Motors are key to any mechanical system working correctly and are always a major component of any TAB job or Commissioning project. So, it is important that the technician understands the basics of motor operation and control so that if any installation deviates from accepted standard practices the technician will know how to correctly respond.
Let’s review the basic characteristics of electric motors and the conditions that affect their correct operation.
1. Motor Types
HVAC single-phase motors are normally Permanent Split Capacitor, Split Phase, Shaded Pole motors, or Electrically Commuted Motors (ECM).
The Permanent Split Capacitor motor (PSC) also has a cage rotor and the two windings similar to that of a Capacitor Start Motor but the capacitor is always in the circuit and does not contain any starting switch. The auxiliary winding is always in the circuit and the motor operates as a balanced two-phase motor. The motor produces a uniform torque and has noise-free operation.
A Split-Phase motor has a secondary startup winding that is 90 electrical degrees to the main winding, always centered directly between the poles of the main winding, and connected to the main winding by a set of electrical contacts. The position of the winding creates a small phase shift between the flux of the main winding and the flux of the starting winding, causing the rotor to rotate. When the speed of the motor is sufficient to overcome the inertia of the load, the contacts are opened automatically by a centrifugal switch or electric relay. The direction of rotation is determined by the connection between the main winding and the start circuit.
Shaded-pole motors are used in devices requiring low starting torque, such as electric fans, small pumps, or small household appliances. In this motor, small single-turn copper “shading coils” create the moving magnetic field. Part of each pole is encircled by a copper coil or strap; the induced current in the strap opposes the change of flux through the coil. This causes a time lag in the flux passing through the shading coil, so that the maximum field intensity moves higher across the pole face on each cycle. This produces a low-level rotating magnetic field which is large enough to turn both the rotor and its attached load. As the rotor picks up speed, the torque builds up to its full level as the principal magnetic field is rotating relative to the rotating rotor.
A capacitor start motor is a split-phase induction motor with a starting capacitor inserted in series with the startup winding, creating a circuit that produces a greater phase shift (and so, a much greater starting torque) than both split-phase and shaded pole motors.
The Electronically Commutated Motor (ECM) are single-phase motors that are electronically controlled and are equipped with integral electronic controllers that know when to turn on the motor or when to apply more or less power. The controller allows the motor to receive feedback from its environment or external controls. ECM motors can provide variable speed operation. ECM motors are 70% to 83% more efficient than shaded pole or PSC motors.
HVAC 3-phase motors are normally Squirrel Cage Induction Motors which have windings for each phase that induce current from the stator coils into the rotor coils causing the rotor to turn. Direction is determined by how the stator phase order is wired.
2. Voltage
In the United States (US), most of the Americas and some parts of Asia, the 60Hz voltages are used while most of the rest of the world uses 50Hz voltages. There is no easy explanation why the US uses 120V for receptacle power and the rest of the world uses 220 volts except transformers and motors are less expensive for 60 cycles rather than 50 cycles but transmission is easier at 50 cycles.
There is not really a direct connection between frequency and delivered power since power is dictated by the formula P=E x I or Watts=Volts x Amps. When dealing with alternating currents, power is affected by circuit impedance and reactance caused by circuit resistance and motor slip. This gets really complicated and leads to very little usable information for us less than physicist types.
My easiest explanation is that if the integral of the sign wave, which is the area under the curves is the same for both a 60Hz and a 50Hz circuits, power will be the same, if they are not then one will have to change the voltage to keep the power equal. (Store this away for when we talk about VFD’s)
For motors in the US prior to 1965 NEMA standards used 110, 208, 220, 440 and 550 volts which are 10% above the lower operating limits. Since 1965 NEMA has used 115, 208, 230, 460 and 575V, but sometimes you will see 120, 208, 240, and 460 volts which are at the top of the operating limits. Single-phase motors are normally 115, 208, or 230 volts, and three-phase motors are 208, 230, or 460 volts.
Of course, there are also higher voltage motors but are not common on HVAC projects. It is important to understand that multiple voltage motors operate at different torque ratings for each voltage; the issue is you must be careful when using a multi-voltage three-phase motor at 208 volts, you may have to use a true 200 volts motor or use a larger multi-voltage motor at 208 volts to achieve the needed torque.
3. Voltage and Temperature
Motor damage can occur when the utilization voltage is significantly different than the voltage for which a device is rated. Overvoltage is a condition that, per the listed standards, begins with a voltage 10 percent above the rated motor voltage. Higher output or efficiency cannot be achieved by supplying a higher than nominal voltage to the motor. The motor will convert this extra energy into heat instead of usable output such as torque. The heat accelerates the degradation of the insulation and bearing systems.
While overvoltage can degrade equipment, constant undervoltage does more harm and greatly impacts equipment performance and reliability. To drive a load, a motor must have enough power to overcome the torque required by that load. Motor power is calculated by voltage x current, otherwise known as Ohm’s Law. So, if voltage drops, the current must increase to maintain enough power to transfer the required torque to the driven load. This increase in current causes excess heat to be generated which, over time, can lead to premature motor failure like that seen in overvoltage.
A good rule of thumb, per NEMA, is that for every 10-degree rise in motor temperature, motor life is reduced by half. Motor insulation is rated by standard NEMA classifications according to maximum allowable operating temperatures. 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.
4. Speed
Motor speeds are dictated by the number of poles for each phase in a complete rotation of the rotor. A two-pole, three-phase motor actually has 6 poles or 3 sets of poles at 120 degrees apart. This creates a rotating field at the same frequency as the AC power, if operating on 60Hz, the field rotates around the motor 60 times per second. The synchronous speed (no slip, so no current induced in rotor so no torque) of a 60-cycle motor would be 3,600 rpm. A 4-pole, 60 cycle single phase motor has 4 poles and a synchronous speed of 1,800 rpm.
But in order to create initial torque to start the motor, the single-phase motor must have another set of windings with lower inductance creating poles in between the stator windings that are slightly out of phase so as to create a rotating field at startup. These auxiliary starter windings are not counted as poles.
The equation for synchronous speed is Speed = (Frequency x 120) / Poles
Please note the actual speed is the results of the motor losses subtracted from the synchronous speed which include friction losses (Bearings) and motor slip from the total torque. The difference between the synchronous speed of the electric motor magnetic field, and the shaft rotating speed is slip – measured in RPM or frequency. This value also represents the efficiency loss of the motor; thus, each efficiency grade of motor will have a different slip or actual speed. That’s why you may see nameplate data of 1,780, 1,750, 1,740 or 1,725 for an 1,800 RPM motor.
5. 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.
6. Horsepower & Torque
Torque is equal to the force applied, its distance from the axis of rotation (radius), and the angle (θ) at which the force is applied and is derived by the formula: t = f (r sin θ) Torque indicates how much work is performed, but it shows nothing about how quickly that work is completed.
Power is the rate at which torque (work) is performed over time and is derived by the formula: p = t/time
Horsepower is related to torque by the equation: HP = Torque ((Lb/Ft) x RPM) / 5,250
Based on this relationship, torque must double if HP is to remain constant when speed is reduced by half. To produce the same HP at the lower speed, a motor has to do twice as much work per rotation, which requires twice as much torque. That is why the shaft and frame of a 900-rpm motor are usually larger than those of an 1,800-rpm motor of the same HP. 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.
7. Variable Speed with a VFD
When the speed of an AC motor is controlled by a VFD, HP or torque will change depending on the change in frequency. Figure 2 provides a graphical illustration of these changes. The X-axis is motor speed from 0 to 120 hertz. The Y-axis is the percent of HP and torque. At 60 hertz (base motor speed), both HP and torque are at 100 percent. When the VFD reduces frequency and motor speed, it also reduces voltage to keep the volts/hertz ratio constant. Torque remains at 100 percent, but HP is reduced in direct proportion to the change in speed.
At 30 hertz, the HP is just 50 percent of the 60-hertz HP. The reason this occurs is because the total torque produced per unit of time is also reduced by 50 percent because of fewer motor rotations. You can use the HP and torque equations to verify this relationship. When a VFD increases frequency above 60 hertz, HP and torque do a complete flip flop. HP remains at 100 percent, and torque decreases as frequency increases. The torque reduction occurs because motor impedance increases with increasing frequency. Since a VFD cannot increase the voltage above its supply voltage, the current decreases as frequency increases, decreasing the available torque.
Theoretically, torque is reduced by the ratio of the base speed to the higher speed (60 hertz / 90 hertz = 67 percent). In real applications, other factors can reduce the actual available torque well below the theoretical values shown in Figure 2. These include increased bearing friction, increased fan loading, and additional rotor windage. A motor’s full-load torque must be derated when operated at speeds above 60 hertz.
Typical manufacturers’ derating guidelines suggest using the base frequency to maximum frequency ratio for speeds up to 90 hertz. At speeds above 90 hertz, the square of the ratio is often used. What this means is as you speed up a motor above 60 HZ the motor torque goes down, motor horsepower stays the same but the load of the fan or pump and friction is actually going up and will require more torque or horsepower to operate.
There are other concerns as well for overspeeding a motor:
- Standard motor bearings are rated for a maximum 3,600 RPM so unless your motor has magnetic bearings or special high-speed bearings do not speed up a 3,600 RPM motor.
- Overspeed motors may have rotor dynamic balance issues or critical speed vibrations that did not appear at or below 60 HZ.
- Operating at overspeed RPM’s will reduce the life of the motor bearings, the driven machine bearings, drive belts, and increase maintenance of the system.
- Operating above 60 HZ speed will increase static electrical discharge on the motor shaft
8. VFD Damage to Motor Bearings
VFD’s induce shaft voltages on the motor shaft caused by the extreme voltage spikes from the insulated gate bipolar transistors (IGBTs) which produce the pulse width modulation used to control AC motor. The higher the carrier frequency (1,250 to 15,000 cps) of the drive the more voltage is induced to the shaft. Eventually the voltage buildup on the shaft exceeds the dielectric properties of the bearing grease and arcs across the bearing, micro machining the bearing race. Eventually causing the bearing to fail, this happens much faster if the motor speed is not varied very often. I have seen new motors fail within three months from this issue.
No matter what your drive vendor tells you this cannot be mitigated by any drive feature; it must be corrected at the motor. There are four common techniques that can minimize or eliminate this bearing damage caused by these ground currents and every VFD controlled motor should have one of the following devices to protect the motor:
- Faraday shield is an electrostatic shield built inside the induction motor (ESIM) which reduces voltage levels on the shaft below the dielectric breakdown.
- Insulated bearings or ceramic bearings. Insulated or ceramic bearings eliminate the path to ground through the bearing for current to flow.
- Shaft Ground Ring. A shaft grounding ring (SGR) uses a conductive microfiber brush, creating a low-impedance path from the motor shaft.
- High Dielectric Bearing Grease. Though not an actual solution to the problem of high voltage on the motor shaft, high dielectric grease can help prevent serious damage from the voltage build-up if it is not significantly high or as an interim repair until other solutions can be implemented.
Hope this information helps some of you when dealing with motor issues in the field.
