Frequently Asked Questions

Subfractional, Synchronous, and Stepper Motors

1. How is torque measured on small gear motors?
2. How is torque measured on motors without gear units?
3. What do the terms pull-in, pull-out, full-load, break-down and locked-rotor mean concerning torque?
4. How do variations in voltage affect motor torque?
5. What is the difference between motor torque ratings and gearing torque ratings?
6. How do various types of loads affect the gear torque rating?
7. What is meant by the temperature rise of a motor and how does it apply?
8. What is the maximum temperature at which standard motors can be operated?
9. What is the minimum operating temperature of a standard motor?
10. What does "Impedance Protected" mean?
11. Can motors designed for 60 Hz be operated on 50 Hz?
12. Can synchronous motors be operated with a variable frequency controller to provide speed control?
13. Are there other methods of obtaining variable output speeds?
14. What are the characteristics of the hysteresis synchronous motors?
15. How can the speed of a synchronous motor be checked to verify synchronous operation under load?
16. What is a reluctance synchronous motor?
17. Where do permanent magnet synchronous motors fit in?
18. How does the high-slip induction motor differ from the normal-slip?
19. What is a stepper motor?
20. What is a permanent magnet DC motor?

Q - How is torque measured on small gear motors?

A - The simplest and most direct method is to secure a pulley to the output shaft which winds up a cord with a weight attached. The torque developed is the product of the weight and the radius of the pulley and is usually expressed in ounce-inches.

Back to Top

Q - How is torque measured on motors without gear units?

A - Due to the higher shaft speeds, the weight and pulley method is impractical and either a version of the pony brake or a dynamometer must be used to measure torque.

Back to Top

Q - What do the terms pull-in, pull-out, full-load, break-down and locked-rotor mean concerning torque?

A - Pull-in torque is the maximum torque at which a synchronous motor can accelerate its load to synchronous speed. Pullout torque is the maximum torque that a synchronous motor can develop and still maintain synchronous speed. Full-load torque is the torque developed by a non-synchronous motor at its rated full-load speed. Break-down torque is the maximum torque that the motor can develop. With a synchronous motor, this will be at a speed slower than synchronous speed and with a non-synchronous motor this will be at a speed slower than the full-load speed. Locked-rotor or starting torque is the torque developed with the rotor at standstill.

Back to Top

Q - How do variations in voltage affect motor torque?

A - Torque will vary with changes in voltage. Torque ratings are given at rated voltage.

Back to Top

Q - What is the difference between motor torque ratings and gearing torque ratings?

A - The motor torque rating is the torque that the motor can produce at the output shaft of a particular gear reduction. At very high reductions (low output speeds), the output torque can reach very high values exceeding the strength of the gearing. For this reason, a maximum torque value (gear torque rating) must be specified to prevent gearing damage.

Back to Top

Q - How do various types of loads affect the gear torque rating?

A - Gear torque ratings are based upon a uniform steady torque load. Dynamic or shock loads impose stresses which can exceed gearing strength or cause early fatigue failure. To a certain extent, this can be compensated by reducing the gear torque rating (see Service Factor Table). However, dynamic braking with inertial loads or locking of the output shaft can force the gearing to absorb destructive amounts of kinetic energy stored in the momentum of the load or the rotor with immediate or rapid failure.

Back to Top

Q - What is meant by the temperature rise of a motor and how does it apply?

A - It is the difference between the measured temperature of the motor winding and the temperature of the air surrounding the motor. Electrical insulation systems are limited in the degree of temperature that they can withstand. Standard motors have Class "A" insulation systems, which are rated at 105°C for the hottest-spot temperature. A hot-spot allowance must be made for the difference between the measured temperature of the winding and the actual temperature of the hottest-spot within the winding, usually 5°C to 15°C depending upon the type of motor construction. The sum of the temperature rise, the hot-spot allowance, and the temperature of the ambient must not exceed the temperature rating of the insulation.

Back to Top

Q - What is the maximum temperature at which standard motors can be operated?

A - The life of the electrical insulation and of the lubricants are adversely affected by high temperatures. A generally accepted "rule of thumb" is that for every 10°C increase in operating temperature, life is halved. NEMA standards normally limit the maximum ambient temperature to 40°C. A probable life of approximately five years under normal operating conditions 8 hours/day, 5 days/week, (ambient temperature below 40°C, and uniform rated load) can usually be expected. Due to the many variables, life can only be estimated before testing under actual operating conditions.

Back to Top

Q - What is the minimum operating temperature of a standard motor?

A - When a motor is started the operating temperature will be the same as the ambient temperature. Frictional losses in bearings and gearing increase as the temperature of the lubricants decrease due to the changes in lubricant viscosity. This change is progressive (although not uniform) over the entire temperature range so that there is no fixed point of demarcation. Practically, when the torque of the motor has been reduced by frictional losses to equal that required by the load, the minimum temperature has been reached.

Back to Top

Q - What does "Impedance Protected" mean?

A - Many motors in the subfractional horsepower sizes can be designed with enough impedance (opposition to AC current) in the windings to limit the locked rotor currents to values that do not cause the motor to overheat beyond a safe temperature.

Back to Top

Q - Can motors designed for 60 Hz be operated on 50 Hz?

A - Not without changes in performance. Some types are extremely sensitive to frequency change and the factory should be consulted before 50/60 Hz operation is planned.

Back to Top

Q - Can synchronous motors be operated with a variable frequency controller to provide speed control?

A - Yes, but with certain power supply requirements and only over a limited range. The factory should be consulted for recommendations.

Back to Top

Q - Are there other methods of obtaining variable output speeds?

A - If fixed ratios of synchronous speeds are required, mechanical dual speed motors (Model PA, etc.) offer a broad range of slower speeds. If speed ratios are 2 to 1, electrical pole-changing motors are available in some speeds. Where continuously variable non-synchronous speed is required, an induction or D.C. motor should be considered. Certain applications might be adaptable to the use of stepper motors as well.

Back to Top

Q - What are the characteristics of the hysteresis synchronous motors?

A - One of the outstanding features is its ability to accelerate high inertia loads to synchronous speed. Operation is smooth and quiet. The external rotor designs, demonstrate a "flywheel effect" that provides uniform angular velocity.

Back to Top

Q - How can the speed of a synchronous motor be checked to verify synchronous operation under load?

A - Observation of the rotor under a stroboscopic light source synchronized with the supply voltage will show a stationary pattern if the rotor is running at synchronous speed. A continuously moving pattern indicates that the rotor is "slipping sync," probably from loading more than pullout torque capability. An oscillating pattern indicates a variation of torque angle caused by a varying load or voltage.

Back to Top

Q - What is a reluctance synchronous motor?

A - Basically, it is a synchronous version of the more common induction motor. Synchronous operation is attained by salient poles produced by flats or notches on the rotor. Size and cost are generally lower than a hysteresis motor of comparable power, however they have limited ability to accelerate inertial loads and have more inherent vibration due to the salient poles.

Back to Top

Q - Where do permanent magnet synchronous motors fit in?

A - Recent developments in magnet materials have raised this type of motor to new levels of efficiency featuring high torque to size ratio and moderate cost. Additional features are; inherent dynamic braking, impedance overload protection, and low rotor speed for quiet operation and rapid acceleration. The motor's limited ability to start and synchronize inertial loads and its sensitivity to parameters of voltage and phasing capacitance should be noted. With a modification of the winding and the proper electronic drive circuitry, this motor type will operate as an excellent stepper motor.

Back to Top

Q - How does the high-slip induction motor differ from the normal-slip?

A - The high-slip motor has a higher resistance rotor, which provides higher starting torque and slower full-load speed. In addition, the shape of the speed vs. torque curve is much more linear which gives wider range and more stable operation where speed control is applied.

Back to Top

Q - What is a stepper motor?

A - A stepper motor could be defined as a brushless DC motor with a fixed number of steps or stable positions per revolution when driven by sequential excitation of its windings. There are several types of steppers available with the permanent magnet rotor stepper being one of the most widely used. Performance depends largely upon proper selection and matching of the electronic driver. Steppers can produce an error free ratio of rotational position to signal pulses up to a maximum speed with the ability to start, stop or reverse direction. Beyond this point (known as the slew range) the motor cannot be started stopped or reversed and still maintain pulse and step integrity. PM steppers provide inherent detent torque when not energized, an asset in many applications.

Back to Top

Q - What is a permanent magnet DC motor?

A - These motors use permanent magnets to provide field flux instead of wound field coils. Reliability is improved, efficiency is higher, and the speed-torque curve is linear. Due to the absence of the wound field however, field control is not possible with this type.

Back to Top

Service Factors For Geared Motors

Hurst Mfg. Corporation reserves the right to make changes without notice.

For additional information on these products, visit www.hurstmfg.com