Note: Many of the generalities that are made for brush-type motors can be applied to brushless motors. However, as brushless motors are electronically commutated and are powered by driver electronics, the following sections dealing with brush commutator systems and torque-speed curves do not apply. There is a separate section for brushless motors here.
Q: Can your motors be driven at something other than their nominal voltage? A. Yes. In fact, if you can design your device to run the motor slower (lower than nominal voltage) this is a very good thing. Running at lower voltages (and therefore lower speeds) means less brush bounce and less brush/commutator wear for brush type motors, lower current consumption, and longer motor life. On the other hand, if size restrictions and performance requirements demand additional torque and/or speed, overdriving the motor is possible. You must, however, be willing to sacrifice product lifetime if you overdrive the motor.
Q: Why are the gearheads not correctly matched to the motor from a torque standpoint? A: If the maximum continuous torque produced by the motor through the gearhead is considered for each gearhead ratio, many ratios would far exceed the gearhead torque rating. If we were to design each gearhead to withstand the full torque produced by the combination, the gearhead's internal gears would have to be modified dramatically (larger face width, larger pitch diameter, different material, etc.). All these would contribute to a much larger and more expensive product that defeats the intent of having the "power and performance in the smallest package."
Q: Are all of your gearheads reversible? A: Yes.
Q: How long do your motors and gearheads last? A: This varies according to each application. Factors such as operating environment, duty cycle, input power, and how the motor or gear motor is coupled to the load all directly affect product life. Mechanical design factors of the overall mechanism, such as running the motor into hard stops or back-driving the gearheads, affect product lifetime. Generally speaking, brush type motors can run for several thousands hundreds of hours when run at nominal conditions. If long lifetime is one of your design criteria, you should consider using brushless motors. These motors are typically limited in their life only by ball bearing wear. If you have detailed questions on this point, it would be best to call one of our Application Engineers, toll-free from the USA or Canada, at 1-800-807-9166.
Q: What is the maximum continuous current the motor can be exposed to? A: This can be calculated from the specifications shown on the motor data sheet. Here's how: Maximum rotor temperature - Ambient temperature = Allowable temperature rise Allowable temperature rise divided by thermal resistances (add up rotor-to-case and case-to-ambient) = Continuous power that can be dissipated in Watts. Set this power = to the current squared x armature resistance. P = I x I x R Solve for I
Q: What's the difference between a "servo" motor and a regular motor? A: The term "servo" implies that there is a feedback loop which adjusts one or more operating parameters of the motor such as motion, velocity, position, and/or torque. Servomotors are used in closed loop systems where accuracy and repeatability are important. "Regular" motors (without feedback) are run "open loop" where positional accuracy is not an important factor.
Q: Can you assemble a motor I want with a specific gearhead I want? What about adding cables and terminations? A: Yes. Micro-Drives has a Class 100,000 clean room that is used for motor and gearhead assembly, cable making, custom circuit board assembly, special soldering operations, and other value-added processes. If you have a special requirement, call toll-free 1-800-807-9166 for additional capabilities information and help with your design.
Q: Can you fax me a speed-torque curve for one of your motors? A: Yes. We can do that.
Q: I notice there are two shaft ends on this motor. Can I get only one? A: In most cases yes. You can select almost all of our motors (both brush, and brushless with either a single output shaft or a thru- (double) shaft. Call one of our Sales Associates toll-free at 1-800-807-9166 if you want specific information on pricing and product availability.
Q: Is it possible to have a flat on this shaft? A: Yes. You can specify where you want a flat on a shaft. You can also specify if you want a hole through the shaft, a pinion or pulley attached to the shaft, or a special sized shaft. For some motor types, thru- (hollow) shafts are also offered.
Q: Can I get feedback devices and other components put on our motor? A: Yes. Micro-Drives brand products are designed to accommodate a large variety of supplemental devices. Some of these are spur, planetary and hybrid gear heads; power-on and power-off brakes; DC tachometers; optical and magnetic encoders; and resolvers. Call Applications Engineering toll-free at 1-800-807-9166 for more detailed information or to review your design.
Q: Can your motors be used as generators? A: Yes. Most Permanent Magnet brush type DC motors can be used as generators. However, they put out only small voltages. If you are looking for something like a vehicle generator or alternator, or something to power a household appliance, you need to contact a company which specializes in these types of higher output devices.
Q: How do I know whether to use a planetary or spur gearhead? A: Planetary gearheads are typically used when high torques are needed in a limited space. Spur gear systems are used where low current consumption, low noise, and high efficiencies are needed. The negative tradeoffs for using planetary gearboxes are higher current consumption, lower effiency, and higher audible noise.
Q: How can I calculate the final operating conditions (current, speed, etc.) for a motor+gearhead combination given the torque load at the output shaft of the gearhead? A: To give you a generalized example, assume that the motor+gearhead combination M2232U12GS000+M22P24YS000 24:1 is being used with 12 Volts applied to the motor terminals, and that a torque of 10 oz-in is desired at the output shaft of the gearhead.
Gearhead M22P24YS000 with a 24:1 gear ratio has a data sheet efficiency value of 70%. This means that 30% of the power developed by the motor will be lost in the gearhead. The simplest method of accounting for gearhead losses is to increase the torque requirement by the appropriate amount and make the calculations as if the gearhead were 100% efficient. In this case, we increase the torque requirement at the gearhead output by 30% resulting in a torque (for calculation purposes) of 13 oz-in.
Total torque = 10 oz-in x 1.3 = 13 oz-in.
The torque reflected back to the motor is then simply the total torque divided by the gear ratio:
Motor torque = 13 oz-in / 24 = .542 oz-in.
The motor torque constant is the proportionality constant which defines the relationship between the torque at the motor shaft and the current in the motor windings. In this case, the torque constant for the motor M2232U12GS000 is 1.86 oz-in / Amp. That is, for every 1 Amp in the motor windings, the motor will produce 1.86 oz-in of torque. The reciprocal of the motor constant in this case is .538 Amp / oz-in. Since we have already calculated the torque at the motor shaft to be .542 oz-in, we can use the reciprocal of the torque constant to calculate the motor current due to the external load:
Current = (.538 Amp / oz-in) x .542 oz-in = .292 A = 292 mA
The motor has a small amount of internal friction which requires a proportionate amount of current to drive it. This current is defined as the motor no-load current. In this case, the value is 60 mA (taken from the data sheet). Since the motor requires 292 mA to drive the external load and 60 mA to drive its own internal friction, the total current required for this application would be .352 mA.
The speed of a DC motor is a linear function of the load which it is driving. The proportionality constant relating motor speed to the motor torque load is the slope of the torque versus speed curve. This slope is calculated by dividing the listed no-load speed (actually a negative value) of the motor (maximum speed and zero external load) by the stall torque (zero speed and maximum torque). In the case of motor M2232U12GS000, the slope of the torque versus speed curve is given by the following:
Note that the slope of the line is a negative value, indicating that the speed losses will be greater with increasing motor load. In this case, we calculated a motor load of .302 oz-in. Therefore, the motor speed loss due to this external torque load will be:
Speed loss = (-2710 rpm / oz-in) x .542 oz-in = -1469 rpm
With no load on the motor shaft, the motor speed will be 8400 rpm. With a load of .302 oz-in, the motor will lose 1469 rpm from the no-load value. Therefore, in this application the motor speed is rendered by:
Motor speed = 8400 rpm - 1469 rpm = 6931 rpm
The speed of the motor at the output shaft of the gearhead under load is simply the motor speed divided by the gear ratio. In this case:
Output speed = 6931 / 24 = 289 rpm
We accounted for the power losses in the gearhead at the beginning of this exercise, so we need not be concerned about this factor again.
If you want help working out your particular application, please call one of our Applications Engineers, toll-free from the USA or Canada, at 1-800-807-9166.
