speed.
If the load on the conveyor changes, like it is being overloaded due to additional weight of product added to the conveyor, the controller should notice a change in pulses from the encoder, for the speed of the conveyor slows down from this additional weight, and the controller will send a signal to the motor to speed up to compensate for this load change.
Once the load has been returned to the standard expected load, the control will again see a signal from the encoder and will slow the motor down to the needed speed.
There are two main types of Encoders, Rotary and Linear and each type can use different sensing technologies. They include Optical, Magnetic or Inductive. Optical Rotary encoders are the most common type used.
- Thermal Protection (Overload)
A thermal protector, automatic or manual, mounted in the end frame or on a winding, is designed to prevent a motor from getting too hot, causing possible fire or damage to the motor. Protectors are generally current and temperature sensitive. Some motors have no inherent protector, but they should have protection provided in the overall system’s design for safety.
Never bypass a protector because of nuisance tripping! This is generally an indication of some other problem, such as overloading or lack of proper ventilation.
Never replace nor choose an automatic-reset thermal overload protected motor for an application where the driven load could cause personal injury if the motor should restart unexpectedly. Only manual-reset thermal overloads should be used in such applications.
Basic types of overload protectors include:
- Automatic Reset: After the motor cools, this line-interrupting protector automatically restores power. It should not be used where unexpected restarting would be hazardous.
- Manual Reset: This line-interrupting protector has an external button that must be pushed to restore power to the motor. Use where unexpected restarting would be hazardous, as on saws, conveyors, compressors and other machinery.
- Resistance Temperature Detectors: Precision-calibrated resistors are mounted in the motor and are used in conjunction with an instrument supplied by the customer to detect high temperatures.
- Shaft Grounding Devices
Shaft grounding is recommended (NEMA MG1 188.8.131.52) as an effective means of bearing protection for motors operated from inverter power. Shaft voltage occurs in motors powered by variable speed drives (VSDs). These VSDs induce shaft voltages onto the shaft of the driven motor because of the extremely high speed switching of the insulated gate bipolar transistors (IGBTs) which produce the pulse width modulation used to control AC motors.
The presence of high frequency ground currents can cause sparks, arcing and electrical shocks and can damage bearings.
One grounding device is adequate to bleed down inverter-sourced shaft voltages, thereby protecting both bearings for motors as large as 6085 frame.
There are four common techniques that can minimise or eliminate this bearing damage caused by these ground currents:
- Faraday shield,
- Insulated bearings or ceramic bearings,
- Ground brush or
- Grounding ring
Shielding the cable or wire between the motor and the VSD can also significantly improve these spikes as well.
13.1 Faraday Shield
An electrostatic shielded induction motor (ESIM) is one approach to the shaft-voltage problem, as the insulation reduces voltage levels below the dielectric breakdown.
This effectively stops bearing degradation and offers one solution to accelerated bearing wear caused by fluting, induced by VSDs.
13.2 Grounding Brush
Grounding the shaft by installing a grounding device provides an alternate low-impedance path from the motor shaft to the motor case. This channels the current away from the bearings.
It significantly reduces shaft voltage, and therefore bearing current, by not allowing voltage to build up on the rotor.
13.3 Shaft Grounding Ring
A shaft grounding ring (SGR) is similar to a grounding brush, except that this brush makes use of conductive micro fibers, creating a low impedance path from the motor.
13.4 Insulated Bearings
Insulated or ceramic bearings eliminate the path to ground through the bearing for current to flow.
- Torque-speed Characteristics of Motors
The amount of torque produced by a motor generally varies with speed. This Torque-Speed characteristic depends on the type and design of a motor, and is often shown on a Torque-Speed graph.
Figure 2 – Typical Torque-Speed Graph
Some important factors indicated by the graph include:
- Starting torque – the torque produced at zero speed
- Pull-up torque – the minimum torque produced during acceleration from standstill to operating speed
- Breakdown torque – the maximum torque that the motor can produce before stalling
Source Reference: Electrical Engineering Portal (EEP)